to affect the rate of travel of the sound beam
significantly, and may be ignored.
Since sound travels faster in water under pressure,
the speed of sound in the sea increases proportionally
with depth. This difference in speed is also very small
and has little effect for the operator.
Temperature is the most important of the factors
affecting the speed of the sound beam in water. The
speed will increase with increasing temperature at the
rate of 4 to 8 feet per second per degree of change,
depending on the temperature.
The temperature of the sea varies from freezing in
the polar seas to more than 85°F in the tropics. The
temperature can also decrease by more than 30°F
from the surface to a depth of 450 feet. Thus, the
temperature is the most important factor because of
the extreme differences and variations. Remember,
the speed of sound in water increases as the
temperature increases.
Depth and Temperature
Except at the mouths of great rivers where salinity
may be a factor, the path of the sound beam will be
Figure 4-2.-Bending of a sound beam away from a
high-pressure area.
determined by the pressure effects of depth and by
temperature. The pressure effect is always present
and always acts in the same manner; it tends to bend
the beam upwards. Figure 4-2 illustrates the situation
when the temperature does not change with depth.
Even though the temperature does not change, the
speed of the sound increases with depth. The speed
increase is due entirely to the effect of pressure.
Notice in figure 4-2 that the sound beam bends
upward.
Figure 4-3 shows what happens when temperature
increases steadily with depth. When the surface of
the sea is cooler than the layers beneath it, the
temperature increases with depth, and the water has a
positive thermal gradient. This is an unusual
condition, but when it does happen, it causes the
sound beam to be refracted sharply upwards.
When the sea gets colder as the depth increases,
the water has a negative thermal gradient. In this
situation, the effect of temperature far outweighs the
effect of depth, and the sound beam is refracted
downward.
If the temperature remains the same throughout
the water, the temperature gradient is isothermal
Figure 4-3.-The effect of a positive thermal gradient.
4-4
