Environmental Engineering Reference
In-Depth Information
SPL (dB) = 20 log
10
(
p
/
p
0
)
where
p
is a pressure fluctuation caused by a sound source, and
p
0
is the reference
pressure, defined in underwater acoustics as 1 µPa at 1 m from the source (Thomsen
et al., 2006). Using the above formula, doubling the pressure of a sound (
p
) results
in a 6-dB increase in SPL.
The sound pressure of a continuous signal is often expressed by a root-mean-
square (rms) measure, which is the square root of the mean value of squared
instantaneous sound pressures integrated over time (Madsen, 2005). Like SPL, the
resulting integration of instantaneous sound pressure levels is also expressed in dB
re 1 µPa (rms). An rms level of safe exposure to received noise has been established
for marine mammals; the lower limits for concern about temporary or permanent
hearing impairments in cetaceans and pinnipeds are currently 180 and 190 dB re
1 µPa (rms), respectively (National Marine Fisheries Service, 2003; Southall et al.,
2007). However, Madsen (2005) argued that rms safety measures are insufficient and
should be supplemented by other estimates of the magnitude of noise (e.g., maximum
peak-to-peak SPL in concert with a maximum received energy flux level).
Sound intensity is greatest near the sound source and, in the far field, decreases
smoothly with distance. As the acoustic wave propagates through the water, inten-
sity is reduced by geometric spreading (dilution of the energy of the sound wave as
it spreads out from the source over a larger and larger area) and, to a lesser extent,
absorption, refraction, and reflection (Wahlberg and Westerberg, 2005). Attenuation
of sound due to spherical spreading in deep water is estimated by 20 log
10
r
, where
r
is the distance in meters from the source (National Research Council, 2000).
Assuming simple spherical spreading (no reflection from the sea surface or bot-
tom) and the consequent transmission loss of SPL, a 190-dB source level would be
reduced to 150 dB at 100 m. Close to the source, changes in sound intensity vary
in a more complicated fashion, particularly in shallow water, as a result of acoustic
interference from natural or man-made sounds or where there are reflective surfaces
(seabed and water surface).
DID YOU KNOW?
According to several articles published in
Electrical Engineering
and the
Journal of the Acoustical Society of America
, the decibel suffers from the fol-
lowing disadvantages (Chapman, 2000; Clay, 1999; Hickling, 1999; Horton,
1954):
•
The decibel creates confusion.
•
The logarithmic form obscures reasoning.
•
Decibels are more related to the area of slide rules than that of mod-
ern digital processing.
•
Decibels are cumbersome and difficult to interpret.
Hickling (1999) concluded, “Decibels are a useless affectation, which is
impeding the development of noise control as an engineering discipline.”
