3: Setting up the Edge via DMS
Views on the Edge
Time-weighted average—is the sound level (in dB) that is accumulated for any time period but
with its average level computed over an 8-hour period. TWA is usually measured with A-
weighting, Slow response, and a 5 dB exchange rate.
All sound levels at or above the threshold level (cutoff level) are averaged into the calculations
relating to noise exposure. TWA is calculated with no cutoff, with cutoff 1 (typically 80 dB), or
with cutoff 2 (typically 90 dB).
If the time period is less than 8 hours, the TWA will be less than the Lavg. If the time period is
more than 8 hours, the TWA will be more than the Lavg.
Sound Exposure Level (SEL)—the sound exposure level averages the sampled sound over a
one second period. Assuming the sampled run time to be greater than one second, SEL is the
equivalent one-second noise that would be equal in energy to the noise that was sampled. SEL is
typically measured using a 3 dB exchange rate without a threshold. (L
AE
is not used by OSHA.)
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). (Please see Glossary of Terms for more
details).
Projected Dose—this is similar to Dose. However, instead of an 8-hour period, a specific hour is
entered. (See dose for more information.)z
Upper Limit—is the amount of time that the sound level exceeds a preset level (the Upper Limit).
It is the result of subtracting an A-weighted Lavg from a simultaneously collected C-weighted
Lavg. This value is used to determine the following equation:
Estimated Exposure (dBA) = Workplace Noise Level (dBA) – (NRR- Lc-a
AVG
)
Typically, noise is not constant. Using a response of Slow or Fast simply smooths the noise
fluctuation and makes the sound level easier to work with.
Slow – 1 second (1,000 milliseconds)
Fast – 0.125 second (125 milliseconds)
Run Time - The overall time of your noise study.
A and C weighting are types of frequency filters that cover the frequency range of human hearing
(20 Hz to 20 kHz).
“A” weighting—the “A” weighted filter attempts to make the dosimeter respond closer to the
way the human ear hears. It attenuates the frequencies below several hundred hertz as well
as the high frequencies above six thousand hertz. “A” weighted measurements are often
reported as dBA.
“C” weighting—provides a fairly flat frequency response with only slight attenuation of the
very high and very low frequencies. “C” weighting is intended to represent how the ear
perceives sound at high decibel levels and is often used as a “flat” response when LINEAR is
not available. “C” weighted measurements are often reported as dBC.
Table 3-1: Dosimeter parameters and definitions (continued)
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