Handbook for Sound Engineers

Delay 807

setups where another loudspeaker can act as the refer-

ence.

It is not generally desirable for loudspeakers in a sys-

tem to appear to be generating echoes as this will have a

detrimental effect on the intelligibility of the system.

Whether the image shift effects are important depends

on the application. In a stage system, it is desirable to

have the apparent sound source at the stage, regardless

of the placement of the loudspeakers. In a distributed

announcement system, the creation of a coherent source

image is not as important as the intelligibility.

Sound travels at 334 m/s or 1130 ft/s. A sound trav-

eling 33 ft will be delayed by 30 ms, so with sound

sources greater than 33 ft apart, delay should be used to

avoid the creation of echoes.

24.2.2 Setting Delay Times

In Fig. 24-6 the sound from the source, a person talking,

is to be augmented by a loudspeaker and the apparent

source of the sound is to be kept on the stage. To

achieve this, the sound from the source must arrive at

the listener before the sound from the loudspeaker. The

time taken for the signal to arrive at the listener from the

loudspeaker is a combination of the distance traveled in

air from the loudspeaker to the listener and the negligi-

ble time taken for the signal to arrive electrically at the

speaker. We must delay the signal to the loudspeaker by

an amount that allows the direct sound traveling more

slowly though the air to catch up and overtake the sound

from the loudspeaker. The delay should slightly exceed

the time taken for the sound to travel the difference in

distance between the source and the loudspeaker so that

the direct sound will be heard first and localized to the

source. The loudspeaker can then add up to 10 dB of

level 5 to 10 ms later to increase the level of the sound

without changing its apparent position.

A graphical method for setting delays is shown in

Fig. 24-7. The positions of the source and loudspeakers

are plotted and a series of concentric circles drawn

around them at 30 ms (33 ft) intervals. The SPL level

from the polar response pattern of the loudspeaker can

also be plotted, but for simplicity in this example, omni-

directional sources are used where the level decreases

by 6 dB per doubling of distance.

If we look at point A, where source and loudspeaker

are in a direct line, the time difference is 30 ms. If we

add a small amount to this to allow the direct sound to

be heard first, we come up with a delay setting of

Table 24-1. Perception Curves of Figure 24-5
Tabulated

Echo Image shift Spacious No effect

Ms after direct dB dB dB

00  10  20

1  6

2  17

44  5

5  17

7  2  14

10  17

11 8

17 6  5

20 5  17

25  21

30  13  21

40  25

50  7

60  10  36

77  14  32  38

Figure 24-6. A delay in the sound system corrects for the

differences in path length between the source and listener

and the loudspeaker and listener.

Figure 24-7. Graphical method of setting delays.

Source

Loudspeaker

Listener

Delay

Loudspeaker

Source

30 ms 70 dB

45 ms 68 dB

60 ms 64 dB

15 ms 76 dB

A. Direct = 60 ms

Delayed = 30 ms

B. Direct = 60 ms

Delayed = 45 ms

C. Direct = 60 ms

Delayed = 60 ms