4.2. Attenuation
There are a number of factors that have been proposed to influence sound
attenuation (Urick 1983; Jensen et al. 1994). Most important here is what
is referred to as absorption, or the transformation of acoustic energy into
heat energy either in the transmission medium or at its boundaries (see
Kinsler and Frey 1962). Boundary losses will be mentioned below in the
context of reflection (Section 4.3). Kinsler and Frey (1962, pp. 217–218) recognize three basic types of loss in the transmission medium. (1) Loss due
to viscosity arises from the “relative motion occurring between various portions of the medium during compressions and expansions that accompany
transmission of a sound wave.” (2) Loss due to heat conduction arises from
the “tendency for heat to be conducted from regions of condensation where
temperature is raised to neighboring regions of rarefaction where the temperature is lowered. In the process of this heat transfer there is a tendency
towards pressure equalization, which reduces the amplitude of a wave as
it is propagated through the medium.” (3) Loss due to the “molecular
exchanges of energy” arise from “the finite time required for a portion of
the compressional energy of the fluid to be converted into internal energy
of molecular vibration, then correspondingly during the expansion cycle
some of this energy will be delayed in restoration so as to be returned to
the fluid during a time of rarefaction. Such a delay will result in a tendency
towards pressure equalization and an attendant reduction in pressure amplitude of the wave.”
The attenuation coefficient, sometimes referred to as the absorption coefficient, a, because this factor dominates most of the frequency band of
interest, describes how much energy of the incident wave is absorbed by
the transmission medium per unit distance (for use in transmission-loss
equations, see Kinsler and Frey 1962). Overall, attenuation, expressed in
terms of dB per unit distance, increases linearly with frequency. Attenuation coefficients in seawater are very small; for example, they are (see Urick
1983; Spiesberger and Fristrup 1990): 0.0002 dB/km for 10 Hz, 0.0015 dB/km
for 100 Hz, 0.025 dB/km for 500 Hz, 0.06 dB/km for 1,000 Hz, and 0.2 dB/km
for 4,000 Hz. (For comparative purposes, we note that the attenuation coefficient at 500 Hz in air is 20–30 times greater for typical conditions.) Thus,
absorption is not a major factor for many whales and fishes that produce
sounds in the low (<100 Hz) and intermediate (100–1,000 Hz) frequency
regions.
4.3. Reflection
Reflection occurs when a sound wave encounters a medium with a different impedance. Of particular interest here would be the reflection of sound
waves at boundaries such as the surface or bottom. In shallow water, where
a sound wave will interact with the surface and bottom multiple times,
28
A.H. Bass and C.W. Clark
There are a number of factors that have been proposed to influence sound
attenuation (Urick 1983; Jensen et al. 1994). Most important here is what
is referred to as absorption, or the transformation of acoustic energy into
heat energy either in the transmission medium or at its boundaries (see
Kinsler and Frey 1962). Boundary losses will be mentioned below in the
context of reflection (Section 4.3). Kinsler and Frey (1962, pp. 217–218) recognize three basic types of loss in the transmission medium. (1) Loss due
to viscosity arises from the “relative motion occurring between various portions of the medium during compressions and expansions that accompany
transmission of a sound wave.” (2) Loss due to heat conduction arises from
the “tendency for heat to be conducted from regions of condensation where
temperature is raised to neighboring regions of rarefaction where the temperature is lowered. In the process of this heat transfer there is a tendency
towards pressure equalization, which reduces the amplitude of a wave as
it is propagated through the medium.” (3) Loss due to the “molecular
exchanges of energy” arise from “the finite time required for a portion of
the compressional energy of the fluid to be converted into internal energy
of molecular vibration, then correspondingly during the expansion cycle
some of this energy will be delayed in restoration so as to be returned to
the fluid during a time of rarefaction. Such a delay will result in a tendency
towards pressure equalization and an attendant reduction in pressure amplitude of the wave.”
The attenuation coefficient, sometimes referred to as the absorption coefficient, a, because this factor dominates most of the frequency band of
interest, describes how much energy of the incident wave is absorbed by
the transmission medium per unit distance (for use in transmission-loss
equations, see Kinsler and Frey 1962). Overall, attenuation, expressed in
terms of dB per unit distance, increases linearly with frequency. Attenuation coefficients in seawater are very small; for example, they are (see Urick
1983; Spiesberger and Fristrup 1990): 0.0002 dB/km for 10 Hz, 0.0015 dB/km
for 100 Hz, 0.025 dB/km for 500 Hz, 0.06 dB/km for 1,000 Hz, and 0.2 dB/km
for 4,000 Hz. (For comparative purposes, we note that the attenuation coefficient at 500 Hz in air is 20–30 times greater for typical conditions.) Thus,
absorption is not a major factor for many whales and fishes that produce
sounds in the low (<100 Hz) and intermediate (100–1,000 Hz) frequency
regions.
4.3. Reflection
Reflection occurs when a sound wave encounters a medium with a different impedance. Of particular interest here would be the reflection of sound
waves at boundaries such as the surface or bottom. In shallow water, where
a sound wave will interact with the surface and bottom multiple times,
28
A.H. Bass and C.W. Clark