Installation manual

Basic theory

Noise and reverberation

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 m ost 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

thinkarecausedbynoise,trytoﬁnd 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. When the bubbles collapse, they cause very strong

local shock-waves in the water. These may be audible and

can also damage the propeller blades. In addition to this, the

cavitation will generate very strong reverberation, commonly

seen as a red stripe of echoes behind the vessel.

If you have big problems w ith cavitation, try to adjust the

propeller pitch and/or the number of revolutions.

Reverberation

Reverberation is generated when the echoes from your sonar

transmission are reﬂected not only by ﬁsh and objects in the

water, but also from the bottom and the sea surface.

Figure 32 Reverberation

(CD015005E)

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