Specifications

The velocity of sound transmission changes only about one percent for a temperature
change of l0°F. However, the resultant 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 rather sharply downward. Therefore, the
sound energy may not reach a shallow detector positioned l000 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, he 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 (Fig. 13).
7.2.4 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—whether man-made, animal, or environmental (waves or rain)—can interfere with
good communications. Such background noise can be suppressed through the use of the
squelch function and thermoclines.
7.2.5 Zones of Silence: Large natural or man-made objects can block acoustical
transmission under certain conditions, in much the same way as a rock blocks a fast-moving
current of water. Close to the back side of the rock, in this example, the current is absent and
Figure 13: Thermoclines affect the ultrasonic signal. Divers must report thermocline
depth(s) so the surface transceiver and/or other divers can be as close to the same
depth as possible.
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