User Guide

Sound System Design Reference Manual
Thus, the direct-to-reverberant ratio will be
83-92, or -9 dB. This is still not good enough, but we
must remember that more than half the listeners will
be closer to a loudspeaker than 4 meters. Another
very important point we have not yet considered is
the fact that the distributed loudspeakers are aimed
almost totally into the audience, with its absorption
coefficient considerably greater than α of .12. This is
the appropriate time to use R’ instead of R in our
calculations.
Calculating R’ based upon an α’ of .95 for the
audience area in the 1 kHz band:
R’ = Sα/(1 - α’) = 375/.05 = 7500 m
2
.
Recalculating the reverberant level from Figure
5-21, we get 80 dB-SPL. The new direct-to-
reverberant ratio is 83 - 80, or +3 dB, and the system
will be quite workable.
Will the reverberant level really be only 80 dB?
In actuality, we might observe something a little
higher than 80 dB, but not enough to alter our
analysis significantly.
We can also ask the question of whether our
analysis using R’ would have materially affected the
performance of the central array system. A rigorous
analysis would be a little tedious, but we can make a
simplifying assumption. Let us assume that half of
the direct sound from the central array was incident
on the audience with its .95 absorption coefficient.
Let us round this off and call it 1.0 instead, resulting
in no sound at all being reflected from the audience.
This would only lower the reverberant level in the
room by 3 dB, hardly enough to make the direct-to-
reverberant ratio workable.
More than any other we have carried out in this
chapter, this analysis points up the multi-dimensional
complexity of sound system design. Again, we state
that there are no easy solutions or simple equations.
Instead, there is only informed rational analysis and
thoughtful balancing of many factors.
Figure 6-15. A distributed system in a large church
6-15