User Guide

Sound System Design Reference Manual
L
REV
= 126 + 10 log W
A
- 10 log R,
where W
A
is the continuous acoustical power output
from the transducer and R is the room constant in m
2
.
In using this equation, we assume that the
space is fairly reverberant at very low frequencies
and that the value of absorption coefficient at 125 Hz
(the lowest value normally stated for materials) will
be adequate for our purposes.
Some design engineers prefer to make actual
direct field calculations for one or more subwoofer
units at a distance, say, of two-thirds the length of the
enclosed space. In large motion picture spaces, both
sets of assumptions yield results that are usually
within 5 dB of each other.
The phenomenon of mutual coupling always
comes to our aid in increasing the power output of
combined subwoofer units. Figure 7-6A shows the
7-7
Figure 7-4. Details of a 70-volt transmission system
Figure 7-5. Details of a typical 70-volt distribution transformer