sound wave. The frequency (in hertz) of the sound
wave is the number of wavelengths that occur every
FACTORS AFFECTING THE SOUND
The particular sound waves of interest to the
sonar operator are the waves that leave the sonar
transducer in the form of a beam and go out into the
water in search of a submarine. If the sound beam
finds a target, it will return in the form of an echo.
The use of sonar equipment depends on the
presence and the recognition of an echo from a target.
Detection of the echo depends on the quality and
relative strength (loudness) of the echo compared to
the strength and character of other sounds, since they
tend to mask or cover it.
The sonar operator should know what factors can
weaken the sound beam as it travels through water,
what factors in the seawater determine the path and
speed of the sound beam, and what factors affect the
strength and character of the echo. Any signal
strength lost during the beams travel through the
water is known as transmission loss. Some of the
factors determining transmission loss are discussed in
the following paragraphs.
Absorption and Scattering
Some of the sound energy emitted by the source
will be absorbed while passing through the water.
The amount absorbed this way depends on the sea
Absorption is high when winds are great
enough to produce whitecaps and cause a
concentration of bubbles in the surface layer of the
water. In areas of wakes and strong currents, such as
riptides, the loss of sound energy is greater.
Therefore, echo ranging through wakes and riptides is
difficult because of the combined effect of false
echoes, high reverberations, and increased absorp-
tion. Absorption is greater at higher frequencies than
at lower frequencies.
Sound waves are weakened when they reach a
region of seawater that contains foreign matter, such
as seaweed, silt, animal life, or air bubbles. This
foreign matter scatters the sound beam and causes
loss of sound energy.
The practical result of
scattering is to reduce echo strength, especially at
Echoes occur when the sound beam hits an object
or a boundary region between transmission mediums
in such a manner as to reflect the sound or to throw it
back to its origin.
Reflection of sound waves
sometimes happens when a wave strikes a medium of
different density from that through which it has been
This will occur in cases where the two
mediums are of sufficiently different densities, and
the wave strikes at a large angle. This happens
because the sound wave travels at different speeds
through the two different densities. For example, a
sound wave traveling through seawater is almost
entirely reflected at the boundary of the water and air.
The speed of sound in seawater is about four times
greater than the speed of sound in air, and the density
of water is more than 800 times greater than that of
air. Therefore, practically all of the sound beam will
be reflected downward from the sea surface.
Similarly, when a sound wave traveling through
the seawater strikes a solid object like a submarine,
the difference in the density and the sound velocity in
the two mediums is such that all but a small amount of
the sound beam will be reflected. That portion of the
beam that strikes surfaces of the submarine
perpendicular to the beam will be reflected directly
back to the origin as an echo.
In calm seas, most of the sound energy that strikes
the water surface from below will be reflected back
down into the sea. A scattering effect occurs as the
sea gets progressively rougher.
circumstances, part of any sound striking the surface
is lost in the air, and part is reflected in scattering
directions in the sea. In water less than 600 feet deep,
the sound may also be reflected off the bottom. Other
factors being equal, the transmission loss will be least
over a smooth, sandy bottom and greatest over soft
mud. Over rough and rocky bottoms, the sound is
scattered, resulting in strong bottom reverberations.
When sound waves echo and re-echo in a large
hall, the sound reverberates. Reverberations are
multiple reflections. Lightning is an example of this
from nature. When lightning discharges, it causes a
quick, sharp sound; but by the time the sound of the
thunder is heard, it is usually drawn out into a
prolonged roar by reverberations.