Appendix B
B: Glossary of Terms
Example: OSHA uses an Exchange Rate of 5 dB. Suppose the TWA is 100 dB. The dose would
double for each 5 dB increase over the Criterion Level of 90 dB. The resulting dose is therefore 400%.
If the TWA was instead equal to 80 dB then the dose would halve for each 5 dB below the Criterion
Level. The resulting dose would be 25%. When taking noise samples less that the full workday, dose
is an easy number to work with because it is linear with respect to time. Example: If a 0.5-hour sample
results in 9% dose and the workday is 7.5 hours long, then the dose for the full workday would be a
135% dose (7.5 / 0.5 x 9%). This is computed making the assumption that the sampled noise will
continue at the same levels for the full 7.5-hour workday.
Exchange Rate (ER – also known as the Doubling Rate)
Exchange rate refers to how the sound energy is averaged over time. (Also, referred to as Doubling
Rate.) Using the decibel scale, every time the sound energy doubles, the measured level increases
by 3 dB. This is the 3 dB exchange rate that most of the world uses. For every increase of 3 dB in the
time weighted average, the measured DOSE would double. Some organizations such as OSHA in the
U.S. have argued that the human ear self-compensates for changing noise levels and they felt that
the 3 dB exchange rate should be changed to more closely match the response of the human ear.
OSHA currently uses a 5 dB exchange rate, which would mean that the reported DOSE would double
with every 5 dB increase in the time weighted average. The exchange rate affects the integrated
reading Lavg, DOSE, and TWA but does not affect the instantaneous sound level.
Field Calibration (vs. Factory Calibration), pre-calibration, post-calibration
To be sure the dosimeter is functioning within specified tolerance limits, it should be returned to the
factory annually for a factory recalibration. To verify that the dosimeter is measuring properly during
normal use, field calibrations should be performed. Pre-calibration is performed prior to any data
being recorded by the dosimeter. During a pre-calibration, the dosimeter level may be adjusted to
match the calibrator output level. Post-calibration is a simple verification that the instrument has
remained in calibration throughout the measurement period.
Free field setting (microphone)
Measurements made in an area without sound reflections, diffractions and absorptions caused by
nearby objects (including the operator). Sound waves are free to spread out continuously, like ripples
on a pond, without reflection. When making free field measurements with a dosimeter set up for use
as a sound level meter, the microphone is typically pointed directly at the sound source.
Hearing Conservation (HC)
The Occupational Safety and Health Administration (OSHA) and Mine Safety and Health
Administration (MSHA) provide regulatory standards for hearing conservation programs. In addition,
the American Conference of Governmental Industrial Hygienists (ACGIH) provides industrial
standards for hearing conservation programs. http://www.osha.gov/ , http://www.msha.gov/ ,
http://www.acgih.org/home.htm.
Hertz (Hz)
Unit of vibration frequency in cycles per second.
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