This is important for a number of sound applications: for area pickup applications, such as choir lofts and stages, each section or area should be covered by only one mic; for lectern applications, only one mic should be used; when a lavalier (lapel) mic wearer talks into a fixed mic, one of the mics should be turned down of off at the mixing console.

The third form of interference, reflection pickup, may occur whenever there are nearby sound-reflecting surfaces.

This is often true in church settings, which often have hardwood or stone floors, brick or glass walls, wood or plaster ceilings, and solid lecterns and altars.

Increase Direct Sound
Remember, reflected sound is always delayed relative to direct sound.

When the delayed, reflected sound arrives with the direct sound at the mic, acoustic comb filtering is again the result.

With the three-to-one rule, the mic-to-mic distance should be at least three times the source-to-mic distance.

One solution is to increase the direct sound level by placing the mic as close as practical to the sound source, so that the direct sound is much stronger than the reflected sound.

Interference effects only become noticeable when the reflected sound is comparable in level to the direct sound. However, close placement may not be possible in the case of area coverage or moving sound sources.

Another solution is to decrease the reflected sound level. The mic may be moved away from the reflective surface, or re-oriented for minimum pickup of sound from that direction.

The acoustically reflective surface may also possibly be moved away, re-oriented, or treated with some sound-absorbent material. Note that sometimes this just isn’t feasible for economic or aesthetic reasons.

Yet another solution is to minimize the delay. Because the delay is due to the difference in the paths of the direct and reflected sound, it can be lessened by moving the mic close to the reflective surface, so that the direct sound and the reflected sound have nearly the same path.

This raises the frequency at which comb filtering starts. If the mic can be brought very close to the surface (within one-quarter inch), any comb filtering will occur above the audible range.

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