Handbook for Sound Engineers

Sound System Design 1239

34.1 Introduction

There are a multitude of different types of sound sys-
tems with purposes as diverse as artificial ambience and
voice paging, yet most share common design criteria.
This chapter covers many of these design criteria using
sound reinforcement systems as examples. Included are
discussions of other types of systems such as foldback
(stage) monitor systems and some types of playback
only systems as well as some of the practical aspects of
sound system design such as equipment choice and
installation techniques.
Since the third edition of the Handbook for Sound
Engineers, digital audio products, driven by DSP chips,
have become mainstream choices for signal processing.
Packaged loudspeaker systems and line arrays have
replaced individual components for most loudspeaker
cluster designs. And, most system designers use EASE
or Bose Modeler or some other system design software
in place of the traditional sound system analysis equa-
tions and graphical cluster design methods.
Yet, the original system analysis equations and
graphical cluster design methods are the foundation for
software programs like EASE and Modeler. An under-
standing of the original tools offers a modern designer a
better understanding of the value, and the limitations, of
these software tools.
For these reasons, and for historic completeness, this
chapter retains its focus on the original sound system
analysis equations, with revisions where appropriate. In
addition, while the cluster design discussions use pack-
aged loudspeaker systems or line arrays for most exam-
ples, component cluster designs are mentioned where
they offer advantages. Finally, this chapter discusses
how digital audio is changing system design as it
replaces analog technology.
Most of the equations presented in this chapter can
be used with either U.S. or metric distances. Just be
consistent throughout. In a few cases, one or more
constants change depending on the choice of units. The
needed changes are noted near the equation. All of the
examples use U.S. units.

34.2 Sound Reinforcement System Models

34.2.1 The Four Questions (and a Fifth)

The system design concepts presented in this chapter
help to answer four simple questions:

• Question 1: Is it loud enough?


• Question 2: Can everybody hear?
  
  
  • Question 3: Can everybody understand?
  
  
  • Question 4: Will it feed back?
  
  
  
  

The sound reinforcement system models and equa-

tions in this chapter provide precise answers to these

four questions and, in so doing, help the designer predict

the success of the sound system in meeting its goals.

It is important to answer a fifth question, “Does it

sound good?” The answer to this question may seem very

subjective. However, good sound quality depends very

much on favorable answers to the first four questions and

these questions have objective answers. Also, good

sound quality depends on other objective factors such as

low distortion and smooth frequency response. Thus,

while there are no equations to answer Question 5, it is

possible to answer this question using objective criteria.

34.2.2 The Simplified (Outdoor) Sound Reinforce-

ment System Model

Fig. 34-1 shows a simplified sound reinforcement system

with a talker or sound source, a listener, a microphone,

and a loudspeaker. (The somewhat awkward term, talker,

replaces the term speaker to avoid confusion between the

person talking and the loudspeaker system.) The talker

could be replaced by a musician playing an instrument

with no changes in the following discussions (although a

very loud, amplified instrument or direct box connection

would change things somewhat).

The primary simplification for this model is to

ignore echoes and reverberation. This is a reasonable

assumption for an outdoor system, away from any large

buildings or other sources of echoes. As discussed later,

this simplified description can be readily modified to

include the effects of indoor reverberation.

Figure 34-1. A simplified sound reinforcement system.

Courtesy Bosch/Electro-Voice.

Ds

EAD

Do

D 1 D 2

Farthest

listener

Desirable

listening

position

TalkerMicrophone