However, this 6 dB increase will not occur for frequencies that are not reflected, that is, frequencies that are either absorbed by the surface or that diffract around the surface.
High frequencies may be absorbed by surface materials such as carpeting or other acoustic treatments. Low frequencies will diffract around the surface if their wavelength is much greater than the dimensions of the surface: the boundary must be at least 5 ft. square to reflect frequencies down to 100 Hz.
The second case occurs when two closely spaced microphones are wired out of phase, that is, with reverse polarity. This usually only happens by accident, due to miswired microphones or cables but the effect is also used as the basis for certain noise-canceling microphones.
In this technique, two identical microphones are placed very close to each other (sometimes within the same housing) and wired with opposite polarity. Sound waves from distant sources which arrive equally at the two microphones are effectively canceled when the outputs are mixed.
However, sound from a source which is much closer to one element than to other will be heard. Such close-talk microphones, which must literally have the lips of the talker touching the grille, are used in high-noise environments such as aircraft and industrial paging but rarely with musical instruments due to their limited frequency response.
It is the last case which is most likely in musical sound reinforcement, and the audible result is a degraded frequency response called “comb filtering.” The pattern of peaks and dips resembles the teeth of a comb and the depth and location of these notches depend on the degree of phase shift.
With microphones this effect can occur in two ways. The first is when two (or more) mics pick up the same sound source at different distances. Because it takes longer for the sound to arrive at the more distant microphone there is effectively a phase difference between the signals from the mics when they are combined (electrically) in the mixer.
The resulting comb filtering depends on the sound arrival time difference between the microphones: a large time difference (long distance) causes comb filtering to begin at low frequencies, while a small time difference (short distance) moves the comb filtering to higher frequencies.
The second way for this effect to occur is when a single microphone picks up a direct sound and also a delayed version of the same sound. The delay may be due to an acoustic reflection of the original sound or to multiple sources of the original sound.
A guitar cabinet with more than one speaker or multiple loudspeaker cabinets for a single instrument would be examples. The delayed sound travels a longer distance (longer time) to the mic and thus has a phase difference relative to the direct sound.
When these sounds combine (acoustically) at the microphone, comb filtering results. This time the effect of the comb filtering depends on the distance between the microphone and the source of the reflection or the distance between the multiple sources.