User Guide

Sound System Design Reference Manual
Also, local acoustical conditions may exist
which are not taken into account by statistical theory
and, therefore, not covered by the Peutz findings or
any of the other equations we have studied. Such
localized dead spots or zones of interference may
not be discovered until the sound system is installed.
In large reverberant spaces, sufficient flexibility
should always be built into the sound system design
to allow for such surprises.
The effect of masking by unwanted background
noise has been touched on only briefly in this
section. Such unwanted noise may be produced by
sound from the outside environment, by noisy air
handling equipment, by noisy backstage mechanical
equipment or by the audience itself. For good
listening conditions, the level of ambient noise as
measured on the “A” scale should be at least 10 dB
below the desired signal. Since the optimum level for
reproduced speech in the absence of strong
background noise is 65 - 70 dB(A) this means that
background noise with a full audience should not
exceed 55 dB(A). In auditoriums and concert halls,
acoustical designers normally attempt to reduce
background noise in an empty house to a level not
exceeding 25 dB(A). In a church or meeting hall, the
maximum tolerable background noise for an empty
room is about 40 dB(A).
A sound reinforcement system cannot be
turned up indefinitely. In many situations it is difficult
enough to achieve a useful operating level of 60 - 65
dB(A) without feedback. It is easy to see, therefore,
that the presence of excessive background noise can
render an otherwise good sound reinforcement
system unsatisfactory.
As an example of how the Peutz analysis can
dictate the type of sound system to be used, let us
consider a reinforcement system to be used in a large
reverberant church. Details are shown in Figure 6-14.
Let us assume that the reverberation time is 4
seconds at mid-frequencies and that the designer’s
first choice is a single-point loudspeaker array to be
placed high above the chancel. Coverage
requirements pretty much dictate the directional
characteristics of the array, and let us assume that
the array will consist of two JBL Bi-Radial horns: 20°
by 40° for far coverage, and 90° by 40° for near
coverage. What we wish to calculate is the direct-to-
reverberant ratio at selected points in the audience
area to determine if the Peutz criteria for acceptable
intelligibility can be met. The most direct way of doing
this is to calculate the total reverberant level in the
room for a given power input to each horn and
compare it with the direct sound coverage provided
by each horn over its coverage angle.
Figure 6-13. Probable intelligibility as a function of reverberation time
and direct-to-reverberant sound ratio
6-13