A similar case often arises in connection with
sonar. Sound waves often strike small objects in the
sea, such as fish or air bubbles. These small objects
cause the waves to scatter. Each object produces a
small echo, which may return to the transducer. The
reflections of sound waves from the sea surface and
the sea bottom also create echoes. The combined
echoes from all these disturbances are called
Since they are reflected from
various ranges, the y seem to be a continuous sound.
Reverberations from nearby points may be so loud
that they interfere with the returning echo from a
There are three main types of reverberation, or
backward scattering of the sound wave. They are as
1. There is reverberation from the mass of water.
Causes of this type of reverberation are not
completely known, although fish and other objects
contribute to it.
2. There is reverberation from the surface. This
is most intense immediately after the sonar
transmission; it then decreases rapidly. The intensity
of the reverberation increases markedly with
increased roughness of the sea surface.
3. There is reverberation from the bottom. In
shallow water, this type of reverberation is the most
intense of the three, especially over rocky and rough
Just as the beam from a searchlight spreads out
and becomes weaker with distance, so does sound.
The farther the target is from the sonar transducer, the
weaker the sound waves will be when they reach it.
This is known as spreading or divergence.
If there were no temperature differences in the
water, the sound beam would travel in a straight line.
This happens because the speed of sound would be
roughly the same at all depths. The sound beam
would spread and become weaker at a relatively
Unfortunately, the speed of sound is not constant
at all depths. The speed of sound in seawater
increases from 4,700 feet per second to 5,300 feet per
second as the temperature increases from 30°F to
85°F. Salinity and pressure effects on sound speed
are not as extreme as the large effects produced by
temperature changes in the sea. Because of the
varying temperature differences in the sea, the sound
beam does not travel in a straight line, but follows
curved paths. This results in bending, splitting, and
distorting of the sound beam.
When the sound beam is bent, it is said to be
refracted. A sound beam is refracted when it passes
from a medium of a given temperature into a medium
with a different temperature. An example of this is a
sound beam traveling from an area of warm water into
an layer of cold water. The sound beam will bend
away from the area of higher temperature (higher
sound velocity) toward the lower temperature (lower
As a result of refraction, the range at which a
submarine can be detected by sound may be reduced
to less than 1,000 yards, and this range may change
sharply with changing submarine depth.
Speed of the Sound Beam
As mentioned previously, sound travels much
faster in seawater than in the atmosphere. Near sea
level, sound travels through the atmosphere at
approximately 1,080 feet per second. In seawater,
that same sound beam will travel at approximately
4,700 to 5,300 feet per second.
There are three main characteristics of seawater
that affect the speed of the sound wave traveling
through it. These characteristics are as follows:
1. Salinity (the amount of salt in the water)
2. Pressure (caused by increased depth)
3. Temperature (the effect of which is calculated
in terms of slopes, or gradients)
There is a high mineral content in seawater. The
density of seawater is approximately 64 pounds per
cubic foot, while fresh water has a density of about
62.4 pounds per cubic foot. This difference is caused
by the salt in the seawater. Salt content in seawater is
called the salinity of water.
The overall effect of increasing the salinity is an
increase in the speed of the sound beam in the water.
This means that as the sound travels through water of
varying salinity, it travels faster through the water
with more salt content. Such a change in salinity is
considerable at the mouth of a river emptying into the
sea. Elsewhere, the difference in salinity is too small