Miking Basics
Sound that gets mixed for an audience has to come from somewhere, and a lot of it isn't created in an electronic instrument. Even in our digital world, where we plug in USB or FireWire or other cables to move a signal from one place to another, there's still only one way to transfer the human voice or acoustic instruments to a PA or recording gear: a microphone.
How a microphone works
A microphone is sort of like a speaker working in reverse. A speaker
takes in electrical signals and makes its diaphragm (the speaker's
paper cone) vibrate, which produces sound. A microphone has a diaphragm
that vibrates when a sound hits it, and it converts that sound into an
electrical signal.
Your main mikes
There are two main types of microphones you are likely to deal with
when running a PA: dynamic and condenser. A dynamic microphone is
usually built to be very durable, both in terms of how you treat it and
how much sound it can handle without creating a distorted signal.
Condensers are great for acoustic instruments because their sound is
bright and crisp. However, they're also more fragile than most dynamic
mikes, so you have to baby them a bit.
You probably want to know what makes these mikes tick. Okay, here goes...
Dynamic mikes use a coil of wire mounted on the diaphragm to generate their signals. Dynamic mikes tend to be inexpensive, shock-resistant, and difficult to distort even at excessively high volumes. They're rugged, and because they can handle loud sounds, they're an excellent choice for placing in front of a bass drum or an amplifier. The trade-off is that dynamics aren't the most sensitive mikes.
Instead of a wire coil and a magnet, a condenser mike uses two tiny electrically charged plates, and one of these is the diaphragm. When the diaphragm moves in response to a sound, the distance between the plates changes, so the voltage changes, and an electrical signal that represents the sound is sent out. A power supply (often built into a mixer and called "phantom power") is necessary to charge the microphone's plates, although some condensers use batteries.
Matching the mike to the job
Microphones are designed to be sensitive either to everything around
them (called omnidirectional) or mostly what's right in front of them
(directional).
An omnidirectional microphone is great for recording one instrument by itself because it picks up everything, including the instrument and the sound of the room. Because it picks up everything, though, an "omni" isn't a great choice for multiple instruments onstage for two reasons: (1) because it picks up everything, it's hard to keep one instrument's sound out of another instrument's mike, and (1) because it picks up everything, including audience noise and the sound of the PA's speakers, you can introduce unwanted sounds to the mix, as well as feedback, a squealing sound that occurs when the mike "hears" the speakers and recycles the sound.
A directional microphone often has what's called a "cardioid pattern" ("cardioid" means "heart-shaped"). Without getting into the science of a microphone's pickup pattern, let's just say that a cardioid mike is the most generally useful. It is most sensitive to sounds directly in front of the mike, with less sensitivity to sounds coming from the sides, and no sensitivity to sounds coming from behind it. Since the cardioid pattern blocks sounds from behind the mike, it's a good choice when you're miking multiple instruments in the same room or on the same stage and want to keep them separated in your mix.
Impedance
Okay, put on your science hat. But just for a minute. Impedance refers
to a circuit's tendency to resist the flow of electrons through the
wire. Microphones can be high-impedance or low-impedance, which means
that you have to make sure that if your mixer requires high-impedance
microphones, you have high-impedance microphones, and if it needs
low-impedance microphones, you have low-impedance microphones. Why does
this matter? It has to do with the way the mixer deals with the
electrical signal, and a mismatch can result in bad sound and extra
noise. This shouldn't be a big problem, since most low-impedance
microphones use a 3-pin plug called an XLR connector. A high-impedance
microphone usually has a 1/4" phone plug like you'd find on the end of
an electric guitar's cord. Match these to the appropriate jacks on the
mixer, and you don't have to worry about the science part of it.
What if you want to use a low-impedance mike, but your mixer or tape deck doesn't have 3-pin XLR, low-impedance jacks? If that's the case, then you need an impedance-matching transformer, available from just about any music store or electronic supply store that sells microphones.
Phase
Sometimes two microphones are close enough together onstage that they
hear the same thing, except slightly differently. What happens is that
the sound waves hit the microphones at different times, and if their
timing is off, then it's said that the signals are out of phase. How
can you tell if the mixture of two microphones is out of phase? The
best thing is to listen to one mike by itself and then the other. When
both mikes are on, if you hear a hollow sound, or if the sound is
suddenly weak, then the mikes may be out of phase. Some microphones
have a phase switch that you can flip to fix this. Some mixers also
have phase switches.
Feedback
You're familiar with the howl of feedback, that squeal and howl when
the principal leans too close to the microphone at an assembly.
Feedback occurs when the speaker output is allowed to leak back into
the mike, and the system just keeps amplifying and re-amplifying it
until it's out of control. Uncontrolled feedback can damage speakers,
amplifiers, and ears.
One solution is to keep the speakers off (and in the case of the principal, not a bad option). But the practical approach is to make sure microphones aren't picking up the sound of the speakers, and the simplest way to assure this is to point the speakers away from the mikes. Otherwise, there are sophisticated boxes that you can connect to your mixer so that the feedback is fought electronically.