Handbook for Sound Engineers

Delay 805

24.1 Delay

Delay is relative. For a delay to have an effect on a

sound it must be heard in conjunction with the original,

nondelayed sound. There are two ways that this can

occur. A single sound can arrive at the listener via two

different length paths, such as a direct sound and a

reflected sound, or two signals with different delays can

be added electrically and then heard from a single loca-

tion, Fig. 24-1.

24.1.1 Comb Filter

Two copies of the same signal at different delay times
combine to add or subtract depending on the relative
phase of each frequency as shown in Fig. 24-2. If the
waves are a whole period apart, they combine to give a
peak in level, if they are half a period apart, they cancel
out to an extent controlled by their relative levels. This
effect sets up a comb filter, so named for its appearance
on a frequency plot as shown in Figs. 24-3 and 24-4.
The series of peaks in the response fall first at dc, then
at every frequency whose period is equal to an integer
multiple of the delay time. The cancellation notches
occur at the exact midpoints between these frequencies.

24.1.2 Directional Perception

In the case of sounds traveling through the air, the path

lengths with their corresponding travel times are differ-

ent for every point in space, resulting in a different

comb filter for every location.

The brain uses the results of the different comb filters

that are in effect at the location of each ear and combines

this information in conjunction with the arrival times,

relative levels, and directional filtering due to the shape

of the pinnae to determine the originating direction of a

sound. Other cues such as the ratio of direct to reverber-

ant energy are used to help determine distance.

A completely dry sound heard in a set of headphones

will appear to originate inside your own head. Gradu-

Figure 24-1. Different sound paths through air.

Figure 24-2. Effects of adding signals of different frequen-

cies with the same delay.

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Figure 24-3. A reflection that will alter the perceived direc-

tion of the sound.