An important consideration in microphone use is acoustic interference, which can occur whenever delayed versions of the same sound are mixed together, acoustically or electrically.
With mics, this may happen in several ways: mics of reverse polarity picking up the same sound, multiple mics picking up the same sound from different distances, a single mic picking up multiple reflections of the same sound, or any combination of these.
The results are similar in each case, and include audible peaks and dips in frequency response, apparent changes in directionality, and increased feedback problems.
The first situation - reverse polarity - will result in severe loss of sound, especially low frequencies, when a mic with reverse polarity is placed next to another of correct polarity and set to the same level.
Signals from the mics are then of equal strength but of opposite polarity. When these signals are combined in a mixing console, the cancellation is nearly total.
Although there is an international standard for mic polarity (pin 2+, pin 3-), a reversal may be found in an incorrectly wired mic cable.
It can be identified by checking each mic and cable against a mic and cable that are known to be correct. The bottom line is that in any system, all mics and cables must have the same polarity.
The second form of interference is the result of multiple mic pickup and can occur whenever more than one mic is used.
If the mics are at unequal distances from the sound source, the sound picked up by the more distant mic will be delayed relative to the near mic.
One of the most common (and easily solved!) sources of mic interference is caused by multiple mics picking up the same source, and it becomes even more problematic if the mics are at unequal distances from the source. The chart at the bottom shows actual measurement of comb filtering interference.
When these signals are combined in a console, peaks and notches occur at multiple frequencies which are related to the delay time, and thus, to the distances between the mic.
This effect is called comb filtering (because the resulting frequency response curve resembles the teeth of a comb.
As the delay time increases, comb filtering starts at lower frequencies. It’s especially noticeable at middle and high frequencies, and creates a “hollow,” distant sound.
The solution to this problem is to use the three-to-one rule: for multiple mics, the mic-to-mic distance should be at least three times the source-to-mic distance.
For example, when using individual mics on a vocal group, if a singer’s mic is one foot away, then the next nearest microphone should be at least three feet away from the first.
This insures that direct sound from the singer will not be strong enough to cause a noticeable interference when picked up by the more distant mics. As the source-to-mic distance increases, the distance to adjacent mics must also be increased.
An implication of the three-to-one rule: avoid picking up the same sound source with more than one mic. Mics should be placed and aimed to minimize areas of overlapping coverage.