Common noise dosimeter terms
Criterion Level (CL)
- expressed in decibels (dB),
it is the maximum allowable accumulated noise
level that results in 100% dose. Regional noise
standards specify criterion level.
Dose
- expressed in percent, it is the percent of
the maximum exposure that has accumulated
over the run time. 100% is the maximum
allowable exposure. 100% dose occurs for an
average sound level equal to the criterion level for
an 8 hour period.
Exchange Rate (Doubling Rate) (ER)
- the
decibel level that would double or halve the sound
exposure. For instance with a 3 dB exchange
rate the sound exposure doubles with every 3 dB
increase, and the sound exposure is halved every
3 dB decrease. Regional noise standards specify
the exchange rate.
L
avg
- stands for “level average” and is the average
sound level measured over the run time.
L
eq
- stands for “level equivalent” and is the
average sound level measured over the run time
but is calculated with a 3 dB exchange rate with
no threshold.
Max Level
- the highest weighted sound level
that occurred, also allowing for the response time
that the meter is set to. If the meter is set for A
weighting with Slow response then the Max level
is the highest A weighted sound that occurred
applying the Slow response time.
Peak level
- the highest instantaneous sound
level that the microphone detects. Unlike the Max
Level, the peak is detected independently of the
slow or fast response time the unit is set for.
Response Time (Fast, Slow, Impulse)
- how
quickly the circuitry responds to changing noise
levels. These are ANSI/IEC defined response times.
Most occupational noise standards require slow
response time.
Threshold Level (Cut Off)
- noise levels below
the threshold are integrated as zero decibels. This
will affect L
avg
, L
eq
, TWA, and Dose values. Most
regional noise standards specify the threshold
level, if any.
TWA (Time Weighted Average)
- takes the
noise exposure accumulated in the run time and
applies an eight hour time period. If the meter was
in run for 5 minutes, the TWA takes that 5 minutes
of noise input and averages it into an 8 hour run
time. The TWA in this case would be much lower
than the L
avg
.
Weighting (A,C,Z)
- frequency filters that
cover the frequency range of human hearing.
A weighting greatly attenuates high and low
frequency noise to mimic how the human ear
hears noise. C weighting also attenuates high
and low frequency noises, but not nearly as much
as A weighing. Z weighting does not apply any
attenuation, or weighting, to any frequency.
Most regional noise standards require a weighted
measurements.
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