Page 18 - Electronic Navigation Cyber Book

More on the principle of Echo Sounding
Echo sounder equipment makes use of sound (or sonic) waves, which
differ from radio waves in several ways. Sound waves vibrate in the
direction of travel of the wavefront Sound wave velocity is comparatively
low (in comparison with radio waves). Sound waves can travel through a
material medium such as a gas, a liquid or a solid, but not a vacuum.

Sound waves can be produced over a very wide range of frequencies,
and these waves exert definite pressure variations, which under certain
circumstances can be measured. The audible frequency or frequencies
that human beings can detect varies, but an approximate range of
detection (also called as the normal range) can be considered as lying
between about 20 Hz to 20,000 Hz. Those above the normal range of the
human ear are termed ultrasonic or supersonic frequencies.

There is a loss of energy when any waves are transmitted through a
medium and the losses increase as the frequencies increase. Sound
waves also suffer losses, however, in water, such losses are of the order
of 1000 times less as compared to the loss in air. Supersonic vibrations
are therefore much more suitable for transmission in water than in air.
The losses do increase due to high frequency, however, they do not
become serious unless very high frequencies are used.

Water is an excellent sound transmitting medium as the velocity of sound
in the water is known accurately and it does not vary more than about 3%
if temperature and salinity change.
The speed of sound increases as water temperature, salinity and water
pressure increases, and all of the above vary with depth. The speed of

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