Environmental Engineering Reference
In-Depth Information
Sound exposure level (SEL) is a measure of the cumulative physical energy of the
sound event which takes into account both intensity and duration. SELs are com-
puted by summing the cumulative sound pressure squared ( p 2 ) over time and nor-
malizing the time to 1 second. Because calculation of the SEL for a given underwater
sound source is a way to normalize to 1 second the energy of noise that may be much
briefer (such as the powerful, but short impulses caused by pile driving), SEL is typi-
cally used to compare noise events of varying durations and intensities.
In addition to intensity, underwater noise will have a range of frequencies (Hz or
cycles per second). For convenience, measurements of the potentially wide range of
individual frequencies associated with noise are integrated into “critical bands” or
filters; the width of a band is often given in 1/3-octave levels (Thomsen et al., 2006).
Thus, sounds can be expressed in terms of the intensities (dB) at particular frequency
(Hz) bands.
There are four fundamental properties of sound transmission in water that are rel-
evant to consideration of the effects of noise on aquatic animals (National Research
Council, 2000):
1. The transmission distance of sound in seawater is determined by a combina-
tion of geometric spreading loss and an absorptive loss that is proportional
to the sound frequency. Thus, attenuation (weakening) of sound increases
as its frequency increases.
2. The speed of a sound wave in water is proportional to the temperature.
3. The sound intensity decreases with distance from the sound source.
Transmission loss of energy (intensity) due to spherical spreading in deep
water is estimated by 20 log 10 r , where r is the distance in meters from the
source.
4. The strength of sound is measured on a logarithmic scale.
From these properties, it can be seen that high-frequency sounds will dissipate faster
than low-frequency sounds, and a sound level may decrease by as much as 60 dB at
1 km from the source. Acoustic wave intensity of 180 dB is 10 times less intense than
190 dB, and 170 dB is 100 times less than 190 dB (National Research Council, 2000).
Noise Produced by Ocean Energy Technologies
There is very little information available on sound levels produced by construction
and operation of ocean energy conversion structures (Michel et al., 2007). However,
reviews of the construction and operation of European offshore wind farms provide
useful information on the sensitivity of aquatic organism to underwater noise. For
example, Thomsen et al. (2006) reported that pile-driving activities generate brief,
but very high, sound pressure levels over a broad band of frequencies (20 to 20,000
Hz). Single pulses are about 50 to 100 ms in duration and occur approximately 30
to 60 times per minute. The SEL at 400 m from the driving of a 1.5-m-diameter
pile exceeded 140 dB re 1 µPa over a frequency range of 40 to 3000 Hz (Betke et
al., 2004). It usually takes 1 to 2 hours to drive one pile into the bottom. Sounds
produced by the pile-driving impacts above the water's surface enter the water from
the air and from the submerged portion of the pile; they then propagate through the
Search WWH ::




Custom Search