Environmental Engineering Reference
In-Depth Information
TABLE 2.2
Permissible Noise Exposures per 29 CFR 1910.95
Duration per Day (hours)
Sound Level dBA Slow Response
8
90
6
92
4
95
3
97
2
100
1.5
102
1
105
0.5
110
0.25 or less
115
Note:
When the daily noise exposure is composed of two or more
periods of noise exposure of different levels, their com-
bined effect should be considered, rather than the individual
effect of each. If the sum of the following fractions
C
1
/
T
1
+
C
2
/
T
2
+
C
n
/
T
n
exceeds unity, then the mixed exposure should
be considered to exceed the limit value.
C
n
indicates the
total time of exposure at a specified noise level, and
T
n
indi-
cates the total time of exposure permitted at that level.
Exposure to impulsive or impact noise should not exceed
the 140-dB peak sound pressure level.
analyzers can assist users in determining the adequacy of various types of frequency-
dependent noise controls. They also can be used to select hearing protectors because
they can measure the amount of attenuation offered by the protectors in the octave
bands responsible for most of the sound energy in a given situation.
Noise Units, Relationships, and Equations
A number of noise units, relationships, and equations that are involved with control-
ling noise hazards are discussed below.
Sound Power
Sound power of a source is the total sound energy radiated by the source per unit
time. It is expressed in terms of the sound power level (
L
w
) in decibels referenced to
10
-12
watts (
w
0
). The relationship to decibels is shown below:
L
w
= 10 log
10
w
/
w
0
(2.2)
where
L
w
= Sound power level (decibels).
log
10
= Logarithm to the base 10.
w
= Sound power (watts).
w
0
= Reference power (10
-12
watts).
