User Guide

Sound System Design Reference Manual

The Role of Time Delay in Sound

Reinforcement

The preceding example mentioned time delay

as a means of preserving naturalness in a distributed

system. This comes about by way of the Haas (or

precedence) effect (5), which is illustrated in Figure

6-16. If two loudspeakers are fed the same signal, a

listener mid-way between them will localize the

source of sound directly ahead (A). At

, we have

introduced a delay in one of the otherwise identical

channels, and the listener will clearly localize toward

the

earlier

loudspeaker. At

. the leading signal has

been reduced in level, resulting in an effect of equal

loudness at both loudspeakers. This has the

approximate effect of restoring the apparent

localization to the center. While this tradeoff is not an

exact one, the values shown in the graph at

indicate the approximate trading value between level

and delay for equal loudness at both loudspeakers.

Figure 6-16E shows how delay is typically

implemented in sound reinforcement. Here, that

portion of the audience seated under the balcony

does not get adequate coverage from the central

array. Small loudspeakers placed in the balcony soffit

can provide proper coverage only if they are delayed

so that the sound arrives at the listeners in step with

that from the central array. In this way, the listener

tends to localize the source of sound at the central

array — not at the soffit loudspeakers. If the soffit

loudspeakers are not delayed, listeners under the

balcony would localize sound directly overhead, and

those listeners just in front of the balcony would be

disturbed by the undelayed sound. In practice, the

delay is usually set for an additional 20 msec in order

to minimize comb filtering in the overlap zone

between direct and delayed sound fields.

The ready availability of solid state digital delay

units has made time delay an indispensable element

in sound system design.

6-16

Fugure 6-16. The Haas, or precedence, effect