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P is the pressure in Pascals, and 20μPa (0.00002 Pascals) is the “reference value” that we are comparing
to. Because the reference value is so small, all sound pressure levels that we encounter in everyday life are
positive:
Time
Amplitude
Sound waves are modulation of air pressure. The metric unit of pressure is a Pascal, a force of one newton over
an area of one meter squared. The dynamic range of audible sound is extremely large, from 0.00002 Pascals
(the threshold of hearing) to 20 Pascals (threshold of pain).
Because we also perceive the loudness of sound approximately logarithmically, it makes sense to use decibels
when working with audio signals, for both acoustic and electrical signals.
Sound pressure level is defined by:
11.3 Sound Waves
SOUND PRESSURE EXAMPLES
dB SPL
PASCALS
1883 eruption of Krakatoa
310
63,245,553,203
Maximum unclipped SPL in Earth’s atmosphere
194
100,237
Large calibre rifle at 1m
171
7096
Jet engine at 1m
150
632
Maximum continuous output of most PA speakers (1m)
130
63
Threshold of pain/risk of instantaneous hearing damage
120
20
Chainsaw at 1m
110
6.32
Amplified live music or club (A-weighted, at FOH)
90-100
0.63-2
Risk of noise induced hearing damage (over time)
85
0.35
Home music listening or TV
60-70
0.02-0.06
Normal conversation
40-60
0.002-0.02
Quiet “silent” room
20-30
0.0002-0.0006
Auditory threshold at 1kHz
0
0.00002
Anechoic chamber at the University of Salford (A-weighted)
-12.4
0.0000048
Summary of Contents for SB10A
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