Page 198 - Master Handbook of Acoustics

polarized and the talker is dead center, there would be a helpful 6-dB boost in level. Assume also
that the microphones are 24 in apart and the talker’s lips are 18 in from a line drawn through the
two microphones and on a level with the microphones. If the talker moves laterally 3 in, a 0.2-msec
delay is introduced, attenuating important speech frequencies. If the talker does not move, the

speech quality would probably not be good, but it would be stable. Normal talker movements shift
nulls and peaks up and down the frequency scale with quite noticeable shifts in quality.

FIGURE 10-9 Comb filtering is produced with two microphones feeding into the same monaural
amplifier with a sound source that moves about.

Example 3 Figure 10-10 shows comb-filter possibilities in a singing group with each singer
holding a microphone. Each microphone is fed to a separate channel but ultimately mixed together.

Each singer’s voice is picked up by all microphones but only adjacent singers may create
noticeable comb filters. For example, the voice of singer A is picked up by both microphones and
mixed, and may produce a comb-filter response resulting from the path difference. However, if
singer A’s mouth is at least three times farther from singer B’s microphone than from A’s own
microphone, the comb-filter effects are minimized. This “3:1 rule” works because maintaining this
distance means that delayed replicas are at least 9 dB below the main signal. Comb-filter peaks and
nulls are 1 dB or less in amplitude and thus essentially imperceptible.