All too often videomakers will seek out and purchase the camcorder model with every whiz-bang, superduper digital video effect available. They have great plans for video masterpieces, but when it comes time to shoot, they quietly settle for the sound from their built-in mike. They don’t realize there are alternatives available to them which will greatly enhance the quality of their audio.
Most of the better-quality camcorders have an input for an external mike. There’s a reason for this. The internal mike has to deal with a lot of problems. These include camcorder noise from drive motors and handling, the distance to the sound source and even wind.
For example: the greater the distance between your camcorder and your subject, the greater effect ambient sound (natural environmental sound) has on your soundtrack. In cases where ambient sound is high, it could override and eliminate the sound you desire. Wind creates similar problems with much the same results.
So, while a camcorder’s internal mike is adequate for family events in which capturing the moment is the main concern, for high-quality audio, you need to consider some alternatives. This month we’ll find out how basic miking techniques allow you to improve on the sound available from your camcorder’s internal mike. We’ll look at different microphone types and how pickup patterns apply when choosing a mike. We’ll also learn to use mikes in recording stereo.
Choosing a Microphone
There are many kinds of mikes available. Knowing the advantages and disadvantages of each type can help you make the proper choice for a given situation.
No matter what that choice, however, it is rarely an advantage to use a cheap external mike in place of the internal mike in your camcorder.
Crystal mikes, which generally use a tin foil diaphragm glued to a crystal to generate a signal, have almost no low frequency response. They are cheap replacement mikes for toy tape recorders and have no business in videomaking. Also avoid ceramic mikes, which are just one step above crystal mikes in quality. Once state of the art, ribbon mikes in some cases still provide a good fidelity range; but these mikes are very fragile and cannot stand up to the rigors of videomaking.
Worthy of interest are dynamic and condenser mikes. Such mikes provide both the fidelity range and the ruggedness you need for video studio and field work.
The sturdy dynamic microphone is the workhorse of the audio industry. The design of this mike (a small coil of wire moving within a magnetic field) limits its frequency range a bit; however, it very accurately reproduces the human voice. This makes it a good choice overall. These tough mikes stand up to rough treatment well.
Some dynamic mikes use an omnidirectional pickup pattern, which means that they pick up sounds equally from any direction. Thus they prove an excellent choice when shooting crowds of people outdoors. When working with crowds, mount a dynamic mike on a fish pole (a long boom pole held by hand); keep the mike just out of the shot.
Some dynamic mikes have a cardioid pickup pattern which helps to eliminate unwanted background noise. In most conditions you can place a cardioid dynamic mike farther away from the subject than an omni mike.
The condenser microphone (sometimes called a capacitor mike) uses two electrically charged plates separated by a small air gap. One of these plates moves in response to sound, affecting the voltage between them. This voltage becomes the audio signal.
Because the plates require a charge to work and the tiny signal requires amplification, a condenser mike must carry a DC voltage source with it to operate. This usually comes in the form of a single “AA” battery mounted in the body of the mike. Because the drain on this battery is low, it will last a long time; check them monthly.
Some condenser mikes don’t have an on-board battery. These mikes work with professional audio mixers, which supply power to the mike through the mike cable. This is called phantom power.
Condenser mikes provide the flattest response, widest fidelity and most realistic sound. These mikes are an excellent choice for close, on-screen subjects, as well as voice overs. They also work well for stage shows, and for distant subjects when the ambient sound level is low. You can mount condenser mikes on the camcorder, but they’ll often provide better results if mounted on a boom and placed close to the subject.
Two problems with both dynamic and condenser mikes: 1) sensitivity to wind and 2) handling noise. To minimize wind noise, try “socks.” These foam wind breakers fit over the front of the mike.
The lavaliere microphone is an excellent choice for interviews. This small condenser mike attaches by a clip to a shirt or jacket collar. Try to avoid mounting the mike where it will rub against a surface and make noise.
The mike cable passes through the subject’s clothing to a small battery container. Worn clipped to the talent’s belt, this box or cylinder also contains the mikes electrical circuit. A cable connects this box to your camcorder, mixer or recording deck. Since you place a lavaliere mike at the source of the sound, ambient sound (background noise) becomes less of a problem.
Lavaliere mikes are also available as wireless mikes. In this case the talent actually wears a small VHF transmitter hidden on a belt clip. This sends a signal to a small receiver placed on or near the camcorder. The wireless lavaliere mike is an excellent choice for distant and moving subjects, and where ambient sound levels are a problem.
The shotgun microphone is a specialized condenser mike which resembles a gun barrel. Its long, fluted and chambered body is highly sensitive to sounds in front of it. The mike does not amplify sounds coming from directly in front; rather it cancels out sounds coming from other directions.
The shotgun is a good choice for distant subjects in noisy locations. It’s also useful when you cannot place the subject too close to the camcorder, such as at a zoo or a public event.
You aim a shotgun mike at the sound source just like aiming a gun. This eliminates unwanted sounds. Remember, shotgun mikes may produce a distorted sound when placed too close to the subject.
Pressure zone mikes (PZMs) are condenser mikes with the element suspended just above a plastic or metal plate. This causes it to sense pressure changes within a hemispherical (half sphere) pickup pattern. This makes the mike a good choice for tabletop group discussions, interviews and staged events–or any kind of shoot in which sound will come from widely spread out directions. Simply place the mike in the center of the sound source.
PZM mikes are not a good choice for noisy areas, or any of the types of shots already discussed for other mikes. Because the size of the backing plate affects its low frequency response, the PZM will often sound tinny unless mounted to large board or wall.
The pickup pattern of a mike is often more important than its type. Say you’re recording dialogue between several people, with a mike hidden near them. If the people are close together, a directional mike will work well. But if the people are more than a few feet apart, an omnidirectional mike is the better choice.
OK, so understanding pickup patterns is important in choosing a mike. Ask yourself the following questions:
- Will you shoot mostly outdoors, where it’s loud, or mostly indoors, where it’s quiet?
- Will you shoot from long distances, or mostly close up?
- Will you need more than one type of mike?
- Are you recording stereo audio?
You will need a basic understanding of pickup patterns to answer these questions.
As already stated, an omnidirectional mike is equally responsive to sounds from all directions. A unidirectional mike tends to reject sounds from any source not directly in front of the mike.
We have already discussed the advantages of these mikes; here are the disadvantages.
The main disadvantage of the omnidirectional mike: you, and everyone not directly involved in making the desired sounds, must be quiet! Omnidirectional mikes pick up all sounds including camcorder noise, the rustling of clothes, people walking or talking behind the set and every ambient sound imaginable.
The main disadvantage of the unidirectional mike: it limits what it picks up. This will reduce the sound from talent to the side of the camera. It will limit ambient sound, which may be more noticeable in other shots recorded with a different mike. This makes the overall sound between shots to sound unbalanced. If not carefully aimed, a unidirectional mike may also concentrate on undesirable sounds.
The most popular unidirectional mike pattern is the cardioid, named for its heart-shaped pickup pattern. There are also supercardioid and hypercardioid mike patterns. These mike patterns progressively narrow the cardioid shape, thereby forcing the mike to reject sounds not directly in front. Oddly, due to the design of highly directional mikes, holes appear in the patterns which are sensitive to side and rear sound; unless you’re aware of them, they can defeat the purpose of these mikes.
The shotgun mike represents the extreme in unidirectional mikes. It will pick up sound even in noisy areas and at considerable distances–but you must aim it carefully at the source.
Its accuracy is its main disadvantage. Aim it properly, or it may pick up the busy woodpecker in the tree down the road, instead of your talent’s dialogue.
The bidirectional mike picks up sound in a pattern much like a figure 8 set at right angles to the front of the mike. This mike is most useful for two-person interviews and stage events.
Zoom mikes feature adjustable pickup patterns. Warning: unless properly designed, zoom mikes offer poor sound quality. So don’t consider a zoom mike an all-in-one miking solution.
Stereo Miking Techniques
Stereo recording and playback gives the effect of placing sound sources at specific points within a field of sound. To record stereo sound requires two mikes.
When recording stereo you may be tempted to take two mikes, space them far apart and record away. For all its obvious simplicity, this method causes a nasty phenomenon called phase cancellation.
Phase cancellation occurs when the same sound waves reach separated mikes at slightly different times. The time difference causes a canceling effect which partially or fully eliminates certain frequencies.
Placing the mikes near each other eliminates this cancellation, because there is no longer a time difference. There are several ways to place mikes for optimum stereo recording:
Mid-side (M/S) miking. In this miking arrangement, you position two mikes at 90 degrees to each other at the center of the sound source. The first mike is bi-directional; you position the elements to pick up the sides of the sound source. The second mike is unidirectional; you point it to the center of the source. Both mikes combine their signals and special electronics control right and left channel separation. Many built-in stereo camcorder mikes use the M/S technique.
X-Y miking. In this miking arrangement, you place two unidirectional mikes together, each aimed 45 degrees left and right of center. This method eliminates cancellation and provides good basic stereo separation.
Both the M/S and the X-Y methods also eliminate the “hole in the middle” effect caused when you place mikes too far apart. This happens when the sounds from each speaker on playback are so different due to mike placement that the center sounds empty and the full sound field does not exist.
When the sound source is big and covers a wide area, you can use a stereo recording technique called spaced pair. You place a pair of same-type mikes a foot or two apart, aiming them at the center of the sound source to help eliminate cancellation.
Professional sound studios use a method called close miking, in which every sound source has its own specially-adjusted mike. Each mike connects to a mixer input assigned to the right or left channel using the mixer’s pan-pot knob. If you have a mixer or access to one and enough mikes for each sound source, you can use this technique. The number of inputs on your mixer will limit the number of mikes you can use.
Also, observe the 3:1 Rule: there should be three feet of space between mikes for every one foot of space between the mike and its sound source. For example: two mikes each placed two feet from their sound sources should be at least six feet apart. Following this rule further helps to eliminate cancellation between mikes.
These tips will help you achieve audio quality far beyond that achieved with only your built-in mike. And any improvement in your audio immediately improves your video, reflecting positively on your reputation as a videomaker. Which is just what you want, right?
Doug Polk is Videomaker’s technical editor. Send e-mail to 71161, firstname.lastname@example.org.