Not all manufacturers acknowledge processing delays in their specifications, but you must
take them into account when synchronizing your system. Make sure all digital equipment is
on and not bypassed when synchronizing. Also, be careful to make an appropriate adjust-
ment in your delay lines if you later add any type of digital equipment to the system.
Center Cluster Speakers
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 almost equal, so most listeners hear about the same
level. Center clusters also offer two other advantages regarding the 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 Prece-
dence Effect below.)
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 amplitude of the waves from each
source is 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. 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 micro-
phones, 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.
4
Fig. 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.)
