SPL in air
at 1 meter
noise levels cited in air do not equal underwater levels
for reasons that will be described in the following sections.
the amplitude of a sound is increased in a series of equal
steps, the loudness of the sound will increase in steps which
are perceived as successively smaller. Because sound "loudness"
varies exponentially, we'd normally have to deal with a lot
of zeros when doing computations involving the parameters
of sound, and we'd have to multiply numbers rather than simply
add and subtract them. Therefore,
sound intensity is usually measured with the decibel scale
in which the steps are not equal, but get progressively louder.
By using the decibel scale, calculations are simplified and
relative values relate more closely to perception. A pressure
in decibels expresses a ratio between the measured pressure
and a reference pressure (see SPL and
On the decibel scale, everything refers to power, which is
amplitude squared.; 0.0 dB corresponds to about the normal
threshold of hearing and 130 dB to the point where sound becomes
painful to humans.