User Guide

Sound System Design Reference Manual
A typical example of combining delayed
coherent signals is shown in Figure 1-5. Consider
the familiar outdoor PA system in which a single
microphone is amplified by a pair of identical
separated loudspeakers. Suppose the loudspeakers
in question are located at each front corner of the
stage, separated by a distance of 6 m (20 ft). At any
distance from the stage along the center line, signals
from the two loudspeakers arrive simultaneously.
But at any other location, the distances of the two
loudspeakers are unequal, and sound from one must
arrive slightly later than sound from the other. The
illustration shows the dramatically different frequency
response resulting from a change in listener position
of only 2.4 m (8 ft). Using random noise as a test
signal, if you walk from Point B to Point A and
proceed across the center line, you will hear a
pronounced swishing effect, almost like a siren. The
change in sound quality is most pronounced near the
center line, because in this area the response peaks
and dips are spread farther apart in frequency.
1-4
Figure 1-5. Generation of interference effects (comb filter response) by a split array
Figure 1-6. Audible effect of comb filters shown in Figure 1-5