Specifications
SECTION 7
BASICS OF SOUND IN WATER
7.1 BACKGROUND
If a diaphragm submerged in water is caused to vibrate by electrical means, it has
mechanical energy of motion communicated to the water. If another diaphragm is
submerged in the water near the vibrating diaphragm, the acoustic energy in the water will
excite mechanical vibrations in the second diaphragm. These vibrations may be detected
electrically to complete a ow of mechanical energy from the rst diaphragm to the second.
The rst diaphragm is called the source or transducer, and the second is called a receiver or
hydrophone. With the Magnacom
®
, the transducer and hydrophone are one and the same.
7.2 FACTORS THAT AFFECT SOUND IN WATER
Many factors affect the propagation of sound in water, depending upon location, depth, and
time of day. As a net result, communication in water can be affected by local conditions and
the dive’s type and depth. Fluctuations in range and intelligibility can be expected.
7.2.1 Distance: The sound intensity from a source varies inversely with the square of
the distance from the source. This sort of variation is referred to as spherical spreading.
Other factors also inuence the variation of sound intensity with distance. As the
sound passes through the water, some of the energy is absorbed and converted to heat
(attenuation) and some of the energy is scattered by sh, pilings, seaweed, bubbles, etc.
(diffraction). In addition, both the surface and bottom may affect the sound intensity by
reecting sound back into the water. The sound reected by the surface and bottom may
raise the intensity above normal levels (reinforcement) or may introduce destructive
interference. The bending of the sound waves by temperature variations also has a great
effect on the sound intensity at points remote from the source.
If the source of the sound is near the surface, there is some point beyond which sound is not
received from the source. This point is said to be in a shadow zone. The distance from the
source to the shadow zone is determined by the rate of change of temperature with depth,
the depth of the source, and the depth at which the reception is made (Fig. 13).
7.2.2 Water Density: In addition to these factors, water density is also important. Because
the density of sea water varies with the temperature, the salt content, and the static pressure,
the effect on sound of each of these three factors is usually considered separately.
7.2.3 Water Temperature: Variations in water temperature affect sound transmission
most. In some areas of the ocean, the temperature changes at a xed rate over large
ranges of depth. If the temperature increases with depth at a xed rate, the velocity of
sound increases at a rate constant with depth and sound waves are refracted toward the
surface. If, however, the temperature decreases with the depth (as is frequently the case),
the velocity of sound decreases with depth and the waves of sound are bent downward.
There are also areas in the sea where, at some depth, temperature changes rapidly over a
small depth range. Such a layer is referred to as a thermocline or thermal layer. Such layers,
in addition to producing rather sharp bending of the sound waves by refraction effects, can
serve as reecting surfaces.
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