Ocean Technology Systems MilCom 6000D User Manual page 25

WATER TEMPERATURE
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Variations in water temperature affects sound transmission most. In some
areas of the ocean, the temperature changes at a fixed rate over large
ranges of depth. If the temperature increases with depth, the velocity
of sound increases and the sound waves will be refracted toward the
surface. If, however, the temperature decreases with the depth, the
velocity of sound decreases and the waves of sound are bent downward.
There are also areas in the sea where temperature changes rapidly over
a small depth range called a thermocline. Thermoclines can to produce a
sharp bending of sound waves and may serve as reflecting surfaces.
The velocity of sound transmission changes only about one percent for a
temperature change of l0°F. However, the bending of the sound path has
great effect over a distance of several hundred yards.
If the temperature of the water decreases with depth at the rate of 1°F
for each 30 feet (starting at the surface), most of the sound energy
originating at the source near the surface will travel along paths that are
bent downward. Therefore, the sound energy may not reach a shallow
detector positioned l,000 yards from the source but may reach a deeper
detector position further from the source. Greater temperature variations
can cause these paths to bend more sharply. The best method to deal
with thermoclines is to bring the divers and/or transducers as close to
each other as possible.
If a diver enters a thermocline, the diver should report it to everyone
(surface and divers) so they know the depth of the thermocline. All
divers should stay within that depth, and the surface station should try to
position the surface transducer below or above, whichever is the case.
WATER DENSITY
Water density is important for the propagation of the sound. Because
the density of sea water varies with temperature, salt content, and static
pressure, the effect on sound of each of these three factors is usually
considered separately.
BACKGROUND NOISE
Marine organisms play an important role in underwater acoustics.
They are important primarily because of the effect they have on sound
transmission, but they often serve as sources of underwater noise as
well. High background noise can interfere with good communications.
Such background noise can be mitigated with the squelch function.