Handbook for Sound Engineers

1322 Chapter 34

tion, the wood grille work, which may be fine for organ
music, can cause severe acoustic problems (both from
cancellations and from vibrations) in the sound system
loudspeakers.

34.6.2.4 Reverberant Room Problem

It is said that the types of chanting services employed
by some religious groups developed in large reverberant
cathedrals before sound systems were invented. Chant-
ing helped carry an intelligible message to the congre-
gation. All too many religious organizations still have
the problem of reverberation, yet many have given up
the chanting type of service and now want intelligible
speech in their facilities.

The problem of intelligible speech in a reverberant
religious facility is no different than in any other rever-
berant room, except, perhaps, for the desire to hide the
loudspeakers. One other consideration, however, is that
pipe organs and much religious choir music depend on
high levels of reverberation, a criteria that conflicts with
the desire for lower reverberation times for speech.
Because even a good compromise may be expensive,
the services of a qualified consultant are invaluable
when this situation arises.

There is no magic way to design a loudspeaker
system for a reverberant room. However, there are two
ways to maximize the intelligibility of a sound system
in a reverberant room. First, get the loudspeakers as
close to the listeners as possible. This maximizes the
direct-to-reverberant ratio at the listener’s ears, which
improves intelligibility. A distributed system (ceiling
type, on building pillars or pew-back) is the easiest way
to accomplish this goal. Second, use directional loud-
speakers and point them at the listeners and away from
walls, ceiling, and other hard surfaces. Sometimes,
these two concepts can be combined. For example, in a
distributed system on building pillars, use a line-array-
type column loudspeaker to direct sound at the listeners.

34.6.3 Sports Stadiums and Other Outdoor
Systems

The discussion of outdoor system design presented in
Section 34.2.2 was limited to very basic, theoretical
considerations. The following discussions include many
of the practical aspects of the design of an outdoor sys-
tem. Also see Chapter 7 for a thorough treatment of
sound systems in stadiums and other outdoor venues.

34.6.3.1 Excess Attenuation of High Frequencies in Air

The friction of air molecules rubbing against each other

causes attenuation of sound that adds to the loss caused

by the inverse-square law. This frictional loss is nor-

mally insignificant in indoor systems (except in very

large rooms) but can become a problem in large outdoor

systems because of the long distances involved. The

problem is considerably worse at the high frequencies,

which are important for speech intelligibility. This is

because the molecules of air are moving faster than at

low frequencies. The problem also increases at lower

relative humidity as shown in Fig. 34-81.

The problem shows up in outdoor systems with long

distances between loudspeakers and listeners. It often

cannot be solved with simple equalization or even by

adding additional high-frequency horns. The reason is

that the attenuation may be 10 to 20 dB or even more,

depending on frequency, distance, and relative humidity.

One potential solution is to add additional loud-

speakers (or high-frequency horns) at a position nearer

to the listeners and, of course, to place these loud-

speakers on delay. Since the attenuation of

low-frequency information is much less, it is normally

unnecessary to add additional low-frequency loud-

Figure 34-81. Absorption of high frequencies in air. Cour-

tesy Bosch/Electro-Voice.

100K 50K 20K 10K 5K 2K 1K 500 200 100

0.001

0.002

0.005

0.001

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0.05

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1.0

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Frequency–Hertz

Attenuation constant–decibels per meter

Temperature–20° C

20% RH

40% RH

60% RH

80% RH