User Guide

Sound System Design Reference Manual
Chapter 4: An Outdoor Sound
Reinforcement System
4-1
Introduction
Our study of sound reinforcement systems
begins with an analysis of a simple outdoor system.
The outdoor environment is relatively free of
reflecting surfaces, and we will make the simplifying
assumption that free field conditions exist. A basic
reinforcement system is shown in Figure 4-1A. The
essential acoustical elements are the talker,
microphone, loudspeaker, and listener. The electrical
diagram of the system is shown at B . The dotted line
indicates the acoustical feedback path which can
exist around the entire system.
When the system is turned on, the gain of the
amplifier can be advanced up to some point at which
the system will “ring,” or go into feedback. At the
onset of feedback, the gain around the electro-
acoustical path is unity and at a zero phase angle.
This condition is shown at C , where the input at the
microphone of a single pulse will give rise to a
repetitive signal at the microphone, fed back from the
loudspeaker and which will quickly give rise to
sustained oscillation at a single frequency with a
period related to ∆t.
Even at levels somewhat below feedback, the
response of the system will be irregular, due to the
fact that the system is “trying” to go into feedback,
but does not have enough loop gain to sustain it.
This is shown in Figure 4-2. As a rule, a workable
reinforcement system should have a gain margin of
6 to 10 dB before feedback if it is to sound natural on
all types of program input.
Figure 4-1. A simple outdoor reinforcement system