the mechanism as a volume-driven monopole source (Aroyan et al. 2000).
The swimming motion of aquatic animals and non–swim bladder soundproducing mechanisms in fish have been compared to dipole and higherorder sources (Harris 1964; Kalmijn 1988; Hawkins 1993).
3.1. Monopole Sound Source
Close to the surface of a pulsating sphere, the motion of the sphere’s surface
forces the medium to move (that is, accelerate) away from the sphere. The
displacement of water molecules adjacent to the sphere’s surface establishes the “local hydrodynamic flow” of an “acoustic near field” (Kalmijn
1988, pp. 85–86). All molecules in the near field essentially move in phase
with the source itself (Fig. 2.3A(a)). The pulsating sphere also produces a
“propagating sound wave” (Kalmijn 1988, p. 88) or “pressure wave” (van
Bergeijk 1964, p. 283) that also originates at the source but has a greater
range, extending beyond the near field into a far field. In the far field, the
mechanical disturbance generated by a pulsating sphere spreads in a periodic, sinusoidal manner as alternating bands of compression (increased
density of water molecules) and rarefaction (decreased density of water
molecules) (Fig. 2.3A(b)). The use of the terms “near field” and “far field”
implies whether the spreading sound wave is dominated by either the nonpropagating, hydrodynamic flow or by the propagating sound wave. In
theory, both could extend to infinity in an ideal environment. Hence, it is
essential to remember that there is no clear boundary between the near and
far fields because they are “physically inseparable components of one and
the same acoustic field” (see Kalmijn 1988, p. 89).
With increasing distance from the pulsating sphere, acoustic energy is distributed over a greater and greater area so that the amount of energy per
unit area dissipates with increasing range. However, the rate of decrease in
net fluid displacement, velocity, and acceleration with increasing source
distance differs between the nonpropagating, hydrodynamic flow and the
propagating sound wave. Thus, the magnitude of these quantities falls off
quickly as the square of the radial distance from the source or 1/r
2 for
the hydrodynamic flow, but only as 1/r for the propagating wave. Pressure
decreases at the rate of 1/r for both the local flow and the propagating sound
wave.
3.2. Dipole Sound Source
Now consider a vibrating sphere that is modeled as a dipole, a motion that
more closely resembles many forms of vibration in water that are of biological origin (Fig. 2.3B). A dipole source is represented by a sphere vibrating mainly along one axis. It is essentially “the equivalent of two
equal-strength monopole fields of which the sources are separated only by
a short distance and that pulsate 180° out of phase” (Kalmijn 1988, p. 93).
22
A.H. Bass and C.W. Clark
The swimming motion of aquatic animals and non–swim bladder soundproducing mechanisms in fish have been compared to dipole and higherorder sources (Harris 1964; Kalmijn 1988; Hawkins 1993).
3.1. Monopole Sound Source
Close to the surface of a pulsating sphere, the motion of the sphere’s surface
forces the medium to move (that is, accelerate) away from the sphere. The
displacement of water molecules adjacent to the sphere’s surface establishes the “local hydrodynamic flow” of an “acoustic near field” (Kalmijn
1988, pp. 85–86). All molecules in the near field essentially move in phase
with the source itself (Fig. 2.3A(a)). The pulsating sphere also produces a
“propagating sound wave” (Kalmijn 1988, p. 88) or “pressure wave” (van
Bergeijk 1964, p. 283) that also originates at the source but has a greater
range, extending beyond the near field into a far field. In the far field, the
mechanical disturbance generated by a pulsating sphere spreads in a periodic, sinusoidal manner as alternating bands of compression (increased
density of water molecules) and rarefaction (decreased density of water
molecules) (Fig. 2.3A(b)). The use of the terms “near field” and “far field”
implies whether the spreading sound wave is dominated by either the nonpropagating, hydrodynamic flow or by the propagating sound wave. In
theory, both could extend to infinity in an ideal environment. Hence, it is
essential to remember that there is no clear boundary between the near and
far fields because they are “physically inseparable components of one and
the same acoustic field” (see Kalmijn 1988, p. 89).
With increasing distance from the pulsating sphere, acoustic energy is distributed over a greater and greater area so that the amount of energy per
unit area dissipates with increasing range. However, the rate of decrease in
net fluid displacement, velocity, and acceleration with increasing source
distance differs between the nonpropagating, hydrodynamic flow and the
propagating sound wave. Thus, the magnitude of these quantities falls off
quickly as the square of the radial distance from the source or 1/r
2 for
the hydrodynamic flow, but only as 1/r for the propagating wave. Pressure
decreases at the rate of 1/r for both the local flow and the propagating sound
wave.
3.2. Dipole Sound Source
Now consider a vibrating sphere that is modeled as a dipole, a motion that
more closely resembles many forms of vibration in water that are of biological origin (Fig. 2.3B). A dipole source is represented by a sphere vibrating mainly along one axis. It is essentially “the equivalent of two
equal-strength monopole fields of which the sources are separated only by
a short distance and that pulsate 180° out of phase” (Kalmijn 1988, p. 93).
22
A.H. Bass and C.W. Clark