Title JBL Sound System Design2 558.8 KB 44
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Sound System Design Reference Manual

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Chapter 6: Behavior of Sound Systems Indoors

Introduction

The preceding five chapters have provided the
groundwork on which this chapter is built. The “fine
art and science” of sound reinforcement now begins
to take shape, and many readers who have patiently
worked their way through the earlier chapters will
soon begin to appreciate the disciplines which have
been stressed.

The date at which sound reinforcement grew
from “public address by guesswork” to a methodical
process in which performance specifications are
worked out in advance was marked by the

publication in 1969 of a paper titled “The Gain of a
Sound System,” by C. P. and R. E. Boner (4). It
describes a method of calculating potential sound
system gain, and that method has since become a
fundamental part of modern sound system design.
The following discussion is based on the Boner
paper. Certain points are expanded, and examples
are given that require calculations more complicated
than those in the original study. Also discussed is the
relation between theoretically achievable system
gain and practical operating parameters of typical
indoor sound systems.

Figure 6-1. An indoor sound system

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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 = 1022/20 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

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Figure 7-1B. detailed level diagram showing noise levels, nominal operating levels,
and maximum output levels of each device