A low underwater noise level is an essential design feature for operation of certain ship types. This particularly applies to offshore survey vessels, fishery research vessels, ocean research vessels, seismic vessels, fishing vessels and military vessels. Such vessels are extremely sensitive to underwater noise radiation because a high noise level will directly interfere with their operational ability.

For these ships the underwater noise control will have to be given high priority throughout the design and construction. DNV is currently developing underwater noise criteria to assess the noise level depending on type and task of vessel.

Noisy vessels Sound is easily transmitted in water and is therefore used for many of the functions that electromagnetism have in air. However, background noise can limit the operational range, induce errors and even completely block the acoustical instrumen- tation. The most significant noise sources is typically propellers, diesel engines / gas turbines, gears, electric propulsion motors, shafting systems, water flow along the hull, hydraulics, ventilation systems, HVAC, exhaust systems, pumps, auxiliary machinery and equipment.

Noise criteria may be issued by national authorities, in which case they usually are health, safety or environmentally related. Noise limits may also be specified by ship owners or users in order to ensure that the vessel will be well suited for the intended tasks or by independent third parties to verify that the vessel is as good as needed to accomplish their duties. Examples of the latter may be classification societies or professional societies applying knowledge or experience to set standards that are technically feasible. Military authorities or organisations issue criteria and standards applicable to military vessels, these criteria may govern personnel health and safety as well as vessel acoustic signatures. Many military standards are confidential.

Underwater noise radiated from commercial vessels operating in environmentally sensitive areas is becoming a concern and recently noise as a pollutant has received considerable attention. Airborne noise exposure has been shown to have harmful health as well as psychological influence on people. Similar detrimental health effects also affect life in the sea. In dark ocean waters, marine mammals such as whales and dolphins rely on sound to communicate with each other, locate prey and find their way over long distances. All these activities, critical to their survival, are being interfered with by the increasing levels of underwater noise from ocean-going ships, sonar devices and seismic exploration. Sound travels nearly five times faster in water than in air and will cover large areas in seconds. Scientists have become increasingly aware of this threat to biodiversity and rate underwater noise pollution as the next global treat after climate change and chemical pollution. This is a rather diffuse and not very well understood field. However, it is already attracting significant attention from environmentalists and conservationists and it is likely that the field will receive increasing attention in the years to come.

measuring methods In order to be able to control and regulate noise emission from vessels, it is important to have clear and consistent measuring methods. As of today, no standardised measurement methods are available for underwater noise from civilian vessels or equipment. Consultants and others have carried out underwater noise measurements from various types of underwater equipment and vessels, though the lack of standards can limit the comparison of measurements and thereby reduce the usefulness of the measurements when evaluating the suitability of a vessel to complete its tasks. Also, the methods employed are rather resource intensive and frequently requires the vessel to be decommissioned and made available to the measuring team for some time.

It is necessary to understand which operating conditions and systems control the vessel's underwater noise signature in order to optimally design quieter ships or to abate the noise from existing ships. DNV has wide experience in calculation and measurement of noise and vibration with an experienced team possessing state of the art methods for predictions and equipment for measurements. In order to assess the underwater noise level from ships a new set of criteria is now being developed by DNV. The criteria specify requirements to maximum underwater noise levels on various ship types and required operational capability.

DNV can also assist yards and designers to ensure that the design includes necessary low noise features and that noise control measures are integrated in the construction if necessary. However, noise control is not a purely theoretical subject. Practical follow up during the building phase is very important for noise critical projects to ensure that all necessary details are carried out correctly. An extensive noise control effort may be severely degraded by minor mistakes or "short-circuits" during the construction phase.

For seismic ships it is of paramount importance to have a controlled level of noise emission in order to fully utilise the hydro-acoustic equipment.

Waterborne noise may frighten fish, hence excessive noise emission from fishing vessels is undesirable. These vessels also employ advanced hydro-acoustic equipment for fish search and to control and monitor fishing gear. For both modes a low noise level will be an advantage.