12
also have defined allowable noise (undesirable sound energy) level exposure
limits for persons in industrial environments. Selected and applicable parts of
those currently in force are cited in this manual. Standards and government regu-
latory laws are subject to modification or expansion. Check for latest available
documents and information.
The specification for a Type S2A instrument takes into account the average re-
sponse of the human ear which varies as a function of the intensity of sound. It
groups intensity into three weighting curves: "A", "B" and "C". The "A" weighted
curve more closely corresponds to the human ear at the low sound levels.
As indicated in Figure 3-1, the "B" and "C" weighting curves include more of the
low frequency information. Thus, at any level, measurements taken with various
weightings can be compared in determining the frequency content of the noise.
This is done by indicating the level in dB, accompanied by the weighting letter
notation "A", "B" or "C", preferable in parenthesis; e.g., the 100 dB(A), etc.
The allowable employee noise level exposure limits are established by the Oc-
cupational Safety and
H
ealth Act of 1970 (OS
H
A), the Walsh-
H
ealy Act (Federal
Register, Volume 34, No. 96) and paragraph 1910.9
5
of the Occupational Safety
and
H
ealth Standards (Federal Register, Vol. 36, No. 10
5
, May 29, 1971) and the
OS
H
A
H
earing Conservation Program Amendment, Federal Register, Vol. 46,
No. 162, August 21, 1981. Table 3-1 summarizes these limits.
*Table 3-1. Permissible Noise Exposure Limits
Duration per Day
Sound Level, dB(A) SLOW
In Hours
Response
32
80
16
8
5
8
90
6
92
4
9
5
3
97
2
100
1-1/2
102
1
10
5
1/2
110
1/4 or less
11
5
*From Federal Register, Vol. 46, No. 162, August 21, 1981.
3.3
Sound Fields
3.3.1
Free Field
A free field contains no reflecting objects. Ideally it prevails around a point sound
source located in free space.
H
owever, it can be considered to exist whenever the
sound level being measured is caused mainly by sound waves coming directly
from the sound source. This can occur even if there are reflecting objects, if
measurements are made close to the sound source.
3.3.2
Reverberant Field
A reverberant field contains reflecting objects and the sound level being mea-
sured is caused mainly by reflections of the sound source. This usually occurs at
a moderate distance from the sound source.
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