Comb Filter Distortion; Center Cluster Speakers - SABINE Power-Q Operating Information Manual

Center cluster speakers offer several advantages over systems that have speakers mounted on
the sides. The most obvious advantage is that the distance to the closest and most distant
locations in the audience is often almost equal, so most listeners hear about the same level.
Center clusters also offer two other advantages regarding visual imaging.
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
below.)
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 Figs. 8 & 9). There are a number of common
situations that cause comb filters. For example, when the program is played through two loud-
speakers, 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.
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