User Guide

Sound System Design Reference Manual
The analysis shown in Figure 6-14 indicates
that when each of the two horns is powered by one
watt, the reverberant field in the room (read directly
from Figure 5-21) is 94 dB-SPL. The direct field level
provided by each horn over its coverage angle is
about 85 dB-SPL. This produces a direct-to-
reverberant ratio of -9 dB, and an inspection of
Figure 6-13 tells us that the system will have
marginal intelligibility. Note that for 4 seconds of
reverberation time, the direct-to-reverberant ratio
should be no less than about -7 dB if acceptable
intelligibility is to be expected. This simple analysis
has told us that, on paper, we have designed a
sound system which will likely fail to satisfy the
customer.
Had the system consisted of a single horn,
knowledge of its on-axis DI and Q could have led
quickly to a determination of critical distance, and the
direct-to-reverberant ratio could have been scaled
from D
C
. However, for the composite array analyzed
here, there is no single value of DI or Q which can be
used, and a direct calculation of the overall
reverberant level, using what we know about the
efficiency of the transducers, and making a
comparison with the direct field, based on the
sensitivities of the transducers, is the quickest way to
solve the problem.
But the question remains: What kind of system
will work in this large resonant room? Clearly, a
distributed system is called for. In such a system, a
number of lower-powered loudspeakers are placed
on columns on each side of the church, each
loudspeaker covering a distance of perhaps no more
than 5 or 6 meters. In this way, the direct-to-
reverberant ratio can be kept high. If such a system
is further zoned into appropriate time delays, the
effect will be quite natural, with subjective source
localization remaining toward the front of the listening
space. Details of this are shown in Figure 6-15.
Again, we calculate the total reverberant level
and compare it with the longest throw each
loudspeaker will be called upon to handle. There are
14 loudspeakers, 7 on each side. Let us assume that
the efficiency of these loudspeakers is 1.2% and that
their sensitivity is 95 dB, 1 watt at 1 meter. Feeding
one watt into each loudspeaker results in a total
acoustical power of 14 x .012, or 0.17 watt. Again
using Figure 5-21, we observe that the reverberant
level will be 92 dB-SPL. The longest throw each
loudspeaker has to cover is, say, 4 meters. Since the
1-watt, 1-meter sensitivity is 95 dB, the direct field for
each loudspeaker will be 12 dB lower, or 83 dB.
Figure 6-14. Analysis of intelligibility criteria
6-14