siliconchip.com.au Australia’s electronics magazine June 2019 17
The use of sound to detect objects in the water was first
recognised by Leonardo da Vinci in 1490. He is said to have
placed his ear to a tube which was immersed in water and
listened for distant vessels.
The fact that sound waves travel at a known velocity
in water and are reflected from solid surfaces such as the
seabed is the basis upon which echo sounding and sonar
were later developed.
The basic principle of echo sounding to determine depth
is that an acoustic pulse is emitted from the device and it
travels through the water column at a predictable speed.
It strikes the seabed and is reflected to a receiver (micro-
phone). At a basic level, the depth of the water is then com-
puted by taking half of the return time for the pulse and
multiplying by the speed of sound in water.
For example, if a pulse took 0.8 seconds to return and
the speed of sound in water was 1500m/s, the water depth
would be 0.8s x 1500m/s ÷ 2 = 600 metres.
In practice, sound velocity can vary slightly in water due
to differences in salinity, temperature and depth. These ef-
fects can and usually are taken into account. In general, a
1°C increase in temperature results in a 4m/s increase in
the speed of sound, an increase in depth of 100m results
in an increase of 1.7m/s and an increase of one part per
thousand of salinity results in an increase of 1m/s.
Note that temperature usually decreases with depth,
causing the speed of sound to decrease, but at the same
time the speed increases with depth (or pressure). The
combination of the two effects can result in a sound veloc-
ity profile that decreases in the first few hundred metres,
then increases at greater depth.Early echo-sounding devices
The earliest acoustic depth measuring devices were
known as echo ranging devices or fathometers. Today it is
known as sonar (“SOund Navigation And Ranging”). These
devices used a single acoustic ‘beam’ to measure the seabed
depth and as a consequence, can only measure the depth
directly beneath a vessel, just like the lead line (see Fig.5).
In 1912, Canadian Reginald Fessenden developed the
first electronic or electromechanical acoustic echo rang-
ing device (Fig.6). It used a mechanical oscillator that was
similar in design to a voice coil loudspeaker. It could gen-Fig.7: the ocean floor between Newport, Rhode Island
(USA) and Gibraltar, as determined by the USS Stewart in
- This survey used the Hayes Sonic Depth Finder and
found what was thought at the time to be the lost continent
of Atlantis. From Popular Science, May 1923.
Fig.8: a map of the soundings taken by the USS Stewart
across the Strait of Gibraltar in 1922.
Fig.9: the Dorsey Fathometer as installed on the SS John W.
Brown, a US Liberty Ship during World War II.
Fig.10: an internal view of the head unit of a Dorsey
Fathometer from the 1925 operator’s manual. Note the
electromechanical nature of the componentry. There were
also other electronics boxes.