The type of water you're using the sonar in affects its
operation to a large degree. Sound waves travel easily in a clear fresh water environment,
such as most inland lakes.
In salt water however, sound is absorbed and reflected by suspended material in the
water. Higher frequencies are most susceptible to this scattering of sound waves and can't
penetrate salt water nearly as well as lower frequencies. Part of the problem with salt
water is that it's a very dynamic environment - the oceans of the world. Wind and currents
constantly mix the water. Wave action creates and mixes air bubbles into the water near
the surface which scatters the sonar signal. Micro-organisms, such as algae and plankton
scatter and absorb the sonar signal. Minerals and salts suspended in the water do the same
thing. Fresh water also has wind, currents, and micro-organisms living in it that affect
the sonar's signal - but not nearly as severe as salt water.
Mud, sand, and vegetation on the bottom absorb and
scatter the sonar signal, reducing the strength of the return echo. Rock, shale, coral and
other hard objects reflect the sonar signal easily. You can see the difference on your
sonar's screen. A soft bottom, such as mud, shows as a thin line across the screen. A hard
bottom, such as rock, shows a wide line on the sonar's screen.
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Soft Bottom |
Hard Bottom |
You can compare sonar
to using a flashlight in a dark room. Moving the light around the room, it's easily
reflected from white walls, and bright, hard objects. Moving the light onto a darkly
carpeted floor returns less light because the dark color of the carpet absorbs the light,
and the rough texture scatters it, returning less light to your eyes. Adding smoke to the
room (children, don't try this at home!), you'll see even less. The smoke is equivalent to
salt water's effect on the sonar signal.
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