Handbook for Sound Engineers

1246 Chapter 34

also, as explained previously, improve the ability to
understand speech by effectively making it louder.

34.2.3.1.4 When Reverberation Is a Problem

While some reverberation is useful, too much reverbera-
tion causes muddy or boomy sound quality in a musical
performance and makes speech very difficult to under-
stand. In this case, the reverberation is a problem simi-
lar to that of too much ambient noise except that the
reverberation gets louder as the signal gets louder so
that a reverberation problem cannot be solved by simply
turning up the volume control.

34.2.3.2 Reverberation and the Sound Reinforcement
System

The indoor sound system model includes two important
assumptions:

1. The room has no distinct echoes.


2. The room has a well-developed and statistically
   random reverberant field, Fig. 34-5.

The first assumption limits the model to rooms with
few or no distinct echoes. This is an acceptable limita-
tion since a room with distinct echoes needs acoustic
treatment before a sound system can be applied. The
second assumption is acceptable for the purposes of this
section, although it should be understood that differ-
ences in reflecting and absorbing surfaces in most
rooms prevent true randomness.
The reason the reverberant field must be considered
is that it will help the system maintain a more consistent
LP from seat to seat, even though it will hinder, to some
extent, the attempt to provide intelligible sound to every
seat.

34.2.3.2.1 Direct/Reverberant Ratio

Direct/reverberant ratio is a ratio of the direct sound at
some point in a room to the reverberant sound, which is
assumed to be the same everywhere in the room. A high
direct/reverberant ratio means good speech intelligibil-
ity (if all other factors are favorable).

34.2.3.2.2 Loudspeaker Q

Q is a measure of the directional properties of a loud-
speaker (also see Chapters 17 and 18). An omnidirec-
tional loudspeaker has a Q of 1. A loudspeaker radiating
into a hemisphere has a Q of 2. A loudspeaker radiating

into half a hemisphere has a Q of 4 and so on, as shown

in Fig. 34-6. A related term, DI (directivity index), is

simply ten times the log (base 10) of Q. That is

(34-9)

A loudspeaker with a high Q will have a narrow

coverage pattern and will, therefore, concentrate its

sound energy on fewer seats than a low-Q loudspeaker.

Thus, the high-Q loudspeaker can provide higher levels

of direct sound and, likewise, higher direct/reverberant

ratios. This leads to better intelligibility, at least in the

single loudspeaker case.

Figure 34-5. Direct and reverberant fields in a room.

Courtesy JBL Professional.

D. Directional loudspeakers.

A. Nondirectional loudspeakers.

B. Loudspeaker in "live" rooms.

C. Loudspeaker in "dead" rooms.

Reverberant field

Direct field

DI= 10 logQ