User Guide

Sound System Design Reference Manual
will simplify things by considering only a single
microphone path through the system to a single
loudspeaker.
For the moment, let us consider only the
abbreviated console flow diagram shown in the
upper part of Figure 7-1A. Microphone ratings in use
today state the unloaded output voltage when the
unit is placed in a sound field of 94 dB SPL. Normal
speech level at an operating distance of .5 meter is
about 72 dB SPL; If we are using a microphone with
a sensitivity of 10 mV/Pa, the microphone’s nominal
voltage output in the 72 dB sound field will be:
E = 10
22/20
x 10 mV = .8 mVrms
Step One:
Set a reference input of .8 mVrms at 1000 Hz at
one of the microphone inputs on the console. With
the input and output faders at their nominal “zero”
markings, set the microphone’s input trim control for
a console output of 0.4 Vrms. (Alternatively, a stable
sound pressure level of 72 dB may be generated at
the microphone, and the microphone trim setting
adjusted for 0.4. Vrms output.) In making this setting,
the trim potentiometer marker will normally be
somewhere between 10 o’clock and 2 o’clock. This
setting represents a nominal operating point for the
microphone/console combination, and there is ample
flexibility for operating the system above or below
this setting, as may be required by weak or loud
talkers. Frequency division and system equalization
are to be carried out by a digital controller, the JBL
model DSC260. The loudspeaker to be used is the
JBL model SR4726A, and the recommended
amplifier is the JBL model MPX600. Typical
operating levels are as shown in the lower portion of
Figure 7-1A.
The level diagram shown in Figure 7-1B shows
that, at the power amplifier’s output, the noise level
of the microphone is about 3 dB greater than the
noise contributed by the power amplifier. Both of
these noise sources will be swamped out by the
acoustical noise level in the acoustical space,
however. The electrical noise floor is transformed
over to an equivalent noise level of -2 dB(A) at a
distance of 20 meters, some 25 dB lower than the
acoustical noise floor of a typical space. With this
calibration procedure, the maximum output level
possible in the house is limited by the dynamic range
and nominal operating point established for the
DSC260. If more output level is desired, the nominal
operating points must be reset accordingly.
Figure 7-1A. Signal flow diagram for a simple reinforcement system
7-2