Introducing the
Microphone
While there are lots of exotic ways to synthesize audio
signals, much of what we listen to starts out as a live sound, and that means it has to be
converted from the acoustic to the electronic realm by a microphone. Microphones can be
among the most intimidating of audio components, but you'll have to have some
understanding of them if you're going to try your hand at any sort of creative recording.
The job of a microphone is to sense the variations in air
pressure that make up the original sound and turn them into corresponding changes in
electrical voltage. There are various ways to do this, each of which has its own
advantages in terms of size, weight, performance, or cost.
In the early days, tape recorders were usually supplied
with a crystal or ceramic microphone, in which the sound flexed a diaphragm attached to a
piezoelectric element that twisted or bent, producing a voltage. These devices were
inexpensive, but they were poor performers and have mostly disappeared.
Much better are "dynamic" microphones, in which
the diaphragm is attached to a fine coil of wire that moves back and forth in the field of
a strong permanent magnet, exactly like a moving coil speaker in reverse. These
microphones are quite inexpensive, virtually indestructible, and can offer excellent
performance. They tend to be fairly bulky, which can be a plus if you're singing into one
on stage but which makes them less desirable where size and weight are concerns.
Even heftier, although potentially better performers, are
ribbon microphones, which have a slender metal ribbon suspended in a very powerful
magnetic field. These mikes are very sensitive to subtle sounds, but the magnets have to
be big and the low output necessitates a built-in transformer, so the package is usually
too unwieldy for anything but stationary applications. The ribbons are extremely fragile
as well
Nowadays, ribbons have been replaced largely by condenser
microphones, in which the metal (or metal-coated) diaphragm and a fixed plate close behind
it form the elements of a variable capacitor. As sound moves the diaphragm, the
capacitance changes in step with the changes in distance between the plates. The best
studio microphones tend to be of this type, but their disadvantage in consumer use is
that, to work, they must have an externally generated polarizing voltage. Pro equipment
provides that, home taping gear does not.
Still, most of the benefits of the condenser microphone can
be obtained from its cheaper offspring, the electret microphone. In this case, the
capacitor plate has a fixed charge, so an external power source is unnecessary. Electrets
can be extremely small with little sacrifice in quality, which is why they are the most
appropriate microphones for built-in applications. Virtually all microphones available to
consumers are electret or dynamic designs.
One of the most important characteristics of a microphone
is its directional pattern: its relative sensitivity to sounds coming from different
directions. An omnidirectional model is equally sensitive to sounds from all angles, while
the rarer bidirectional mike has a figure-eight pattern, sensitive to front and back but
very dead to the sides.
A microphone that is significantly more sensitive in one
direction than the others is sometimes called "unidirectional" but is more often
referred to as "cardioid" because a circular plot of its sensitivity, looking
from above (its "polar pattern"), is roughly heart-shaped. More extreme versions
are sometimes called "supercardioid" or "hypercardioid."
Such directionality can be useful for favoring the wanted
sound source and rejecting sounds from other directions, either ambient noise or
reflections off the room surfaces. The main drawback is what's called "proximity
effect": as you get closer to the mike, the bass is emphasized more and more. This is
fine if you want to snuggle right up to make it sound as though your voice has more body
than it really does, but it is far from accurate. Omnidirectional microphones don't have
that problem.
Two specialty microphones are increasingly common. One is
the stereo microphone, which is really two separate devices in a single case; it's also
called a coincident pair as the two elements are, in effect, in the same place. They are
cardioids, however, and their patterns are splayed by, say, 60 degrees (sometimes it's
adjustable) to pick up the real stereo spread of a performance.
The other type is the lapel microphone, which is almost
always an electret because unobtrusiveness is a major virtue in this sort of device. They
are widely used in television and can be practical in your videos as well. Usually they
are omnidirectional microphones, but they sometimes have heightened sensitivity to treble
to compensate for the fact that the wearer is usually talking away from the mike. It's an
effective solution, but it may make lapel models unsuitable when sound is being directed
toward them.
Microphones are only the first step in the recording chain;
they have to feed into something that can make use of their output. If a microphone is an
integral part of a piece of gear, there's little fear that it will be incompatible with
the rest of the equipment. But microphones are often separate components that have to be
connected to some external piece of electronics, or maybe several from time to time, so
compatibility is important.
There are some standards for connectors, but there are
several different ones, so your microphone will either have to match the rest of your gear
or you will have to use adapters. Most home tape recorders, if they have microphone inputs
at all, use quarter-inch "phone jacks," usually one for each microphone.
Portable equipment, including camcorders, generally economize on space by using
mini-jacks, stereo if the device is capable of stereo recording. Attaching a stereo
microphone to a mono jack, or vice-versa, is not likely to damage anything, but it will
probably result in your missing part of the signal you want to capture.
Choosing a microphone and ensuring that it will work with
the rest of your equipment are only the first steps, of course. How you use that mike is
vital too; we'll look at some recording tricks another time.
...Ian G. Masters
ian@mastersonaudio.com
|
