The exchange rate is the increase or decrease in decibels (db) corresponding to twice
(or half) the noise dose. For example, when using a 5 dB exchange rate, a dose of 90 dB is twice the dose of 85 dB, assuming that the
duration of exposure is the same.
- Only instruments using a 5 db exchange rate may be used for OSHA compliance measurements.
- Noise dosimeters used by the Department of the Navy use a 4 dB exchange rate.
- Instruments used by the Department of the Army, the Department of Air Force, the National Institute for Safety and Health (NIOSH), the
Environmental Protection Agency (EPA), and most foreign governments use a 3 db exchange rate. Additionally, the American Conference of
Governmental Industrial Hygienists (ACGIH) Physical Agents Threshold Values (TLV) Committee revised its noise TLV to also use the 3 dB
exchange rate.
The threshold level is the A-weighted sound level at which a personal noise dosimeter
begins to combine noise into a measured exposure.
- For example, if the threshold level on a sound level meter is set at 80 dB, it will capture and integrate all noise in the employee's
hearing zone that equals or exceeds 80 dB into the dose computation. Sound levels below this
threshold would not be included in the computation of noise dose.
- Differences between sound-measuring instrument settings must be taken into account when measuring employee noise exposure. A dosimeter
with an 80 dBA threshold integrates all noise above 80 dB into the dose, and so the dosimeter will report a higher noise dose than the dose
reported by a dosimeter with a 90 dBA threshold if both instruments are used side-by-side to evaluate the same noise exposure. (Note: In high
noise levels (all noise above 90 dBA), the two dosimeters will read exactly the same.)
The criterion level is the permissible exposure limit (PEL).
- For OSHA purposes, the value for the criterion level is 90 dB, averaged over an 8-hour period on the A scale of a standard sound level
meter set on slow response. Noise measurements taken with an instrument set on the A weighting scale are expressed as dBA.
Additional factors to consider include frequency weighting and
instrument response.
- For compliance measurements, use the A-weighted network and slow response setting exclusively. The computation of noise dose for
compliance with OSHA noise standards includes the following types of noise:
- Continuous
- Intermittent (aircraft flyovers, automobile traffic, trains, mobile equipment, sporadically operated tools and machines)
- Impulse (drop forge hammer, dog barking, pistol shot, door slamming)
- Repeated impulse (riveting, pneumatic hammers, machine guns)
- The standard requires that all continuous, intermittent, and impulsive sound levels from 80 dB to 130 dB be included in the measurement
of dose.
Additional information
(App III:A-5) about dosimeter settings is also available. |
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