User Guide

Sound System Design Reference Manual
Level at listener = L - 20 log (D
ct
/D
s
), where D
ct
is the critical distance of the talker. The assumption
made here is that the level at the listener is entirely
made up of the talker’s reverberant field and that that
level will be equal to the inverse square component
at D
ct
.
Now, the system is turned on, and the gain is
advanced until the loudspeaker produces a level L at
the microphone. At the same time, the loudspeaker
will produce the same level L at the listener, since
both microphone and listener are in the
loudspeaker’s reverberant field.
Subtracting the levels at the listener between
the system on and the system off, we have:
Difference = L - [L - 20 log (D
ct
/D
s
)]
or:
Gain = 20 log D
ct
- log D
s
Finally, adding a 6 dB safety factor:
Gain = 20 log D
ct
- 20 log D
s
- 6
Note that there is only one variable, D
s
, in this
equation; D
ct
is more or less fixed by the directivity of
the talker and the acoustical properties of the room.
Of course there are many systems in which the
microphone may be placed in the transition zone
between the talker’s direct and reverberant fields, or
where the listener is located in the transition region
between the loudspeaker’s direct and reverberant
fields. In these more complicated cases, the
foregoing equation does not apply, and the designer
must analyze the system, both on and off, pretty
much as we went stepwise through the three
examples at the start of this chapter.
Measuring Sound System Gain
Measuring the gain of a sound system in the
field is usually done over a single band of
frequencies. It is normally specified that system gain
shall be measured over the octave-wide band
centered at 1 kHz. Another common technique is to
use pink noise which is then measured with the
A-weighted scale. A typical specification for sound
system gain might read as follows:
“The lectern microphone shall be used in its
normal position. A small loudspeaker shall be
mounted on a stand to simulate a person talking
approximately .6 meters from the microphone. The
response of this test loudspeaker shall be reasonably
flat over the range from 250 - 4000 Hz.
“With the system turned off, the test
loudspeaker shall be driven with a pink noise signal
to produce a sound level of about 80 dB(A) at the
system microphone. This level shall be measured
with a precision sound level meter, using the “A”
scale, with its microphone immediately adjacent to
the sound system microphone.
“After noting the sound level at the system
microphone with the sound system turned off, the
sound system shall be turned on and its gain advanced to
a point just below sustained oscillation. The amplified
sound level shall be measured with the same sound
level meter in the central part of the auditorium.
“The ∆ of the sound system shall be calculated
by subtracting the measured SPL at the microphone
(system off) from the measured SPL in the auditorium
(system on).”
The gain of the system is of course measured at
some point in the auditorium and is the level difference
at that point produced by the test loudspeaker before
and after the system has been turned on. Details of the
measurements are shown in Figure 6-11.
Figure 6-10. Conditions for the indoor system gain equation
6-10