Furuno CSH-5 MARK-2 - Manual (Page 67)

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The drawing below shows how temperature variation affects sound
propagation with respect to different emitting directions (tilt angles).
Beams tilted five and ten degrees bend upward at 400 m and 600 m
points respectively. Beams tilted down more than 15 degrees travel
in almost straight lines. Between the two beams, a blind zone is cre-
ated beyond the 600 m point. In this zone nothing can be detected.
The shown drawing is only an example calculated by a computer,
based on the temperature with depth as shown in the left column of
the figure. In actual fishing grounds, the temperature distribution and
subsequently the behavior of the sound beam is much more compli-
cated. It is, therefore, for effective use of sonar, necessary to know at
least roughly how the temperature is distributed in various waters.
0 10 20 0 200 400 600 800 1000 1200 1400 1600 1800
0
2000
50
0
100
150
Sea depth (m)
-35
-30
-25
-20
-15
-10
-5
0 (DEG)
Blind zone
Adverse Effect of Air Bubbles
Even infinitesimal air bubbles in sea water (liquid medium) affect
propagation of ultrasonic sound. This is because the cubic elasticity
of gas is extremely small when compared with that of liquid; the air
bubbles violently vibrate (contract and expand) by the action of sound
pressure, diffusing the ultrasonic wave and dispersing part of the
acoustic energy. In a liquid which contains a large amount of air
bubbles, attenuation of an ultrasonic wave increases and the wave is
reflected at the boundary of waters which contain and do not contain
air bubbles.
From the above it can be said that reflection occurs in the boundary
where the density (P) of the material (medium) that is, the velocity of
the ultrasonic wave changes. The velocity of an ultrasonic wave with
respect to its medium is 200 thru 400 m/s in gas, except for hydrogen
and helium; 900 thru 2000 m/s in liquid (several times higher than in
air) and 2000 thru 6400 m/s in ordinary metal.