Tip #6:
The 3 to 1 Rule
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Suppose you have set up several microphones to pick up sound sources. Each sound source has its
own close-placed mic. You are mixing the mic signals through a mixer.
Sound from a single source arrives at each microphone at a different time. So, a mic that is distant from the
source is picking up the source with a delay, which causes variable phase
shift vs. frequency. When you combine the close and distant mic signals in
your mixer, certain frequencies cancel out due to phase interference,
creating a "comb-filter" effect. The frequency response of a comb
filter has a series of peaks and dips (see figure below.) This
response often gives a thin, hollow, filtered tone quality.
Audible comb filtering can occur whenever two or more mics pick up the
same sound source at about the same level but at different distances, and
are mixed to the same channel.
This problem can be minimized or eliminated by following
the 3:1 rule: Separate the mics by at least 3 times the mic-to-source
distance. This creates a level difference of at least 9 dB between
microphones, which reduces the comb-filter dips to an inaudible 1 dB or
less.
In general, place mics close to their sources and keep the mics far apart
to prevent audible comb filtering.
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This figure shows how to mike two sound sources
with two mics while following the 3:1 rule. If the mic-to-source distance
were 2 feet, the mics should be at least 2x3 or 6 feet apart to prevent
audible comb filtering.
The left-side frequency response
results when two mics are mixed to the same channel at equal levels, and
you followed the 3:1 rule. The right-side response results
when you don't follow the 3:1 rule.
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