BRUEL & KJAER 2204 - Page 12

Conservation of Hearing.
Determine N for each of the three octave bands with centre frequencies
500, 1ooo and 2ooo Hz. The noise rating number is the highest of these
numbers . N = 85 is proposed as a limit for conservation of hearing because
habitual exposure to such a noise for 10 years may be expected to result
in a negligible loss in hearing for speech of an average individual .
Speech Communication.
The above noise rating number may also be used in order to determ~ne the
probable interference with speech communication in noisy surroundings .
Procedure: Determine the noise rating number N as for conservation of
hearing above. Then use Table 1.5 to determine if the noise rating number
is permissible for the case considered.
Distance at which everyday
Distance at which everyday
Noise speech of conversational
speech of raised voice level
Rating voice level is considered to is
considered to be
Number
be intelligible intelligible
m ft
m ft
40 7 23 14
46
45
4 13 8 26
60
2.2 7.2
4.5
15
55
1.8 4.1 2.5
8.2
60
o.7 2.3 1.4
4.6
65
0.4 1.3 0.8 2.6
70
0.22
o.74 o.45
1.5
75
o.13
0.41
0.25
0.82
80
0.07
o.23 0.14
o.46
85
-
-
0.08 o.26
Table 1. 6. Permissible noise rating numbers for speech communication.
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