User Guide

Sound System Design Reference Manual
Calculations for a Distributed
Loudspeaker System
Figure 6-8 shows a moderate-size meeting
room or lecture room. Its volume is 485 m
3
, surface
area is about 440 m
2
, and α is 0.2 when the room is
empty. For an unaided talker in the empty room, R is
110 m
2
. However, when the room is fully occupied, α
increases to 0.4 and the corresponding room
constant is 293 m
2
. We calculate the critical distance
for the unaided talker (directivity index of 3 dB) to be
2 meters in an empty room and 3.4 meters when the
room is full.
The room is provided with a sound system
diagrammed in Figure 6-9. Forty loudspeakers are
mounted in the ceiling on 1.5 meter centers to give
smooth pattern overlap up into the 4 kHz region.
Coverage at ear level varies only 2 or 3 dB through
the entire floor area.
The usual definitions of critical distance and
direct-to-reverberant ratio are ambiguous for this kind
of loudspeaker array. Here, however, we are
interested only in potential acoustic gain, and the
ambiguities can be ignored. We already have stated
that the loudspeaker array lays down a uniform
blanket of sound across the room. The relative
directional and temporal components of the sound
field do not enter into gain calculations.
An omnidirectional microphone is located
.6 meters from the talker, less than 1/3 D
C
. No matter
how many people are present, the microphone is in
the direct field of the talker.
The farthest listener is 9 meters from the talker,
more than three times D
C
when the room is empty,
and just about three times D
C
when the room is full.
If the unaided talker produces 70 dB sound
level at the microphone with the system off, and if the
amplified sound level can be no greater than 70 dB
at the microphone with the system on, then the
maximum level is 70 dB everywhere in the room
.
Figure 6-8. A moderate-size lecture room
Figure 6-9. Sound system in a medium-size lecture room
6-8