Lexicon PCM 42 - 4.5 Delay Lines in Distributed Sound Reinforcement Systems

4.5 DELAY LINES IN DISTRIBUTED SOUND
REINFORCEMENT SYSTEMS
SOURCE
Figure 4.12 — SOUND REINFORCEMENT WITH ONE REMOTE ZONE OF SPEAKERS
The delay for the remote speakers in milliseconds should be equal to the distance of the listener from the main
speakers (in feet), minus his distance from the remote speakers, times 0.885, plus 20.
In sound reinforcement systems, delay lines can be
used not to create echoes but to eliminate them. Time
delay cannot help reflections from walls or ceilings, but
it can be used to avoid the confusion that results when
the sound arrives at the listener from different
loudspeakers at different times.
ideally the sound from a reinforcement system should
arrive at each listener first from a main cluster of
speakers over or near the stage, and then, between 5
and 30 milliseconds later, from any other auxiliary
speaker(s}. This can be achieved by connecting the
PCM 42 as shown in Fig. 4.12 and dialing in the proper
amount of delay. Use the DLY X1 setting for full band-
width.
The correct amount of delay for a given setup may be
calculated exactly with the algorithm given in Lexicon
Application Note AN-2, “Application of Digital Delay
Units to Sound Reinforcement.’ The general principle
is to calculate the difference in path length to the
listener from the main and remote speakers in feet,
multiply the result by 0.885, and add 20. The result is
the number of milliseconds of delay required for the
remote speakers.
As an example, suppose that the remote speakers are
100 feet closer to the listener than the main cluster. The
required delay for the remotes would be 88.5 + 20 =
108.5 ms. All speakers with the same path length dif-
ference may be fed from a single delay; speakers whose
distance differs by more than about 40 feet must be fed
from a separate delay line (or tap) and a separate power
amplifier.