spreading wave may fill the entire available depth, hit the surface and bottom of
the pond and therefore only be able to spread sideways. This is called cylindrical
spreading. With sound spreading in a cylindrical shape, sound intensity (power
transmitted through a unit area in a specified direction) decreases more slowly than
in spherical spreading. We therefore see, that the chemical and physical make-up,
as well as the morphology of the waterbody (or any other body that sound travels
in) influences propagation and properties of sound. Different types of sound
propagation can therefore be harnessed for the study of the ocean and for remotesensing purposes.
Sound is also impacted by absorption and scattering. When sound travels
through a medium, it interacts with the molecules of the medium. If it has enough
power to overcome the molecules’ resistance to movement, these will start
vibrating and thus absorbing some energy from the acoustic wave. The molecules
get the energy to vibrate from the acoustic wave, and the higher the frequency of
the wave, the faster the molecules in the medium will vibrate, thus taking more
energy from the acoustic wave. This is why, under the same conditions, a high
frequency wave will not travel as far as a low frequency wave. The ocean also has
dissolved salts that absorb sound and convert it to heat, thus decreasing the
amplitude of the acoustic wave. Further, when a sound wave interacts with suspended microorganisms, bubbles, suspended particles in the ocean, etc., it will
scatter. The amount of scattering will depend of the size of the scatterer and the
wavelength of the sound. If the size of the object is comparable to, or bigger than,
the wavelength of the sound, the amount of scattering will be significant.
The general goal of SONAR is to obtain signal-to-noise ratios above a certain
threshold (i.e., where the signal features can be effectively distinguished from any
noise). Noise, either ambient, due to reverberation or self-noise, inherently exists
in the environment and interferes with the signal. Ambient noise is the noise that
exists in the water whether SONAR transmitters are present or not. Its sources can
be biological, thermal, resulting from surface processes such as wind, waves, and
rain, or may be due to shipping. Ambient noise is highly dependent on frequency,
location and depth, and is often highly directional; however, the simplest models
of dealing with ambient noise are isotropic. Noise from reverberation is defined as
the echo of a transmitted signal off the environment. The cause for the generation
of such echoes can be boundaries like the water surface (which is a near-perfect
acoustic reflector due to the great impedance difference between air and water), the
seafloor, or the presence of scatterers in the water itself, in which case we speak of
volume reverberation. Reverberation is directly proportional to signal energy and
duration. Self-noise is the noise the SONAR and its transporting vehicle make in
the environment, which is of important consideration when small signal amplitudes need to be evaluated. The sources can be manifold, such as from electrical,
machinery, or the most commonly dominant source of self-noise, flow through
water. The target itself can also emanate noise. While inconvenient for active
detection, since the energy emanating from the SONAR and the target can
interfere, it is very convenient for passive detection.
8 Acoustic Methods Overview
201
the pond and therefore only be able to spread sideways. This is called cylindrical
spreading. With sound spreading in a cylindrical shape, sound intensity (power
transmitted through a unit area in a specified direction) decreases more slowly than
in spherical spreading. We therefore see, that the chemical and physical make-up,
as well as the morphology of the waterbody (or any other body that sound travels
in) influences propagation and properties of sound. Different types of sound
propagation can therefore be harnessed for the study of the ocean and for remotesensing purposes.
Sound is also impacted by absorption and scattering. When sound travels
through a medium, it interacts with the molecules of the medium. If it has enough
power to overcome the molecules’ resistance to movement, these will start
vibrating and thus absorbing some energy from the acoustic wave. The molecules
get the energy to vibrate from the acoustic wave, and the higher the frequency of
the wave, the faster the molecules in the medium will vibrate, thus taking more
energy from the acoustic wave. This is why, under the same conditions, a high
frequency wave will not travel as far as a low frequency wave. The ocean also has
dissolved salts that absorb sound and convert it to heat, thus decreasing the
amplitude of the acoustic wave. Further, when a sound wave interacts with suspended microorganisms, bubbles, suspended particles in the ocean, etc., it will
scatter. The amount of scattering will depend of the size of the scatterer and the
wavelength of the sound. If the size of the object is comparable to, or bigger than,
the wavelength of the sound, the amount of scattering will be significant.
The general goal of SONAR is to obtain signal-to-noise ratios above a certain
threshold (i.e., where the signal features can be effectively distinguished from any
noise). Noise, either ambient, due to reverberation or self-noise, inherently exists
in the environment and interferes with the signal. Ambient noise is the noise that
exists in the water whether SONAR transmitters are present or not. Its sources can
be biological, thermal, resulting from surface processes such as wind, waves, and
rain, or may be due to shipping. Ambient noise is highly dependent on frequency,
location and depth, and is often highly directional; however, the simplest models
of dealing with ambient noise are isotropic. Noise from reverberation is defined as
the echo of a transmitted signal off the environment. The cause for the generation
of such echoes can be boundaries like the water surface (which is a near-perfect
acoustic reflector due to the great impedance difference between air and water), the
seafloor, or the presence of scatterers in the water itself, in which case we speak of
volume reverberation. Reverberation is directly proportional to signal energy and
duration. Self-noise is the noise the SONAR and its transporting vehicle make in
the environment, which is of important consideration when small signal amplitudes need to be evaluated. The sources can be manifold, such as from electrical,
machinery, or the most commonly dominant source of self-noise, flow through
water. The target itself can also emanate noise. While inconvenient for active
detection, since the energy emanating from the SONAR and the target can
interfere, it is very convenient for passive detection.
8 Acoustic Methods Overview
201