8--Multiple Microphone Use
Care has to be taken when using multiple microphones in the same location. Serious side effects occur when the sound of a single source arrives at two or more microphones. Sound travels at 1120 feet per second---very fast---so it is easy to understand that with one source and multiple microphones, sound will reach the closer microphone before the farther microphone. When these two signals arrive at the mixer, one slightly behind the other, you get a weird, hollow, "comb effect" sound or total cancellation! This side effect is called "phase cancellation" and is a condition that must be avoided because it will make good sound turn suddenly bad.
So here's the rule to memorize and never forget:
3:1 Ratio Rule
Whenever two microphones must be used in nearby locations and they can "hear" the same source, apply the 3:1 Rule. If microphone A is 1 foot from the sound source, Microphone B should be at least 3 times the distance away (3 feet) from Microphone A. If you would have Mic A at 2 feet, the minimum distance for Mic B must be 6 feet from Mic A---a 3:1 ratio. The sound picked up by the more distant microphone is then at least 12 dB less than the sound of the closer one. Any closer and you get the dreaded phase cancellation that causes things to sound hollow and empty for no apparent reason!
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