Appendix B
B: Glossary of Terms
Crest Factor
Crest Factor is the ratio of the instantaneous peak value of a wave to its RMS value. This is a
performance specification of a meter’s ability to process signals that have peaks that are substantially
higher than their RMS averages.
Criterion Level (CL)
Criterion level is the average SPL that will result in a 100% dose over the Criterion time, usually
8 hours. The Criterion Level is typically set by a regulating agency, such as OSHA, and is not usually
applicable for community noise monitoring. Examples: OSHA mandates the Criterion Level (maximum
allowable accumulated noise exposure) to be 90 dB for 8 hours. For an 8-hour sample, an average
level (Lavg) of 90 dB will result in 100% dose. For the OSHA HEARING CONSERVATION
AMENDMENT, the “action level” is 85 dB for 8 hours. This would result in a 50% dose reading. Note
that the Criterion Level has not changed. (If the Criterion Level were changed to 85 dB then an 8-hour
average of 85 dB would result in 100% dose.)
Criterion Time
The time over which the Criterion Level is established, generally 8 hours.
Decibel (dB)
Sound Level Meters use the decibel as the unit of measure known as Sound Pressure Level (SPL).
SPL uses the ratio between a reference level of 20 microPascals (.00002 Pascals) and the level being
measured. SPL = 20 log (measured level/reference level). Example: the SPL for 1 Pascal is 20 log
(1 Pascal/.00002 Pascal) = 94 dB 20 microPascals (.00002 Pascals) is considered the average
threshold of hearing. A whisper is about 20 dB. A normal conversation is typically from 60 to 70 dB,
and a noisy factory from 90 to 100 dB. Loud thunder is approximately 110 dB, and 120 dB borders on
the threshold of pain.
Dose
Related to the Criterion Level, a dose reading of 100% is the maximum allowable exposure to
accumulated noise. For OSHA, 100% dose occurs for an average sound level of 90 dB over an
8-hour period (or any equivalent exposure). By using a TWA reading rather than the average sound
level, the time period is no longer explicitly needed. A TWA of 90 dB is the equivalent of 100% dose.
The dose will double (halve) every time the TWA increases (decreases) by the Exchange Rate.
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