Introduction to Acoustics

23.4.4 Criteria for Noise Immission
In this section, criteria are divided into criteria for hearing
damage, criteria for annoyance, and other currently
used criteria.
Hearing Damage Criteria
In the United States, permissible levels for industrial
noise exposure are set by the US Department of Labor,
and are described in 29 CFR 1910.95. In summary, an
employer shall administer a hearing conservation program
when the eight hour time-weighted average level
exceeds 85 dB. The requirements of such a program are
described in detail in the above document – as is the
requirements on audiometers for the determination of
hearing loss, personnel conducting the tests, and hearing
protectors when used to satisfy the requirements below.
More details may be found in the Hearing Conservation
Manual [23.149].
Permissible noise exposures are given in the regulation
in Table G-16, reproduced below (Table 23.9) with
an explanation of how the permissible levels are to be
determined.
Currently, engineering controls are required only if
the eight hour noise exposure exceeds 100 dB. For lower
levels, hearing protection is an alternative.
The problem of specifying upper limits for noise
in the workplace has been studied by a technical study
group of the International Institute of Noise Control Engineering,
and a report has been prepared and approved
by the member societies of that organization [23.150].
Most countries in the world allow a time-weighted average
sound level over eight hours of 85–90 dB as the
upper limit, and an exchange rate of 3 dB. Selection
of this exchange rate is widely believed to be the best
alternative, and it greatly simplifies the measurement
of levels that fluctuate during the working day. Table
23.10, from [23.150], shows hearing damage criteria
in a number of countries as of 1997.
I-INCE publication 97-1 [23.150] makes specific
recommendations with respect to allowable levels, exchange
rate, and hearing conservation programs:
1. It is desirable for jurisdictions without regulations,
or with currently higher limits, to set a limit on
the level of exposure over a workshift, A-weighted
and normalized to eight hours, of 85 dB as soon as
may be possible given the particular economic and
sociological factors that are pertinent.
2. This exposure level should include the contribution
from all sounds that are present including short-term,
Noise 23.4 Noise and the Receiver 1001
high-intensity sounds. If such sounds are further limited
in regulations to a maximum sound pressure
level, then regulations should set a limit of 140 dB
for C-weighted peak sound pressure level.
3. An exchange rate of 3 dB per doubling or halving of
exposure time should be used. This exchange rate is
implicit when the exposure level is stated in terms
of eight-hour-average sound pressure level;
4. Efforts should be made to reduce levels of noise
in the workplace to the lowest economically and
technologically reasonable values, even when there
may be no risk of long-term damage to hearing. Such
action can reduce other negative effects of noise such
as reduced productivity, stress and disturbed speech
communication.
5. At the design stage of any new installation, consideration
should be given to sound and vibration
isolation between noisier and quieter areas of activity.
Rooms normally occupied by people should have
a significant amount of acoustical absorption in order
to reduce the increase of sound due to excessive
reverberation.
6. The purchase specifications for all new and replacement
machinery should contain clauses specifying
the maximum emission sound power level and emission
sound pressure level at the operator’s position
when the machinery is operating.
Table 23.9 Criteria from Table G-16 of 29 CFR 1910.95.
Permissible noise exposures (1)
Duration in hours per day Sound level (dBA)
slow response
8 90
6 92
4 95
3 97
2 100
11/2 102
1 105
1/2 110
1/4 or less 115
Notes:
1. When the daily noise exposure is composed of two or more
periods of noise exposure of different levels, their combined
effect should be considered, rather than the individual effect of
each. If the sum of the following fractions: C1/T1 + C2/T2 +
...Cn/Tn exceeds unity, then, the mixed exposure should be
considered to exceed the limit value. Cn indicates the total time
of exposure at a specified noise level, and Tn indicates the total
time of exposure permitted at each level. Exposure to impulsive
noise should not exceed 140 dB peak sound pressure level
Part G 23.4