Simrad ES80 Reference Manual page 467

Cavitation
Cavitation is the formation of small air bubbles
close to the transducer face. The bubbles appear
because the local pressure becomes negative
during parts of the acoustic pressure cycles.
The cavitation threshold increases with the
hydrostatic pressure. The noise is made when
the bubbles implode.
Cavitation noise may appear near extruding
objects at higher speeds, but more often it is
caused by the propellers. Propeller cavitation is a severe source of noise. The cavitation
starts when the water flows in the same direction as the propeller blades. This is where
the propeller blades move downwards.
In some cases a resonant phenomenon is set up in a hole near the hull. This sound will
have a discrete frequency, while all other flow noise will have a wide frequency spectrum.
(Image from U. S. Navy in the public domain.)
Flow noise
The upper water layers of the sea contain a myriad of small air bubbles created by the
breaking waves. When the hull moves through water it will cause a disturbance, and this
will generate friction. The friction zone is called the flow boundary layer. The flow in
this boundary layer may be laminar or turbulent.
• The laminar flow is a nicely ordered, parallel movement of the water.
• The turbulent flow is a disorderly flow pattern, full of eddies.
A Turbulent flow
B Laminar flow
C Air bubbles
Air bubbles absorb and reflect the sound energy, and they may in worst cases block the
sound transmission altogether.
The boundary layer increases in thickness when it becomes turbulent. The boundary
layer is thin in the forward part of the vessel hull, and increases as it moves aft. The
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Concept descriptions
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