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Figure 8.6
Most microphones include a polar chart that diagrams their directionality and sensitivity. The polar chart shows a crosssection of the mic’s pickup pattern, with the mic laying in the middle of the chart, pointed toward the top. This chart diagrams a supercardioid mic.
Finally, parabolic mics are extremely directional mics that use a large parabolic dish to gather and focus sound onto the head of a unidirectional microphone. Parabolic mics are sensitive to sounds over 200 feet away and are not practical for most feature shoots. However, for difficult shooting situations where a subject is extremely far away (climbing the side of a mountain, for example), a parabolic mic might be the most reasonable way to record sound.
How a Mic Hears
All microphones work by converting sound waves into electrical impulses. There are different mechanisms for performing this conversion, and each has its own advantages and limitations. Typically, different mechanisms are used for different types of microphones.
A dynamic microphone is the most basic mechanism, consisting of a simple diaphragm attached to a coil of wire that is surrounded by a magnet. (It is, literally, the exact opposite of a speaker.) When the pressure of a sound wave hits the diaphragm, it moves the coil of wire within the magnetic field of the magnet. This generates a small electrical current that is fed out of the microphone.
Because of their simple design, dynamic microphones are incredibly rugged and require no external power or batteries. The dynamic mechanism has a very short range, and so is typically only used in handheld mics. Their short range makes dynamic mics well suited to narration and voice-over recording (they tend to pick up only the voice of the speaker), but frequently impractical for on-set recording.
In a condenser mic, a metal plate and a metal diaphragm are separated by a thin gap of air. Both plates are electrically polarized by an external power supply. Because of a property called capacitance, an electric charge is created between the plates. As incoming sound waves move the metal diaphragm, the amount of charge between the plates changes. The electrical charges
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produced by this mechanism are tiny and must be amplified using power from either a battery or a phantom power supply housed externally. Because the diaphragm in a condenser mic is so small, it requires very little acoustic pressure to make it move. Consequently, condenser mics are much more sensitive than dynamic mics.
An electret condenser is a somewhat cheaper, lower-quality version of the condenser mechanism. Electret condenser mics use a small battery to create a polarizing voltage, and so don’t require an external power supply.
Because their pickup mechanisms are so tiny, condenser mics can be made very small—most clip-on mics use electret condenser mechanisms. Most condenser mics also include a small preamplifier, either built into the mic itself, or located outboard in a small box.
Ideally, you’ll want to choose a condenser mic simply because it delivers the best quality. However, for recording voice-overs or narration, or for doing interviews in more harsh conditions (bad weather, extreme humidity, etc.), a handheld, dynamic mic might be a better choice.
Types of Mics
No matter what type of pickup element is used, mics typically fall into three categories: handheld, lavalier (clip-on), and shotgun. Which to choose depends on your set, and you might find yourself using multiple mics of different types. Obviously, for a dramatic production, keeping the mic hidden is your primary concern (after getting good audio!). However, even documentary producers might want unobtrusive mics, both for aesthetics and to record their subjects more clandestinely. (Note that we’re not advocating illegal wiretapping! Rather, we’re simply trying to remind you that a microphone can be a very intimidating object to someone who hasn’t spent a lot of time “on-camera.” To keep an interview or documentary subject calm and more spontaneous, a less-intrusive type of mic might be preferable.)
We’ve all seen the handheld mics used by talk show hosts, rock stars, and karaoke singers. Usually omnidirectional dynamic mics, these are the least useful mics for onset use. Because they typically operate at short range, they have to be held close to the speaker’s mouth, making it nearly impossible to keep them hidden (see Figure 8.7).
Figure 8.7
A handheld mic on a microphone stand.
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Lavaliers
Lavalier, or clip-on mics, are the small condenser mics that you see clipped to the front of newscasters. Lavaliers are usually omnidirectional mics, but because they are kept so close to the speaker’s mouth, they rarely pick up extraneous sound, making them well suited to record individual performers. Moreover, because of their small size, they can easily be hidden under clothing (see Figure 8.8).
Figure 8.8
A clip-on, or lavalier, mic.
Although laveliers are omnidirectional, they might still have some directional characteristics. A somewhat directional lavalier will produce a very clear recording, but might lack some ambient sound that makes for a full, realistic sound. (These lavs are typically used for newscasting where the sound of an authoritarian voice is all you’re interested in recording.) A more omnidirectional lav can record a more natural sound, but you’ll have to worry about controlling extra ambient noises on your set.
For scenes involving many actors, you’ll probably need a separate lav for each performer, which means that you’ll have to deal with the requisite logistical troubles, such as wiring multiple performers and managing lots of mic cables. (You’ll also need a mixer, which we’ll discuss later in this chapter.) Because keeping cables hidden (and keeping actors from tripping over them) can be a problem, wireless lavaliers are a good solution for complex shoots involving lots of mics. A wireless lav system consists of a normal lav microphone attached to a small transmitter worn by the actor. A receiver picks up the audio and routes it on to your recording device.
Although a great solution to the “obtrusive cable” problem, wireless lavaliers have complications of their own. First, you’ll want to be sure you get a system that uses an FM transmitter, for higher quality. In addition, you’ll have to worry about radio interference from other devices, cell phones, overhead power lines, and so on. These problems can be easy to solve, but they can take time.
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Shotgun Mics
Those long, skinny microphones that you often see sticking off video cameras are referred to as shotgun mics. Although varied in pickup mechanism and directional characteristics, shotgun mics provide the greatest flexibility for miking. Most shotgun mics record stereo audio, usually by having two pickups inside, one each for left and right channels (see Figure 8.9).
Figure 8.9
A typical shotgun mic.
If your camera already has a shotgun mic attached, odds are it’s not very high quality. Whatever its quality, the front of your camera is not the best location for a mic. Ideally, you’ll want a supercardioid or hypercardioid shotgun mic attached to a boom pole. We’ll discuss using a boom later in this chapter.
A Little of Each
The ideal audio setup involves several microphones, frequently of different types. If you can only afford one mic, then you’ll want to get a directional shotgun mic and attach it to a boom. However, for better sound and greater postproduction audio editing flexibility, fitting actors with lavaliers while simultaneously miking them with a boom mic provides your sound editor with more options for mixing and correcting difficult-to-understand audio.
No matter what types of mics you choose, you’ll ideally want to listen to them before you buy. Just like loudspeakers, different mics provide different sounds. Some will boost low frequencies to help create authoritative, booming voices. Others will have more of a “flat,” realistic response. Although you can look at frequency response charts for a mic, such stats don’t reveal everything about how a mic will sound. Your ear is the best judge of whether or not a mic is right for you.
Mixing
As you might have noticed, most cameras and tape decks only have one place to plug in a microphone. Therefore, if you’re using multiple microphones, you’ll have to plug them into a mixer to mix their signals down to two or four signals that can be plugged in to the inputs on your camera or deck.
When choosing a mixer, you’ll need to pick a unit with enough inputs for the number of mics you’ll be using (see Figure 8.10). You might also want to select a unit that has a built-in graphic equalizer. As we’ll discuss later, an equalizer will let you slightly modify the sound coming through a mic to add more bass or treble. Equalizers are a great way of improving the sound of a weak voice or removing certain problems such as “shushing” S sounds or hisses.