Page 201 - Master Handbook of Acoustics

FIGURE 10-13 With a flush-mounted microphone, sound from the source S that strikes the surface
does not reach the microphone, thus avoiding comb-filter effects. Another advantage of this mounting
is a level increase due to the pressure buildup at the reflecting surface.

Estimating Comb-Filter Response

A few simple relationships can be used to estimate the effect of comb filters on the response of a

system. If the delay is t seconds, the spacing between peaks and the spacing between nulls is 1/t Hz.
For example, a delay of 0.001 second (1 msec) spaces the peaks at 1,000 Hz (1/0.001 = 1,000 Hz)
and the nulls will also be spaced the same amount, as shown in Table 10-3.

TABLE 10-3 Comb-Filter Peaks and Nulls

As noted, the frequency at which the first null (i.e., the null of lowest frequency) will occur is
1/(2t) Hz. For the same delay of 1 msec, the first null will occur at 1/(2 ×0.001) = 500 Hz. For this 1-
msec delay, the first null is at 500 Hz, peaks are spaced 1,000 Hz, and nulls are spaced 1,000 Hz
apart. Of course, there is a peak between each adjacent pair of nulls at which the two signals are in

phase. Adding two sine waves with the same frequency and the same amplitude, in phase, doubles the
amplitude; yielding a peak 6 dB higher than either component by itself (20 log 2 = 6.02 dB). The nulls
will be at a theoretical minimum of minus infinity as they cancel at phase opposition. In this way, the