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21
WATER TEMPERATURE
Variations in water temperature affect 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 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
(5.6°C). However, the bending of the sound path has great effect over a distance
of several hundred yards (meters).
If the temperature of the water decreases with depth at the rate of 1°F for each
30 feet (.56°C for every 9 meters), most of the sound energy originating at the
source near the surface will travel along paths that are bent downward. There-
fore, the sound energy may not reach a shallow detector positioned 1000 yards
(914 meters) 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 thermo-
cline, 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 im
-
portant 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.
