Handbook for Sound Engineers

Stadiums and Outdoor Venues 203

8.1 Overview

Stadiums and outdoor venues present designers with a
set of challenges which are not usually encountered in
interior spaces. The leading challenge is the immense
distance over which sound of an appreciable level must
be projected. This is followed by the fact that the sound
is not propagating in a stable medium—i.e., outdoors
the air temperature and relative humidity are erratic
variables and the air is hardly ever still. Lastly, in addi-
tion to the normal 6 dB loss for doubling of distance
from a point source in a free field, there exists an addi-
tional attenuation from atmospheric absorption whose
value is a function of frequency and depends on both
temperature and relative humidity. These challenges
will be addressed in turn.

8.2 Sound Projection Distances

The values of the distances required between reinforce-
ment loudspeakers and observers in stadiums hinge on
both the stadium geometry and on whether the reinforce-
ment system is to be of the single source or distributed
loudspeaker type. A distributed system avoids large
throw distances and major atmospheric effects but is
more expensive to install and maintain. The sound
quality of distributed systems in stadiums is somewhat
unnatural in that the nonlocal loudspeakers are sources
of apparent echoes. A single source system is less expen-
sive to install and maintain but requires special tech-
niques to achieve adequate levels at large throw
distances. Advocates exist for both system types. The
problems associated with the single source system are
more interesting and are the ones initially discussed here.

Throw distances for a central source system in a
typical stadium range from 15 m to 200 m (50 ft to
650 ft). Sports stadiums, with the possible exception of
baseball stadiums, have playing surfaces in the shape of
elongated rectangles with audience seating being
peripheral to the playing area. Single source loud-
speakers are located at one end of an axis of symmetry
along the long dimension of the playing surface as illus-
trated in Fig. 8-1. This allows the coverage of the
seating spaces to fall into a number of zones for which
the axial throw distances vary by no more than a factor
of two. For example, in the stadium of Fig. 8-1, there is
a near, intermediate, and far zone with axial distances of
approximately 50 m, 100 m, and 200 m, respectively.
Thus, the single source system is actually a splayed
array of short, intermediate, and long throw devices.

8.3 Source Level Requirements

For a point source in a free field without atmospheric

absorption, the acoustic pressure varies inversely with

the distance measured from the source, i.e., there is a

6 dB loss for each doubling of the distance. The pres-

sure level at 200 meters from such a source is 46 dB

less than it is at one meter. If one assumes a noise level

of 85 dB and a signal level at least 6 dB above the noise

level, then the sound level at one meter must be at least

85 + 6 + 46, or 137 dB, even without any headroom

consideration. If one imposes a modest headroom

requirement of 6 dB, an impressive 143 dB level is

required even before considering atmospheric

attenuation.

While this is not attainable from one loudspeaker, it

is easily attainable by multiple loudspeakers.

8.4 Atmospheric Effects

Sound propagation is subject to the vagaries of the

medium in which it exists. The air in outdoor venues

has variable temperature, wind, and relative humidity.

The effect of wind is twofold. Wind speed near to the

ground is ordinarily less than at a higher elevation. This

causes sound waves propagating in a direction into the

wind to be diffracted upward while sound waves propa-

gating in the same direction as the wind to be diffracted

downward. Cross-winds shift the azimuth of the propa-

gation direction towards that of the wind. Thus wind

can cause shifts in apparent loudspeaker aiming points.

Additionally, sound will propagate greater distances

with the wind than against the wind. A gusting or vari-

able wind introduces a temporal quality to these proper-

ties. The effect on a listener is that the sound intensity

Figure 8-1. Plan view of typical stadium.

Source

Far

throw

Intermediate

throw

Near

throw