User Guide

Sound System Design Reference Manual

3-1

Chapter 3: Directivity and Angular

Coverage of Loudspeakers

Introduction

Proper coverage of the audience area is one of

the prime requirements of a sound reinforcement

system. What is required of the sound contractor is

not only a knowledge of the directional

characteristics of various components but also how

those components may interact in a multi-component

array. Such terms as directivity index (DI), directivity

factor (Q), and beamwidth all variously describe the

directional properties of transducers with their

associated horns and enclosures. Detailed polar

data, when available, gives the most information of

all. In general, no one has ever complained of having

too much directivity information. In the past, most

manufacturers have supplied too little; however,

things have changed for the better in recent years,

largely through data standardization activities on the

part of the Audio Engineering Society.

Some Fundamentals

Assume that we have an omnidirectional

radiator located in free space and that there is a

microphone at some fixed distance from it. This is

shown in Figure 3-1A. Let the power radiated from

the loudspeaker remain constant, and note the SPL

at the microphone. Now, as shown in

, let us place

a large reflecting boundary next to the source and

again note the SPL at the microphone. At high

frequencies (those whose wavelengths are small

compared to the size of the reflecting boundary), the

increase in SPL will be 3 dB. The power that was

radiating into full space is now confined to half

space; thus, the doubling of power at the

microphone. Moving on to the example at

, we

place a dihedral (2-sided) corner next to the source.

Power that was confined to half-space now radiates

into quarter-space, and the SPL at the microphone

Figure 3-1. Directivity and angular coverage