Handbook for Sound Engineers

Sound System Design 1307

Keep the Cables Short. Rack mounting can help here,

as can simple neatness.

Keep Cables of the Same Type Close Together. Group

cables that carry the same signal level. Especially when

they form an unavoidable ground loop, keeping cables

close together will help reduce noise pickup.

Keep Cables of Different Types as Far Apart as Pos-

sible. This means keep the microphone cables away

from loudspeaker cables. And keep all audio cables

away from the ac power cables. On long cable runs,

keep line-level cables and microphone cables sepa-

rated. It’s a common, but risky, procedure to run micro-

phones through a snake (a multimicrophone cable) to a

mixer and then run the outputs from the mixer back to

the power amplifier through the same snake. This

mixing of levels, in a long cable run (greater than about

25 ft) can cause a form of electronic feedback that could

cause harmful oscillations in the system mixer.

Keep the Wiring Neat. Carefully made cables, of the

proper length (not too long), that are carefully laid out

on a stage or in an installation are probably the best way

of all to reduce external noise pickup, Fig. 34-71.

34.5.2 Testing and Adjusting

34.5.2.1 Signal Delay in Sound Reinforcement

There are two uses for signal delay in sound reinforce-

ment. The first is to delay one loudspeaker system to

allow the sound from a remote loudspeaker system to

catch up. This avoids the creation of an artificial echo.

The second purpose of signal delay in sound reinforce-

ment is to line up the wave fronts from the components

in a packaged loudspeaker system or, similarly, to line

up the wave fronts of the various loudspeakers in a

loudspeaker cluster.

34.5.2.1.1 Signal Delay for Loudspeaker Clusters

To calculate the delay for a rear cluster in a two-cluster
system, choose a typical listener in the coverage pattern
of the second cluster who can still hear the first cluster.
Calculate the distance from this listener to the first clus-
ter and subtract the distance from this listener to the sec-
ond cluster. Perform this calculation for several listeners
in the coverage of the second cluster who can also hear
the first cluster. Choose an average value, biased toward
those listeners who can hear the first cluster best. Multi-
ply this average value times 1.13 for distances in feet or

3.71 for distances in meters, to obtain the starting point

for delay in milliseconds. Add 6 to 20 ms (the exact

amount is best determined on site by listening) to take

advantage of the localization known as the Haas effect.

34.5.2.1.2 Signal Delay for an Under-Balcony

Distributed System

Choose a listener near the front of the area under the

balcony, one who can hear both the central cluster and

the under-balcony distributed system. Then, follow the

instructions in Section 34.5.2.1.1 above.

34.5.2.1.3 Signal Delay for a Loudspeaker or

Loudspeaker Cluster

See Section 34.3.2.11 for a discussion of this topic.

34.5.2.2 Equalization

34.5.2.2.1 The Concept of Equalization

Sound system equalization is a process of adjusting the

electronic frequency response of a system to compensate

for uneven loudspeaker response and room acoustics.

The goals of equalization are to provide a natural-sound-

ing system with good intelligibility and to minimize

feedback that might be caused by peaks in the frequency

Figure 34-71. Cable routing in equipment rack. (After

Reference 4.)

Line level & dc control circuits

Microphone

circuits

(local option)

24 Vdc

emergency

source

Ac power

conduit

High level

loudspeaker

circuits

Ground

lug

24 Vdc emergency

power & ground circuits