Page 196 - Master Handbook of Acoustics

disregards these distortions; this is called binaural suppression of differences in timbre. However, no
generally accepted theory exists to explain how the auditory system accomplishes this. Distortion can
be heard by plugging one ear; however, this destroys the stereo effect. Comparing the timbre of
signals from two loudspeakers (producing comb-filter distortion) to one loudspeaker (that does not)

will demonstrate that stereo comb-filter distortion is barely audible. The timbre of the two is
essentially the same. Furthermore, the timbre of the stereo signal changes little as the head is turned.

Reflections and Spaciousness

A reflected sound reaching the ear of a listener is always somewhat different from the direct sound.

There are numerous causes for this. The reflecting sound will be changed by the frequency
characteristics of the reflecting wall. By traveling through the air, both the direct and reflected
components of a sound wave are altered slightly, due to the air’s absorption of sound, which varies
with frequency. The amplitude and timing of the direct and reflected components differ. The human
ear responds to the frontal, direct component somewhat differently than to the lateral reflection from

the side. The perception of the reflected component is always different from the direct component.
The amplitudes and timing will be related, but with an interaural correlation less than maximum.
Weakly correlated input signals to the ears contribute to the impression of spaciousness. If no
reflections occur, such as when listening outdoors, there is no feeling of spaciousness. If a room
supplies “correct” input signals to the ears, the perception of the listener is that of being completely
enveloped and immersed in the sound. The lack of strong correlation is a prerequisite for the

impression of spaciousness.

Comb Filters in Microphone Placement

When two microphones separated in space pick up a single sound at slightly different times, their
combined output will be similar to the single microphone with delayed reflections. Therefore, spaced
microphone stereo-pickup arrangements are susceptible to comb-filter problems. Under certain
conditions the combing is audible, imparting phasiness to the overall sound reproduction, interpreted
by some as room ambience. It is not ambience, however, but distortion of the time and intensity cues
presented to the microphones. It is evident that some people find this distortion pleasing, so spaced
microphone pickups are favored by many producers and engineers.

Comb-Filter Effects in Practice: Six Examples

Example 1 Figure 10-8 shows three microphone placements that produce comb filters of varying
degree. A hard, reflective floor is assumed in each case; other room reflections are not considered.
The numerical results of these placements are given in Table 10-2. In a close source-to-microphone
setup, the direct component travels 1 ft and the floor-reflected component travels 10.1 ft. The
difference between these (9.1 ft) means that the floor reflection is delayed 8.05 msec (9.1/1130 =

0.00805 second). The first null is therefore at 62 Hz with subsequent null and peak spacing of 124