User Guide

Sound System Design Reference Manual
room constant with some confidence, then we can
estimate the sound pressure level that will be
produced in the reverberant field of the room for a
given acoustical power output.
Figure 5-22 enables us to determine by
inspection the room constant if we know both α and
the total surface area. This chart can be used with
either SI or English units.
If both room constant and directivity factor of a
radiator are known, the critical distance can be
solved directly from the following equation:
D = .14 QR
C
This equation may be used with either SI or English
units, and a graphical solution for it is shown in
Figure 5-23. It is helpful to remember that the
relationship between directivity index and critical
distance is in a way very similar to the inverse square
law: an increase of 6 dB in directivity (or a “times-
four” increase in Q ) corresponds to a doubling of the
critical distance. One might think of this as the “direct
square law”.
A second useful factor to keep in mind is that
the directivity index of a person talking, taken in the
5-19
1 kHz range along the major axis, is about 3 dB.
For convenience in sound system calculations, we
normally assume the Q of the talker to be 2.
These two facts can be used to make
reasonably accurate acoustical surveys of existing
rooms without equipment. All that is needed is the
cooperation of a second person — and a little
experience. Have your assistant repeat a word or
count slowly in as even a level as possible. While
he is doing this, walk directly away from him while
carefully listening to the intensity and quality of his
voice. With a little practice, it is easy to detect the
zone in which the transition is made from the direct
field to the reverberant field. Repeat the experiment
by starting at a considerable distance away from the
talker, well into the reverberant field, and walking
toward him. Again, try to zero in on the transition zone.
After two or three such tries you may decide,
for example, that the critical distance from the talker
in that particular room is about 4 meters. You know
that a loudspeaker having a directivity index of 3 dB
will also exhibit a critical distance of 4 meters along
its major axis in that room. To extend the critical
distance to 8 meters, the loudspeaker must have a
directivity index of 9 dB.
Figure 5-23. Critical distance as a function of room constant
and directivity index or directivity factor