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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.
Summary of Contents for PowerCom 3000D
Page 1: ...User Manual...
Page 6: ...v OTS POWERCOM...
