Specifications

This leads to the first and most important rule of
microphone placement: Place the microphone as close
as practical to the desired sound source. It has several
corollaries: 1) place the microphone as far as possible
from loudspeakers and other undesired sources; 2) use
directional microphones to minimize ambient sound
pickup; 3) aim directional microphones toward the desired
sound and/or away from undesired sound; and 4) keep
the system gain to a minimum.
Ultimately, the position chosen should be consistent
with the characteristics of both the sound source and the
microphone: larger sources such as an audience may
require greater distance, depending on the microphones’
directionality; extremely loud sources may require greater
distance to avoid overload of some sensitive condenser
microphones; very short distances may cause proximity
effect (low frequency boost) in some directional micro-
phones; close-up speech use requires adequate “pop”
filtering. In any case, following the above rules will give the
best pickup of the desired sound, the minimum pickup of
ambient noise, and least likelihood of feedback.
Another important acoustic interaction is called
interference. Interference effects may occur whenever
delayed versions of the same sound are mixed together,
acoustically or electrically. With microphones, this may
happen in several ways: 1) microphones of reverse polarity
picking up the same sound, 2) multiple microphones
picking up the same sound from different distances, 3) a
single microphone picking up multiple reflections of the
same sound, or 4) any combination of these. The effects
are similar in each case, and include audible peaks and
notches in frequency response, changes in directionality,
and increased feedback problems.
The first situation, reverse polarity, will result in severe
loss of sound, especially low frequencies, when the micro-
phones are placed next to each other and set to the same
level. Signals from the microphones are then of equal
strength but of opposite polarity. When these signals are
combined in a mixer the cancellation is nearly total.
Although there is an international standard for
microphone polarity (pin 2 +, pin 3 -), a reversal may be
found in a mis-wired microphone or, more commonly,
in a mis-wired microphone cable. It can be identified
by checking each microphone and cable against a
microphone and cable that are known to be correct.
In any installation, all microphones and microphone
cables must have the same polarity.
The second form of interference, multiple microphone
pickup, can occur whenever more than one microphone is
used. If the microphones are at unequal distances from
the sound source, the sound picked up by the more distant
microphone will be delayed relative to the nearer micro-
phone. When these signals are combined in a mixer,
peaks and notches occur at multiple frequencies which
are related to the delay time, and hence, to the
distance, between the microphones. This effect is called
comb filtering because the resulting pattern of notches in
the frequency response resembles the teeth of a comb.
As the delay time increases, comb filtering begins at lower
frequencies. It is especially noticeable at mid- and high-
frequencies, and creates a “hollow”, distant sound.
17
MEETING FACILITIES
Audio Systems Guide for
Multi-microphone pickup and the resulting “comb filtering”
Reverse polarity