Handbook for Sound Engineers

1250 Chapter 34

.

The value LP in the first line of the equation is the
desired LP at D 2. The value H is a desired value for head
room, usually assumed to be 10 dB. That 10 dB, of
course, may be changed for a particular system. Dc is
the critical distance given in Eq. 34-12. It is instructive
to note that this equation is the same as the simplified
system (outdoor) EPR equation (Eq. 34-6) except for
the term

which can be interpreted as a contribution from the
reverberant field.
EPR can also be found from an equation that is more
common in the literature

(34-17)

where,

where,

x is any distance,
other terms are as before,
For metric distances, replace the constant 3.28 with the
constant 1.00.

Example:
Let

LP = 90 dB,
H = 10,
D 2 = 128 ft,

Ls = 113 dB/1 W/1 m,
Q = 5,
S = 28,000,

= 0.35.

then

and

and

Eq. 34-17 may be more familiar to some readers.

However, it and Eq. 34-16 are mathematically equiva-

lent and will produce the same answers from the same

data. Eq. 34-16, however, may be easier to understand

and to use in a computer program.

34.2.3.4.2 Question 3: Can Everybody Understand?

In the simplified (outdoor) system, Question 3 was

answered by considering the required SNR and making

certain that the sound system output was sufficiently

above the ambient noise level to provide intelligible

sound (speech). Indoor intelligibility also depends on

the reverberation time and the direct/reverberant ratio,

and an unfavorable reverberation time or direct/rever-

berant ratio cannot be made better by merely increasing

the SPL from the loudspeakers, since that will also

increase the reverberant field level!

If the reverberation time or direct/reverberant ratio is

unfavorable, one or more of the following may help:

1. Decrease the reverberant field by adding absorp-
   tion to the room (usually a costly process).


2. Move the listener closer to the loudspeaker (in a
   reverberant church with the pews half filled, people
   sitting near the loudspeakers will hear and under-
   stand better than those farther away from the loud-
   speakers).


3. Move the loudspeakers closer to the listeners
   (adding additional loudspeakers, as described later,
   is a common way to improve direct-to-reverberant
   ratio but this is not a panacea since the new loud-
   speakers will add to the reverberant level as well as
   the direct level).


4. Use a loudspeaker with higher Q (this is ideal
   provided the required Q doesn’t mean that you
   have a very narrow coverage pattern that cannot
   cover all the listeners).

How is a direct/reverberant ratio determined? What

is a favorable reverberation time or direct/reverberant

ratio? It is possible to determine this ratio directly, but it

is more common to use the articulation loss of conso-

nants concept. If the Alcons is 10% or less and the SNR

is favorable (+15 dB or greater), there is every reason to

believe the answer to Question 3 will be yes.

Note that some texts suggest an Alcons of 15% or

less is acceptable. Also note that this Alcons equation is

10

gD (^2)
g 3.28
log------------------
10
gD (^2)
g 3.28
log------------------
EPR 10
Lp+HLS+– 'D 2 '3.28– –LS
10

=
'x 10 Q
4 Sx
----------- 2 -
4
Sa
–= log©¹§·----- -+
a
'D 2 =33.6
'3.28 14.3=
EPR 10
90 ++ 10 33.6–11314.3–
= ------------------------------------------------------------------^10
=4.33 W