Kongsberg SBP 29, Installation Manual

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Machinery noise
The main contributor to machinery noise is usually the main engine on board the vessel.
The contribution from auxiliary machinery may, however, be considerable, especially if
it is in poor shape. The machinery noise can be transmitted to the SBP 29 as:
• Structure-borne noise through the ship structure and the SBP 29 mountings
• Water-borne noise through the hull into the water to the SBP 29
Electrical noise
Modern vessels are normally equipped with a lot of electric instruments such as
hydroacoustic systems, radars, navigation systems, and communication equipment.
Any electric instruments may in some cases cause electrical interference and noise.
International regulations and certifications are used to control and reduce this, but even
these are limited if the electrical systems are poorly installed and/or maintained.
Propeller noise
Propeller noise is often the main source of noise at higher vessel speeds. Variable pitch
propellers or fast moving propellers usually make more noise than fixed propellers or
slow moving propellers.
Propeller noise is usually water-borne. In some cases, however, shaft vibrations or
vibrations in the hull near the propeller may be structure-borne to the SBP 29. If a
propeller blade is damaged, this may increase the noise considerably.
Propeller cavitation is a severe source of noise. "Singing" propellers might be a source of
noise, which interferes at discrete frequencies. In some cases static discharge from the
rotating propeller shaft may be quite disturbing.
Cavitation
Cavitation is the formation of small air bubbles
close to the SBP 29 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.)
SBP 29 Installation Manual