Page 197 - Master Handbook of Acoustics

Hz. The level of the reflection is –20 dB referred to the direct component (20 log 1.0/10.1 = 20
dB). The direct component is thus 10 times stronger than the floor reflection. The effect of the comb
filter would be negligible in this case.

FIGURE 10-8 Common microphone placements can produce comb-filter effects (see Table 10-2). At

a distance of 1 ft, the reflection level of –20 dB yields minimum comb-filter problems. At a distance
of 4 ft, the reflection level of –8 dB may yield some comb-filter problems. (C) At a distance of 10.3
ft, the reflection level almost equals the direct level, and thus comb-filter problems are certain.

TABLE 10-2 Comb-Filter Effects from Microphone Placement (Refer to Fig. 10-8)

The two other microphone placements (see Fig 10-8) yield lower reflection levels (see Table 10-

2). A source-to-microphone distance of 4 ft presents an intermediate case, where the reflection
level is 8 dB below the direct-signal level; the comb-filter effect would be marginal. With a
source-to-microphone distance of 10.3 ft, the reflection-level difference of only 1 dB; the reflection
is almost as strong as the direct signal; the comb-filter effect would be considerable. In contrast to
these microphone placements, consider what would happen if the microphone was placed directly
on the floor some distance from the source. This technique would essentially eliminate the
difference between the direct and reflected path length.

Example 2 Figure 10-9 shows two microphones, for example, on a podium. Stereo reproduction
systems are relatively rare in auditoriums; the chances are very good that the two microphones are
fed into a monaural system and thus become an excellent producer of comb-filter effects. The
common excuse for two microphones is to give the speaker greater freedom of movement or to
provide a spare microphone in case of the failure of one. Assuming the microphones are properly