Handbook for Sound Engineers

1658 Chapter 48

48.5 Sound System Quantities and Design
Formulas

Various quantities used for sound system design are
defined as follows:

D 1. D 1 is the distance between the microphone and the
loudspeaker, Fig. 48-7.

D 2. D 2 is the distance between the loudspeaker and the
farthest listener, Fig. 48-7.

Do. Do is the distance between the talker (sound source)
and the farthest listener, Fig. 48-7.

Ds. Ds is the distance between the talker (sound source)
and the microphone, Fig. 48-7.

DL. DL is the limiting distance and is equal to 3.16Dc
for 15%Alcons in a room with a reverberation time of
1.6 s. This means that D 2 cannot be any longer than DL
if Alcons is to be kept at 15% or less. As the RT 60
increases or the required %Alcons decreases D 2
becomes less than DL.

EAD. The equivalent acoustic distance (EAD) is the
maximum distance from the talker that produces

adequate loudness of the unamplified voice. Often an

EAD of 8 ft is used in quiet surroundings as it is the

distance at which communications can be understood

comfortably. Once the EAD has been determined, the

sound system is designed to produce that level at every

seat in the audience.

Dc. Critical distance (Dc) is the point in a room where

the direct sound and reverberant sound are equal. Dc is

found by the equation

(48-45)

where,

Q is the directivity of the sound source,

R is the room constant,

M is the critical distance modifier for absorption coeffi-

cient,

N is the modifier for direct-to-reverberant speaker

coverage.

It can also be found with the equation

Figure 48-5. Relationship between VU and dBm at various
impedances.

dBm

Ohms

Figure 48-6. Sound pressure level versus acoustic pressure.

200

100

80

60

50

20

40

30

10

8

6

5

2

3

4

1

0.8

0.6

0.5

0.4

0.3

0.2

0.1

0.08

0.06

0.05

0.03

0.02

0.01

0.04

30 40 50 60 70 80 90 100 110 120 130 140

Sound level–decibels above 0.0002 dyne/cm^2

Acoustical pressure–dynes/cm

2

Dc 0.141QRM

N

= -------------