Noise And Reverberation; Cavitation - Simrad SH90 - REFERENCE MANUAL REV A Reference Manual

Noise and reverberation

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If you use a very small tilt (C) you will not be bothered by bottom
echoes. If the sea surface is gusty, you will however easily see
echoes from the waves. These echoes will normally be rather
weak, and will hardly move in relation to your vessel.
By means of a medium tilt angle (B) you will most likely be able
to pick up scattered fish or schools if these are located at some
distance above the bottom. The upper school in the illustration
will appear on the sonar picture before the bottom echo, while the
lower school will easily be disguised by the bottom echoes. Once
again you can activate the RCG function to improve the reading.
Every time you use the sonar the quality of your search
operations will depend on numerous error sources caused by the
natural environment and physical properties of your vessel. We
distinguish between the two terms noise and reverberation.
• Noise is generated by the vessel: engine, propellers, winches,
refrigerating plants etc.
• Reverberation is caused by natural conditions in the sea:
temperature, salinity, sea state and bottom conditions.
The most common cause of acoustic noise is the propeller and
the cavitation caused by its rotation. Other typical noise sources
on board your vessels are winches, refrigerating plants, power
generators, pumps and cranes. If you experience problems you
think are caused by noise, try to find out which system that causes
the noise, and contact your dealer for advice.

Cavitation

As the propeller blades move through the water, low pressure
areas are formed as the water accelerates around and moves past
the blades. The faster the blades move, the lower the pressure
around it will become. As it reaches vapour pressure, the water
vaporizes and forms small bubbles of gas. This is cavitation.
Basic theory
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