Appendix B
Sound Examiner SE-400 Series
34
Glossary of Terms
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.
Dynamic range
The range of input amplitudes on any given range setting over which the instrument can produce a meaningful
response. The bottom of the dynamic range is the instrument’s Noise Floor for that range setting, and the top of the
dynamic range is the maximum input signal that will not overload the instrument on that range setting.
Exchange Rate (ER)
Also known as the Doubling Rate, this refers to how the sound energy is averaged over time. 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 L
AVG
,
Dose, and TWA but does not affect the instantaneous sound level. (Please refer to
www.osha.gov
for more
information.)
L
AVG
Average sound level measured over the run time. This becomes a bit confusing when thresholds are used. Any
sound below the threshold is not included in this average. Remember that sound is measured in the logarithmic
scale of decibels therefore the average can not be computed by simply adding the levels and dividing by the number
of samples. When averaging decibels, short durations of high levels can significantly contribute to the average level.
Example
: Assume the threshold is set to 80 dB and the Exchange Rate is 5 dB (the settings of OSHA’s Hearing
Conservation Amendment). Consider taking a one hour noise measurement in an office where the A- weighted
sound level was typically between 50 dB and 70 dB. If the sound level never exceeded the 80 dB threshold during
the one hour period, then the L
AVG
would indicate a reading of zero. If 80 dB was exceeded for only a few seconds
APPENDIX
B
