User Guide

Sound System Design Reference Manual
Once the critical distance is known, the ratio of
direct to reverberant sound at any distance along
that axis can be calculated. For example, if the
critical distance for a talker is 4 meters, the ratio of
direct to reverberant sound at that distance is unity.
At a distance of 8 meters from the talker, the direct
sound field will decrease by 6 dB by virtue of inverse
square law, whereas the reverberant field will be
unchanged. At twice critical distance, therefore, we
know that the ratio of direct to reverberant sound
must be -6 dB. At four times D
C
, the direct-to-
reverberant ratio will obviously be -12 dB.
Statistical Models vs. the Real World
We stated earlier that a confidence level of
about 10% allowed us to simplify our room
calculations significantly. For the most part, this is
true; however, there are certain environments in
which errors may be quite large if the statistical
model is used. These are typically rooms which are
acoustically dead and have low ceilings in relation to
their length and width. Hotel ballrooms and large
meeting rooms are examples of this. Even a large
pop recording studio of more regular dimensions
may be dead enough so that the ensemble of
reflections needed to establish a diffuse reverberant
field simply cannot exist. In general, if the average
absorption coefficient in a room is more than about
0.2, then a diffuse reverberant field will not exist.
What is usually observed in such rooms is data like
that shown in Figure 5-24.
Peutz (9) has developed an empirical equation
which will enable a designer to estimate the
approximate slope of the attenuation curve beyond
D
C
in rooms with relatively low ceilings and low
reverberation times:
∆
0.4 V
h T
60
≈ dB
In this equation, D represents the additional fall-
off in level in dB per doubling of distance beyond D
C
.
V is the volume in meters
3
, h is the ceiling height in
meters, and T
60
is the reverberation time in seconds.
In English units ( V in ft
3
and h in feet), the equation
is:
∆
0.22 V
h T
60
≈ dB
As an example, assume we have a room
whose height is 3 meters and whose length and
width are 15 and 10 meters. Let us assume that the
reverberation time is one second. Then:
∆
0.4 450
3 1
= 2.8 ≈
()
dB
Thus, beyond D
C
we would observe an additional
fall-off of level of about 3 dB per doubling of distance.
5-20
Figure 5-24. Attentuation with distance in a relatively dead room