Some means to reduce acoustic noise
Several factors are contributing to the performance of the hydroacoustic equipment used on board a vessel. Careful planning of the installation may reduce the acoustic noise.
Unfortunately, it is impossible to simply provide a number of specific procedures to reduce the noise.
The physical location of the installation trunk and the hull unit is important. This depends on the vessel's design and construction, how the hull is shaped, and how the water runs along the hull. Other factors deal with other equipment mounted on board, and this will also be vessel dependant. At moderate ship speeds the machinery noise is usually dominant. At medium speeds the flow noise increases more rapidly and takes over, while at higher speed the propeller noise will be the main contributor.
The information here must be considered as general advice. Each system installation must be handled separately depending on the hull design and the other electrical and mechanical systems installed on the vessel.
Reducing flow noise
The shape of the hull plating around the installation trunk opening must be as streamlined as possible. It is particularly important that the hull plating in front of the installation trunk is as smooth as possible.
Be especially aware of bilge keels and sacrificial anodes. The keel must be rounded off without sharp edges. Neither extruding objects nor abrupt transitions must be present.
The fairing around the installation trunk must be streamlined. The transducer must be protruding far enough down to reach out of the boundary layer.
Reducing machinery noise
The hull unit must be installed far away from the engine room. The main engine and relevant auxiliary engines and equipment must be fixed to rigid foundations to avoid vibrations. Any hull structure that may vibrate must be damped or coated to reduce the vibrations.
The use of shock absorbers or floating rafts may sometimes reduce this noise. The structure-borne noise may be reduced by isolation, for example by providing vibration clamping between the transducer and the hull structure.
Reducing propeller noise
The hull unit must be installed far away from the propellers.
Sufficient clearance between the propellers and the hull, the rudder and the keel must be provided. Place the sacrificial anodes in places where the water flow is the least disturbed.
Ensure that the propellers blades are correctly designed and without damages.
The use of a baffle between the propellers and the transducer may reduce noise appreciably. Static discharges caused by the rotating propeller shaft may be removed by proper grounding or by mounting a coal brush from the shaft to vessel ground.
Reducing rattle noise
Ensure that no parts near the transducers can rattle as a result of water flow or vibrations.
Reducing interference
Interference from the transmission pulses from other hydroacoustic instruments on board the vessel is difficult to avoid. The problem may be reduced by choosing the working frequencies carefully and to some extent by separating the different transducers. On vessels with a large number of separate hydroacoustic systems installed and in simultaneous use, a separate synchronizing system (for example the K-Sync) should be considered.
Reducing electrical noise
Make sure that all units are properly grounded. This is important to avoid electrical noise.
Make sure that all units are properly grounded, including shielded cables.
Separate the cables used by the ST90 system from other cables with high voltages, large currents or transients. Place all high voltage power cables in metal conduits.