Handbook for Sound Engineers

Sound System Design 1325

34.6.3.5 High Noise Outdoors (or Indoors)

How much LP is required at the listener’s position? The
answer depends almost entirely on the expected ambient
noise at the listener’s location. Crowd noise in a sports
stadium, for example, can easily exceed 90 to 95 dB LP
on the A scale. Noise from aircraft in an airport paging
system or noise from race cars in a racetrack paging
system may exceed this figure by a considerable mar-
gin. At a grand prix race, for example, spectators seated
near the racetrack may be subjected to potentially
ear-damaging short-term peak LP levels of as high as
140 dB! In the case of the sports stadium, it may be pos-
sible to overcome the crowd noise (budget permitting).
In the case of the racetrack, ask the announcer to avoid
making announcements when the race cars pass the
stands and to repeat announcements whenever possible.
Also, put the announcer in an acoustically isolated
booth so the announcement microphone does not pick
up race noise as the cars pass the booth.
In the indoor system design equations in Section
34.2.3, a 15 dB SNR was assumed. For an outdoor clas-
sical music reinforcement system, this 15 dB SNR is
still desirable. In many speech reinforcement systems,
both indoors and out, a 15 dB SNR is simply not

possible, and fortunately, it is not necessary. Except in

the rare case where the frequency content of the noise is

concentrated in the speech band, a 10 dB SNR will

usually result in acceptably intelligible speech rein-

forcement. It is possible to achieve intelligible speech

reinforcement with a SNR of lower than 10 dB but it is

difficult to predict the success of such a system prior to

its installation. When this type of system is contem-

plated, use a compressor to keep the voice level as

constant as possible. Use a good-quality microphone

with a bandpass filter to reduce unneeded low and high

frequencies and avoid the use of a telephone handset as

a paging microphone. Use equalization to boost the

intelligibility frequencies slightly to further improve

intelligibility. Use a smooth curve from about 1000 Hz

to about 8000 Hz peaking in the 2000–4000 Hz bands.

Training the announcer in speech articulation will

help, as will repeating announcements. In a plant or

airport paging system with high noise levels, alerting

the listener to an impending announcement by using a

prepage tone can also help improve the effective intelli-

gibility of a paged announcement.

34.6.3.6 Dealing with Varying Noise Levels

A manufacturing plant may have noise levels that vary

widely with time and at different local areas within the

building. Dealing with noise that varies in different

areas is as simple as varying the quantity, type, and

power delivered to the loudspeakers in the different

areas. For noise levels that vary with time, there are two

primary tools. If the noise varies predictably with time,

as for an assembly line that is running or stopped on a

predictable schedule, devices are available that vary the

audio power fed to a loudspeaker line (usually by vary-

ing the input level to a power amplifier) depending on

the time of day. Some of these devices will vary several

different loudspeaker lines at several different times.

For noise levels that vary unpredictably with time,

such as the crowd noise at a sports stadium or the noise

of an airplane entering a waiting area at an airport,

devices are available to measure the ambient noise level

and adjust the paging level accordingly, Fig. 34-85.

These devices work quite well, although they may have

Figure 34-84. A large outdoor horn. Courtesy

Bosch/Electro-Voice.

Figure 34-85. A device that increases or decreases sound system level with varying ambient noise. Courtesy Symetrix.