Sound Advice Audio Tips

Do Not Use The Built-In Mic - Always Shoot Room Tone - Silence The Wind - Shoot With Headphones
Low Cost Shotgun Microphone Comparison -

Location Sound: The Basics and Beyond

Basic Tips:

Do Not Use The Built-In Mic

Resist the urge to go the easy route and use your onboard mic. Unless you are only interested in “nat sound” (natural background), use an external microphone to capture the audio during taping. In most cases I recommend a mic with a XLR cable, but you could also use a wireless mic.

lavs The onboard mic will pick up noise from the camera’s drive mechanism, and the camera operator. It is most often not in the best position to capture the sound you want and the mic it self will usually be of lesser quality than an external mic.

For interviews documentary interviews I prefer a lav mic I can hide on the clothing of the person being interviewed. For interviews that have the interviewer I like to use a good stick mic.

RE50B About the only time I use my wireless is when the person being taped has to move about a lot or it is a situation where a mic cable in inconvenient or dangerous.

When using wireless mics, select UHF instead of VHF to avoid frequency conflicts. All sorts of fun stuff can go wrong with wireless mics. Your receiver can pick up other sources on your channel, such as radio stations, electrical sources, pizza delivery guys, or other wireless mics from local commercial TV stations. The UHF wireless mics have multiple channels at the higher MHz frequency range, so there is less chance of interference. Always keep fresh batteries on hand. As the batteries grow weak, reception problems occur.

If you must use the built in mic get a good mic muff. You can purchase one for $40, or make your own. Just sew, or have someone that can sew a bag out of the fuzziest material you can find at Jo Anne fabrics. It need to be big enough to slide all the way over your camera mic. Line the bag with a soft porous foam, (you can get that at Jo Ann fabrics too) and sew an elastic band around the opening of the bag, tight enough so that it will not slip off. You can even sew on a small velcro band to wrap around the handle so it can not fall off. The soft fake fur really cuts wind noise.

Total cost about $5. If you get a yard of the fuzzy foam and inside foam you will also have enough to make a nice shotgun muff. OK, it's not quite as good as the pro model and if you got the bucks I recommend you go for it, but it's hard to tell the difference looking at it and it works almost as good. Best of all you save $45 you can put into craft services!

Always Shoot Room Tone

At the beginning and end of your shoot tell everyone to be quiet and always record one minute of quiet ambient sound. This two minutes can be the difference between a good sound track and a bad one for two reasons. First, put your headphones on and listen. You might hear sounds, hums, and other problems your ears may miss. This will give you the chance to solve the problem before you hit the editor.

Silence The Wind

Although it may not sound like much to you, wind can wreak havoc on an unprotected microphone, leaving you with a video in which the only audible sound is rushing air. Some camcorders include a wind cut feature that attempts to filter out this noise. But it can’t work miracles, voices will still be drowned out. Whenever possible, try to shield the microphone from wind by blocking it with your body or by covering it with a wind screen. In a pinch a thin piece of fabric, such as a T-shirt will work.

Shoot With Headphones

If you don’t have a sound person, always shoot with headphones. If your camera has a meter all it can tell you is that there is sound there and the level that sound is coming in. It can not tell you the quality of the sound. A good pair of over the ears, sound isolating studio headphones can save your film, especially in noisy environments. Make sure your headphones are “studio” headphones, without any audio conditioning properties. One time I shot with a guy that was using a pair of fancy headphones that cleaned up the audio, took out the buzz and adjusted the modulation of the audio. It sounded great to him but was not so good on the tape.

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Content Geared for the Media Professional, The DV Show

The following quick and easy Audio Tips from The DV Show. This is just a taste of the ton of information on the Web site.

A Little Info About Dynamic Microphones

Dynamic microphones used to be the basic workhorses of the film and video industries, although in more recent years there has been the changeover to electret condenser and condenser microphones. Nevertheless, dynamic microphones remain as important tools to the production sound mixer.

For starters, dynamic microphones can be counted on to work when other mics won't. Dynamics require neither batteries nor powering of any sort. They are built extremely rugged, and are resistant to harsh climatic conditions including RF and electromagnetic interference.

Dynamic mics offer less sensitivity and reach, than electrets and condensers. This makes the dynamic mic unsuitable for most overhead dialogue applications.

On the other hand, this same lack of broad reach becomes a valuable asset in non- theatrical applications, such s when recording spokesperson commentary in high ambient noise environments. Dynamics are recommended for recording location narration, since they isolate voice extremely well from background sounds.

Dynamic microphones are also very useful for recording loud and sudden sound effects, such as crashes and explosions. Not only are these microphones virtually impervious to damage from high noise levels, but they also tend to compress or
dampen the audio in such a way s to make these sounds easier for the recording electronics to handle.

When recording dynamic scenes, in which the actor is using a handheld microphone as a prop, it is better in most cases not to take the dialogue feed form that same mic. Actors may use their hand mics as objects to accentuate their visual performance, without regard for what their broad gesturing and handling may to do the sound. Instead, mic your pretend "stage performers" either from overhead from a fishpole or boom, or off a concealed lavaliere.

The outward appearance of most handheld mics can be easily and temporarily change merely by the use of colored foam windscreens and the addition of color shrink tubing over the sleeve of the mic.

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Alternative Mic Placement

During a recent roundtable meeting, I needed to mic a stenographer who was recording the proceedings; however, she didn't want to hear the lavaliere, and I had no table mics. I folded an index card several times to create a stiff tab, slid it under the protective glass on the table where the stenographer was sitting, and attached her mic's tie clip to it. To Keep the cords from her mic and the others from being tangled, all were taped to the underside of the table.

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Attaching a Microphone to the Body

The problem: An actor in a live performance has to make several very fast costume changes. There's not enough time to remove and replace his wireless body-pack microphone with each change.

The tip: The body pack can be hidden in the small of the actor's back and held in place by an elastic bandage wrapped around his body. (Commercial holders with a pocket for the transmitter and Velcro-type closures are also available.) The microphone can be run up the back of his neck and hidden in the hair just above the forehead. Most lavaliere microphones are deigned with a built-in roll-off to compensate for the excess low-end frequencies that contact with the chest produces. Contact with the head produces almost the same low-end excess, and the microphone works very well.

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How Do I Maintain My Microphones?

Microphones are precision, delicate instruments that require periodic maintenance much in the same way as Nagra recorders or wireless microphones. Various microphones have unique functional characteristics that require attention. Failure to maintain your microphones properly can result in poor frequency response, improper level, distortion, poor noise readings and a poor pick-up pattern.

All microphone should be checked regularly for reliable ground connections. Similarly, connector and contact surfaces should be inspected and cleaned. Routine inspection, cleaning and calibration are your best assurance to continued, trouble-free operation of your microphones.

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How Do I Select and Use My Lavalieres?

A lavaliere is generally defined as being any small microphone designed to be worn on a performer's body. In the majority of cases. Lavalieres are omni directional in pattern they pick up sound in all directions). Modern lavalieres tend to be electret condenser in design, which allows them to be highly sensitive to a full frequency of sounds, and also facilitates miniaturization.

Up until recently, lavalieres could best be described as "proximity" oriented – that is, they work best when the sound source is close to them. Thus, when these lavalieres are employed in the normal manner, they tend to favor the close, or dominate source, and downplay background ambiance.

This "close -up" sound, emphasizing the voice and holding back the ambiance, has long been associated with lavalieres, and is both their strength and their weakness.

Some of the problems that may be encountered with lavalieres include the difficulties of hiding them under clothing, clothing noise, wind noise and the loss of audio perspective in relation to the camera (the dialogue always sounds "close-up" regardless of framing).

Lavalieres may be used as either "hardwire" or "wireless". Hardwire means that the actors are physically connected via audio cable to the mixing panel or recorder. Wireless refers to plugging the lavaliere into a small transmitter and broadcasting the signal back to the receiver, which, in turn, feeds an audio signal into the mixing panel or recorder.

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How do I make lavaliere microphones "noise free"?

One of the ever-present difficulties in hiding lavalieres under wardrobe is clothing noise. In actuality, there are two different causes of "clothing noise": Contact noise and Acoustic noise.

Contact noise is the result of garments rubbing against either the mic capsule itself or the leading few inches of cable (equally sensitive to friction). Contact noise can usually be controlled – if not completely eliminated – by careful positioning and taping down of the mic and cable.

Begin by securing the clothing on both sides of the mic capsule. This can be done by sandwiching the mic between two sticky triangles of cloth, camera, or gaffers tape. Form these triangle by folding a few inches of 1" wide tape corner over corners, similar to folding a flag.

By immobilizing the mic between both layer of clothing, you have eliminated the possibility of either layer of clothing rubbing against or flapping onto the microphone.

If the lavaliere must be positioned between skin and clothing, or attached directly to the skin, then a professional medical/surgical tape should be used against the skin.

Once the mic capsule has been secured, the next step is to for a strain relief for the thin cable. Make a small loop just under the mic capsule. In the case of very sensitive mics, such as the Sony ECM-77, the Sennheiser MKE2, and the SankenCOS-11, make the loop goes around twice. Tie a small thread or use a thin strip of camera tape (sticky
side out) to preserve the loop. Tie the loop loose enough so that it can "Breathe" (change diameter to absorb tugs).

Apply a few inches of tape along the cable below the loop. Any tension on the cable
will be absorbed by the garment, rather than by the microphone (which is somewhat
isolated by the floating loop).

The remainder of the mic cable can be run under clothing and can terminate either at the waist or the ankle. The end of the mic connector should be secured so that it does not dangle freely.

During a take it becomes a simple matter to plug in an extension XLR cable. Afterwards , the talent can easily be disconnected so that he or she is free to roam around.

When using an external "tie clip", it is still important to think in terms of creating a strain relief. Loop the thin cable up and under the tie clip, forming a semi-circle , and passing through the wide hinge of the clip. Continue the loop behind the garment, and bring the cable around downward, thus completing the circle. As the cable loops downward, it should be inserted between the jaws of the tie clip and the back of the garment Hide the balance of the cable behind the wardrobe.

Not only is this arrangement more pleasing to the eye than a dangling cable, but the floating loop of the cable isolated the mic while the grip of the tie clip serves as a strain relief.

Acoustic clothing noise is the sound generated by the clothing itself as garments or layers rub against each other when the actor moves. Noise is much more prevalent from synthetic fabrics than from natural cottons or wools. There is no simple remedy, only prevention, so it is wise to consult early with the wardrobe department.

However, there are a couple of tricks that may help. Anti-static sprays, such as Static Guard, will reduce static electric discharge, clinging, and reduce friction. Dry silicon spray lubricants sometimes help, but be careful of staining. Stiff or starched clothing can be softened with water or alcohol (make sure the colors don't bleed). Saddle soap, silicon, or light oil can take the bite out of hard leather.

Another noise problem common to lavalieres is that of wind noise. Manufacturers usually supply small foam or metal mesh windscreens with their lavalieres, but these are usually more effective against breath pops than against outdoor gusts of wind.

Lavalieres used under clothing have the advantage of being partially shielded from the wind, but may still require added protection.

Clothing rubbing against windscreens can be extremely noisy, so great care must be taken when using hidden lavalieres out of doors. Surrounding the windscreen with sticky tape and securing it to both layers of clothing, as you would a bare mic, will reduce the friction noise. However, the tape may destroy a foam windscreen with it is removed! Inexpensive, expendable windscreens can be make by wrapping the mic in acoustafoam; or by pulling the foam booties off of video cleaning swabs.

Cheesecloth over a mic works very will against wind. Another Hollywood variation is to snip the fingertips off children's woolen gloves, and pull the wool tips over lavaliere wrapped in foam or cheesecloth.

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Selecting a Microphone

A majority of the production dialogue recorded in major "Hollywood" theatrical productions and television series is miced from overhead, utilizing either a fishpole or studio boom.

Overhead micing provides a natural sound. Normal sound effects and some background ambiance are also picked up, and at lower relative level than the dialogue, thus rounding out the total track.

Perspective matches camera angles, since the boom mic is able to get in closer on tight shots, and is further away on longer angles.

In contrast, the use of lavalieres and radio mics produces dialogue that is often sterile in texture – lacking natural sound effects and ambiance. Perspective is always forced and "close-up" – regardless of camera angle. ?Audio is often subject to abrupt changes in presence, such as those caused by talent turning their head off-axis to the lav, or leaning over a hard surface (such as a desk or podium). Last, though surely no least, lavalieres are prone to distracting clothing noise and other interference.

A good technique is to follow the same approach towards dialogue recording as practiced by feature mixers: Use lavalieres with discretion and take advantage of overhead micing as much as possible.

Microphone selection plays an important role in overhead micing technique, along with choosing a skilled and experienced boom operator. Just like camera lens focal length, there is no one choice of specific microphone that will be right for all situations A professional package should include an assortment.

It cannot be over stressed that, for best results, only the highest quality condenser microphones – such as those discussed in this section – should be used in capturing dialogue. Although most electret condensers are very good microphones for their price and features, they simply do not perform as well as condensers for professional or theatrical dialogue applications. Top of the line condensers offer superior reach and sensitivity over the electrets, and that can spell out the difference on those more demanding shots between getting 'rich' dialogue versus 'weak' or 'thin' audio.

Allow the extra room in the budget to purchase or rent a package of true condenser microphones along with the proper accessories (power supplies, shock mounts, blimp windscreens) to make them work.

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About Foam Windscreens

Foam windscreens and pop filters are intended to provide protection against low velocity moving air, such as would be encountered from performers exhaling onto the mic. Foam windscreens also protect sensitive condenser mics against the motion of room air caused by normal ventilation ducts as well as from the physical act of moving the microphone while mounted on a boompole.

Although some handheld performance mics come equipped with built-in wind or pop protection, condenser microphones should never be used without a protective windscreen!

Do not expect a foam windscreen to function adequately out of doors even on calm days, there will occur occasional soft gusts of wind that will penetrate the foam and cause audio overload. Employ a blimp type windscreen, such as a Rycote. However, it is beneficial to use a slim foam windscreen inside of the blimp, providing that at least ½" or more of airspace remains between the foam and the inner walls.

Foam windscreens also provide a quick means of altering the color and appearance of microphones. Some sound mixers place a white band of tape around the front tip of the windscreen. This facilitates the camera operator identifying the microphone through even a dim viewfinder, where otherwise it might go unnoticed until brightly projected during dailies.

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Free Production Music!

If you desire an easily recognizable song for your video, but you don't have a budget for using copyrighted music, you might want to consider music from the public domain.

For more information about the public domain, a list of songs in the public domain, and links to other related sights, go to: http://www.bright.net on the web. The site contains a wealth of information. If you still have questions after reading all the information available, you can always leave questions via e-mail.

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Lavaliere Logic

Does the cable of the lavaliere microphone that you have clipped to your subject's tie keep popping out from behind the tie? Try running the cable through the loop formed by the label that is sewn onto the back of the tie.

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Audio Backup

If you work as a one-person video crew and need to record audio delivered at widely varying volumes, Joe Salerno of Bellaire, Texas, suggests running a normal audio level on one track and setting a second track considerably lower. (Most professional video recorders have more than one audio track.) If the first channel peaks too high, you'll still have a good chance of getting a usable signal from the backup channel.

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Sticky Situation

Wig tape is a very thin, double-sided adhesive tape normally used –you guessed it – to keep wigs on people's heads. But it is also handy for videographer's. Mike Saxton of Lutz, Florida, uses it to secure small lavaliere mics in actors' clothing. He says the tape also keeps clothing layers from rubbing against each other an making airborne noise that could be picked up by the mic.

I've also found that a piece of wig tape will keep an actor's wayward necktie centered an looking neat.

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The Handheld Mic and tips how to use it

The most common kind of microphone is the handheld type. This style is the most flexible, because it can be held by the user, mounted on a floor or desk stand, or attached to a flexible "gooseneck" on a lectern. A good quality handheld mic should have an internal shock mount which will minimize handling noise (thumping sounds transmitted through the handle and picked up by the microphone cartridge), and it should be ruggedly constructed to withstand physical abuse. If you can have only one microphone in your kit of audio gear, it should be a handheld mic. Models at the upper end of the price scale will usually offer clearer, wider-range sound, better shock mounting, and more durable construction.

Tips:

Whether held in the hand or mounted on a stand, the microphone should be positioned about 6-12 inches from the talker's mouth, pointing up at about a 45-degree angle. With some types of microphones, holding the microphone very close (3-6 inches) will cause additional emphasis of the lower frequencies (known as proximity effect), resulting in a "warmer," bass-heavy sound.

From:
Shure – Guide to Better Audio
Written by Christopher Lyons
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The Lavalier Mic and tips how to use it

Another popular mic for video use is the lavaliere type. Historically, the word "lavaliere" refers to microphones which are hung on a cord around the wearer's neck, but in recent years the term has grown to include models which mount with a tie-clip, tie-tack, stick-pin, or other similar arrangement.

Lavalier microphones leave the talker's hands free to gesture, hold notes, or demonstrate a product. In addition, they are usually very small and therefore tend to "disappear" on camera. Also, using a lavaliere will keep the distance from the
microphone to the talker's mouth fairly constant, reducing the need for frequent mixer adjustment once levels have been set.

A disadvantage of lavaliere mics is the fact that they tend to be "single-purpose" microphones – they rarely sound good if handheld or used away from the body. While the lavaliere mic's small size makes it easy to conceal behind lamps or other objects, an equalizer is usually necessary to make the mic sound "natural" when it is not attached to the person talking.

Tips:

For best results, lavaliere mics should be clipped to the tie or lapel at the breast pocket level. Try to avoid placing the mic behind the tie or any material having more than one layer—this reduces pickup of high frequencies, which results in a flat, "muddy" sound. In addition, noise from the movement of clothing against the mic or its cable can be severe; experiment before risking the quality of your audio track. On women, the mic may often be attached to a stickpin or small chain with good results.

From:
Shure – Guide to Better Audio
Written by Christopher Lyons

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The Surface Mount Mic and tips how to use it

These are microphones that are designed to work on a flat surface. They are usually physically contoured to look less intrusive on a conference table or desktop. The microphone element is located very close to (but not touching) the surface, which allows it to take advantage of the reflected sound as well as the direct sound. This effectively doubles the sensitivity of the microphone over a free-standing handheld type at the same distance.

Tips: Surface mount microphones work best when positioned on a smooth, flat surface, such as a table or desk. If table vibrations are a problem, try putting a very thin piece of soft foam rubber underneath the mic. In some situations surface mount mics can even work well when mounted on a wall. Keep in mind that the sound quality of this type of microphone is affected by the size of the surface it is placed on. For best results, use a surface at least 3 feet square; using a smaller surface will tend to reduce pickup of low frequencies.

From:
Shure – Guide to Better Audio
Written by Christopher Lyons

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The Shotgun Mic and tips how to use it

The shotgun microphone is so named because the long, slotted tube in front of the microphone cartridge makes it resemble a shotgun. This "interference tube" helps to reject sounds coming from more than about 30 degrees off to the sides, while still picking up sounds from the front. This extremely directional pickup pattern (called a Line/gradient pattern) makes shotgun mics popular for TV news and on movie sets.

Shotgun microphones are not "telephoto Lenses" for sound. They do not allow you to "zoom in" on a conversation from 100 feet away. Here's a much more accurate analogy: imagine looking through a long tube at a person standing 20 feet away. The person's image does not appear to be any larger or closer, but it is somewhat easier to see, because the eye is not distracted by things happening off to either side. This is exactly what shotgun mics do best – screen out sounds coming from the sides. You can use this advantage in two ways: position the mic at the same distance as you would a handheld mic, and enjoy clear pickup of the source with very little background noise; or position the mic farther away, and get the same amount of background noise as the handheld microphone would have given you.

Tip:

Shotgun mics can be positioned either slightly above, below, or to the side of the sound source, so that the mic does not appear in the camera frame. Try to avoid moving the mic rapidly, since shotguns are sensitive to wind noise, and use a foam windscreen if possible. Larger "zeppelin" or "blimp" type windscreens are usually necessary outdoors. Also, it's a good idea to use a rubber-isolated shock mount to minimize handling noise.

From:
Shure – Guide to Better Audio
Written by Christopher Lyons
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Leaving mics open

When we record panels, we don't close mics because we don't want to miss the first word of a speaker who suddenly decides to chime in. But, as at least one former President can attest, there is nothing worse than having an under-the-breath comment heard by the world. To help prevent embarrassment, Richard Johns, a freelance producer in Chicago, puts a note in front of each participant that says, "Mics are live at all times."

I find that with wireless mics, it's best to have the audio person manually turn off the mic at the transmitter, any time a break is called. I make it their job, the talent will not remember. Not only will it save batteries, but before I started doing this I had recorded sound and conversations ranging from the talent relieving themselves in the bathroom to conversations about everything from the breast size of the cute new PA to good and bad comments about the production, crew, director and even private information about the business.

One time on a film shoot, recording audio in a Nagra, I had no idea that we had recorded a bathroom clip until I picked up the full coat from the lab and the tech said, "Funny clip at the end." As the production was a fairly dry presentation by a medical person I had no idea what he was talking about until I got it on the rewinds. It did however; make a great addition to the wrap party blooper reel. Yea funny, but I had to pay for the 10 min full coat.

I also find that with non professional talent, such as the owner of the car dealership who we had in a cherry picker in the middle of his 4 acre car lot always forget they have a wireless on. I always make sure they know we can always hear them, and if they do say anything that their secrets are safe, helps build trust and may get you another job.

10 Tips for Using Wireless Microphones

1. Every wireless microphone system must operate on a specific frequency. Because of government-mandated frequency sharing, there is always at least a small chance that someone else in the area is using the same frequency as your wireless system.

2. There must be one transmitter and one receiver to make a complete wireless system, and they both must be on the same frequency.

3. If any two transmitters are operating on the same frequency, severe interference will result and the wireless system will be unusable. Two transmitters cannot be used with one receiver at the same time. You can use a handheld and a body-pack
transmitter with one receiver — one at a time.

4. If the frequencies of any two wireless systems are too close together, interference is likely, and one or both systems will
probably be unusable.

5. The practical maximum operating range of a wireless system will vary from as little as 100 feet in heavily crowded indoor situations to approximately 1,000 feet outdoors.

6. Diversity wireless systems will almost always have better operating range than similar non-diversity systems.

7. Weak or worn-out transmitter batteries are a common cause of wireless problems, including complete failure, poor range, distorted audio, and interference.

8. High-quality alkaline batteries will provide several hours (8 to 16) of transmitter operation. Most other types of batteries will have much shorter life, and some may cause other problems.

9. Because it's easier to accidentally walk near speakers, feedback problems are slightly more common with wireless
microphones than with wired microphones.

10. The power output of wireless microphone transmitters is very low — only about 10 percent of that of a typical cellular
phone — and they're completely safe to use. However, any source of RF energy may interfere with the normal functioning of implanted cardiac pacemakers or AICD devices. A body-pack transmitter should not be worn where it is immediately adjacent to such a medical device. If you're attaching a transmitter to an older person, it doesn't hurt to ask. Note also that any medical- device disruption will cease when the RF transmitting source is turned off.

Here is one of the best articles on sound I have ever seen on sound. It's written by Dan Brockett, a film and video director who co-owns a film and video production company, Big Little Films™ , Inc. and a sound design company, Big Little Sounds™ . Dan is also a guide on the premier Final Cut Pro information source, 2-Pop.com and he serves as Vice President of the Los Angeles Final Cut Pro Users Group. I thank him for letting me reprint this here.

White Paper - Location Sound

October 21, 2002
Location Sound: The Basics and Beyond
By Dan Brockett

The First Link - Sound Itself
The Second Link - Microphones
The Third Link - Cables and Adaptors
The Fourth Link - Mixing and Routing Devices
The Fifth Link - The Recording Device
The Sixth Link - The Monitoring Circuit of the Recording Device
Let's Discuss Sound For Picture

The concepts of location sound recording that we will discuss in this article are basically the same, whether you are shooting your tenth independent film or your first project with your first new camcorder. Audio seems to be one of the most challenging areas for beginners and even experienced filmmakers alike. Video professionals typically find sound one of the most challenging aspects of production. Ten years ago, producing professional quality film or video was a much more cut and dried process. If you wanted decent sound for your picture, you either had the knowledge and equipment to record it yourself or you hired a location sound team. This is still true today but the differences are that there are a lot more people producing video today who may not have experience and skill in recording location sound than there were ten years ago.

DV users with many different experience levels and widely diverse backgrounds are producing their own projects. The fact is that almost all of the tools needed to produce broadcast quality video and DV "films" have become relatively inexpensive and widely accessible. Final Cut Pro and AVID Express DV both have about 90% of the capability of a $100,000.00 AVID Media Composer system at a minute fraction of the cost. Camcorders like the Sony PD-150, Panasonic AG-DVX100 and the Canon XL-1S are capable of producing extremely high quality images.

PD-150                                    AG-DVX100B                                   XL-1
The immense popularity of digital video means that a large majority of users today have access to the most advanced communications medium society has ever seen. We have small, relatively affordable, high quality camcorders that can make amazing images with far less light than ever before. We have very sophisticated and capable video editing tools available on both platforms. Assuming we all want to produce work of quality, what's missing from this equation? You guessed it, the sound. The fact is that most independent, low/no budget projects that are produced today seem doomed to suffer with sound that ranges from merely average to barely usable. Whether you are producing video for your friends and family, to view, weddings and events, corporate audiences, or for broadcast or theatrical release, no matter which category your audience falls into, they expect "transparent" sound from your project's soundtrack. Let's define what "transparent" sound is.

Audio conveys almost all of the emotional impact in the visual medium. It's a fact. If you watch your favorite scene from any film or TV show with the sound off, you soon discover that moving images on their own are typically not very emotionally involving. Don't forget, silent films could be scary, sad, happy, dramatic or interesting BECAUSE they were conceived without sound. To be totally fair, most silent films were viewed with either live or pre-recorded music. Obviously, most of us want to produce projects that will emotionally involve our audience. For most of us, video has become the single most common collective vocabulary in our lives. It is also a given that video is great for communicating almost any message to almost any audience, if done well.

The Experience Economy
What may be less obvious to you if you are new to film and video making, is that audiences of all kinds now expect to be entertained while you are conveying your message. If you are in the entertainment end of this business, this is understood, but for those of you who want to create home video, weddings, events or video for business, your content must also be entertaining and compelling. Emotional involvement from your audience is what defines good entertainment. You may not feel that the Shop Safety training video you are producing can or should be very entertaining, but if the production values and concept are not very high quality, your training video will bore your audience. If it's done well, even a Shop Safety training video can be entertaining. Your sound is largely what will determine if your project is entertaining to your audience. Unless you want to conceive your project as a "silent film", you have to be concerned ('obsessed' might be a better term) with your project's sound.
One of the toughest concepts for many newer DV users to grasp is that the better job you do with your project's sound, the less it will be noticed. I feel that this concept is one of the reasons why most projects don't end up with very high quality soundtracks. We are very used to spending time, effort and money on a better camera, lens, bigger and better lighting, crew and visual effects and seeing an immediate "payoff" when our images are viewed. It's instantly recognizable if a scene is lit effectively or if a visual effect is done well. We feel "justified" in shooting on a higher quality, more expensive format or with a bigger crew because the end result is usually easily identifiable on-screen. Most of us can immediately recognize if a project was shot on 35mm film versus DV or if a project's motion graphics or visual effects were well executed. If we notice a sound mix though, it is usually because the sound was done incompetently. This is the central concept of "transparent" sound. If your location sound is recorded correctly, the easier it will be to work with the basic audio during the post-production process. The better job you do with the sound during video and audio editing, the less the audience will notice it. The only sound that is noticed in a visual medium is usually poorly executed. Great sound works on a subconscious level with the viewer by drawing them into what they are viewing. Great sound supports and enhances the stories you are trying to tell. Now that we have a basic understanding of the goal for your project's soundtrack, let's review what we have covered before we dive headlong into equipment and technique.

Four points to remember about sound for picture

1 . The principles of location sound are the same for almost everyone shooting anything.
2 . No matter who the audience is, at the very least, they expect "transparent" sound
3 . Sound conveys emotion - picture conveys information
4 . The better your soundtrack, the less it is consciously noticed

It's All Just a Chain
The easiest way to discuss location sound is to think of the entire audio path as a chain. In the case of location sound, the "links" are:

The sound itself

The microphone(s) that capture the sound

The cables and connectors that carry the signal from the microphone to the mixing or routing device and from the mixing or routing device to the recording device

The mixing or routing device that carries the signal from the microphone to the recording device

The recording device itself (typically a camcorder but could also be a VTR, Hard Disc Recorder, MD or DAT recorder)

The monitoring circuit of the recording device

Just as in an actual chain, the audio path is only as strong as it's weakest link. This means that a high-quality, accurate recording device paired with a low-quality microphone will not be able to record anything better than what the microphone is capable of picking up. It means that a great microphone and audio mixer paired with a substandard recording device will only be able to record to the limitations of the device's recording circuit. While it is not practical for most DV users to acquire and use the best quality location sound equipment made, it should be the goal of every user to acquire and use components that match each other in features and quality. Don't buy the best mixer you can afford and then skimp on cables and connectors. Don't buy the best shotgun microphone on the market and then skip using a mixer because you spent your entire budget on the microphone. You get the idea. We'll discuss rental versus buying later in the article but needless to say, buying one or two high quality pieces and then renting the rest as needed is a viable option.
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The First Link - Sound Itself
It is not possible to try to give even the basic principles of sound within the confines of this article but let's talk about some basic concepts of what sound is and why sound behaves the way it does. At the most basic level, sound can be described as waves moving through air. The farther apart these sound waves are, the lower the frequencies. The closer the sound waves are to each other, the higher the frequency. In the most basic terms, sound is "messy". It bounces, reflects and behaves in ways that seem mysterious to most us. It cannot be seen and all of us perceive sound differently. There is an entire branch of study and academia called "Psychoacoustics" which is the study of how humans perceive, process and react to sound. Sound is definitely a case of "perception being reality."
Sound waves cannot be seen in most cases, but effects of sound waves are evident if you know where to look. Although not actual sound waves, the ripples produced when a rock is dropped into water produce a nice visual approximation of what sound waves would look like if they were visible to us.

If you place something lightweight, like a piece of paper in front of a typical transducer, like a two way audio speaker (a two way speaker has only a woofer for generating low frequency sounds and a tweeter for reproducing high frequency sounds), you will probably see the paper physically move if placed in front of the woofer while the speaker outputs sound at a decent volume level. However, if you place the paper in front of the tweeter only, you will probably see either very little or no perceptible movement. This is because the high frequencies generated by the tweeter are much closer together and occur at much more rapid intervals. Too rapidly to perceptibly affect the mass of even something as lightweight as the paper unless the amplitude of the sound is increased to very high levels. Understanding this concept is central to understanding how sound waves behave.
Low frequency sound waves (bass sounds), because of their larger physical size, tend to interact with their surrounding environment in much more perceptible ways than high frequency sound waves (treble sounds) seem to. All sound waves reflect off of physical objects but because of their larger size, lower frequency sound waves reflect and penetrate objects more than higher frequency sound waves do. If you can grasp this idea, you will begin to have an understanding of how to help modify or eliminate undesirable sounds when you shoot in any location. If your apartment or hotel neighbor is cranking his stereo loudly in the room next door, which frequencies are you hearing through the walls? Obviously, the higher frequency sound waves do not have the strength to penetrate the wall. Few of the mid range sound waves will penetrate the wall. What you will mostly hear through the wall are mostly the low frequency sound waves.
"Preventative Maintenance"
What we deem as "poor quality" sound can be manipulated and adjusted all along the audio path in different ways but in most cases, it is best to adjust and compensate for the "poor quality" sound before it ever enters the audio path. Although this sounds like a simple concept, the reality is that it is much more common for people to hear what will obviously be a sound problem (undesired noise, excess ambient sound, technical sound problems like hum or buzzes, etc.) and to just go ahead and shoot, rather than determining the source of the undesirable sound and fixing it. In some cases, it's just sheer laziness. It's a pain to hunt down an audio problem and fix it before rolling. When shooting with a crew or in limited access or time-constrained situations, this is somewhat understandable but you should know that despite all of the great post production technology available, basically, what you hear is what you get. If you record poor quality sound, it will always be poor quality, no matter what types of filters, plug-ins or processes you run it through. It pays to spend a few extra minutes to fix audio problems before you shoot.
Below are the top five most common causes of location sound audio problems that most people will run into when shooting. You will notice that several of the categories kind of overlap each other in definition. Such is the nature of sound. Included are some suggestions for mitigating, reducing or correcting the problems before you shoot:
Excessive ambient noise.
Too much background noise. Too much traffic noise. People speaking or making noise in the background or in the room next door. Dogs barking in the background. Sirens in the background. Aircraft flying overhead.

Possible solutions.
Excessive ambient noise is one of the toughest issues to work with because often, the excess ambient sound is beyond your control. The solution can be as simple as repositioning your microphones or waiting until the excess ambient sound dies down or goes away to more elaborate steps such as soundproofing a location. Obviously shutting doors and windows can help when shooting interiors. Exteriors are tougher. In the past, I have resorted to "paying off" a tree trimming crew using chainsaws to take a "long" lunch or knock off early. In that instance, I paid to record better sound. You must be creative in your thinking and do whatever is within your power to limit ambient noise when shooting. Microphone choice is extremely important here as well. More on that later.
Building noise.
HVAC (heating, vacuum and air conditioning) noise. Building creaking or settling. Elevator noise. Clocks ticking. Doors opening and slamming. Noisy lighting fixtures.
Possible solutions.
HVAC can be tricky to deal with. If you have done your homework, when scouting a location to shoot, you should always determine where the access is for the HVAC controls for the areas you'll be shooting in. Unfortunately, many newer buildings have "zone climate" HVAC systems where there is one control for an entire area or even a floor of a building. So if you turn off the HVAC system to shoot, you may be fine in the room or area you are shooting in but people in other areas may be dying of heat or freezing to death. If you cannot obtain access to HVAC controls or personnel to control the HVAC system, you can also reduce the "whoosh" of vents by bringing sound blankets or foam and temporarily blocking a single HVAC vent. Blocking one or two vents at a time is rarely an issue for an HVAC system. We typically chill down a room to just above "teeth chattering" point before beginning an interview, and then turn the air conditioning off to get rid of the low HVAC system rumble. Video lights and shutting all of the doors and windows will typically heat the room right back up fairly rapidly. The added benefit is that shooting in a cool room will also keep everyone alert and awake.
As far as creaking and settling of a building, there is not much you can do other than listen carefully as you shoot and shoot alternate takes. You can do the same with elevator noise.

Carefully worded signs (quiet please - video crew shooting) on the doors can be of help or stationing a crew person at the offending adjacent door to act as a "doorman", carefully and quietly opening and closing the door can help.
Typically, the noisiest lighting fixtures are fluorescent light banks. As the ballasts age, they typically become more and more noisy. The obvious solution, if you are lighting the scene is to just turn the offending lights off. Many newer buildings have motion sensor light switches though and taping black gaffer tape over the light's sensor can take forever for the light to turn off. Always bring a stepladder and if necessary, you can just remove one tube and the bank or at least part of it will go out. As in HVAC systems, in many newer buildings, you may encounter a single set of lighting control switches that control all lighting in a large area or even an entire floor of a building. Learn how to quickly and safely remove fluorescent tubes from fixtures. Many times, it's the quickest, easiest option.
Machinery.
Fan/hard drive noise from computers. Carrier tone from computer and video monitors. Refrigerator/freezer noise.
Possible solutions.
Beware of computers. The obvious solution is to turn the CPU off, if possible. When it is not possible, another solution is to isolate the CPU within it's own nest of sound blankets (a.k.a. known as furniture pads). Make sure to leave room around the CPU so that its fan system still has access to adequate airflow. We typically use two or three C stands with sound blankets grip clipped to the extension arms.
Beware of the "carrier" tone, the high-pitched whine that all CRT's emit when turned on. It varies from monitor to monitor but if it is loud enough and at the correct frequency, it can be recorded and can be difficult to EQ out.
Refrigerators are commonly encountered in many locations. They can be tricky to hear because as the unit's compressor cycles on and off, you may hear the sound and not know what it is, then as you look around, the unit's compressor, may cycle off. It's not usually a good idea to unplug a refrigerator, unless you obtain permission from its owner. We usually will close doors and or isolate the refrigerator using sound blankets ands C stands. If you do unplug a refrigerator, it's a good idea to place your car keys inside so that it's impossible to leave without remembering to plug it back in.
Talent & Crew Noise.

Talent/crew fidgeting or moving. Creaking floorboards. Squeaks from shoes. Clothing rustling/rubbing. Microphone (usually lavalieres) rubbing on skin or clothing. Stomach rumbles. Heavy breathing. Mouth sounds (clicking or sticky mouth)
Possible solutions.
Make sure that your talent and crew holds still and breathes softly during takes. This can be tough during long takes and having an assistant director on the crew can really help enforce this. Most crews are fairly savvy about this but at times, observers, crowds and even clients can cause a lot of off-set noise that can be a nightmare to deal with in post so deal with it before you roll.
Creaking floorboards can be challenging. If the talent's feet are not seen in shot, carpet or "dance floor", a portable type of flooring system can be laid down for crew and talent. Squeaking shoes can be remedied by removing them if they are out of frame, using baby powder or even a lubricant like WD-40 or silicon spray although these can stain shoes. Shoes squeaking on floors can be tougher to deal with than squeaking shoes themselves. A bit of sawdust or even once again, removing the offending shoes can help although it's a little strange to see a crew and talent doing scenes in their socks. I've done it though and it can lighten the mood although it's probably not the safest way to shoot a scene. Grip equipment can be heavy and dangerous.
Clothing rustling and rubbing are also one of the most common challenges. A lot of this can be mitigated I the correct types of clothing are used. The "sound mixer's nightmares" are silk, rayon and corduroy. Trying to mic a female talent wearing a silk blouse will test the patience of even the most experienced sound person. The other half of this equation is just learning basic mic techniques. There are many different "tricks of the trade" when it comes to learning how to put a mic on talent and no having it rub and pickup extraneous noise. More on this later in the article.
Mouth sounds are more of a problem when doing voiceover's but can be a problem on set as well. Water helps. Always make sure that all talent has easy access to water as they shoot scenes. Depending on the person, water alone may not remedy "sticky" mouth. One of the best solutions for sticky mouth is, believe it or not, green (Granny Smith) apple slices. The acidity and sugar content balance of this particular variety tends to stabilize the saliva and make it so that mouth clicking and smacking is reduced or eliminated. I work with several VO talents who have been using this method for years and swear by it. It works.
Sound/Video Gear.
Ground loop hum/buzz. Loose or defective connector/cable crackle. "Hits" when using wireless microphones. Camera motor noise. Video monitor carrier tone. Substandard microphone mounting/handling. Wind buffeting.
Possible solutions.
Ground loops are perhaps the most common equipment problem as far as location sound. Many of these issues occur because professional balanced gear is interfaced with consumer-unbalanced gear. Using the proper gear will remedy most ground loop issues. In general, using Radio Shack style adaptor plugs and jury-rigged cable adaptor plug combinations are a recipe for hum or buzzes. Always use professional balanced XLR gear as much as possible. More on this in the equipment segments later in this article.
Cables are easy. If you experience bad or loose connectors or cables, mark them with a piece of tape and either repair them, if you are talented with soldering cables and connections or replace them.
Wireless microphones are susceptible to noise. From the very cheapest consumer models to the top of the line pro units that sell for $4,000.00 to $7,000.00 per system, all wireless systems can experience interference. The more expensive the unit, generally, the less susceptible to extraneous noise, plus the higher the sound quality. Only use wireless when you must. Do not shoot sit down interviews with wireless systems because you are too lazy to run a cable on the floor. With the advent of new high-end digital wireless systems, noise is becoming less of an issue than with analog units. Eventually, all wireless systems will probably be digital and we'll be free to use them anywhere we want for the most part.
Camera motor noise is almost always a by-product of using an on-camera microphone. It's really simple to fix this one. Don't use an on camera microphone unless it's ONLY for ambient sound. NEVER try to record talent with an on-camera microphone; it's the perfect recipe for bad sound. The on-camera microphone will pickup very little of the talent's sound and lots of background ambient sound. If you must use an on-camera mic, buy a high quality microphone mounting system and you should eliminate most camera motor noise.
Video monitor carrier tone can be picked up if it occurs near the microphone. Move the monitor or turn it off.
Microphone mounting or handling. Booming technique is an acquired skill. You don't just buy a boom and start using it successfully. It takes training and practice to become a good boom operator. As far as the microphone mount itself, beware of using cheap, rubber band mounted microphone mounts. These are simple metallic ring systems where the microphone is suspended in the ring using rubber bands. These types of mounts are okay for stationary use but for hand booming, many of these mounts are inadequate. A high quality microphone mount can take a lot of handling and not transmit it to the mic element.
Wind buffeting can be very tough to prevent when shooting outdoors. The easiest solution is to use a quality microphone mounting system, zeppelin and windsock. More details on these later.
Four points to remember about the "first link" in the audio chain and
"preventative maintenance"

1 . The audio chain is only as strong as it's weakest link

2 . Low frequency sounds, because of their larger physical size, tend to interact with their surrounding environment in more perceptible ways than high frequency sounds seem to.

3 . Fix as many sound "issues" as possible BEFORE shooting
4 . If you record poor quality sound, it will ALWAYS be poor quality

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The Second Link - Microphones
In our discussion of sound, we talked about some basic concepts of what sound is (and sound waves) and a few basic characteristics of how sound waves behave, (they are 'messy', the differences between how low frequencies behave versus high frequencies). Microphones are, at their most basic, transducers that that convert input energy of one form into output energy of another. Microphones are broken down into two separate categories; Dynamic and Condenser. Most Dynamic microphones are the moving coil type. Moving coil microphones use a magnet, a coil wrapped with wire and a diaphragm that sits over the top of both. Sound pressure hits the diaphragm and moves the coil across the magnet. This creates the voltage that travels out and along the mic cable on the way to the mic preamp. With Condenser microphones, Phantom power from a battery inside the mic, or from mic cable from the mixer or recording device, or a separate power device is used to power the microphone. The phantom power charges a capacitor. This capacitor holds a charge in the microphone's fixed back plate. In front of the back plate, a thin diaphragm is located. When the diaphragm moves in relation to the fixed back plate, a charge is created in proportion to how much movement the diaphragm makes. Unlike the signal created by the dynamic mic, a condenser's signal is very weak and must be amplified before it gets to the mixer or recording device. Condenser microphones contain a small amplifier that boosts the signal before it leaves the mic
It may help you to visualize what is happening inside the microphone by imagining how a speaker works. A microphone works the exact same way that a speaker works, only in reverse. When a speaker receives an electronic signal, it moves its transducer in response to the amplitude and modulation of the signal that it is receiving from the amplifier. When a microphone element is moved by the sound waves, it generates the same kind of electronic signal. The main difference is that the microphone SENDS the signal out to a monitoring or recording device, whereas the speaker RECEIVES a signal from an amplifier.
Balanced versus Unbalanced
What do the terms "balanced" and "unbalanced" mean? Without going into a lengthy explanation of impedance and electronic signal measurement, balanced line-level equipment operates at a much higher nominal level (+4dB). Besides a positive and negative lead, it also uses a third grounded lead (or 'earthed' lead as our UK friends say), and usually, but not always, uses three-pin, locking XLR connections. Unbalanced line-level equipment operates at a lower nominal level (-10dB) and has only a positive and a ground lead (no negative) and mostly uses non-locking RCA connections, although unbalanced connections are also occasionally 3.5 mm mini plugs, 1/4" (tip-ring TR) plugs and spade lugs.

Balanced XLR

Unbalanced 3.5 mm mini-plug

Balanced 1/4" or unbalanced stereo1/4"

Unbalanced RCA

The basic difference between balanced and unbalanced microphones, connectors and devices are that professional audio and video equipment predominantly uses balanced cables and connections. Consumer equipment almost exclusively uses unbalanced connections. Why should you care? Improperly wiring to unbalanced equipment is probably the single most common cause for ground loop problems, interference, hum and buzz. I suggest you use balanced professional equipment whenever and wherever in your audio chain that you can. Balanced equipment is much less susceptible to interference, but can still pick up ground loop hums especially if there are grounding problems in the electrical system the equipment is plugged into. However, you will still have fewer problems with interference in your audio if you use balanced equipment. Almost all professional microphones and mixers operate using balanced connections for this reason.
Microphone types and purposes:

Although there are probably more than a dozen different microphone types, it probably makes sense to discuss the most common types of microphones used in film and video production. Just as a golfer uses a specific type of club to "tame" specific areas of the golf course, a discerning location sound mixer might use several types of microphones during the typically varied shooting situations encountered in a typical project. Let's discuss the various microphone types that are used in 90% of shooting situations, Shotgun/hyper cardioid, cardioid microphones and lavaliere microphones. Besides physical construction and size, the main factors that categorize microphones are their pickup patterns and characteristics. Do you recall our discussion of what sound waves look like and how they behave? Certain pickup patterns are better than others for picking up different subjects and specific portions of the audio frequency spectrum. Certain pickup patterns have specific desired characteristics. Add up these factors and you will see that there is not always a "right" or "perfect" microphone for any given situation. You could give three different cinematographers the same lighting and grip package and each of them will use a different combination of lights and support gear to realize each of their individual interpretations of how the scene should look and feel. You could give three different location sound mixers a sound kit and each of them might possibly use a different combination of microphones and techniques to shoot a scene. It should be apparent to you by now that there is no such thing as a "general use" microphone. This should also prepare you for the fact that you will need to eventually rent or own multiple microphones.
Shotgun/Hyper Cardioid & Cardioid Microphones

These are pickup patterns for three types of cardioid microphones.
Notice how even the narrowest pickup pattern (hyper cardioid) still picks up sounds from the rear axis.

Some of the terminology in classifying microphones can become confusing. The term "shotgun" describes a type of microphone that is a long narrow tube, not unlike the barrel of a shotgun, hence the term. You will hear terms described below like "long shotgun" and "short shotgun". In simple terms, a longer shotgun usually has a narrower angle of acceptance of sound and rejects more sound from the sides, also referred to as "off-axis". Shorter shotguns will usually pickup more sound from the sides and will not isolate a single element as much as a longer shotgun will. The term "shotgun" is sort of slang for any long, narrow tubed microphone. The official terms are variants of the term "cardioid" If you refer to the above illustration, you can see the differences between cardioids, hyper cardioids and super cardioids. These types of microphones are used in about 90% of sound for picture recording. What this means for us is that these kinds of microphones can be aimed at talent as they record dialogue and somewhat isolate the talents sound from most of the extraneous background sound. A shotgun or cardioid mic is almost exclusively operated mounted to a microphone boom so that it can be usually suspended about two to three feet above the talent, depending on framing.

A common misconception is that a shotgun microphone has a longer "reach" than other microphones, as if it can "magically" record sounds that originate a considerable distance from the microphone element. This is the wrong way to think about how microphones and sound behave. Longer microphones don't have any "reach"; they have a narrower angle of acceptance than other microphones. Even the longest shotgun microphone will pick up lots of extraneous ambient sound in the right situation. Remember that sound is "messy"? This means that the undesirable sound is reflecting and bouncing into the area where you are trying to record clean sound mixed with just a slight amount of ambient sound. In order to understand the reason that overhead booming is used whenever possible, think about what lies beyond the talent. If the mic is pointed at the talent at mouth level from near the camera, not only will the microphone pickup the talent, the microphone will also pickup whatever is behind the talent. By pointing the microphone element down toward the talent from above, it picks up the voices of the talent and mainly, the floor or ground as the case may be, rather than all of the sound of the activity behind the talent. Shotgun microphones are more directional than other types of microphones. Hyper cardioid and cardioid microphones have a slightly wider angle of acceptance than shotguns but narrower than most other microphone types. Some seem to prefer the more open sound of a cardioid or hyper cardioid instead a shotgun, but it can be difficult to get enough isolation on talent when using a cardioid, especially when shooting exteriors. In my experience, the single most typical problem encountered when shooting on location is excessive ambient sound creeping into the recording.
Above all other factors, the distance between the mic and the sound source will have the largest influence on the overall sound in your recordings. Regardless of pickup pattern, cost or quality of your mic, if it is placed too far away or aimed improperly, then your sound quality will be diminished.
Which shotgun/cardioid microphone should I buy?
I wrote a review of inexpensive shotgun microphones that is also posted on this web site so if you are in search of a good, low cost shotgun microphone, I suggest you read this article. In the interest of not adding all of the copy of that review to this article, here is the link to the review Low Cost Shotgun Microphone Comparison In a nutshell, plan on spending from $250.00 to $450.00 for a low cost shotgun microphone.
If you are interested in a higher quality shotgun, hyper cardioid or cardioid mic, I will list a few of my favorites but just realize that these are by no means the only good microphones of this these types available. Probably the most common shotgun microphones used in sound for picture production are the Sennheiser MKH series. The Sennheiser MKH-416 ($999.95 street) is an industry standard short shotgun and for good reason. It is very flexible, reliable, simple and efficient. This MKH-416 was slated to be replaced totally by the newer MKH-60 ($1,299.00 street) years ago but the continuing demand for the MKH-416 has been such that Sennheiser has continued manufacturing and selling both models for years. The MKH-60 is a more modern, sophisticated take on the MKH-416 and both are excellent short shotgun microphones. Two other favorites of mine are the Neumann KMR-81i ($1,199.00 street) and the Sanken CS-3 ($1,499.00 street). The Neumann line of microphones are world renowned for their superb German construction quality and sound and KMR-81i is a beautiful sounding example. The Sanken CS-3e was developed in conjunction with NHK, Japan's largest broadcaster and has fast become very common on film and television sets all over the world. It is an excellent sounding microphone.

Sennheiser MKH-416 ($999.95 street)
MKH-60 ($1,299.00 street)

Neumann KMR-81i ($1,199.00 street)
Sanken CS-3 ($1,499.00 street)

As far as long shotguns, The Sennheiser MKH-70 ($1,399.00 street) is a great workhorse long shotgun that I find very useful for exterior interviews and dialogue as it's narrow angle of acceptance reduces exterior ambient to a low level. It is also quite rugged and reliable. The Neumann KMR-82i ($1,599.00 street) is basically a longer version of the KMR-81i and also features that smooth sound quality that all Neumann microphones are known for.

Sennheiser MKH-70 ($1,399.00 street)
Neumann KMR-82i ($1,599.00 street)

There are also numerous other shotgun, hyper cardioid and cardioid microphones available from Schoeps, AKG, AudioTechnica, Beyer Dynamic, ElectroVoice and others that are great microphones. As we discussed earlier, plan on using, renting or owning several different microphones. There are no absolutes when it comes to microphones; it's just that certain brands may offer a sound quality that is more appealing or more effective in various situations. Certain brands and models may be more rugged or have a more delicate sound but also a delicate construction. I personally own eleven microphones and will probably acquire more of them in the future. Each is a unique tool that is better suited to certain situations, budgets and uses. The bottom line here is to rent, borrow, demo and listen before you buy. Certain microphones will be better for your situation.
What Else Do I Need to Use Shotguns/Hyper Cardioids and Cardioids?
If you are serious about recording high quality sound, you soon realize how futile mounting a microphone on a camera is. The time will come when you will finally decide to get serious and go with a microphone mount, zeppelin, windsock and boom pole. You should remember that all of these tools that are necessary to work in conjunction with shotguns, hyper cardioids and cardioids, in total, may cost as much or more than the microphone itself. Allocating your budget intelligently here comes into play once again. It would make much less sense to buy a top of the line Neumann shotgun, then skimp by purchasing a rubber band mount only and not purchasing a zeppelin, windsock and real boom pole. Buy a more reasonably priced microphone and all of the support gear needed to use it correctly. You should budget for purchasing all of these items at once. Let's break down all of these support items, one at a time:
Microphone Mounts
As we discussed earlier in this article, there are many different models of "rubber band" mounts. Even if you are on a tight budget, this is not a place to try to skimp. If all of your sync sound shooting is stationary interviews, you may be able to get away with using a rubber band mount, but if you are planning on doing any "walk & talks" you must invest in a more sophisticated mounting system. Both Lightwave Systems and Rycote make excellent microphone mounting systems. The mount that we use that I am quite familiar with is the Universal Mini Mount from Lightwave Systems. The reason we like this mount so much is because it is extremely effective, it is very versatile and it is well engineered. The beauty of the Universal is that with the proper adaptors, the same mount can be used on a boom pole, on a camera or mounted on a handgrip for sound gathering. You will want to budget approximately $200.00 for a microphone mounting system if you want to obtain top quality results.

"rubber band"

Universal Mini Mount
Lightwave Systems

Zeppelins and Windsocks
A Zeppelin is a protective enclosure that totally encapsulates a microphone. The name evolved from the microphone Zeppelin's obvious resemblance to the German gas dirigibles of the 1920's and 30's or from their less obvious resemblance to the seminal band Led Zeppelin. The jury is still out on that point. The function of the zeppelin is to not only protect the somewhat delicate microphone element but also to filter out extraneous wind and HVAC air movement. The ideal situation is to have separate sizes of Zeppelin for both long shotguns and shorter shotguns and cardioids although in practice, we have been satisfied in using shorter shotguns in our longer size Zeppelin.

Zeppelin

Windsocks (dead kitties or road kill due to their usual gray or black fur appearance) are synthetic fur-covered sleeves that are designed to slip on over a Zeppelin. Most offer Velcro tabs and or zippers to snugly fit a Zeppelin. Windsocks offer a much higher degree of wind resistance than just using a Zeppelin alone. Some of the manufacturers even offer two different length of fur on their windsocks, a shorter hair for moderate wind and a longer hair for strong wind. Besides diminishing wind noise and buffeting, using a furry wind sock will cut out some of your high frequency response so you should not use one all of the time, you need to listen to what the microphone is picking up and choose to use or not use a windsock in addition to the Zeppelin accordingly. There are also socks that are "velvety" instead of furry and work very well in winds up to 15 mph and give minimal high-frequency attenuation.

Windsock

Another alternative to a fully encapsulated Zeppelin are the slip-on windscreen systems. Lightwave refers to their system as "The Equalizer" and Rycote refers to their system as a "Softie Windjammer". The advantage to these systems over using a full Zeppelin is that they are both less expensive and are smaller and quicker to use. This can be handy for more "run & gun" ENG (electronic news gathering) style shooting. The disadvantage of the slip on windscreens is that they do not provide the same amount of isolation and wind buffeting protection since by design; they only cover the front portion of the microphone element. We usually use the slip-on Equalizer when traveling, when space is at a premium or when shooting indoors. When shooting outdoors though, you will obtain better sound using a full Zeppelin and windsock. You should also know that you might sometimes need to use windsocks indoors, particularly when you cannot turn off HVAC systems or in when shooting large auditoriums.

"The Equalizer"

"Softie Windjammer"

Boom Poles
Now that you have at least one good shotgun, hyper cardioid or cardioid microphone and a microphone mount, zeppelin, windsock or a slip on windscreen, how do you use it? The first step is to obtain a boom pole. Boom poles are available in a variety of lengths, styles and materials with the most popular poles being made of aluminum and carbon fiber. Generally, it would be better to own two lengths of boom pole, a short one for travel and small setups and a longer one for larger wide shots. We own one boom pole, A Gitzo G1553 carbon fiber model that can extend to about 11'. This unit was very reasonably priced (for a carbon fiber model) at about $330.00 street. Aluminum boom poles are generally heavier than carbon fiber but are cheaper and easier to repair. I have seen decent quality 9' aluminum poles start at around $150.00. If you shoot more narrative style projects, it may be worth it to spend the extra money for a lighter carbon fiber model.

Aluminum

Carbon fiber

You will notice that as you begin to look at boom poles, there are a lot of different models on the market. The other decision for you to make besides material construction and length is whether or not you want to pay for an internally cabled model or not. Internal cabling is much handier and a LOT more expensive. The K-Tek version of our Gitzo with a coiled internal cable system runs almost three times as expensive in price than as our Gitzo. We live without an internally cabled pole and it's fine. We do mostly stationary booming setups with an occasional hand boomed shot. If we did more narrative or ENG/reality type work that was largely hand boomed, I would spend the extra money for a cabled pole. When you are doing a lot of different setups, hand booming a lot or when you travel, a cabled pole is more convenient. You just have to decide if the extra money is worth it to you.
Lavaliere Microphones
A lavaliere is defined as "a pendant worn on a chain around the neck" which should give you a good idea about what a lavaliere microphone ('lav' for short) is. In the early days of broadcasting, the smaller microphones were actually worn around the neck in the same manner as a necklace. You may not realize that today's lavaliere microphone is an incredible feat of engineering. The most popular lavaliere microphones today are incredibly small. Smaller than a match head. So small that they can easily be hidden in the talent's hair of even behind a tiny button on a shirt. Lavaliere microphones come in usually one of two flavors, omni-directional and uni-directional, although unidirectional lavalieres are rare and are limited mostly to newscaster style applications. An omni directional unit has a pickup pattern that picks up fairly well from all sides. A unidirectional lavaliere has a pickup pattern much more like that of a shotgun or hyper cardioid microphone; it must be "aimed" at the talent's mouth. Because a unidirectional microphone must be aimed at the talent's mouth, the opportunities for hiding the microphone element are mostly eliminated, limiting the use to mostly planting the microphone in the center of the talent's chest. Besides being used on talent, certain lavaliere microphones are also handy for using as a "plant" microphone. A "plant" microphone is typically placed in a hidden spot on the set to pickup talent as a supplement to or instead of a boom microphone. So if you see a "roundtable" or "dinner table" setup, the sound mixer may be covering the scene with one or more "plant" microphones hidden in the centerpiece on the table. Plant microphones can also come into play when scenes are shot with a Steadicam or large dolly moves where it may be impractical to have the boom operator try to follow the camera. Another instance could be that wardrobe restrictions make using body mounted lavalieres tough or impractical.
These Are a Few of My Favorite Lavs
The Tram TR-50B ($250.00-$300.00 street) has been a workhorse industry standard for many years. It was the first lavaliere microphone to use a "vampire" clip mount (a small plastic clip that holds the microphone and has two small 'needle-like' barbs) and it sounds very neutral. The Tram is available in several different configurations and a frequency-modified version is sold as a Sonotrim ($350.00 street). Another similar mic with a slightly different sound but a similar look and construction is the PSC MilliMic ($300.00 street). All three of these microphones share the same basic rectangular form factor and all Sonotrim and PSC seem to be variants of the basic TR-50B design.
For a completely different approach, the Countryman B6 ($350.00 street) is the smallest lav in the world. You must hold one in your hand to comprehend exactly how small this microphone is. The really cool part is that besides being the smallest, it is also, in my opinion, one of the nicest sounding as well as one of the most flexible designs. This mic is so miniscule, that it can be easily used as a hair or wardrobe microphone. The microphone is also available with small plastic caps that can alter the basic frequency characteristics in case a different response is desired or if the microphone's response from being buried under wardrobe is too muffled. Very cool! The microphone is also sweat-proof and waterproof so if the talent will be in scenes with water or rain (the mic element itself is waterproof, not the power supply although it could be made waterproof with the addition of gaffer tape and silicon sealant) or if the talent will be sweating a lot. The Countryman B6 is also available in different flesh tones as well as black and white.

The Sanken COS-11 ($400.00 street) is another popular lavaliere that is slightly larger than the Countryman B6. The Sanken offers what some people feel is a slightly smoother response than the Countryman B6 at the sacrifice of a slightly larger physical size. The Sanken is also waterproof and available in flesh tone as well as black and gray. The Sanken is another marvel of engineering and is considered a premium, top of the line lavaliere.

Wireless Microphone Systems
There are also two methods that lavalieres are typically used with in production, wired or wireless. While wireless transmitters can also be used with a handheld microphone, they are most commonly teamed with lavaliere microphone elements. Some sound mixers also use wireless systems on their boom mic (this is very typical in reality type 'run & gun' style shows like 'COPS') Also, some location sound mixers use wireless systems to "cut the tether" from the sound mixer to the camera although this strategy can be risky unless the camera operator is always monitoring the camera's audio. In general, use cables whenever you can although for certain shooting styles and locations, wireless must be used. A wireless system consists of a single transmitter and a single receiver. For some strange reason (costs?), many new DV users seem to think that a single receiver can simultaneously receive the output signals of more than one transmitter. I have been asked several times if a single receiver can work three or four transmitters. The reverse is actually true, more than one receiver CAN receive the output of a single transmitter simultaneously. So in multi-camera shoots, it is possible to have a single transmitter's output go to multiple receivers on multiple cameras.
Wireless systems are improving but the bandwidth that wireless systems operate on is becoming more and more crowded. Diversity (dual) antenna systems are considered far superior to single antenna systems because in a diversity system, when the reception between transmitter and receiver is weak or encounters interference on one antenna, the system can switch over to the other antenna. The UHF (ultra high frequency) band is generally considered more effective and less crowded than the VHF (very high frequency band). There are brand new digital systems hitting the market that have great potential to make wireless usage more reliable than it ever has been. Unfortunately, the first digital systems available are on the pricey side for DV users, costing considerably more than most high-end camcorders. That's the bad news. The good news is that the digital technology will trickle down the price scale just as it did with digital camcorders. In few years, probably almost all wireless systems will be digital. When that occurs, wireless microphone system usage will undoubtedly skyrocket, as wireless systems are so much more convenient to use than dealing with long runs of cable. But until we are all using digital wireless systems, my advice is to only use wireless when you must.
Another term to look for is "frequency agile". If your wireless system is "frequency agile", it means that when your system encounters other users, static or interference, it can be switched over to another operating frequency. I am amazed that all wireless systems DON'T have this feature, but they don't.
What Are The "Hot" Wireless Systems For DV Users?
I will admit to being quite biased when it comes to wireless microphone systems. I have rented and used numerous wireless systems during my career and in my opinion, Lectrosonics, hands down, are the best values in wireless systems. There are systems available that perform as well as the Lectrosonics UHF systems, but the problem is that they cost a lot of money. $4,000.00 to $7,000.00 per system. The Lectrosonics 210 systems ($2,800.00 to $4,000.00 street) are another industry standard that deserves their excellent reputation. This system is probably used on more than half of all of the feature films and television shows made. Lectrosonics' product is built incredibly well and performs reliably. No, I don't work for Lectrosonics, have never received and compensation from them. I just love their product.
If you cannot afford a Lectrosonics 210 UHF system, you can check out some of their lower cost systems, but not all of them are frequency agile.
The Sennheiser Evolution 500 series ($600.00 to $1,000.00 street depending on options) is another viable, low-cost option for a wireless system. This system is available with several different options and performs reasonably well, especially considering it's cost.

When using wireless systems, consider renting over purchasing. This is another situation where you probably will not need wireless systems all of the time so it probably makes more sense for the average DV user to rent the best rather than buy something "middle of the road". When using wireless systems, it is essential that you feed the system brand new batteries quite often. Wireless systems eat a lot of batteries and begin to perform poorly as the batteries get weaker. If using wireless all day during a typical 10-hour day, plan on at least one and possibly two battery changes.
Other Microphone Types
Although a shotgun and a lavaliere are the standard production microphones, there are certain instances when you will want to consider using or supplementing your audio kit with other types of microphones.
Dynamic
Shotguns, hyper cardioids, cardioids and lavalieres are usually condenser microphones. This means that they each require internal or external power sources to amplify the signal that they output. They must have an internal battery or require phantom power. Phantom power is supplied to the microphone via the XLR connector from the XLR outputs of recording devices or mixers. Phantom power is typically rated at 48 volts, abbreviated as 48V, although some microphones require less voltage. Why all of this discussion about condensers and power? Because a dynamic mic DOESN'T require any power, hence it is more reliable. Another characteristic of dynamic microphones is that dynamic microphones can generally record at much higher sound levels than condensers without distorting. When we discuss a dynamic mic, we are talking about such venerable "institutions" like the Shure SM-57 ($100.00 street) (the standard rock singer stage microphone) and the Electro Voice RE-50 ($165.00 street) (the standard newscaster location microphone). The downside is that dynamics are not very sensitive so they really only pickup sounds that are very close to the diaphragm. Unless you want to have the microphone in your shots, the dynamic microphone is not very useful in most production situations. When you are shooting something that is supposed to look "newsy", the hand held dynamic microphone is the "weapon of choice". Other than this application or recording live, high volume music or sound effects, its use is severely limited in normal video or film production.

Shure SM-57 ($100.00 street)

ElectroVoice RE-50 ($165.00 street)

Boundary
Boundary microphones (PZM - Pressure Zone Microphones) are usually flat or plate shaped microphones that are set on a table or fastened to a wall. These microphones are useful for recording roundtables or audience reactions with certain caveats. These types of microphones use the surface that they are fastened to pickup sounds. When used on a table with talent, these microphones will pickup finger tapping, pencil rolling, cups being picked up and set down and any other physical sounds that reflect off of the surface they are fastened to. Boundary microphones are also effectively used to mic large areas of audience and work well for picking up laughter and applause. They are not effective at picking questions from the audience.

Stereo
Stereo microphones are more commonly used in recording musical or dramatic performances than in typical narrative dialogue situations. Dialogue almost always ends up as a mono element, even in 5.1 surround mixes so there does not seem to be much reason for recording dialogue in stereo since voice is not a stereo element. Another factor to consider is phase cancellation. Without a decent knowledge of stereo recording and postproduction processing and mixing, you may encounter phase cancellation where certain sounds within your mix will be cancelled out. On the other hand, stereo microphones can work very nicely for recording ambient environments. Any environment sounds more realistic in stereo since stereo recording mimics how our ears perceive our actual environments. There are stereo microphones on the market, ranging from inexpensive consumer units that sell for $99.00 to units like the Sanken CS-5, a high-end stereo shotgun. In summary, stereo recording is the exception rather than the rule in sound for picture recording. My rule of thumb for recording in stereo is to determine if stereo recording would enhance the subject matter, live musical or dramatic performance, for instance. In most cases, dialogue would not be enhanced by stereo.
Choir

Choir microphones are designed to be suspended above live choirs. They are good for picking up a large crowd of voices. Although they are good for recording an ensemble of voices, they are not nearly directional enough to pickup individual voices out of a crowd. If you shoot a live event, choir microphones are great for picking up the audience reaction, although once again, choir microphones cannot pickup individual questions or statements from an audience very well. There is no microphone that will pick up an individual sound well unless the microphone element is fairly close to the subject.
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The Third Link - Cables and Adaptors
We have already discussed the differences between unbalanced and balanced connections and cables. Cables and adaptors are one of the most underrated yet most important parts of the audio path. It pays to buy the highest quality cables and connectors. Canare, Mogami and Neutrik all make excellent connectors and cables. While there are literally dozens of different connections and adaptors used in audio and video production, let's go over the most common ones:
XLR

XLR connections and cables use three leads, positive, negative and ground. The XLR connection has two components, a male connector that looks like this and a female receptacle. XLR connectors usually have a locking pin so that they cannot be accidentally pulled out. XLR cables and connectors are the standard audio connections in most professional sound for picture equipment.


1/4"
Quarter-inch connections are more common in music gear than in sound for picture gear although there are popular devices like the Mackie line of audio mixers that are commonly used in both music and video applications that use 1/4" connections. 1/4" connections can be mono or stereo and can be either balanced or unbalanced. These connections are also common as headphone output jacks on larger gear.

1/8" mini plug (3.5mm mini plug)
These connections (usually unbalanced) are very common on consumer camcorders as headphone output jacks and as microphone input jacks. This connection is fairly fragile since it is so small and hanging any kind of weight with cables, etc. off of the jack is not recommended. Plug stress is the most common cause of shorts, grounding problems or failure.

RCA (cinch connections, phono)
RCA connections are the same as the connections used to hook up your home stereo components. Because RCA connectors are always unbalanced, they are almost exclusively used on consumer gear although a lot of pro gear features RCA connectors as secondary connectors, to be used in addition to or instead of XLR connectors.

Impedance Matching
As we discussed above, there are many more types of connections and adaptors to go from any of the above connectors to any other type of connection, but the four listed above will be the most common ones that you will encounter. Special care should be taken when interfacing balanced with unbalanced equipment. There are special devices known as "impedance matching" boxes that are very useful for interfacing pro to consumer gear. You will generally have a better result when using a powered impedance matching device over a passive one. One of my favorites is the Henry Engineering Matchbox ($169.00 street). This is a box that can simultaneously convert balanced XLR connections to unbalanced RCA and unbalanced RCA to balanced XLR. These are very useful for dubbing setups from professional DVCAM and Beta SP decks to consumer VHS decks, for example.
Distance
Another factor to consider is cable distance. Unbalanced connections generally are not very good for distances over about twelve feet because of the lack of a balanced negative connection; noise also has a tendency to creep into the signal. The relative nominal signal strength of -10dB doesn't help either when it comes to distance. Balanced XLR connections generally are safe to use for runs of up to around 200 feet. At distances farther than 200 feet, even balanced lines can use a boost from a line amplifier to restore the signal's strength. Generally, in a location sound kit, you end up using mostly 25-foot cables for the lines from the microphones to the mixer. It does pay to build up a small kit of various adaptor plugs. You never know when you may have to take a sound feed from a telephone to the camera, a CD player to an MD recorder, etc. It pays to be prepared.

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The Fourth Link - Mixing and Routing Devices


The first point I want to make in this part of the article is that most professional productions use an audio mixer. The exceptions are single camera shoots where the audio is just being used for reference or background ambient. But if you are shooting dialogue, you should be using an audio mixer. It's really that simple. If you are shooting with a stationary camera and AC power is accessible, you can get a great result with small desktop mixers. Just remember, with a location audio mixer you get:

1. Tone generator - You must have a tone generator in order to record reference tone so that the editor has a constant point of reference as to the levels that the audio was recorded at the head of each reel.

2. Roll offs and cuts - Location audio mixers often have high and low roll offs and cuts. These are useful for eliminating rumble, hiss and other undesirable sounds before the sound is sent to camera
3. Superior Quality - Almost all location audio mixers have considerably higher quality microphone pre-amps than almost any camcorder, even professional high-end camcorders.
4. Slate Microphone - This is a microphone located on the mixer that lets the operator insert their voice onto the mixer's output. Very useful for audio slates, for example; "room tone :30" when you go to record room tone.
5. Better Monitoring - Location audio mixers usually have a tape return circuit that lets the operator compare the output of the mixer to the output of the recording device by flipping a switch.
6. Panning and mixing - A mixer lets you determine the panning of as many channels of microphones as you have capacity for. Some of the better mixers also have multiple outputs. The PSC M4 MKII mixer that I own lets me route a set of outputs to the camcorder and another set of outputs that we route to a Mini Disc recorder for backup. With a four- channel mixer, you could route three lavaliere microphones to the right channel on the camcorder and the boom to the left channel.
7. Microphone power - Certain mixers can also actually power certain wireless receivers.
8. Phantom power - Almost all location audio mixers can supply phantom power to at least one of their outputs and some to more than one. This saves camcorder battery power, if the camcorder has phantom power. If it doesn't have phantom power, the mixer is able to provide the phantom power instead of having to buy a separate microphone power supply.
9. Better metering - Most camcorders do not have very accurate, high quality audio meters. Most location audio mixers have very high quality audio meters.
10. The ability to "ride" gain - Generally, most location sound mixers "ride" gain, smoothly setting the gain levels being sent to the recording device as the talent's levels rise and fall. This does not mean raising and lowering the volume levels as the talent speaks, it means making sure that the signal being recorded does not clip. Try riding gain as someone is shooting with a typical consumer camcorder. In many models, you cannot even adjust the gain unless you go into a menu

.

Favorite location Audio Mixers
There are actually quite a few high quality audio mixers available but I will just highlight three here that I feel would most likely appeal to DV users and limited budgets. All three of the mixers described are only two channel models but most are available in three, four and five channel versions. Any of the three mixers listed below are an excellent value and you would not go wrong in choosing any of them.
PSC Mjr
The PSC Mjr ($750.00 street) is a two-channel version of the PSC M4 MKII, a very popular four- channel mixer. Valencia, Ca.-based PSC (Professional Sound Corporation) makes excellent products and this mixer has gained considerable popularity as a high quality, basic simple mixer.

Wendt X2
The Wendt X2 ($880.00 street) is a two-channel version of the Wendt X4, also a very popular four- channel mixer. Built by Westlake Village, Ca. based Wendt Audio with military grade parts, rugged construction and high quality assembly; this is an impressive mixer for the money. It is simple to use and provides outstanding sound quality. This mixer has been used to mix MTV's "Road Rules" and on "C.O.P.S." and is a good choice for high-stress, "run & gun" reality television shooting.

Sound Devices Mix-Pre
Ex-Shure audio employees founded Sound Devices, the makers of the Mix-Pre ($850.00 street). This mixer is also sold as the Shure FP-24, but is actually built to Sound Device's specifications and is the same unit. The form factor on this unit is a little different than most of its competitors and its cosmetics and design are sleeker and less boxy than the competition. The Mix-Pre is acknowledged to be a high-quality audio mixer and is often seen on ENG news crews.

Impedance Conversion Devices

Beachtek DX-A4 ($180.00 street)

Studio One XLR PRO ($199.00 street)

I have called this section "routing devices" because both of the devices we will discuss here are not really true audio mixers, they are impedance conversion devices. Both the Beachtek DX-A-4 ($180.00 street) and the Studio One XLR PRO ($199.00 street) adaptors are small boxes that take balanced XLR microphone level inputs and output unbalanced two-channel audio through a 3.5mm mini plug terminated 6" cable that most consumer camcorders can accept. Because the unbalanced portion of the output is only 6" cable, then extraneous noise does not enter the signal path. Both boxes also have internal capacitors that defeat the small voltage that most consumer camcorders output from their microphone input jacks for powering unbalanced consumer microphones. Either box attaches to the camcorder via a screw that connects the box to the camcorder via the camcorders tripod screw-mounting hole. If you own a consumer camcorder that doesn't have XLR inputs, you will need to buy one of these boxes. There are cables and adaptors that can also be used in place of the boxes but only some of these cables/adaptors have the capacitors needed to defeat the powered camcorder microphone jack output. The other advantage of these boxes is that the inputs are microphone/line level switch able and both have separate volume potentiometers for both left and right channel.

6v charge

tripod screw-mount

Since the possibly heavy XLR cables and plugs from the mixer's output also plug into a sturdy box that is mounted to the bottom of the camcorder, there is no strain on your camcorder's fragile 3.5mm mini plug microphone input jack. I have experienced grounding "issues" when using our Sony TRV-900s with the Beachtek when switching over to AC power. The unit sounds perfect when using batteries but a nasty buzz creeps in when using AC power with this particular setup. A grounding wire from the box to the camera's battery connectors solves the ground loop but you may not want to tape a wire to your camcorder. This problem may also be only isolated to the particular combination of camcorder and adaptor I was using. I have not tried the Studio One adaptor but supposedly the Studio One adaptor has a ground lift defeat feature that solves this issue. The latest BeachTek DXA-6 model also has a groundlift switch as well as phantom power on one mic input.
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The Fifth Link - The Recording Device
The most commonly used recording device for most DV users would be your camcorder. For the sake of covering this section completely, we will also discuss some alternative recording devices including Mini Disc recorders, DAT recorders and hard drive recorders and the dual system sound recording workflow.
The Ugly Truth - DV Camcorder Audio - Class of 2002
The unfortunate truth is that if you use a consumer camcorder, you are already beginning your quest for high quality sound with somewhat of a handicap. A few common consumer camcorders have decent sound quality but the sad fact is that the majority of the most popular consumer camcorders have major sound issues straight out of the box. Below are my observations based upon having either used each particular model or having at least edited with tape from each model. Keep in mind, these limitations are exclusive to each model and have nothing to do with the quality of the audio that is fed into each unit from your mixer. These are strictly the limitations of each device from an audio perspective and are definitely my own opinion although searching discussion boards on the web will bear out most of these observations:

Canon XL-1/XL-1S
- Two major audio liabilities. The first liability is the use of either of Canon's XLR/shoulder pad mount combinations, the MA-100 or MA-200. Using either adaptor is not advised if you are trying to record high quality audio as both of them add considerable noise, hum and possible buzzing since they ineffectively transport and transcode the rear balanced XLR inputs to unbalanced input for the camcorder itself. The second issue with both of the Canon XL-1 models is that thy both feature one of the worst headphone output stages ever seen in a consumer camcorder. They both lack sufficient output level and both seem to add a pseudo "limiter/compressor" effect that makes it seem as if the audio levels the camcorder is recording are varying more than they are in actuality. It is difficult to record high quality audio if you cannot hear what the camcorder is actually recording. Other than these two limitations, the XL-1/XL-1S seem capable of recording high-quality audio with a Beachtek or Studio One impedance adaptor box in place of Canon's lousy adaptors. It can be tough "flying blind" though. If you are a Final Cut Pro user, there is also the Canon "sync adjust movie" issue and Canons "interesting" audio sample rates to deal with as well. Audio Grade - C

Canon GL-1/GL-2
- The lower priced Canons seemed to always be capable of recording decent quality audio. The only problem with the GL-1 seemed to be that it had no manual audio input control. Using ALC (automatic level control), it would occasionally react with incorrect levels when the input was overdriven and "pumping" when levels were not hot enough. The GL-2 seems to have remedied this issue by including manual audio input levels. Unfortunately, the flimsy optional cost Canon MA-300 audio adaptor XLR adaptor is guaranteed to break the first time any load or stress is put onto it so if you buy a GL-2, still budget for a Beachtek or Studio One adaptor. Oh, and both still suffer from the Canon sample rate and FCP issues. Audio Grade - GL-1 C GL-2 B-

Sony PD-150

- The first run of the Sony PD-150s had horrendous hiss problems. That problem was subsequently "fixed" during later model runs. All of the PD-150s still have a considerable amount of hiss, although granted, they did reduce the amount of hiss when they "fixed" the issue. The problem is that Sony didn't also "fix" this model's lousy overall audio quality. The PD-150 was blessed with what is probably one of the lowest S/N (signal to noise) ratios ever seen in a consumer DV product. The fact that Sony markets the PD-150 as a "pro" model through their broadcast/industrial division doesn't change the fact that this is one of the sorriest sounding $4,000.00 list DV camcorders ever hoisted on the public. The PD-150 is one of the few consumer camcorders that also will accept line level input. When inputting a clean line level signal, the same hiss and low S/N ratio is still evident. When inputting line level, you are bypassing the unit's microphone preamps, yet you still hear the same audio "issues". I have a couple of Sony's least expensive consumer MD recorders, the MSR-37P that retailed for $129.00. This $129.00 MD recorder is capable of recording sound that blows the sound of the PD-150 out of the water. How can this be unless we hypothesize that perhaps the Sony marketing department was a little paranoid that if the audio was perfect on the PD-150, perhaps they were afraid of is sales cannibalizing too many DSR-300/500 buyers? We will never know for sure, but the circumstantial evidence definitely points to this. I cannot believe that Sony cannot make a $4,000.00 product with such a nice picture yet with totally substandard sound by accident. Once again, these are my opinions. Audio Grade - D

Sony VX-2000

- Same exact story as the PD-150 except all of the poor suckers who were stuck with these are stuck with permanent hiss problems. Since the VX-2000 is a "consumer" model sold through Sony's consumer division, after all of the uproar over this unit's hiss issues, Sony decreed, "this unit's sound specifications are within an acceptable range for a consumer product". Thank you, Sony. I'll remember this next time I buy a camcorder. Audio Grade - F

Sony TRV-900/PD-100A
- Same typical Sony audio issues. Excessive hiss. Poor S/N ratio. Lousy headphone output stage that leaves you guessing. Not as many TRV-900/PD-100A owners complained about the hiss issues as did PD-150 owners. Probably because they only paid about half of the price that the PD-150 owners paid for their cameras. My TRV-900 has been in for audio repairs to Sony service three times. Once, one of the microphone elements went out and had to be replaced and the other two times were to fix the audio hiss issue. Both times, Sony returned the camera, quoting, you guessed it, "this unit's sound specifications are within an acceptable range for a consumer product". I strongly disagree. The audio is acceptable for outdoor interviews and clips that have a decent amount of ambient sound in the background. But try to shoot an interior interview and you will immediately hear the hiss. Even with high quality microphones and a $2,000.00 audio mixer. Audio Grade - D

Sony VX-1000

- This old warhorse was the first three-chip consumer DV camcorder. Released before the DSR-300/500 existed. Interestingly enough, even though it was saddled with a 32KHz only audio recording capability, the audio that it recorded was actually decent, much better than any of the current or new Sony models. I wonder if it's coincidence or the fact that at the time the VX-1000 hit the market, nobody had any idea that "professionals" would want to do real professional projects with these new consumer "toys". We were all shooting with $60,000.00 Betacam SP camcorders at the time. "Cannibalizing" sales back then was a non-issue. Interesting that when the DSR-300/500 hit the market, the audio in all Sony consumer 3 CCD DV camcorders seemed to head downhill. Seems like too much of a "coincidence" to me. Audio Grade - B.

Panasonic AG-DVX100
- If the Panasonic AG-DVX100B were a person, it would be the really good-looking hunk/babe that everyone wants to date, mainly because of it's much lauded 24P capability (IMHO, much 'hyped'). This feature will provide all DVX-100 users the ability to shoot their footage in 24 frame progressive mode and output pseudo "film look" footage via Firewire. Whoopee! I doubt if more than 1% of the DVX-100 users will ever output their footage to film so this feature really turns out to be a validation from a major corporation that yes, all DV users want to pretend that they are shooting 35mm film when they are actually shooting with a consumer DV camcorder. Much more importantly, perhaps this model will actually integrate decent audio recording capability? That would be really much more exciting than 24P! The unit does look promising with dual onboard XLRs integrated into the body, not some cheesy plastic adaptor hanging off of the hot shoe. If the audio is good on this unit, it will automatically be promoted to "head of class" in my book for consumer DV camcorders, mainly because the rest of the class are such audio underachievers.

Dual System Sound

Some DV users utilize separate audio recording devices like these to record their project's soundtrack.
From left to right, a Sharp MD-MT770 Mini Disc Recorder, Tascam DAP-1 DAT Recorder, Creative Labs Nomad hard disk recorder.
For the past few years, there has been much discussion about using Mini Disc, DAT or hard disk recorders for dual system sound recording when shooting DV. In theory, it's really easy. Sure, your VX-2000 may record horrible, unusable audio with a lot of hiss. Just bring along an MD recorder, pop in a disk and life is good, right? Well, it's too bad that in reality, it's just not that simple.
First of all, from a mechanical standpoint, even though both of the devices (camcorder and MD, DAT or hard disc recorder) are both digital, they are also both completely independently synchronized to their own internal master timing device. You would think that if both devices are digital and both are using the same sample rate that it would be a simple matter of lining up the picture and the MD, DAT or hard disc recorded audio on the same timeline and let them go. The problem is that neither device is synced to each other's master "clock" so eventually, the two sources will lose sync with each other. The two may lose sync at 5 minutes; they may lose sync at 45 minutes. There are no hard and fast rules about how long each set of devices can hold sync. There is no way to determine exactly how long it will take unless you test the devices and notate how long it takes for the two to lose sync. Still sounding easy and straightforward? Let's say that your particular combination of DV camcorder and sound recording device will stay in sync for 19 minutes at a time before they lose more than one or two frames sync. You did your homework and determined the number. The next challenges become ones of routing, organization and media management. Let's go over each of these:
Routing
You're stubborn. You have decided to do the dual system sound thing. That's cool. Your first issue to determine is routing. Realistically, you need the mixer's output to go to your camcorder as well as your digital audio recording device. You DON'T want to use the camcorder's microphone to record the audio for the scene because if you are any significant distance from the action with your camera, there is a distance-induced delay that can make it challenging to match your "guide" audio in postproduction. You need the audio from your mixer on your camcorder as a guide for lining up the audio from your digital audio recording device. In this way, both devices will receive an identical signal. I recommend not using the audio from digital audio recording device audio during the actual post process. If you keep comprehensive sound reports and are organized, it is more efficient to edit your show, approve the cut, and then add the "quality" audio at the end. "Post conform" the audio, as it's called. Does your mixer have dual audio outputs, one set for the camcorder and one for the digital audio recording device? If so, great. If not, now you are looking at an "interesting" combination of either an audio distribution amplifier or splitter or some other way of splitting the mixer's output without destroying the quality of the signal. Is using double system sound still sounding simple?
Organization
Organization is the key to making double system shooting work well. The problem is that most DV users don't have a dedicated audio crew, so who is going to keep sound reports? Sound reports are small sheets of paper that are commonly used to keep track of which sound "reel" goes with which film or video "reel". The sound reports are also used for notation about any individual quirks or anomalies. Who is going to slate each take? It's not absolutely essential to slate each take but it sure does make it easier to organize your material when you enter postproduction. What if you are shooting a scene called "Jose enters room"? You do 17 takes of it, have to change videotape and MD during take 8. How are you or the editor going to be able to keep track of which take is which? A slate at the head of each take at least gives you a point of reference about which take is which.
Media Management
Media management is the simple act of making sure that the sound and picture elements are cataloged and kept organized. This is the act of making sure that each time a videotape or film reel is changed on the camera that a new Mini Disc or DAT is inserted and begun. Having a separate sound element for each reel is not the most economical way to shoot but it is the best way to keep the elements organized. If you shoot with a hard disc recorder, this becomes impractical but it works well when shooting with MD or DAT.
You're Really Sure About This?
So are you getting the picture here? The bottom line is that shooting with double system sound and not having it turn into a nightmare means three things. It takes more time (time is money, right?), it takes more crew positions (who would keep the sound reports, slate and roll the MD, DAT or hard disk recorder each take?) and it takes more work in postproduction. I make the conclusion that most DV users are ideal candidates NOT to use the double system sound workflow. Most DV users do not have enough budget and enough crew. Of course, all of these factors are proportional to how long and involved your project is. If you are shooting a :30 commercial and just a few locations and shots, the double system sound may work fine for you. On the other hand, if you are shooting 60-100 hours of footage for a documentary, think long and hard before you commit to the workflow. But as we stated above, let's assume you are stubborn and have decided to go for it. Here is a little breakdown:
Mini Disc
Probably the most popular double system sound format, Mini Disc is a good value for the money. You can read a review I did on the HHB MDP-500 professional MD recorder Professional Mini-Disc Recorder Review that you might want to read for a perspective on how professional MD recorders perform. Mini Disc uses a compression scheme called ATRAC that is fairly good, especially for dialogue. Mini Disc does have a somewhat limited dynamic range so it would not be my first format choice to record highly dynamic sound. There are many Mini Disc recorders on the market and there are several that can even take the optical output of a digital microphone preamp. The media is cheap and easily available. One of the downsides of MD is that most of the non-pro units only have digital input but not output so you end up having to purchase a home MD player with optical output as well as an audio card or Firewire audio interface if you want to transfer the MD's digital signal into your computer and keep it digital.
DAT (digital audio tape)
DAT is probably the choice you would use for higher quality than MD if you were shooting with double system sound. The upside is that DAT is basically uncompressed audio. It sounds better than MD. The downside is that it is linear and the media costs four to five times as much as MD media. DAT recorders, even the cheapest DAT "walkman" cost more than MD recorders with comparable features. DAT will be technologically outdated soon since newer portable hard disk recorders are becoming available. DAT does give you the option of using time code although time code capable portable DAT recorders are quite expensive and are not currently being manufactured. They are still easy to rent from most audio houses though. DAT is still used on most Hollywood features although more and more of them are using an an advanced and very expensive time-code-capable hard disk recording system called Deva by Zaxcom.

Other Hard Disk Recorders

The only inexpensive hard disk I have seen to date is the Creative Labs Nomad. This is a portable CD player-sized hard disk recording system. The advantages are pristine quality and relatively low cost. The Nomads are capable of recording uncompressed but of course, uncompressed recording uses up lot of disk space. There is also the issue of input and output, interface, reliability, verification and the fact that all material must transferred to another format to archive.
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The Sixth and Final Link - The Monitoring Circuit of the Recording Device
The monitoring system on your recording device is the last part of the equation. As we have already discussed, there are several recording devices that have substandard headphone monitoring circuits, including the two most popular consumer DV camcorders, the Canon XL-1 and the Sony PD-150. It can be difficult to judge your audio efforts if you cannot hear what is being recorded properly. The good news is that if you have invested in a decent audio mixer with a tape return monitor circuit, you can at least compare the audio signal you are sending the recording device with what the monitoring circuit in the recording device is reporting back to you. By doing this comparison numerous times over a long period of time, combined with listening to what you recorded on accurate audio monitors that you hopefully have hooked up to your edit system, hopefully you can begin to hear past the deficiencies in the monitoring circuit.
Headphones
The last component in the audio chain are your headphones. The headphones you use to monitor what you are recording serve several very specific purposes:

1. You must be able to judge the characteristics of the room, your talent's voice, sound effects and ambient levels.

2. You will need to be able to determine if your microphones are picking up the traffic outside, the helicopter in the distance, etc.
3. They must be portable enough that you will always bring them and use them on everything that you shoot that has audio
4. They must be efficient enough to output high enough sound levels from weak or inefficient headphone circuits.
5. They must cover your entire ear. You cannot judge what your recording device is recording with headphones that only partially cover your ears. You will be hearing too much of the live sound and not enough of what the recording device is recording to make informed judgments about what you are recording.

My favorite headphones and the industry standard are the Sony MDRV-7506s ($99.00 street). While the Sonys are not the most accurate headphones available, they are extremely efficient, they are small for a full ear design, they fold up to about half of their own size and they come with a screw-on mini to quarter-inch adapter. There is a consumer version called the Sony MDRV-600s ($109.00 street) that sound close to the same. They cost about the same but they are bigger. They still fold up but not as small as 7506s do. They are more comfortable than the 7506s for wearing for long periods of time.
Some location sound mixers favor the more accurate sound of the Sennheiser HD-25s ($199.00 street) but they are twice as expensive and are not as small as the Sonys. It's up to you as to which features and sound will suit your needs.

Sony MDRV-7506s ($99.00 street)

Sony MDRV-600s ($109.00 street)

Sennheiser HD-25s ($199.00 street)

And Your Total Is...

If you have made it this far into the article, you are probably beginning to wonder how much it costs to build a basic location audio kit. Let's total up our little laundry list thus far:



Shotgun, hyper cardioid or cardioid microphone $400.00 to $1,500.00

Microphone mount $40.00 to $200.00

Boom Pole $150.00 to $900.00

Zeppelin $350.00 to $400.00

Windsock $100.00 to $250.00

Wired lavalieres (2) $250.00 to $500.00

Two channel mixer $650.00 to $900.00

XLR adaptor (Beachtek or Studio One) $180.00 to $230.00

Mixer harness $225.00 to $225.00

Cables, accessories $200.00 to $1,000.00

Audio cases and or bags $200.00 to $500.00

Totals $2,745.00 to $6,835.00

As you can see, you are looking at well over $2,000.00 to put together even a low-end basic audio kit. Keep in mind that this kit would have no wireless microphones. To put together a top of the line location sound kit with multiple high-end microphones, lavs and a couple of professional wireless UHF diversity microphone systems can easily run to around $30,000.00. Hopefully you may have read this article before you went out and spent every penny you had on a new camcorder. Hopefully this article gives you some perspective on your budgetary priorities if you hope for your work to be perceived as polished, well-executed film or video. The end result is that you will either have to own or rent most of this equipment to do almost any kind of professional level shooting. In my mind, it makes a lot more sense to own a lower end three chip camcorder and a basic audio kit, than to blow all of your budget for the top of the line DV camcorder with a great lens and a high end tripod and then have to compromise on your sound.
This brings us to our next topic:
Renting Versus Owning
Even though our production company owns its location sound equipment, I am a big believer in renting. We only bought our own gear because we took on a long-term project that easily paid for all of it in one fell swoop. For some reason, probably because of the low relative cost of DV equipment, many DV users somehow arrived at the conclusion that they should own everything that they need for production. I personally feel that this mindset is somewhat misguided. Most new DV users seem to have very little idea about what is involved to produce professional quality work. Sure, you can buy a nice DV camcorder and have the capability to shoot decent looking images. But what about a real tripod instead of that $189.00 special you picked up at Best Buy? Sure, if you are talented, you can try to light with Home Depot $50.00 work lights. What about the audio? Sure your new XL-1S has a microphone, isn't that all you need to shoot? Hopefully, this article thus far has given you some solid information about the equipment that is needed to obtain professional quality sound on your projects. But should you run out and buy all of this gear? Even if you have the money to do so, unless you are somewhat experienced at audio, I vote no. The best way to determine what equipment will best fulfill your needs is to borrow or rent it. Use different microphones and mixers for a few shoots. Get to know how the different models respond and sound. Choose the best gear that will suit your specific needs. Here are a couple of pros and cons to renting versus owning:
Renting Pros

1. To rent the best, state of the art gear is still fairly inexpensive. Usually from $75.00 to $125.00 per day will get you a basic location audio kit. It cost's about the same price range to rent a quality wireless system.

2. You can try out different types and brands of gear before owning to see what works best for you.
3. The rental company takes care of the maintenance and some expendables
4. If it breaks, the rental company has to deal with the repairs, warranties, manufacturer, and dealer

Renting Cons

1. It can get expensive to rent the gear on long-term projects

2. Sometimes, certain rental houses will still try to rent thrashed equipment. Build a good relationship with a reliable, trusted rental house and this shouldn't be a factor.
3. It's not as convenient to have to go pickup the gear and return it
4. Insurance is expensive, especially for "one man bands
"

Owning Pros

1. You get to indulge your equipment fetish

2. If you own it and you use it, you will know who thrashed it or took care of it
3. Convenience. When a shoot comes up, you are ready to go.
4. Cheaper to own on long term projects (if you are shooting enough of them or if they are long term enough)
5. Audio gear doesn't outdate itself very often. Changes in gear are often incremental and not dramatic. Unlike computers and camcorders, rarely will you buy a piece of audio gear and regret it because the "latest thing" just replaced it.

Owning Cons

1. Gear breaks. Gear needs to be maintained. It takes time money and effort to keep everything in top condition and working perfectly.

2. Unless you are willing to spend the big bucks, you will not have the best gear, you will probably have to compromise in order to keep your kit affordable.
3. Eats up storage space in your home or business
4. If you have one location sound kit and end up where you are doing multiple projects at once, you will have to rent anyway for a second or third kit.

Putting It All to Work
By this point, you have either purchased or rented a location sound kit and are ready to do some shooting. Not so fast. As you have hopefully figured out by now, location sound is a craft, not just a crew position. There is no way that you will learn how to record location sound by just reading articles on the Internet. Remember, location sound is somewhat of a paradox in that the better you are at doing it, the easier it will seem. When you sit down to edit the footage you have recorded quality sound on, you will probably not think to yourself, "Wow, I did an awesome job with this sound." You will hopefully hear it, realize that it is fine and proceed with the editing. In a way, doing a really great job with location sound recording is a somewhat thankless pursuit, even if you are editing your own footage. If you do the job well, nobody will complain about the sound. That's about the best you will probably experience with it.
Preparation
One of the most important things you can do to improve your location sound is to be prepared. You cannot record high quality sound without the proper tools to do the job. If you sit down with the director and producer, even if you happen to be both of them, and you determine that you will need eight wireless microphones and two boom operators to successfully record that three camera, live, one-take scene, then so be it, those are the tools that will be needed to successfully record the sound. You may have to be prepared to justify to the producer/client/financier why you will need the tools that you will to get the job done. In order to learn which tools you will need to do a particular job, you must gain experience. The only way to gain experience is to get out there in the field and begin to record.
I am a big believer in location scouts for sound as well as for picture. Go to the location you will shoot at, close your eyes and listen. Listen to the room characteristics or the environmental characteristics, if it's an exterior. Listen to the ambient noises you will have to work around and work with. Which sounds can you eliminate? Which sounds can you minimize and how?
Prepping On The Set
When you arrive to the shoot location, after setting up the sound equipment, DO A SOUND CHECK! If there are actors or just regular non-professionals that will serve as your talent, setup your audio equipment and have the talent rehearse the scene or if it's an interview, setup your microphones and have the talent chat a while hear how their voice projects and what levels you should set. If you are setting up before talent is on the set, have a crewmember stand in and get some levels.

Action!
The location sound mixer's job is to ensure that the best possible sound quality gets onto the tape. Anything else that is expected of the sound mixer is a distraction to that goal. This is the tough part of resolving recording high quality location sound with the entire DV "do it all yourself cheap or free" mentality that is falsely propagated in advertising for DV products and all over the Internet. The big secret that nobody seems to want to admit is that it still takes talent and people to fill certain crew positions to effectively perform all of the functions necessary for producing high quality work. Cinematography is cinematography; it doesn't really matter if it's lighting for DV or 35mm film. There are differences in how to light for each of the two mediums but actually any DP will tell you that DV is harder than 35mm film to make look good. Audio is the same way. It's easier to work with top of the line audio equipment with film cameras or HD/Digital Betacam and obtain a good recording than it is to work with the convoluted signal path of professional front end audio equipment (microphones and mixers) feeding recording devices with unbalanced inputs, inadequate or no audio metering and questionable sound quality. The main point to all of this is to make it clear that the "DV Revolution" really only has to do with equipment price and performance parameters. The DV Revolution has nothing to do with the talent, skills and education required to be effective at the art of film and video making.
Fifteen Tips and Tricks
As previously discussed, it is not realistic within the scope of this article to try to train you, step by step, on how to setup the equipment, microphone technique with talent, how to mix, etc. There are books you can read specifically on technique or I really recommend getting hands on training in a class, workshop or internship to learn the hands on techniques. But there are some fundamentals and tips that should prove helpful in at least getting you started. Here are a few:

1. Channel Redundancy?

When you are running a single microphone source into the two channels on a camcorder, split the output with a mixer or "Y" cable and feed both audio channels of the camcorder. Set one channel to peak a few dB lower than the other channel. In this way, you will have a buffer of a few extra dB in case the talent or subject gets too dynamic. Also, this redundancy is helpful in a more practical way. The audio tracks on a DV tape are smaller than a human hair so having two of them is always better. A chunk of debris can cause a dropout on the tape that may be just during an important part of the scene. Having that extra track of audio, even when using a mono microphone only is cheap insurance.

2. Auto Versus Manual Input Level Control
Should you use auto audio levels if you have manual audio input levels? This is dependent on the specific camcorder and situation you are shooting under. On most consumer camcorders, setting the camera's audio input level to under 50% and controlling the levels with the mixer will get you the best possible sound quality. Each situation is different though.
3. Audio Meters
Generally, when shooting with consumer DV, most of the camcorders used do not have segmented meters. The only way to determine which levels on the ambiguous camcorder unmarked meters means what actual dB level is to run tests with a tone generator and FCP. In that way, you can learn what the "red" segment on your un-marked meters is in dBs.
4. Panning
When recording two channels through an audio mixer, it is generally a good idea to hard pan each channel all of the way to the left and right so that the two signals are recorded separately and independently of each other.
5. Room Tone
What is room tone? Room tone is the ambient sound of the shooting environment that is recorded separately for use in patching, smoothing and matching ambient background levels when editing. It is imperative to record at least :30 of room tone in every sound setup.
6. Phantom or Battery Power
Double powering. Beware of double powering a phantom microphone. If your camcorder has phantom power switched on and your mixer has phantom power switched on, this is not a good situation. Only one phantom power source at a time please.
7. Checking Playback
After the audio is setup, camera is setup, roll tape on a sound check. Then rewind tape and listen to the playback. This verifies that picture, as well as sound is recording okay. You can get burned by tape head clogs or other problems that won't be apparent unless you check playback.
8. Lavaliere Batteries
Most professional lavaliere microphones can be powered off of phantom power from the mixer but if you are using batteries to power your lavalieres, make sure you have a good supply of them on hand. The Tram TR-50B, for instance, takes an unusual size of mercury battery that can be difficult to locate.
9. Connections
When using non-XLR connections, many common audio issues can be traced to plugs not being inserted all of the way into jacks. Make sure that all of your connections are tightly connected when investigating noise issues.
10. Boom Pole Holder
You can purchase a steel device that is designed to hold fishing poles from most sporting goods dealers as well as places like Wal-Mart for around $10.00. These holders, when coupled with a C Stand and sandbag, make great boom pole holders for stationary booming situations.
11. Lavaliere Placement
Hairpiece tape, which can be bought at most beauty supply stores, is excellent for taping lavaliere microphones to skin when setting up lavs under wardrobe.
12. Sound Reports
Most professional audio retailers have sound reports available for free or at a very reasonable cost. Using sound reports, especially on long form or involved shoots is a great habit to build and will help you be more efficient in post production.
13. Clothing
There are professional sound vests available with compartments for lavs, batteries, cables, tape, etc. Very handy. Although wearing one makes you look a bit geeky, when time is of the essence on set, it can be very helpful to have some of your tools at your fingertips as you setup the microphones for each shot.
14. Tools
Most of the grips on set will have a small nylon belt pouch that will contain a Mag Light and a Leatherman or Gerber all-in-one tool. You should have one of these as well. The Mag Light will come in handy when set lighting is not on, for tracing cable runs, checking connections, etc. The Leatherman-type tools are handy for a huge number of tasks.
15. Microphone Level Versus Line Level
Whenever possible run line level from the mixer to the camcorder (some camcorders accept line level, others, only microphone level). Line level is much higher than microphone level, giving your audio a better S/N ratio and less chances of noise creeping into a weak signal.

Location Sound - Beyond The Basics
In this article, we've covered a lot of ground about location sound and the part that it plays in making your project appeal to your audience. We've discussed preparation, planning for the shoot and we've covered the specifications and use for a lot of location sound equipment. Although I am aware that many of you are reading this primarily because you want to know which equipment to buy, I hope that this article has also given you a sense of the skills, talent and patience needed to record good location sound. Although we have covered a lot here, just realize that we have barely scratched the surface and that producing great audio for your projects really begins with the simple desire to communicate with your audience on a more sophisticated and effective level.
There are many great resources available for learning more about the details of recording location sound but really the most effective way to learn about location sound is to get out there and do it. Many of the better sound rental facilities give monthly lectures and presentations on new equipment, technique and the science of sound and how to record it more effectively. The best way to learn how to enhance your location sound is to work with professionals that have a high level of knowledge and skill. You might hire professionals, or be hired by them or work with them side by side on a PSA or a friend's independent film, but the most important aspect of the entire process is to get out there and become an active participant in the process. As we all know, production is a collaborative process and learning from the ideas and skills that your colleagues posses are what will help you grow as an artist. Use your ears and listen to what works and why. Read some books on basic audio engineering. Take a film school or community college sound for picture class. The skills and a trained ear are so much more important than the equipment. Anyone can buy or rent this equipment, but not just anyone can use it effectively. You have to invest the time and effort to learn how to use it correctly.
Believe it or not, one of the most enjoyable aspects of location sound is that it is never perfected. Once you reach a certain level of competence with sound, then the nuances of fine-tuning the sound come into play. It is much more pleasant to think of ways of enhancing the sound on your projects than trying to repair major audio disasters. There are always new challenges to face and new problems to solve. As we move forward into the age of High Definition, digital filmmaking, DVD, multimedia and surround sound, the requirements and appetite for high quality sound will increase. I hope that this article is your first step towards realizing your goal of producing better quality work.
By Dan Brockett

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Low Cost Shotgun Microphone Comparison
Audio-Technica AT835b
Azden SGM-2X
Sennheiser K-6/ME-66/ME-67
Review by Dan Brockett

The Common Question

In order to supplement the amazingly high quality images that DV users are creating with low cost DV camcorders, many users are discovering during the editing process how important audio is in effectively telling their stories. Whether you produce documentaries, feature films, corporate communications or wedding videos, the quality of your soundtrack is what immediately quantifies your work with your audience. It's really pretty simple, bad audio equals amateurish, high quality audio equals professional. Hopefully, this review will serve as the first in series of reviews in helping you assemble a decent quality, relatively low cost professional field audio kit for your productions.

OK, now we're all in agreement that good audio is important, right? In case you haven't figured it out yet, the camcorder-mounted microphones that are standard issue on most DV camcorders are really about as useful as a breadstick in gathering high quality audio. Besides being mounted directly on the camcorder (a big no-no), the sound quality on the microphones that are included with most consumer/prosumer camcorders ranges from bad to worse. A logical first purchase for a field audio package is higher quality main microphone.

Notice how I said "main" microphone? I have been asked many times about which microphone to purchase as "a good all-purpose" microphone. Truth be told, there is no such thing. Microphones are kind of like golf clubs, there are different types of clubs for different purposes and shots. Microphones are the same. More about this later. But in the meantime, a shotgun microphone is the best place to begin in assembling a versatile field audio package that will serve you well under 80 to 90% of the sound situations that occur on most projects. It's also one of the only pieces of a good field audio kit that you can begin to use immediately without many other pieces of support equipment, therefore increasing your audio quality as you build your field audio kit.

The First Step?
"Shotgun mic" is actually a generic term that has come to classify microphones of several types. The "shotgun" mic used to typically be a true condenser (externally powered) long microphone, but has now come to mean almost any long barreled microphone with a narrow acceptance angle.

Cardioid

Super Cardioid

Lobar/Gun

These could include various types of mics termed by their manufacturers as "cardioid", "super cardioid", and "lobar/gun " as well as several other "creative" terms. In a typical field audio kit, the shotgun mic is typically the sound source for around 50 to 70% of what you hear actually ending up in the final mix for films and television. The reason is that, correctly used, a shotgun microphone will pick up the voices of the talent with a comfortable amount of the surrounding ambient sound. Unlike the sterile, close mic'd sound that is typical with a lavaliere microphone, the shotgun mic usually exhibits a fuller, richer and more pleasing sound quality. However, it is a mistake to think of a shotgun mic as a magical device that can only selectively pick up a specific sound while totally rejecting all sounds to the side or rear of the microphone. The way sound behaves is more complex than can be explained in this review, but suffice it to say, a shotgun mic is the correct tool for many audio tasks but it is not an all-purpose solution to every audio challenge. The shotgun mic is the logical place to begin in building a high quality field audio package.

Criteria For The Candidates
The purpose of our review was to examine and test shotgun microphones in a realistic price frame for most DV users, the majority of whom did not pay more than $3,500.00 for their DV camcorders. Most Canon GL-1 owners would not invest $2,000.00 for a high-end shotgun microphone because most DV users tend to view audio acquisition as an afterthought so reviewing high-end shotgun microphones like the Neumann KM82i or the Sanken CS5 that cost almost as much as the average DV camcorder was not an option for this review. Instead, we focused on the low end of the scale. How high quality is the sound reproduced from a $250.00 microphone? As compared to a $400.00 microphone? How much difference is there in the sound quality produced by these low cost units versus their higher priced cousins? Is the sound quality produced by these microphones good enough for television? Theater sound systems? How should these mics be used? We set out find out the answers to these questions and what we encountered in our tests may surprise you. We decided to review shotgun microphones available for a street price of between $250.00 and $400.00 and approached several microphone manufacturers.

Azden Corporation http://www.azdencorp.com provided us with a sample of their SGM-2X Microphone System.

Audio-Technica http://www.audio-technica.com offered their AT835b shotgun.

Sennheiser http://www.sennheiserusa.com provided the modular K6 power supply along with the ME-66 short shotgun capsule and the ME-67 long shotgun capsule.

An MCE86 S.1 shotgun mic was requested from Beyerdynamic for review http://www.beyerdynamic.com but the company refused to provide a test sample so that particular microphone was not reviewed here.

Typical Uses For Shotgun Microphones
After much discussion, we decided that most DV users would probably mainly use a shotgun microphone in one of three typical recording scenarios.

Camcorder Mounted
As the owner of a sound design/audio post facility, I cringe at the phrase, "camcorder mounted mic" but the facts of life are that many DV users do cover live event, documentary type situations, working alone without a sound recordist , and are specifically interested in how well a microphone mounted on their DV camcorder will perform. We tested all three microphones mounted on our Sony TRV-900 DV camcorder with a Beachtek Systems XLR adaptor plate and a Lightwave Systems http://www.lightwavesystems.com Universal Mini Mount with a two inch height extension to remove the tip of the mic from the upper portion of the picture. This extension is needed on most of the smaller camcorders but may or may not be necessary on a larger unit like a Canon XL-1 or Sony PD-150.

Boom Mounted
This is the preferred method of working with a shotgun mic and the method used in 90% of all professional work. The advantage is that the shotgun can be brought much closer to the talent or subject's proximity, thereby resulting in a much better signal to noise ratio, isolating the talent or subject's sound effectively from unwanted ambient sound while still retaining the richness and superior bass response that a shotgun records as compared to most other types of mics. The downside is that you must use a bigger crew, using the shotgun/boompole with at least with a boom operator/mixer or preferably, with a separate mixer and boom operator. It takes a crew of least two and preferably three to really work well with a single camera and a shotgun/boompole. We tested the microphones using a Lightwave Systems Universal Mini-Mount and Zeppelin mounted on a carbon-fiber Gitzo 11 foot boompole on a variety of shooting situations including informal "run and gun" setups, two camera sit down interviews and Steadicam "walk and talks". The mics were recorded through a PSC M4MkII four channel field mixer into several camcorders including a Sony TRV-900 using a Beachtek Systems XLR adaptor plate, a Sony DSR-500WS camcorder and a Canon XL-1 camcorder, as well as an HHB MDP-500 Professional Mini Disc recorder.

Sound and Sound Effect Gathering
For this test, we plugged each microphone directly into the XLR inputs of an HHB MDP-500 Mini Disc recorder. The Lightwave Systems Universal mini mount and Zeppelin were detached from the Gitzo boompole and the mics were used with the handgrip for placement near various specific sound sources and also in and around various sound environments that would typically be recorded for ambient and sound effect gathering including several specific office sound effects, ambient sources including a children's playground, a shopping mall food court and several other locales.

So How Were They Different?
Here is where it got interesting. The Azden SGM-2X is actually a microphone system. When you purchase the SGM-2X, the kit includes a short barrel power supply/base, which is powered by a single "AAA" 1.5V battery as well as a long barrel capsule, which turns the system into a long shotgun mic. That's just the beginning though. Besides the long shotgun capsule, the kit includes short capsule which when combined with the power supply, results in an omni-directional cardioid mic. The kit also includes a long foam windscreen for the shotgun, a short foam screen for the omni cardioid configuration and a camera mounted universal shock mount. The kit even includes extra replacement "elastic bands" for the included shock mount. The system does not ship with a carrying case though. Very nice extras in this offering, especially considering that this mic was the second lowest price of the three units tested. No mention is made in the Azden owner's manual as to whether the SGM-2X can also be phantom powered from a mixer or camcorder's XLR jack.

The Audio-Technica AT835b is the company's low cost line+gradient condenser shotgun. The AT835b came in a nice plastic case and also included a long foam windscreen, a standard 5/8" mic mount (non-suspension) and the required "AA" 1.5V battery. The AT835b may also be phantom powered from any DC 9-52 volt system. The AT835b is not a mic system and is not expandable as are the two other systems we tested.

The Sennheiser K-6/ME-66/ME-67 is also a microphone system, much like the Azden SGM-2X. The K6 module is the power supply, the ME-66 is a short shotgun capsule and the ME-67 is a long shotgun capsule. Unlike the Azden, the Sennheiser system components must be purchased separately though. Just like the Audio-Technica, the K6 module includes a primitive (non-suspension) mic mount, the required "AA" 1.5V battery and a carrying case. The K6 power module can be powered by an internal "AA" 1.5 V battery or by external phantom power 12-48V from a mixer or camcorder. If for some reason, you would like to have the K6 powered only from phantom power with no option for "AA" battery power, the K6P is available and is only phantom powered. More on the significance of using the phantom power versus "AA" batteries later.

Below is a comparison table, detailing the features of each of the three mic systems.

Profiles
Audio-Technica AT835b
$239.99 Street Price
Audio-Technica U.S., Inc.
1221 Commerce Drive, Stow, Ohio 44224
http://www.audio-technica.com

With a street price of just $239.99, the Audio-Technica was the least expensive mic we tested. According to Audio-Technica, "the AT835b is a wide-range condenser microphone with a line + gradient polar pattern designed to provide the narrow acceptance angle desirable for long distance sound pickup." Finished in matte, dark copper finish, the AT835b had a distinct, typically "Audio-Technica" look and feel. The tiny recessed bass roll-off control provides a 180 Hz, 12dB/octave roll-off. The location and design of this important control is a mixed blessing. It's impossible to accidentally turn on or off which is good for the field, but it also makes it exceedingly difficult to A/B compare the mic's sound with and without the bass roll-off. The overall look and feel of the AT835b is one of quality and the mic has the heft and feel of serious pro audio equipment. In our opinion, it is as difficult to quantify the sound qualities of microphones as it is to quantify the sound qualities of audio monitors. Listed below are several comments and observations about the sound qualities of the AT835b

Camcorder Mounted
"At 14.5" long, this is pretty unwieldy on a camcorder like a TRV-900 or a GL-1. It works but is a little long."
"Nice isolation from shock and handling noise"
"Very precise sound with a lot of detail"
"Lower noise than the Azden"
"Sounds pretty clean for the least expensive mic"
"With the bass rolled off, I think it's too thin sounding"

Boom Mounted
"It say right in the literature not to leave it in open sun or in areas where temperatures exceed 110 degrees F for long periods of time or use it in high humidity areas. Kind of like a Schoeps. Perhaps a little fragile or just their lawyers being prudent?"
"Fairly directional with good rejection of off-axis noise"
"Doesn't react at all to handling noise on the boom pole. This is a mixed bag as it's nice but it also can indicate a lack of sensitivity"

Sound Gathering
"Very impervious to handling noise"
"Output seems right on par with the Azden."
"In looking over the frequency response, there is a definite bump at around 8-10KHz"
"Good for overly bassy environments"
"A little thin sounding for my taste"
If you think about it, this mic sounds pretty amazing for less than $240.00. A mic like this ten or fifteen years ago would have cost easily over $800.00 to have the same quality of build and sound. The AT835b comes from Audio-Technica with a standard one year parts and labor warranty. The unit is rated at 1200 ours of continuous use with a standard "AA" 1.5V battery or if you would like to use it as most professionals would, it will run happily on almost any phantom power source you care to feed it.

Azden SGM-2X
$249.99 Street Price
Azden Corporation
147 New Hyde Park Road
Franklin Square, N.Y. 11010
http://www.azdencorp.com

With a street price of just $249.99, the Azden definitely seemed to be an outstanding value in comparison to the other mics we tested. The Azden included a short capsule and extra windscreen that basically turns your shotgun mic into an omni-directional cardioid. The fact that this is included in the $249.99 street price is quite impressive. The included shock mount is the inexpensive, "elastic band" type that frankly, doesn't do a really great job in isolating the mic from bumps and thumps as well as regular handling noise when camcorder mounted. We tested the Azden as well as the other mics with a LightWave Systems Universal Mini Mount that costs nearly as much as this mic, but it did make a huge difference in the sound quality during a typical session with camcorder movement. Although the extras included in the price are nice, we were most interested in the sound quality of the SGM-2X. Other than an informal listening, we didn't test the SGM-2X's omni cardioid capabilities as we felt that most prospective buyers would mainly care mostly about the performance of the shotgun configuration. The SGM-2X does feature an easy to adjust bass roll-off (200Hz 6dB/Octave) control as part of the main power switches operation. This made it much easier to A/B compare the sound with and without the mic's bass roll-off on the SGM-2X than with the other mic's tiny, recessed bass roll-off controls.
Listed below are several comments and observations about the sound qualities of the SGM-2X

Camcorder Mounted
"This thing looks ridiculous on a DV camcorder. It's way too long"
"Great bass response even with the bass roll-off on"
"More 'grit' than either the Sennheiser or Audio-Technica"
"Sounds very 'rich and thick' to my ear"

Boom Mounted
"It's more directional than the Audio-Technica"
"Impressive rejection of off-axis noise"
"Fairly sensitive to handling noise"
"Good basic sound but not a lot of detail"
"I can actually hear some hiss with headphones on"

Sound Gathering
"I really like having the bass roll-off easy to access. There are times when you want to not use it and it's easier to decide whether to use the bass roll-off with this than with the others"
"The output level is decent but a little on the lower end"
"Good for thin sounding voices"
"Constructed OK but you can tell it's little a less expensive"

The SGM-2X can also be converted to a wireless mic unit although we generally avoid wireless mics like the plague unless we have to use them and we have never tested or used an Azden wireless. The SGM-2X comes from Azden with a standard two year parts and labor warranty. The lack of phantom power specifications (I didn't want to chance damaging the unit since it was on-loan) and the fact that the unit requires a "AAA" 1.5 V battery versus the two other brands which use the more common "AA" could be seen as a slight disadvantage. "AAA" batteries are easy to obtain, it's just that every other piece of audio gear seems to use "AA" size batteries so having to keep "AAA"s around as well could be a little inconvenient. This mic is rated at a 1,000 hour battery life from the single "AAA".

The testing staff seemed to be divided about half and half in regards to the sound quality of the SGM-2X. Half felt there was perhaps a little too much self-noise and distortion, while the other half of the testing staff liked the richness and bass response that the Azden seemed to provide that the other units tested seemed to lack. As stated previously, to us, the sound qualities of microphones are akin to audio monitors, each preference is very subjective depending on the listener's preferences.

Sennheiser K6 with ME-66 and ME-67 Shotgun Capsules
$379.99 Street Price for K6 power module and either capsule - $209.99 for extra capsule
Sennheiser USA
1 Enterprise Drive
Old Lyme, CT 06371
http://www.sennheiserusa.com



First, a little clarification is in order here. We tested the Sennheiser K6 with both the ME-66 short gun capsule and with the ME-67 long gun capsule. In our minds, the two capsules are two separate products with different applications. The Sennheiser combos were the most expensive mics we tested. The K6 with either capsule can be bought at a street price of $379.99 and the extra capsule can be picked up for a street price of $209.99. During our time testing the Sennheiser mics, it became apparent to us that if you need to mic more than one talent with one boom operator or if you are shooting in sound controlled environments like sound stages, the ME-66 had a very nice, open sound with medium rejection of off-axis noises. However, if you are shooting in practical locations or need more rejection of off-axis noise than the ME-66 or either of the other two mics tested here can provide, the ME-67 is a superior long shotgun. The construction the two units was typical of products from Germany, minimalist design with very high quality construction. So what does your extra approximately US $130.00 over either of the other two mics buy you? A slightly more refined sound quality with both the ME-66 and the ME-67, although at almost 18" long, the ME-67 was too long to practically mount on a mini DV camcorder in our opinion.

Listed below are several comments and observations about the sound qualities of the K6/ME-66/ME-67 (Each comment is marked as to which capsule was being used)

Camcorder Mounted
"This is the perfect size for a DV camcorder" (ME-66)
"Wow, in comparison to the other mics, this thing has twice the output level of the other two" (ME-66)
"This thing is way too long to use on a camcorder like this (TRV-900)" (ME-67)
"More bass than the Audio-Technica but less full then the Azden" (ME-66)
"This mic works great with the Lightwave Mount and the windsock" (ME-66)
"This is a great pickup for camcorder mounting. Less directional than the Azden or Audio-Technica" (ME-66)

Boom Mounted
"It's more directional than any other mic I've ever used except the 816T"(ME-67)
"It aims like a gun. Very directional" (ME-67)
"This mic has a very nice, smooth open quality. Perfect for two shots and flipping between the two talents" (ME-66)
"I have the recording levels on the camcorder at almost half of the level of the other two brands to get the same dB output. The output is very hot!" (ME-66)
"Very quiet and very little of the off-axis noise from the crew bleeds through. Great isolation" (ME-67)

Sound Gathering
"Without a doubt, the best overall sound of everything we tried" (ME-66)
"Picks up more detail like the whoosh of the lever on this L.E.D. clock" (ME-66)
"They call this a shotgun? It's almost more like an omni" (ME-66)
"Sennheiser makes great stuff. I love this for location or man-on-the-street stuff" (ME-67)
The K6 power module can also be converted to lavaliere mic unit as well as well as a PZM, omni-cardioid and a super cardioid. Very flexible. The K6 system comes from Sennheiser with a standard one year parts and labor warranty.
If you refer to the specs of most dual power system mics, you actually can obtain significantly better performance using phantom power rather than batteries if you have a mixer or camcorder with phantom power output. Everyone who used the K6 system seemed to be impressed with the overall sound quality. It was amazing how different the two capsules sounded and performed. It is nice to have both capsules for maximum flexibility in different shooting situations, but at close to $600.00, it becomes more of a luxury for users on tight budgets.

Cut To The Chase!
So now the question comes up, "which one (or two) to buy?". The good news is that with the right skills and technique, excellent results could be obtained from any of these mics. There was not a bad mic in the bunch. Just like audio monitors, much of the final choice comes down to personal preference.

If money is no object, (relatively speaking), the Sennheiser K6 with both the ME-66 and ME-67 capsules would be a great choice. Based upon this test, this is the system we are going to purchase for our own lower budget DV projects. I would encourage only users who will mostly shoot in sound controlled environments or users who want to camcorder mount a shotgun to go with the K6/ME-66 combo.

If you typically are going to shoot in practical locations versus soundstages, the ME-67 would probably be a superior choice as far boom mounted work. The ME-67 is too long and has too narrow angle of acceptance to be useful as a camcorder mounted mic. In my opinion, the extra $130.00 is justified in higher quality sound and a little more elegant look and feel over the other two microphones. Don't forget though, with audio equipment, the law of diminishing returns applies. What this means is that as you go up in price, the quality of the product and the product's performance go up, but in incrementally smaller steps as the price range rises.

The Audio-Technica AT835b seems to be a great choice if you shoot in many bass heavy environments with high dB levels like live musical performances or have had problems with muddy mixes. The AT835b was rated for significantly higher dB levels than the other two units. The AT835b has a nice clear tone that many will find appealing and the quality and reputation are amazing for a $239.99 mic. An excellent value and a great choice for DV users who just cannot or will not spend $400.00 for a shotgun for their inexpensive DV camcorder but need better quality and more versatility than an on-camcorder mic can provide. No nonsense or frills but gets the job done nicely in the right hands. Not tested here, but available from Audio-Technica are several variants of the AT835b, both longer and in stereo and also higher line, more expensive shotguns

The Azden SGM-2X is a bargain hunters dream. It features decent sound and maximum versatility with the inclusion of the included omni directional cardioid mic at a $249.99 street price. Not to be overlooked is the Azden's standard two year warranty versus a single year on both of the other mics. Several of the testers liked the sound characteristics of the SGM-2X while, conversely, several felt that it had a little more self-noise and distortion than the Sennheiser or the Audio-Technica units. Once again, there is a matter of preference in sound characteristics. I would not recommend the Azden with high sound level projects as much as the other two mics since to my hearing, the SGM-2X distorted earlier than the other two mics with high sound levels. But for average level interview and dialogue work, the SGM-2X tended to slightly exaggerate proximity effect, which has the end result of warming up thin-sounding voices, which can be a big plus for weddings and corporate projects with "real people" who tend to not project their voices or have nasal or un-appealing voice qualities.

And now, to answer some questions I posed at the beginning of this review:

Q- How high quality is the sound reproduced from a $250.00 microphone? How much difference is there in the sound quality produced by these low cost units versus their higher priced "industry standard" cousins?
A- Surprisingly good. The sound reproduced from all of these microphones lacked the fine detail and the pickup from distance was limited in comparison to pricey offerings from Sennheiser, Schoeps, Neumann and Sanken but at less than 1/4 of the cost of most of the industry standard true condenser shotguns, the sound was quite good.

Q- The $250.00 mic's sound as compared to a $400.00 microphone?
A- Per my expectations, the $400.00 mic sounded better than the $250.00 mics. But ask yourself, "where and how will most of my work be reproduced?" If it's on mono VHS with a 2" speaker, the difference will probably not be apparent to your audience. If they cannot hear it, why pay for it?

Q- Is the sound quality produced by these microphones good enough for television?
A- For the most part, yes. I know that the Sennheiser K6 system is used by many news outlets and I have seen the Audio-Technica on some episodic and cable series sets. The Azden is fairly new to the market but I am sure that someone, somewhere is using it for television.

Q- What about in a feature for theater sound systems?
A- OK, let's get real here. We are talking low-end mics. They are amazing for the money but if it were my film or had my name on it as sound recordist, I would buy, rent, beg, borrow or steal to use a higher-end mic. Remember, in a theater, you have a lot more dynamic range, detail and sensitivity than even the best home theaters have so go for something a little better. But if these are all you can afford, (and you're doing a theatrical release project?), you could probably squeak by with these mics. But most of the nuances of the sync sound would be lost on a high dynamic range theatrical sound system. A good compromise would be to rent or purchase a Sennheiser MKH-70 or Neumann KM82i. But that's another article, right?

Hopefully, this review has been of help to you in selecting the best low cost shotgun microphone for your needs. In our next review article in the "Building A Field Audio Kit" series, we will take a look at the choices available for wired lavaliere microphones. Stay tuned for what promises to be another set of tough but fun choices.

by Dan Brockett

Dan Brockett is a film and video director and co-owns a film and video production company, Big Little Films™ , Inc. and a sound design company, Big Little Sounds™ . Dan is also a guide on the premier Final Cut Pro information source, 2-Pop.com and he serves as Vice President of the Los Angeles Final Cut Pro Users Group.
Big Little Films, Inc.
2955 E. Hillcrest Drive, Suite 121 Thousand Oaks, Ca. 91362 USA
Office (805) 496 8130 Fax (805) 496 4027 E-Mail: dan@biglittlefilms.com

copyright © kenstone.net 2002

All screen captures, images, and textual references are the property and trademark of their creators/owners/publishers.

· uni-directional (cardioid) pick-up pattern
· lo-impedence (ohm symbol) 600 ohms or lower
· frequency response range: 50-100 hertz to 10,000-15,000 hertz
· 1/8” mini-plug or a ¼” phone plug with a 1/8” adapter
· 10-20 foot cord (you might find it a pain to wrap up and store such a long cord, but it’s indispensable when you really need it; a detachable cable is OK)
· No battery required

A lavaliere microphone for planned sit-down style interviews is a plus, but it’s not imperative if you’re trying to save money at the beginning. Some stick microphones available in the price ranges detailed above come with those cheesy little plastic stands, and believe it or not, these are fine for getting started. Just place the mic on the cheesy stand outside of the frame and shoot. Try to shoot in a quiet place to minimize audio distractions, and you’ll be surprised how well this will suffice to get you started.

You must use earphones to check that your audio is working. It’s surprising how easy it is to forget to turn on a microphone when you’re busy setting up equipment, trying to look confident and, at the same time, working to put your subject at ease. A simple check, even with the cheapest earphones, can save your entire video.

Cheap mic boom/fishpole idea
A handheld mic boom, or fishpole, is used to hold a microphone above the subject who is speaking. Your mic assistant (fishpole operator) typically positions the mic just above the visible frame, or (depending on camera angle) just under. Pro shooters use this technique to obtain better sound than you could get from a mic fixed at the camera position.

You can buy a fishpole from a pro video source for $100 and up, but one cost-cutting trick is to use a golf ball retriever: a pole with a round plastic part at one end. Wrap foam around the mic handle for isolation, and insert it in the retriever hole. Another great source is your local paint store. You can get a lightweight roller extension that can work great as a fishpole. It might taks some thinking to mount the mic holder but there are many ways to do that.

Tips on the Use of a Mic Boompole

Placing the microphone where the lens is may be convenient but it certainly does nothing to assist in picking up good sound. For one thing, on-camera mics tend to hear zoom motors and other camera created noise. Mounting the mic parallel to the ground is not a good practice, either. Shotgun mics are similar to telephoto lenses in that they both compress planes of action so that very little distance appears to separate foreground and background action. If you point a mic horizontally towards a person, you will pick up the sound of that person as well as the background sound directly behind that person.

Amount of extension

How long a boompole you will need really depends on the type of production you will be doing. Feature films, commercials, and episodic television calls for a long reach, around 12 to 15 feet, in order to cover the set. News gathering and "run & gun" documentary style traditionally requires a shorter reach, around 5 to 8 feet, since the camera crew is more mobile and working close-in. Whenever you extend a boompole, do not lock the pole sections extended all the way to the safety stops.

The proper technique for achieving maximum reach is to slide the pole section to the stop, and then back it in a couple of inches. A slight overlap will make the pole sturdier (no wilting at the locking collars) and quieter. Another good practice is to extend the pole further than what you need for the shot so that you can grip the boompole closer to its center of gravity (think of a circus tightrope walker�s balance pole). By letting the pole counterbalance itself in your hands, your muscles will not be exerting to overcome torque.

Preventing cable noise

Cable noise in a boompole can originate from three problems: conductance, percussion, and loose connections. Conductance is noise or rumble (physical vibrations) that travel along the sheath of the cable. To prevent this, the inside tube section of the boompole should be foam dampened. For instance, in the RoboPole� the cable is fully enveloped in compressed foam rubber for the entire length of the inside section. To maintain the pliability and cleanliness of the mic cable, routinely wipe it down with a restorative solution such as Armor All�. Percussion is noise created by the cable banging against the remaining tube sections of the boompole. Since the pole telescopes, it is impossible to foam dampen any but the innermost tube. The best technique for controlling cable percussion is to keep the cable taut while holding the boom. As the cable exits from the pole, loop it around the little finger or thumb of your supporting hand and keep the line snug. Do not allow the cable to merely exit the pole and drop to the floor! The final cause of cable noise can be the mic connection.. XLR connectors on mics as well as cables have been known to loosen from continuous usage. Place a strip of cloth camera tape over the junction where the microphone connects to the boom cable to protect against intermittent connection occurring when the mic is moved around. Always maintain some slack in the cable connection between boompole and microphone. A taut cable will conduct handling noise. On the same token, excess cable can flap around and cause noise. This excess can simply be wrapped once or twice around the pole beneath the shockmount. The cable on the RoboPole� is cable-tied in a small loop where it exits the tip of the pole in order to reduce conductance as well as to serve as a strain relief. Another useful trick is to use a short jumper cable inside of your blimp windscreens. This cable should terminate at the handle of your shockmount, and be permanently attached with cable ties or tape. It will simplify the process of mounting your shockmount to the pole, because it will no longer be necessary to open up the windscreen and dress the cable every time you need to use the mic.

Holding the boom

To reduce handling noise, grip the pole firmly but not tightly with your fingertips and avoid excess hand or finger movement such as tapping or drumming. Some boom operators wear white editing gloves to reduce finger sticking on excessively cold or hot days. Hold the boom parallel to the floor and high above your head with both arms. If you support the boom underhanded like a flagpole, the boom will enter the scene at a steep angle. Although the mic may be high enough to clear the frame line, the body of the pole may cut across the corner of the frame. Keep your arms close to your head, sort of like a capital "H". When your arm is vertical with the elbow locked, all you are doing is supporting a couple pounds of weight in a straight line with your body. If your arms are extended in a wide "V", your muscles will fatigue quickly. Also, when your arms start from a true vertical, it is possible to quickly reach in or out with the boom to follow the action. Use your front arm as a fulcrum to support the pole above your body. If the situation permits, grip it towards the natural balance point of the boom. Use the rear arm to steer (pan/tilt) the boom, as well as to rotate the pole in order to cue (aim) the microphone.

Microphone Placement

Try to position the mic as close to the action as possible. Depending on the situation and the characteristics of each particular microphone, your mic may be several inches to a few feet overhead of talent. Be aggressive in your mic placement. Ten feet overhead may be very convenient for the camera and lighting crew, but your dialogue will be poor. Remind the director that a wide angle lens can always be tilted downward so that the frame is not filled with ceiling or sky at the cost of his soundtrack! Professional boom operators often place a strip of white tape on the tip of the windscreen so that the camera operator can readily spot if the mic has dipped into the shot. Better to see the mic in the viewfinder than to wait until it shows up on the big screen. To establish a working frame line, dip the mic completely into the shot and slowly raise it up until the camera operator tells you that you’re just barely clear. If you start the boom up high and gradually lower it towards the frame, the camera operator will usually play it very conservative and tell you to stay higher than necessary.

Dr. Fred Ginsburg, CAS has over 15 years experience as a Production Mixer for film and video. He is president of the Equipment Emporium, a supplier of professional audio & video equipment as well as seminars & training.

Microphones

Contents

Introduction

Vibrations and Oscillators

Sound Waves

Microphone Types
The Carbon Granule Microphone
The Piezoelectric Microphone
The Condenser Microphone
The Dynamic Microphone
The Ribbon Microphone
The Hot-Wire Microphone

Microphone Specifications

References

Introduction

A microphone is an electroacoustic transducer. When located in a sound field, its output is an electrical signal that reproduces the sound pressure variations that it senses. There are two fundamental types of microphones, passive and active. The electrical power output of a passive transducer is derived solely from the acoustic power it absorbs, while an active transducer controls an external source of power.

The first application of a microphone was as a telephone transmitter. Bell first tried a passive transducer, simply his receiver used in reverse, but its output was far too feeble for practical use. Then he used a liquid transmitter, with a fine point immersed in a conducting liquid. This was an active transducer, and provided the necessary power by controlling the current from an external battery. The carbon microphone, perfected by Edison, soon became the preferred transmitter, and was used until very recently. It is an active transducer, supplying about a thousand times more electrical power than the acoustical power it absorbs.

The word "transmitter" was originally used for the telephone transducer, and is still so used in telephone technology. The many active transducers proposed as telephone transmitters that used microscopic contacts were dubbed "microphones" by Hughes. In radio, the complete transmitting apparatus is called the "transmitter," so "microphone" was adopted for the transducer in radio to avoid confusion. This usage has become so general that "microphone" is now the word for a general electroacoustic transducer of any type, except with telephones.

A passive transducer is strictly limited by the conservation of energy. It would seem that by using stronger magnets in the Bell receiver used as a transmitter, more energy could be extracted, but magnets are not a source of energy, so all attempts to do this were utterly defeated. There is always some reaction that keeps the energy accounts straight. Most passive transducers have the property that they are reversible: if a transducer converts acoustic energy into electrical energy, the same transducer will convert electrical energy into acoustical energy. This is illustrated by intercom systems that use a moving-coil loudspeaker as a microphone as well. Active transducers are not reversible.

We must deal with three interconnected elements to understand microphones. First is the acoustic input, sound waves in air. These interact with a mechanical system, usually a diaphragm, to excite motion in solid bodies. Finally, the mechanical system interacts electrically to create an electrical generator. Therefore, we shall take up these elements one by one in what follows. There is a large amount of interesting physics and engineering in microphone design.

Vibrations and Oscillators

The diaphragm of a microphone is a mechanical system that vibrates under the influence of the sound waves that reach it. The operation of a microphone is very greatly affected by the motion of the diaphragm, sometimes influenced by air volumes and passages behind it. Therefore, we begin by a thorough examination of the vibrations of mechanical systems.
Small vibrations in gases, liquids and solids are described approximately by linear equations, so that the principle of superposition holds, usually to a very high degree. This means that we can build up any vibration from a superposition of harmonic, or sinusoidal, vibrations. This is a very powerful method in electric circuits, with which the reader is probably quite familiar. The vibrations of even complex systems can be analyzed in terms of normal modes, each representing a harmonic vibration of a definite frequency. We shall generally specify frequency as the ordinary frequency f in Hz, or as the angular frequency ω = 2πf in s-1, calling them both frequency, but identifying which is meant by the units.
Consider an oscillator consisting of a mass m grams and a spring of stiffness s dyne/cm. Let x be the departure of the position of the mass from its equilibrium position in a certain fixed direction. If the mass m hangs from the spring, there will be a certain average displacement xo = mg/s. We will neglect this displacement in what follows, since it does not affect any of our results, and is only mentioned to avoid confusion in mental pictures. If a positive x compresses the spring, then the force on m will be -sx and the equation of motion will be m(d2x/dt2) = -sx. If primes stand for the time derivatives, then the equation of motion is x" + (s/m)x = 0, which is linear. It is also very familiar and easily solved in exponentials, so that x = A sin ωot + B cos ωot, where A and B are arbitrary real constants and ωo2 = s/m. We can write this instead as x = Aejωt, where A is now a complex constant, or a phasor, and we agree to take the real part as our solution. Either way we have the necessary two arbitrary constants for a general solution that will match any boundary conditions (x,x' at t = 0, say).

Such an oscillator can be driven, or forced, by an external force f(t) applied to the mass, so that the force acting on it in the direction of x is f - sx. Forced oscillators are shown in the figure. The external force can be applied either to the mass, or to the point of support. The equation of motion is now mx" + sx = f. Let us assume that x and f depend on time through ejωt, where ω is an arbitrary frequency. Then, we find -mω2 + sx = f, where x and f are now phasors, so that x = f/(-mω2 + s). The velocity is v = jωx, so v = f/j(mω - s/ω), or f = v[j(mω - s/ω)]. The quantity in square brackets is called the mechanical impedance Z', by analogy to electric circuits, where f corresponds to the voltage V, and v to the current I, and V = IZ. In this analogy, m corresponds to the inductance L and s to the inverse capacitance 1/C. Then, ωo = 1/√(LC). The oscillator can be likened to a series circuit driven by a voltage V across it, with the velocity corresponding to the resulting current. This can be a valuable analogy for studying vibrations, or by simulating a vibrating system by an electrical circuit. An oscillator driven at the mass acts like a series resonant circuit, with the velocity of the mass a maximum at resonance.

This is not the only possible analogy. If we assume that I corresponds to the force f, and V to the velocity v, then we have I = VY, where Y is the admittance. If m corresponds to C and s to 1/L, then we have I = V[j(ωC - 1/ωL)]. Now the equivalent circuit is the parallel combination of L and C across which there is the voltage V, giving rise to a current I. This is just as valid as the previous analogy, but we shall stick mainly to the previous analogy.

Another way to force the oscillator is not to apply a force to the mass, but to move the point of support by an amount y, as shown in the figure above. Now the force on the mass will be f = s(y - x), and the equation of motion will be mx" + sx = sy, or x"/s + x/m = y/m. Now, substituting the time dependence ejωt and solving for the phasor x, we have x = sy/(s - mω2). If we use this to find f = s(y - x), we find that f = -mω2x. We can equally well consider f as a force applied to the point of support to drive the oscillator. Using the expression for f in terms of x to replace x in the solution of the equation, we have -(1/mω - ω/s)f = ωy, or jωy = [j(ω/s - 1/mω)] = jωy = v, where v is now the velocity of the point of support, not the mass m. Applying the electrical analogy, with I = jωy and V = f, we have I = VY, where Y = j(ωC - 1/ωL), the admittance of the parallel combination of C and L. That is, when driven at the point of support by a force f, an oscillator looks like a parallel resonant circuit, so that at resonance, the velocity of the point of support is a minimum (zero in the ideal case). If we use the second analogy instead, we find a series circuit in this case, as we might expect.

So far we have neglected frictional dissipation in the oscillator. If we add a force proportional to velocity, -rx', the equation of motion becomes mx" + rx' + sx = f. The frictional force is represented in the diagram by a dashpot below the mass. When exponential time dependence is introduced, we find (-mω2 + jωr + s)x = f, or v = jωx = f/[r + j(ωm - s/ω)]. A real component r has been added to the mechanical impedance. Mechanical impedance Z' relates force f and velocity v by f = Z'v, so the dimensions of Z' are dyne-s/cm or g/s. In the MKS system, this is kg/s, of course. r has the same dimensions, g/s while stiffness s has dimensions g/s2. At resonance, we see that v = f/r, so that v and f are in phase and proportional. A little frictional resistance removes the infinities at resonance, so the quantities vary smoothly through this region.

The Q, or quality factor, of the oscillator is the dimensionless combination Q = ωom/r = √(sm)/r, containing all three parameters. When Q is larger than 1, the resonance curve of v vs. ω is more or less sharply peaked. When Q is smaller than 1, the resonance is not pronounced. When Q is large, it is very closely ωo/Δω, where Δω is the frequency difference between the half-amplitude points of the resonance curve. It is also the ratio of the energy stored in the oscillator to the energy dissipation per cycle.

We found that the natural vibration of an oscillator with r = 0 was a linear combination of ejωt and e-jωt, where ω was the natural frequency √(s/m). When r is greater than zero, if we try a solution of the form eα, we find that α must satisfy the equation mα2 + rα + s = 0. Solution by the quadratic formula gives α = -(r/2m)[1 ± j√(4Q2 - 1)] after a little algebra to show how Q enters in this expression. If Q > 1/2, we find two exponentially-damped solutions of a frequency slightly different from ωo. If Q < 1/2, we find a linear combination of two exponentially decreasing functions, with no oscillation at all. If Q = 1/2, we have a special case called critical damping in which r = 2√(sm). Not only do we have one solution e-ωt, but te-ωt is also a solution. The general solution is then (A + Bt)e-ωt. Although this exact form occurs only on the boundary, it is a good approximation in the neighborhood of Q = 1/2. The general solution for the forced oscillator is the sum of a solution of the unforced oscillator at the natural frequency, and the special forced response we have previously found at the forcing frequency. The first term is called the transient solution and decays with time until finally only the forced oscillation remains. All of these familiar phenomena occur with microphones and loudspeakers.

We generally want the sensitivity of a microphone to be independent of frequency, or "flat." If the microphone is sensitive to pressure, then a force f drives the diaphragm, and we want the displacement x = v/jω to be independent of frequency, or v to be proportional to frequency. Since v = f/[r + j(mω - s/ω)] this will happen, at least approximately, if r and mω are much less than s/ω. In that case, v = -jωf/s, which is what we desire. An oscillator that behaves in this way is called stiffness controlled. The response of a stiffness-controlled oscillator is shown at the right. The response as 20 log [x/(f/s)] is plotted against u = f/fo, where fo is the resonant frequency of the oscillator. The expression for the magnitude of the response x is x = (f/s)[(1 - u2)2 + (u/Q)2]-1/2, which is easily derived from the equations above. This oscillator has Q = 3.16. It is not hard to see that any oscillator is stiffness controlled at frequencies much less than its natural frequency. The response is essentially level for frequencies less than a tenth of the resonant frequency. This is the reason that the resonant frequency of the diaphragm of a microphone is made greater than the maximum frequency at which the microphone will be used. The diaphragm of a telephone transmitter to be used from 300 Hz to 3000 Hz may have a resonant frequency of 10,000 Hz for this reason. The resonant frequency is raised by making the diaphragm stiff and light. It should be noted that increased stiffness s means less response.

A microphone whose output depends on the velocity of the response, such as a moving-coil or dynamic microphone, or a ribbon microphone, is made flat by operating near the resonant frequency with a low Q. In this case, v = f/r is independent of frequency. Here, too, we have a trade-off, since increased r means decreased output. An oscillator operated in this range is called resistance controlled.

An oscillator driven well above its resonant frequency is called mass-controlled, since in this case v = f/jωm. In this region, the displacement drops off very rapidly with frequency, so it is not useful for a pressure microphone. The acceleration a = ωv is the constant quantity in this region. This may sometimes be desired, but not in a microphone.

Before leaving vibrations and oscillators, let's look at modelling an oscillator using the electrical analogy. Most texts mention that this can be done, but give no examples of how. Assume we have an oscillator with m = 100 g and s = 106 dyne/cm (about 1 kg per cm). We will neglect the damping in this case, but it can easily be added. The natural frequency of this oscillator is 100 s-1, relatively low for electrical simulation, but possible. First, we choose the values of L and C to be used in the circuit. Let us try L = 1 H. Since the resonant frequency must be the same as for the mechanical oscillator, C = 100 μF. It is possible to scale the frequency, but we will not get into the complications involved and stick with a direct analogy. Since velocity will be the analogue of current, let us choose 1 mA to represent 10 cm/s. The force exerted on the 100 g by an acceleration of 10 cm/s2 will be 103 dynes. The voltage induced in L for a current change of 1 mA/s will be 10-3 V. Therefore, 1 mV corresponds to 103 dynes, or 1 V to 106 dynes.

To see that this works, suppose we apply a driving force with an amplitude of 106 dynes and a frequency of 50 s-1 to the oscillator. The displacement x will be 106/[-(2500)(100) + 106] = 1.33 cm. [2500 is ω2 = (50)2.] The velocity amplitude will be jωx = j66.7 cm/s. Now we consider the electrical circuit. We connect 1 H and 100 μF in series, and apply 1 V peak at 50 s-1 to them. Z = j(50 - 1/50 x 10-4) = -j150 Ω. The current I will be 1/-j150 = j6.67 mA. Since 1 mA corresponds to 10 cm/s, the analogous velocity of the mechanical oscillator will be j66.7 cm/s, which we have already determined. We can use the circuit to predict the behavior of the mechanical oscillator for any magnitude and frequency of applied force.

If we take L = 10 mH and C = 1 μF instead, the resonant frequency will be 10,000 s-1. If we apply 1 V at 5000 s-1, Z will still be -j150Ω, and I will still be 6.67 mA, which gives the proper result, 66.7 cm/s. This illustrates how the frequency may be scaled to put the simulation into a more convenient frequency range. 5000 s-1 is 796 Hz, much more convenient than 7.96 Hz.

Sound Waves

Sound waves are longitudinal scalar waves in air. The important quantities are the displacement x, the velocity v, and the overpressure p. The air is treated as a continuum, and x is its displacement and v is its velocity, often called the particle velocity to distinguish it from the phase velocity of acoustic waves. The direction of x and v is normal to the wavefront, in the direction of propagation. All of these quantities are exceedingly small for sound of normal intensities, allowing the equations of motion to be linearized to high accuracy. Sound waves obey the principle of superposition, and harmonic (sinusoidal) waves are the basis of analysis. Sound waves are treated in detail in the article Sound Waves.
The density of air at 0°C and 1 atm pressure (STP) is 1.2926 g/cm3. 1 atm is 1.01325 x 106 dyne/cm2. Dry air obeys the ideal gas law with a molecular weight M = 28.97, and the ratio of the specific heats is 1.402. The phase velocity of sound under these conditions is c = (γp/ρ) = √(γRT/M) = 331.5 m/s. At 20°C, c = 343.4 m/s. Since the speed of sound is independent of frequency, propagation is nondispersive, and the group and energy velocities are the same as the phase velocity.

The relations between the quantities in a harmonic plane wave of displacement x = Aej(ωt - kz), where ω is the angular frequency, k is the wave vector 2π/λ, and ω/k = c. A is an arbitrary complex amplitude, are easily expressed. The particle velocity v = jωx, and the condensation s = Δρ/ρ = jkx. The overpressure p = jγpos = (γpo/c) v = rv. The quantity r connecting overpressure and velocity is the acoustic impedance of air. For air at STP, r = 42.6 dyne-s/cm3 or g/cm-s. The power in a sound wave is expressed in terms of the overpressure p by P = p2/2r. The phase relations between these quantities as a function of time at a fixed point are shown in the diagram. ∂p/∂x is shown for k positive; for k negative, it is multiplied by -1. The other quantities are independent of the sign of k (direction of wave).

An overpressure of 10 μbar (a μbar is just a dyne/cm2) or 1 Pa (N/m2) makes a rather strong sound wave. However, p/po is still only about 10-5. The corresponding condensation, or fractional change in density is s = p/jγpo, and even smaller. The particle velocity is p/r = 10/42.6 = 0.235 cm/s, much less than c = 33150 cm/s. The particle velocity is in phase with the overpressure, but the condensation is in quadrature. Finally, the displacement x = v/jω is in phase with the condensation, but in quadrature with the pressure. At 1000 Hz, ω = 6283 s-1, so the magnitude of x will be 3.74 x 10-5 cm, only about 0.4 μm! The energy flow in the wave is 1.17 erg/s/cm2, or 0.116 μW/cm2. Sound is a very small disturbance of air, and it is a marvel that it can be detected by ears and microphones at all.

The threshold of hearing at 3000 Hz is an overpressure of 2 x 10-4 dyne/cm2. Hearing is less sensitive at lower and higher frequencies. Since we are dealing with many orders of magnitude, sound intensity is expressed logarithmically, in decibels. The sound pressure level (SPL) is dB re 2 x 10-4 dyne/cm2, or SPL = 20 log (p/2 x 10-4). Normal conversation is carried out at SPL 50 to 60 dB. An SPL of 60 dB corresponds to p = 0.2 dyne/cm2. An SPL of 120 dB causes discomfort; it corresponds to p = 200 dyne/cm2. Even at this intensity, the acoustic energy flow is only 0.469 mW/cm2. The apparent loudness of a sound increases logarithmically with energy intensity, giving the aural sense a large dynamic range. This is characteristic of all the senses, and is known as Fechner's Law.

The wavelength at 1000 Hz is 33.1 cm, or a little over a foot, something worth remembering. Most microphones, especially the ones popular today, are rather small, and do not disturb the sound field greatly. When the wavelength approaches the size of the microphone, diffraction effects occur that change the distribution of the pressure at the surface of the microphone. For a spherical microphone, diffraction about doubles the overpressure at the point facing the incoming wave. This effect may be relied upon to lift the response of the microphone at high frequencies, when it would otherwise begin to droop. Diffraction effects are important only at high frequencies. In the telephone bandwidth of 300-3000 Hz, they can be neglected.

The acoustic impedance mismatch at the interface between air and water or a solid is very great. The result is that sound is almost perfectly reflected or diffused from a liquid or solid surface. The same occurs for sound generated within water when it reaches the surface. The air and the water are almost perfectly separated acoustically. Sound waves exhibit all the diffraction and interference phenomena that light waves do, and usually more obviously.
We shall usually assume that the pressure at the microphone diaphragm is the pressure in the undisturbed sound wave. This is a good approximation for low frequencies and small microphones, where the microphone disturbs the sound wave only minimally. When the dimensions of the microphone approach the acoustic wavelength, the pressure is affected by diffraction and reflection. If the wave is reflected at the diaphragm, a pressure node is created and the pressure is twice that in the undisturbed wave.

Microphone Types

Bell's invention of the electromagnetic telephone in 1875 set off a vigorous search for a good transmitter, which was lacking to make the telephone a practical system. Passive transmitters could not provide sufficient power for general use in view of the lack of electronic amplification. Therefore, active transmitters of greater output that varied their resistance in time with the acoustic signal seemed the only answer. Many such devices rapidly came to light, using a wide variety of interesting techniques. Some used liquids, such as the Bell liquid transmitter itself, and others that employed jets of electrolytes. Others used uncertain mechanical contacts, or flames whose conductance varied with height, which in turn varied with acoustical pressure. Glow discharges in air were sensitive to acoustic waves as well. These devices are all interesting and curious, but none were satisfactory, and it is difficult now to experiment with them, though their principles may make good demonstrations.

Émile Berliner invented a variable-resistance solid transmitter using the contact between a metal diaphragm and a metal ball 1877. Edison followed with a similar solid-contact transmitter in 1878. In that year, Hughes suggested that the contact between small grains might be superior to that of larger bodies, and named transmitters using this principle microphones to distinguish them. Hunnings devised a microphone using carbon granules from coke, which was the starting point for Edison's search for a better transmitter. Edison perfected a transmitter using anthracite coal granules that became the standard for telephones after 1881. It was small, simple and easy to use, and above all had a large output. The carbon transmitter was a standby from this time until solid state electronics made it possible to put amplification in the telephone set, allowing other kinds of transmitters to be used. It is still the only transmitter that can be used in practice without electronic amplification. It has now been superseded by the electret condenser microphone, which is superior in characteristics and very cheap. Still, it is disappointing that carbon microphones are not still available for experimentation and non-critical use.

The Carbon Granule Microphone

A single-button carbon microphone as a telephone transmitter is shown in the figure. The mouthpiece acts as a horn to increase the acoustic pressure on the diaphragm. The displacement of the diaphragm is transmitted directly to the carbon button, which contains carbon granules between two carbon discs. The front and rear contacts are insulated and brought out to terminals. An external battery drives current through the button, which has a resistance of 30 to 100Ω. The resistance varies slightly when the diaphragm is displaced, causing a change in the current and a consequent change in voltage, which is the output of the microphone. The analysis in this section applies to any device in which the resistance is made to vary by the displacement of a diaphragm, not just to the carbon microphone.

Suppose that the resistance of the button is r = R - ax + bx2 to a sufficient degree of approximation. The constants a and b can be assumed positive, with b much less than a. The resulting current when a constant voltage E is applied will be i = (E/R)(1 - ax + bx2)-1. This can be expanded in powers of x to get i = (E/R)[1 + ax + (a - b)x2 + ...]. The quadratic term is usually negligible. If x varies sinusoidally, then the alternating current variation is i' = (Ea/R)x. This produces an alternating voltage of e = Eax, that can be considered as the Thévenin equivalent voltage source, in series with internal resistance R.

The overpressure p cos ωt in the sound wave will produce a displacement x = (pA/s)cos ωt in a stiffness-controlled diaphragm of stiffness s and area A. Of course, this expression may be generalized as required, but this approximation is good enough for the present purposes. This gives a generator voltage of amplitude e = EapA/s. The constants a and s are usually not well-known, but this at least shows the effects of the most important parameters. In particular, e is proportional to the DC voltage across or the current through the button.

The sensitivity of the microphone is expressed as S = e/p = EaA/s or IaRA/s, normally in decibels: dB = 20 log S. Telephone transmitters with a current of roughly 25-50 mA have S about -15 dB or -20 dB, or 0.18 - 0.10 V/μbar. This can be raised by a factor of 10 using a transformer, which gives 1.0-1.8 V at an impedance level of 5 kΩ, a very creditable output. A transformer with a turns ratio of 1:10, or n = 10, raises the voltage by a factor n, to ne, and the impedance by a factor n2, to n2R.

The power output into a load resistance R' coupled through a transformer of turns ratio n is P = [e/(R + n2R')]2(n2R'). If we let u = n2R'/R, then differentiate P(u) with respect to u and set the result equal to zero, we find that u = 1, or n2R' = R, the very familiar result for maximum power. On the other hand, if we wish the maximum voltage (as for the input to a high input impedance amplifier), then the load resistance should be made as high as possible. If we use a transformer, then the DC current is not affected by such arrangements, and can still be adjusted to any level required. Impedance matching is illustrated in the diagram at the left.
Carbon transmitters of 1920 had sensitivity above -30 dB from about 600 Hz to 1900 Hz, strongly peaked at 1000 Hz. This range contains most of the voice power, but gave the telephone sound a somewhat unnatural quality. By 1934, the -30 dB bandwidth had been extended to 275-3100 Hz and was much less peaked, giving about -15 dB sensitivity from 75-2500 Hz. Further improvements were rather less dramatic, but the 1000 Hz peak was completely removed. This was a result of diaphragm and backing plate design, not changes in the carbon button.

A double-button carbon microphone has a push-pull action that cancels second harmonics. A good microphone with level response from 60 Hz to nearly 10 kHz, except for some wiggles of a few dB near the upper limit, was created. The sensitivity, however, was only about -47 dB, much less than that of a telephone receiver, in microphones optimized for good fidelity. By this time electronic amplification was available, so low output was not a drawback. Such microphones were used in recording and broadcasting in the early days, and gave very good service.

The outstanding disadvantage of the carbon microphone is noise, the so-called "carbon hiss," that could not be eliminated, though it could be reduced by careful preparation of the granules. This noise is inherent in the source of variable resistance, which was the surface properties of the carbon granules. Carbon by itself, even in bulk, exhibits 1/f (pink) noise, and this was exacerbated in the granules. The noise can be represented as a random voltage or current generator in series with the signal generator, in the usual way for noise analysis. The carbon granules could be damaged, and even fused together, by unusually high currents, such as those produced by inductive kicks. If the hermetic seal of the button was damaged, moisture could cause the granules to pack. The resistance of the microphone would decrease in that case, and it would become much less sensitive.

The Piezoelectric Microphone

It was long known that certain crystals, notably tourmaline, would attract light objects when strongly heated. This was the pyroelectric effect, the production of electrical polarization upon heating. While studying this effect, the brothers Pierre Curie (1859-1906) and P.-J. Curie (1855-1941) discovered the direct piezoelectric effect, or the production of electrical polarization when a crystal was strained, in 1880. In 1881 they announced the converse effect, the production of strain when an electric field was applied to a crystal. Much of the pyroelectricity previously observed was simply the piezoelectric effect due to strains caused by thermal expansion, but there is also a primary pyroelectric effect.

The application of an electrostatic field to any substance may cause mechanical strains by the phenomenon of electrostriction. These strains are proportional to the square of the applied field, and do not change if the field direction is reversed. Piezoelectricity is quite distinct; piezoelectric strains are proportional to the electric field, and reverse if the field is reversed. Piezoelectricity, where it exists, is usually much larger than electrostriction.

The description of the piezoelectric effect is made complicated by the many directional quantities and the crystal symmetries that enter. Strain is deformation per unit length, and has six components, three axial and three shear. Stress, force per unit area, also has six components. Stress and strain are related by a symmetrical matrix with, in general, 21 independent components. Electrical polarization, dipole moment per unit volume, has three components, as does the electric field. Therefore, we have 18 quantities, all depending on each other and on the orientation of the crystal. At least we can assume that the dependence is linear, and described by a certain number of coefficients.

The symmetry important here is that of the point group of the crystal, those operations leaving one point fixed. There are 32 possible point groups, each the basis of a crystal class, divided into six or seven crystal systems. Crystals that do not have a centre of symmetry may exhibit piezoelectricity; those with a centre of symmetry cannot, by Neumann's theorem, which states that any property of a crystal must have at least the symmetry of the crystal. Such crystals are called hemihedral. Their axes are essentially one-sided, and opposite directions on them are not equivalent. This is required if the piezoelectric strain is to be proportional to the electric field, and reverse with it. Of the 32 crystal classes, 20 may be piezoelectric. There are, in general 18 coefficients connecting the electric field to the strain in the direct effect, or the polarization to the strain in the converse effect.

If X is a stress, in dyne/cm2, and x is a strain, dimensionless, then the relation between them is of the form X = kx, where k has the dimensions of stress, and is called an elastic modulus. The inverse relation is x = sX, where s = 1/k is called a compliance, with dimensions cm2/dyne. This is really a matrix relation, and the matrix s is the inverse of the matrix k, not a simple reciprocal, though often the actual relations are simple. Analogously, the direct piezoelectric effect can be expressed by P = ex, where x is strain, P the polarization in esu/cm2, and e is a piezoelectric constant with the dimensions of polarization. The converse effect can be expressed by x = dE, where x is the strain, E the electric field in statvolt/cm or erg/esu, and d is a piezoelectric constant, with dimensions of inverse field. The polarization and the electric field are also related by P = ηE, where η is the electric susceptibility. Again, it must be emphasized that these are all tensor relations generally involving many coefficients, and a constant spontaneous polarization Po may also be involved. In that case, the P above is the change due to E.

When the Curies made their initial studies, which included discovering the piezoelectricity of quartz, which has been very important, they found that Rochelle salt, or Sel de Seignette (a pharmacist in La Rochelle who isolated and discovered the medical properties of the salt in 1672), had an extremely large piezoelectric effect. Rochelle salt was used extensively in microphones, and is of considerable interest besides, so the discussion here will focus on it. However, it is typical of all such materials. Rochelle salt is sodium-potassium tartrate tetrahydrate, NaKC4H4O6·4H2O, which easily forms large orthorhombic crystals. Above 55°C, it begins to form separate Na and K tartrates dissolve in the water of crystallization, and disintegrates irreversibly. To preserve the crystal, it should not be heated above 45°C. Its large piezoelectric effect occurs only between -18°C and +24°C, called the lower and upper Curie Points of the substance. Between these temperatures, it is an electrostatic analogue of a ferromagnetic material, called a ferroelectric, with a large spontaneous polarization. Like ferromagnetic materials, it is divided into domains of constant spontaneous polarization, but the domains are quite large, even centimetres in size. The domains are not obvious to the eye. Its crystals are enantiomorphic, like quartz, but only right-handed crystals occur in most cases.

An X-cut crystal plate of Rochelle salt is shown at the right. The x,y,z axes correspond to the crystallographic axes a, b,c. An X-cut plate has the normal to its broad surface parallel to the x-axis. There are only three piezoelectric coefficients for Rochelle salt, which relate the three shear strains to the three components of the electric field. The shear stress Yz is a force per unit area in the z-direction on a surface whose normal is parallel tot the z-axis. For equilibrium, it must be equal to Zy. The opposite faces have the same forces acting, but in the reverse direction. This shear stress gives rise to an electric polarization in the direction shown, with d14 as the coefficient. The other nonzero coefficients are d25 and d36, relating to zx and xy shears, respectively. The three coefficients are different in value, but d14 is the largest. For Rochelle salt, d14 is about 2.6 x 10-4 cm/statvolt, or 1/d = 3850 statvolt/cm (the electric field for unit strain). For a strain of 10-4, an electric field of about 115.5 V/cm is required. The exact value of d depends on the temperature and the circumstances of the crystal, but this gives an idea of its magnitude.

A "45° X-cut rod" is an X-cut with the lateral sides making equal angles with the z and y axes. The shear strain yz is 2ΔL/L, if L is the length of the rod, and ΔL is the change in length. The formula at the right in the diagram for the converse effect serves to find the change in length for any applied field. Of course, the strain and stress are related by the elastic constants of Rochelle salt, but we will not go into that here.

Ammonium dihydrogen phosphate or ADP, NH4H2PO4, as well as the potassium salt potassium dihydrogen phosphate or KDP, are also strongly piezoelectric, resembling Rochelle salt quite closely. The Curie temperature of ADP is 147.9°C, it has no water of hydration, and is quite stable, so it makes more durable devices. The symmetry is about the same, but a little higher, so that d14 = d25. Ceramics like barium titanate, BaTiO3, which are ferrielectrics (two lattices oppositely polarized spontaneously; the observed polarization is the difference), also are strongly piezoelectric. If a microphone is described as "crystal," it usually contains ADP; if it is called "ceramic," barium titanate is the active element. Any piezoelectric device is reversible; if a voltage is applied to a piezoelectric microphone, it will emit sound. It is also strictly a passive electroacoustic transducer, and the output power cannot exceed the acoustic input power.

Rochelle salt first became more than a curiosity around 1917, when it was applied by Langevin to ultrasonic acoustic transducers, or hydrophones, for the detection of submarines. Not only could strong signals be created in water by the converse piezoelectric effect, but the same crystals could be used to detect the reflected waves. This was, in fact, the origin of the important field of ultrasonics, which used acoustic waves of greater frequency than 20 kHz, which were inaudible but very useful.

The impedance mismatch between air and a diaphragm is much greater than the mismatch between water and a crystal hydrophone, so microphones are much more difficult to devise. The first microphones had a 45° X-cut bar, 1-2 cm long, 0.4-1 cm wide and 0.1 to 0.2 cm thick, cemented between a diaphragm and a backing plate. A much more sensitive arrangement was a "bimorph" of two cemented X-cut plates with one thin electrode between them. They could be arranged to bend or twist, and could be operated from a diaphragm through mechanical leverage. A typical inexpensive modern ceramic microphone responds from 30 Hz to 15 kHz, with a sensitivity of -60 dB (1 μV/μbar) and an advertised internal impedance of 8kΩ at an unspecified frequency (Kobitone LM037). The capacitance measures 786 pF, which gives a capacitive reactance of 20.2 kΩ at 1 kHz. Piezoelectric microphones give low output at a moderate internal impedance, and must always be used with amplification.

Piezoelectric transducers were used as analog phonograph pickups, giving a much higher output than dynamic pickups. As driven elements, piezoelectric devices are used as telephone receivers, acoustic transducers and record cutters. They are used for small loudspeakers, although dynamic loudspeakers give much better results. In 1925, G. W. Pierce invented the acoustic interferometer, which uses an X-cut plate and a parallel reflector to measure the speed of sound with great accuracy. He devised an oscillator that was very sensitive to the reaction of the air on the crystal. W. G. Cady developed the quartz crystal resonator at about the same time, which has had widespread application as a frequency-control device.

The Condenser Microphone

The condenser, or capacitor, microphone was perfected by C. H. Wente in 1917 as a much-needed low-noise substitute for the carbon microphone, and as a standard microphone for acoustical measurements. It was then used in broadcasting for a few years, until replaced by the dynamic microphone, which is much easier to use. The capacitor microphone is very simple in principle, and is still used for acoustical measuring instruments. The recent development of the electret capacitor microphone (ECM) has overcome all the inconveniences of the traditional capacitor microphone, and it is now used almost universally in general applications, as in telephones and consumer electronics. An excellent, easy to use microphone can be purchased for a dollar or two, and operated on 5 V at 0.5 mA. The ECM depends on two recent developments, the polymer electret film, and the field-effect transistor. We shall discuss it after looking at the traditional capacitor microphone.
A capacitor microphone consists of a metallized diaphragm that forms one plate of a capacitor, a backing plate forming the other. The diaphragm is tightly stretched to have a high resonant frequency, and is placed very close to the backing plate. Grooves are cut in the backing plate to control the mechanical impedance of the diaphragm. The capacitance of a plane-parallel capacitor of plate area A and separation h is C = 4πκεA/h F, where A is in m2 and h is in m. The dielectric constant is κ, and ε = 8.854 x 10-12 F/m, the MKS constant that masquerades as a physical reality. Since the dielectric is air, we can take κ = 1. The charge on a capacitor charged to a voltage V is Q = CV. If h varies, then dC = -(4πεA/h2)dh = -(C/h)dh. This means that the current will be i = -(CV/h)(dh/dt).

The microphone can be represented as a Norton equivalent circuit with a current generator i in parallel with a capacitance C. This can be transformed to a Thévenin equivalent circuit of a generator e = i/jωC = (V/h)x, where x is the displacement of the diaphragm, in series with a capacitance C. If the diaphragm is stiffness-controlled, then x = pA/s, so e = pVA/hs and the sensitivity S = e/p = VA/hs. The sensitivity is proportional to the bias voltage V and the diaphragm area A, and inversely proportional to the separation h and the stiffness s. The stiffness is raised if h is reduced, so they cannot be varied independently. Taking A = 10 cm2, h = 0.01 cm, and s = 1 x 108 dyne/cm, we find S = 10-5V V/μbar. If V = 200V, then S = 2 mV/μbar, or -54 dB. This happens to be a relatively typical value for a capacitor microphone. The capacitance of the microphone will be about 88.5 pF, which will give a capacitive reactance of 1.8 MΩ at 1 kHz, and 18 MΩ at 100 Hz. The microphone cannot be located far from the amplifier. Furthermore, the bias supply must be extremely well regulated and ripple-free, since every variation will be combined with the acoustic signal.

An electret is a body with a permanent polarization, analogous to a magnet which has a permanent magnetization. Polarization P is dipole moment per unit volume, and has the dimensions of surface charge density. If P is uniform in the electret shown, a surface charge +P appears on the left-hand face of the electret, and a charge -P on the right. The electric dipole moment of the bar electret is p = PAL, where A is the cross-sectional area of the electret, and L its length, just as the magnetic dipole moment of the bar magnet is m = MAL. An electric field exerts a torque on a dipole tending to align the dipole with the field, just as a magnetic field acts on a magnetic dipole. The bar magnet and bar electret shown establish fields in space, and these fields have energy. Some of this field (the "demagnetizing field") acts in the reverse direction to the polarization or magnetization, tending to reduce it. A soft iron bar may be placed over the poles of a magnet as a "keeper" through which most of the field will pass. Magnetic pole strength can be considered to be induced at the ends of the keeper, which will neutralize some of the pole strength of the magnet, reducing the demagnetizing field. Exactly the same thing occurs with an electret when it is placed between the plates of a shorted capacitor. The surface charge due to the polarization is neutralized by the surface charge of opposite sign induced on the capacitor plates. This happens naturally when an electret is exposed to the air, as it collects charged particles floating as ions and dust. A magnet is not neutralized in the same way, because there are no free magnetic charges.

Assume that we have a sheet electret of thickness d', rigid polarization P and permittivity ε'. Let this sheet be placed in a parallel-plate capacitor so that there is a space d between the upper plate and the upper surface of the electret, and the permittivity of this air space is ε. This capacitor is carefully sealed away from floating charges so that the electret does not become neutralized. When the plates are shorted to each other (a "short" in this case can be a resistance of many megohoms) the voltage difference between them becomes zero. The fields in the air space and in the electret must be opposite in direction, and related by Ed = E'd' so the total voltage is zero. A charge density σ will appear on the upper plate, and an opposite charge will appear on the lower plate. The net charge at the bottom of the electret will be σ' - σ, where σ' = P. An opposite charge will appear on the top of the electret.

The field E is given by εE = σ and the field E' by ε'E' = σ' - σ. Since E'd' = Ed, σ = σ'[d'/(d' + ε'd/ε)], and so E = (P/ε)[d'/(d' + ε'd/ε)]. As d becomes small, this approaches E = P/ε. If the upper plate is the diaphragm of a microphone, the electric field E plays the same role as the field produced by the bias voltage in an ordinary capacitor microphone. Elimination of the bias voltage and all its inconveniences makes the electret capacitor microphone a very desirable device.
Since the capacitance is small, even with the dielectric properties of the electret, the ECM still has a very high internal impedance that is almost entirely capacitive. This drawback is eliminated by putting an FET right at the diaphragm. The gate presents a very high impedance to the diaphragm capacitor plate, and the FET is manufactured to have a small drain current for VGS = 0. The output voltage is the voltage across an external drain resistor in the range 2kΩ to 50kΩ, which becomes the output impedance of the microphone. The output voltage also increases proportionately to the drain resistance, of course.

An example of an ECM is the Kobitone LM045. This microphone is remarkably small, only 9 mm in diameter and 6 mm tall. Diffraction effects will be negligible, so this omnidirectional microphone will probe an acoustic field without perturbing it. Its bandwidth is 20 Hz to 12 kHz, and its sensitivity is advertised at -64 dB (without specifying the load resistor, which is unfortunate), or 0.63 mV/μbar. The power supply range is 2 to 10 V, and the current drawn is less than a milliampere. I found that a load resistance of 22 kΩ and supply voltage +5 gave excellent results. Perhaps most remarkable is that is costs only $2.29! The ECM is a worthy successor to the carbon microphone for general uses.

The Dynamic Microphone

The principle of the dynamic microphone was known in 1877 when Bell developed the telephone, but it was impossible to use because of the lack of electronic amplification. In all dynamic transducers, a coil of fine wire is free to move in a strong annular magnetic field. If the coil is moved by a diaphragm, a voltage is induced in the coil. If a current flows through the coil, forces are exerted that cause the coil to move. The equations governing these effects are F = BLi and e = BLv. B is the magnetic field in tesla (webers per square metre), L the length in metre, v the speed in metre/s, i the current in A, and e the voltage in V. From these equations, we find that F/i = e/v, or Fv = ei. Fv is the rate of doing mechanical work, and ei the rate of doing electrical work. The signs are such that when mechanical work is done, an equal electrical work appears, and vice versa. This shows very clearly that we are working with a reversible effect and that the conservation of energy is observed. The word "dynamic" simply refers to the role of motion in the device; it is not a very well-chosen term, but always refers to a moving-coil device.

In somewhat friendlier mixed units, we have F = BLi/10, where F is in dyne, B is in gauss, and i is in A. Also, e = BLv x 10-8 in V, where B is in gauss, L is in cm, and v is in cm/s. The generated voltage in a microphone is e = 10-8BLpA/Z', where p is the overpressure in μbar, A the area of the diaphragm in cm2, and Z' is the mechanical impedance of the diaphragm. It is clear from this that if the diaphragm is a simple oscillator, the output cannot be independent of frequency, since the magnitude of Z' is least at resonance, and increases rapidly for higher and lower frequencies. L is the total length of wire in the coil, L = πdN, where d is the diameter of the coil and N the number of turns.

Dynamic microphones became possible when it was realized that by making the diaphragm oscillator include air volumes within the microphone, the response could be flattened very nicely, at the expense of some sensitivity. The structure of a typical dynamic microphone is shown in the figure. The domed diaphragm acts like a rigid piston, and carries the coil of wire, which moves in the annular gap of high magnetic field. The pole pieces of the microphone are soft iron, with permanent magnets providing the field. The mounting rim of the diaphragm contributes stiffness and a little resistance (s0, r0), while the diaphragm itself contributes mass (m0). It is acted upon by the acoustic overpressure, so that the microphone is a pressure microphone, with omnidirectional characteristics. The air in the small volume beneath the diaphragm acts as another stiffness element (s1), while the kinetic energy of the air moving in and out from under the diaphragm through the silk cloth contributes mass (m1), as well as a larger resistance (r1). The result is two coupled oscillators, whose parameters can be varied to get the best results. The electrical analogy was a help in designing dynamic microphones, since the results of different arrangements could be studied easily without building actual microphones. The port V is to help low-frequency response.

A typical dynamic microphone has a very low internal impedance, seldom as large as 10Ω, and this impedance is approximately resistive over the whole frequency bandwidth. On the other hand, the sensitivity is quite low, no more than -90 dB or -100 dB, so amplification is essential. At least 40 dB can be gained with transformers, bringing the output up to -50 dB when applied to the amplifier, which is not too bad. Dynamic microphones are low-noise, require no bias voltages or other nuisances, and are relatively rugged. They replaced capacitor microphones almost completely in broadcasting and recording. Most high-quality microphones are still dynamic microphones, and are relatively expensive.

A dynamic microphone in reverse becomes a loudspeaker. Of course, the designs are quite different, because they must be optimized for different things. A small loudspeaker makes a very passable microphone, and can be used for this purpose, as in an intercom system. Such small loudspeakers radiate poorly at low frequencies, so this is compensated by making them resonate at a few hundred Hz. This is easily recognized in the oscilloscope traces when the output of a small loudspeaker is used as a microphone, since it tends to ring at this frequency. A transformer of turns ratio 1:10 can be used to increase the voltage output, and to make the internal impedance about 800Ω for the usual 8Ω speaker. The same amplifier that drives the speaker from a line of this impedance can be turned around to drive the line in turn when a button is pressed.

The Ribbon Microphone

All the microphones so far described are operated by the acoustic overpressure acting on one side of a diaphragm, and so may be called pressure microphones. Because pressure is a scalar quantity, these microphones are omnidirectional, except for diffraction effects that depend on frequency. To realize a directional microphone, it is necessary to operate the microphone by some vector quantity. One vector quantity in an acoustic wave is the pressure gradient, which is parallel to the direction of propagation and in phase with the displacement. A microphone operated by the pressure gradient is called a pressure-gradient microphone. Since velocity is also a vector quantity, such microphones are also called velocity microphones, but they are not directly operated by particle velocity.

If ∂p/∂x is the pressure gradient, the force in a direction making an angle θ with the propagation direction, acting on a surface of cross-sectional area A and length L is f = -(∂p/∂x)AL cos θ. For a harmonic travelling wave, f = jkpAL cos θ. This is the desired angle-dependent force. It is in quadrature to the pressure, and proportional to k = cω. If ω is much larger than the resonant frequency of the surface, then the mechanical impedance is jωm, where m is the mass of the surface. The velocity of the surface is the ratio of the force to the mechanical impedance, or v = (pAL/mc) cos θ, where c is the speed of sound. This means that v is proportional to p independently of frequency.
This equation holds when kL << 1, that is, when the size of the microphone is small compared to a wavelength. At high frequencies this is not necessarily true, and the force is smaller, dropping to zero when f = c/L. Instead of kl cos θ, we have 2 sin[(kl cos θ)/2] in the expression for the force on the surface. This falloff is compensated by an increase in pressure difference due to diffraction.

I have spoken of a "surface" to avoid using the word "diaphragm," which gives the wrong idea when used of a pressure-gradient microphone. The most important pressure-gradient microphone is the ribbon microphone. The surface in this case is a corrugated aluminium ribbon supported in a strong magnetic field. The emf generated when the ribbon moves is proportional to v, and so to the overpressure p. That this ribbon acts like a thick surface is harder to realize. The ribbon is exposed to pressure equally on front and back, and the distance L is determined by the size of the baffle in which the ribbon is suspended. L turns out to be roughly equal to the radius of a circular baffle. (not 2L, as might be expected).

The ribbon acts like a coil of only one turn, so the generated emf is very small. On the other hand, not only is the internal impedance very small (less than an ohm), but the velocity can be made higher by reducing the mass m to a minimum value. The sensitivity of a ribbon microphone may be -90 dB or -105 dB, but more than 40 dB can be gained at once with a transformer, so its output of -50 dB is comparable to that of a dynamic microphone. There is usually a transformer at the microphone to match it to a transmission line, and another transformer at the amplifier input.
Most significantly, we now have a sensitivity that is proportional to cos θ. The ribbon microphone is equally sensitive to sound coming from front and back (so two people can face one another across it and both be equally heard), and is completely insensitive to sound coming from 90° or 270°. This pattern is not dependent on frequency, as are diffraction effects. The ribbon microphone discriminates by a factor of 3 against isotropic noise. It can also be turned so that noise sources can be put in a zone of low sensitivity. These features made the ribbon microphone the standard for broadcasting, and the lozenge-shaped shiny microphone a familiar sight.

The cosine sensitivity of the ribbon microphone can be combined with the isotropic sensitivity of a pressure microphone to make a microphone with a response proportional to (1 + cos θ). This curve is a cardioid. Cardioid microphones favor sound coming from 0°, and discriminate against sounds coming from 180°. A cardioid microphone at the front of a stage will pick up sounds originating onstage, and reject those coming from the direction of the audience. Polar plots of the amplitude sensitivity of the ribbon and cardioid microphones are shown at the left.
The amplitude response of a pressure gradient microphone changes sign for a reversal of the direction of the wave, while that of a pressure microphone does not. This has a strange effect in a standing wave, where waves are moving in opposite directions. At a point where the pressure microphone gives a maximum signal, the pressure-gradient microphone gives a minimum signal, and vice-versa.

The Hot-Wire Microphone

The hot-wire microphone is not like the other microphones we have studied. It does not reproduce sound pressure variations electrically, but is more of a detector of sound and an indicator of its energy. Since the name does crop up from time to time, we'll describe it here for completeness. It is specifically used for low frequencies and for infrasonic signals. It was developed during the 1914-1918 war as a sound ranging device, for acoustic location of artillery to aid counterbattery fire. After the war, Tucker and Paris perfected the hot-wire microphone for infrasonic detection, publishing their results in 1921.
An example of a hot-wire microphone is shown at the right. It consists of a very fine platinum wire placed over the neck of a Helmholtz resonator and heated by a current passed through it. The wire is supported by a thin glass rod and a disc of mica. The disc is clamped between silver rings that make the contacts. When a sound wave of the resonant frequency arrives, air rushes in and out of the neck of the resonator at that frequency. This air flow cools the wire by forced convection, so its resistance decreases. The resistance decrease is easily detected by a Wheatstone bridge. The hot wire of a typical device is 6 μm in diameter, with a resistance of 350Ω and requiring about 30 mA to heat.

A Helmholtz resonator consists of a volume V and a neck of length L and cross-sectional area A. Its resonant frequency is given by the formula in the diagram, where c is the speed of sound. A 125-ml Florence flask makes a good Helmholtz resonator. I measured L = 5.5 cm and A = 1.54 cm2, which gave f = 256 Hz (the physicist's middle C). The actual resonance was an A, or 220 Hz, on the musician's scale, not far off. Without the resonator, the sensitivity of the hot-wire microphone is very low, so practical devices are all resonant. The microphone can be applied to frequencies as high as 512 Hz.

In addition to the DC change in resistance, it is also possible to detect AC variations in the hot-wire resistance. These variations are at twice the sound frequency, since the air blows alternately in and out, and the cooling does not depend on the direction of the air velocity. The hot-wire microphone is, accordingly, not applicable to speech or music. As its use in acoustic ranging indicates, it has a rather quick response. It is useful in a frequency range where other microphones are unresponsive.

Microphone Specifications

The most fundamental microphone specifications should allow you to estimate how much electrical output you will get for a certain acoustical input. All microphones, except for carbon microphones, require electronic amplification, and you should be able to select a suitable amplifier. The microphone can be represented by a Thévenin equivalent circuit with a generator in series with an internal impedance. At a minimum, the microphone specifications should supply these two parameters.
The generator voltage is typically specified in dB re 1 V per μbar, or e = 20 log p, where p is the acoustic overpressure in dyne/cm2 or μbar. Sometimes the pascal is used as the reference pressure, 1 Pa = 10 μbar. This adds 20 dB, so manufacturers like to use it to imply that their microphones are more sensitive. The sensitivity is specified at some reference frequency, usually 1000 Hz. A curve of dB vs. frequency, giving the frequency response of the microphone, may be available. A bandwidth of 50 Hz to 5000 Hz between -3 dB points is not bad, but most microphones exceed this, and 30 Hz to 10,000 Hz is often found.

The calibration curve of microphone sensitivity in dB vs. frequency must be measured experimentally. This is usually done in one of two ways. If a constant overpressure is applied at each frequency, a constant-pressure calibration results. If the microphone is placed a reasonable distance from the microphone in an anechoic chamber, so that it is activated by what is essentially a plane wave, the result is a free-field curve. The free-field curve will be more representative of the response of the microphone in actual use, since it includes the effects of diffraction and other influences that change the relation between the acoustic pressure in a wave and the pressure on the diaphragm. On the other hand, the pressure calibration reflects the fundamental properties of the microphone, and is useful in designing measuring instruments.

Equally important is the specification of the internal impedance of the microphone, which varies over an extremely wide range for different microphone types, from less than 1Ω for a ribbon microphone to many megohms for a capacitor microphone. This is more difficult to find, and sometimes you must assume a typical value for the microphone type involved. A microphone with a low internal impedance, say 50Ω or less, is normally used with a step-up transformer that provides 40 dB of gain effortlessly (with a 1:100 transformer) and matches well to the high input impedance of many amplifiers. A microphone with a high capacitive internal impedance cannot be used with a cable of any length, so the amplifier must be very near. In the modern electret condenser microphone, the amplifier is an FET in the microphone cartridge itself.

Sometimes microphones are specified in terms of output power for a given acoustical input, often as dB re 1 mW per pascal. Transformers do not change the power, of course, so they do not affect this specification. With this figure, it is easy to estimate how much amplifier gain will be necessary to bring the electrical signal to the desired level. 0 dB is, as usual, re 1 mW. This is not as useful a specification as that of sensitivity and internal impedance, from which it can easily be derived.
The directionality of a microphone is often of interest in applications. A pressure microphone is fundamentally omnidirectional at lower frequencies (wavelength larger than the physical size of the microphone). A ribbon microphone has maximum sensitivity from front and back, and is insensitive to sound coming from the sides. Its sensitivity has front and back lobes of approximately equal size of r = a cos θ shape. A cardioid microphone has the very desirable characteristic of a large forward-back asymmetry in its sensitivity. Placed at the front of a stage, it can pick up sound coming from the stage, and reject that coming from the audience. The directional sensitivity can be expressed in a polar plot. The directionality can be expressed in dB by subtracting the average sensitivity over all directions from the maximum sensitivity. For a ribbon microphone, this is 4.8 dB.

References

L. E. Kinsler and A. R. Frey, Fundamentals of Acoustics, 2nd ed. (New York: John Wiley & Sons, 1962). Particularly Chapters 1 and 11.
A. L. Albert, Electrical Communication, 2nd ed. (New York: John Wiley & Sons, 1940). Chapter V. Note that "bar" here is really the μbar.
W. G. Cady, Piezoelectricity (New York: McGraw-Hill, 1946). This venerable classic is still the best explanation of piezoelectricity, correct and complete.
A. Wood, Acoustics (New York: Dover, 1966; originally London: Blackie and Son, 1940). Hot-wire microphone, pp. 303-305.
Inexpensive ECM's, small loudspeakers, crystal and ceramic microphones, and dynamic microphones are available from Mouser.

Composed by J. B. Calvert
Created 31 August 2003
Last revised 7 September 2003
http://mysite.du.edu/~jcalvert/tech/microph.htm

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