Handbook for Sound Engineers

Sound System Design 1243

Here, the term Ds is the (reference) distance at which
the LP from the talker was measured. The term Ds (the
microphone to talker distance discussed earlier) is used
because it is a convenient number (normally about 1 m)
and because it will make the next calculation (needed
acoustic gain) easier. LPt is the sound pressure level
from the talker at that reference distance Ds, and LPd is
the desired sound pressure level at the listener
(normally, this will be equal to the ambient noise plus a
25 dB SNR). EAD, then, is the equivalent acoustic
distance number to be used in the next calculation, that
of needed acoustic gain (NAG).

34.2.2.10 Needed Acoustic Gain (NAG)

The next question to be answered is, “How much acous-
tic gain is needed to achieve this desired LP for a given
talker’s voice level?” This needed acoustic gain, or
NAG, is the gain in decibels needed to produce the
desired LP at the listener’s ears, LPd, given an EAD as
calculated previously.

(34-8)

where,

Do is (as before) the distance from the talker to the

farthest listener.

Example:

Let

Do = 128 ft,

EAD = 4 ft.

34.2.2.11 Will the System Feed Back?

If the potential acoustic gain (PAG) from Eq. 34-5 is

greater than or equal to the needed acoustic gain from

Eq. 34-8, there is every reason to believe that the system

will be stable and will not feed back. If, on the other

hand, the potential acoustic gain is less than the needed

acoustic gain, chances are good that the system won’t

work because turning up the volume control enough for

the farthest listener to hear properly will always cause

the system to be at or near feedback.

34.2.2.12 The Effect of Directional Microphones and

Loudspeakers

The PAG and NAG equations assume an omnidirec-

tional microphone and an omnidirectional loudspeaker.

Some improvement in acoustic gain before feedback

Figure 34-4. Nomograph for finding the EAD. Courtesy Syn-Aud-Con.

Meters Feet

6.096 20

4.877 16

3.048 10

2.438 8

1.829 6

1.219 4

0.9144 3

0.696 2

0.3048 1

0.1524 0.5

50 53 60 70 80 90 100 110 120 130

Noise level - dBA + 25 dB SIN

Area where face-to

-face communications

are possible using

"normal voice"

Weak voice

Expected voice level

Shout

Maximum vocal effort

Area where face-to-face

communications

are impossible

Area where

face-to-face

communications

are difficult

Normal voice

Raised voice

Communicating voice

Very low voice

Limit for amplified speech

Distance from speaker to listener

EAD 210

71 65 –

------------------- 20

©¹

̈ ̧

§·

=

=3.99 ft

NAG 20

D 0

EAD

= log------------

NAG 20 128

4

= log---------

=30.1 dB