reflection has an especially important influence on frequency-dependent
propagation.
Reflected waves create even more complicated sound fields, depending
on the relative texture of the surface that is contacted and the time required
for the reflected wave to decay as it moves away from the obstacle’s surface.
For example, the sound reflected off the surface can combine with sound
from the direct path to form an interference pattern representing the cancellation and summation of the two waves. This phenomenon is referred to
as the Lloyd mirror effect and can lead to as much as a 6dB increase in
sound pressure (complete summation and doubling of pressure) or a total
cancellation of sound pressure (see Jensen et al. 1994). Transmission loss
associated with the reflection (and absorption) of sound along the bottom
is even more complicated than that for the surface because the bottom may
vary widely in its composition and layering of different solids.
5. Refraction
In cases where sound moves between media with different specific acoustic
impedances, the direction of the incident sound wave will change because
of changes in the speed of propagation of the sound wave according to what
is known as Snell’s law (Weidner and Sells 1965; see discussion in Speaks
1992). This angular change in the wave’s direction is known as refraction.
Refraction is important when considering the behavior of sound traveling through water in which the sound-velocity profile varies strongly as a
function of depth (Section 2.3). Because water is essentially incompressible, sound speed is the dominant variable when considering underwater
acoustic refraction, and the sound-velocity profile is a critical measure for
predicting refractive effects.
One mechanism for modeling sound propagation is to treat a sound
source as emitting rays of sound. Ray paths in the ocean can be categorized
into four different types (from Jensen et al. 1994): (a) refracted-refracted
rays that propagate only along refracted paths, (b) refracted surfacereflected rays that bounce off the sea surface but refract in the deep water,
(c) refracted bottom-reflected rays that bounce off the sea bottom and
are refracted in the upper layers of the water column, and (d) surfacereflected bottom-reflected rays that reflect off the surface and the bottom with little influence of refraction. The refracted-refracted rays (Fig.
2.4A) suffer the least loss because they are only affected by attenuation
within the water column, whereas the surface-reflected bottom-reflected
rays (Fig. 2.4B) suffer the most because they are affected by the losses
associated with multiple reflections. As discussed later, each of these
conditions can potentially have profound effects on sound communication
among cetaceans in the deep ocean and teleost fishes in very shallow water,
respectively.
2. Physical Acoustics of Underwater Sound Communication
29
propagation.
Reflected waves create even more complicated sound fields, depending
on the relative texture of the surface that is contacted and the time required
for the reflected wave to decay as it moves away from the obstacle’s surface.
For example, the sound reflected off the surface can combine with sound
from the direct path to form an interference pattern representing the cancellation and summation of the two waves. This phenomenon is referred to
as the Lloyd mirror effect and can lead to as much as a 6dB increase in
sound pressure (complete summation and doubling of pressure) or a total
cancellation of sound pressure (see Jensen et al. 1994). Transmission loss
associated with the reflection (and absorption) of sound along the bottom
is even more complicated than that for the surface because the bottom may
vary widely in its composition and layering of different solids.
5. Refraction
In cases where sound moves between media with different specific acoustic
impedances, the direction of the incident sound wave will change because
of changes in the speed of propagation of the sound wave according to what
is known as Snell’s law (Weidner and Sells 1965; see discussion in Speaks
1992). This angular change in the wave’s direction is known as refraction.
Refraction is important when considering the behavior of sound traveling through water in which the sound-velocity profile varies strongly as a
function of depth (Section 2.3). Because water is essentially incompressible, sound speed is the dominant variable when considering underwater
acoustic refraction, and the sound-velocity profile is a critical measure for
predicting refractive effects.
One mechanism for modeling sound propagation is to treat a sound
source as emitting rays of sound. Ray paths in the ocean can be categorized
into four different types (from Jensen et al. 1994): (a) refracted-refracted
rays that propagate only along refracted paths, (b) refracted surfacereflected rays that bounce off the sea surface but refract in the deep water,
(c) refracted bottom-reflected rays that bounce off the sea bottom and
are refracted in the upper layers of the water column, and (d) surfacereflected bottom-reflected rays that reflect off the surface and the bottom with little influence of refraction. The refracted-refracted rays (Fig.
2.4A) suffer the least loss because they are only affected by attenuation
within the water column, whereas the surface-reflected bottom-reflected
rays (Fig. 2.4B) suffer the most because they are affected by the losses
associated with multiple reflections. As discussed later, each of these
conditions can potentially have profound effects on sound communication
among cetaceans in the deep ocean and teleost fishes in very shallow water,
respectively.
2. Physical Acoustics of Underwater Sound Communication
29