Kongsberg EM 712, Installation Manual

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401027/B
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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
thickness depends on ships speed and on the roughness of the hull. All objects sticking
out from the hull, or dents in the hull, will disturb the flow and will increase the thickness
of the boundary layer. When the flow speed is high, the turbulence can be violent enough
to destroy the integrity of the water. Small voids or cavities in the water will occur and
this is called cavitation.
Rattle noise
Rattle noise may be caused by loose objects in the vicinity of the transducer, like fixing
bolts. The rattle may also come from loose objects inside the hull.
Interference
Interference from other hydroacoustic equipment on board the same vessel may be an
annoying source of disturbance. Unless the same frequency is used for more than one
piece of equipment only the transmitted pulse will contribute to the interference.
In physics, interference is the phenomenon in which two waves superpose each
other to form a resultant wave of greater or lower amplitude. Interference usually
refers to the interaction of waves that are correlated or coherent with each other,
either because they come from the same source or because they have the same or
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