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Studies have shown that people can detect even small horizontal changes in the direction of a sound
source, but vertical shifts are much less noticeable. This suggests that the sound from center-cluster
speakers is more likely to be visually aligned with the performer than loudspeakers placed on each side
of the stage.
All those in the audience who are closer to the performer than the center cluster will hear the direct sound
from the performer before they hear the sound from the loudspeakers. This makes the sound seem to
come from the performer, not the loudspeakers. (See the Precedence Effect on page 47.)
9.4.2. Comb Filter Distortion
Many who took high school science may remember ripple tank experiments where waves are generated
from two separate point sources. The waves from each source combine to form visible interference
patterns. In some places the wave crests and troughs are in phase so they combined to make a larger
wave. In other places the crests are out of phase, so the crest of one wave source is canceled by the
trough of the other. Ripple tank experiments show the interference patterns are strongest when the
amplitudes of the waves from each source are equal.
A similar interference occurs in sound systems when a signal is delayed and mixed back into the original
signal. These interference patterns are called COMB FILTERS because their frequency response plots
look like the teeth of a comb (see Delay Figs. 1 & 2). There are a number of common situations that
cause comb filters. For example, when the program is played through two loudspeakers, the loudspeaker
that is farther away interferes with the closer loudspeaker. Comb filters are also created when a performer
is picked up by two microphones, one closer than the other. You even introduce comb filters by mixing
digital effects back into the “dry” signal at the mixer’s effects loop.
Delay Figure 1: COMB FILTERS.
Input signal mixed with a 2 msec.
delayed signal. (Both signals have
the same amplitude. Max. filter gain
is +6dB, and max. depth is -4)
Section Nine: Suggestions for Optimal Use of the GRAPHI-Q
