Handbook for Sound Engineers

806 Chapter 24

ally adding reverberation to it will make the sound
appear to move away out in front of you. The sound can
be made to appear to move from side to side by alerting
the relative levels in each ear, as is commonly done in a
pan control, but the same effect can be achieved by
altering the relative delay of the dry sound to each ear.
The reasons that this delay technique is not commonly
used are that the level control is much simpler to imple-
ment and the result is compatible with monaural repro-
duction when the left and right channels are summed.

A sound is perceived as originating in the location at
which it was first heard. This is generally the correct
location as the direct sound will always arrive before
any reflected sounds. The same sound coming from a

second location will be perceived in different ways

depending on its timing and level relative to the first:

• If the second sound is more than 30 ms after the first
  it will be heard as a distinct echo.


• If the second sound is more than 10 dB louder than
  the first, it will be heard as a distinct echo.


• If the second sound is within 10 dB and less than
  30 ms after the first, it will cause an image shift in
  where the source location is perceived.


• If the second sound is more than 10 dB below the
  first, it will contribute to the spatial feel of the sound
  but will not be heard as a distinct sound or alter the
  apparent location of the first.

These rules of thumb are approximations of the psy-

choacoustic effects in operation. The perception curves

are more complex than the rules of thumb suggest. The

actual values are plotted in Fig. 24-5 and tabulated in

Table 24-1.

24.2 Uses of Delay

Delay is sometimes useful. It should also be noted that

there can be undesirable delays in a system. This is par-

ticularly true with digital recessing equipment where

there is always a conversion delay in and out of the pro-

cessor plus any processing delay. It is not uncommon

for processors to have a minimum delay of a few milli-

seconds, and these delays should be considered when

calculating the amount of delay that you actually want

to use.

24.2.1 Delay in Loudspeaker Systems

A sound amplified through a loudspeaker system will be

subject to image shifts and audible echoes only if there

is a reference point against which to judge it. This is

usually the case in sound reinforcement with the origi-

nal sound being the reference or in multiple loudspeaker

Figure 24-4. A reflection that is too low in level to affect
perception of the sound.

Figure 24-5. Perception curves.

0 20 40 60 80 100 120 140 160 180 200

Time—ms

1024 samples from 1780 Hz to 2220 Hz in 4.0 s, Hamming

70

60

50

40

30

20

10

dB—Pascals