Noise is a significant stressor on board ships. The fact that facilities are located above the propulsion mechanism, increases in engine power and the emergence of significant vibration all mean that noise reduction has become a matter as much of crew health as it is of onboard comfort.
Many authors have studied the problem of noise onboard ships, ever since ships first began to be mechanically propelled (1, 2, 3, 4, 5).
The main noise sources are as follows:
The vast majority of ships are propelled by diesel internal combustion engines. Based on the revolutions/minute of the engine, a distinction can be made between “slow” engines with a relatively low noise level and "high-speed" or "medium-speed" engines, which are more powerful than other types of engine but which create more noise.
At equal power levels, airborne noise produced by these engines is proportional to speed of rotation and maximum combustion pressure. Noise is produced by the scavenger and exhaust housing, as well as by the gear case. As well as noise from the combustion process, there is noise created by turbo blowers (this is high-frequency noise). In addition to noise created by the engine itself, we should take into account the noise transmitted via combustion gas exhaust pipes (funnels).
Apart from noise generated by the main engine, there is also noise from secondary engines, such as electricity generators, reducers, ancillary machinery (e.g. winches, hydraulic motors). Mounting an engine or auxiliary motor on silencers does not affect the amount of noise it produces, but can reduce the level of vibration which is transmitted to the ship’s structure and by extension the noise from the acoustic radiation thus produced.
The largest ships (gross tonnage greater than 60,000), particularly oil tankers, are equipped with steam turbines. In general, steam turbines are much less noisy than internal combustion engines, for equal levels of power production. However, steam valves can cause loud noise, particularly at high frequency, when they are open and/or unsophisticated in shape.
In the future there will be electric motors, which cause considerably less noise than any other type of propulsion system. The idea of generalised electrification of ships began in the United States in the early 1980s and was called Integrated Electric Drive. The term electric ship is ambiguous, and does not imply that diesel engines and gas turbines will disappear, at least not for another 20 years. The term denotes an integrated system of electrical energy production and distribution to all users on board. One remarkable consequence of this is that it becomes possible to remove the drive shaft, which are large parts that entail constraints both in design (installation) and use (alignment, watertightness, noise and vibration). Moreover, a naval architect is more able to optimise equipment positioning, for example by placing the gas turbine away from the bottom of the ship and by choosing the positions of the diesel engines wisely. Electrification also enables removal of various fluid systems that are associated with conventional types of architecture. The principle of electrification has already been adopted in civil shipbuilding, particularly of passenger ships, because of the increased comfort it provides: low noise levels, no vibration. A recent example is the “Star Princess”, a cruise ship carrying 1700 passengers built by Chantiers de l’Atlantique (France) and equipped with a diesel-electric propulsion system. Independent propulsion pods have also been developed, which enable energy savings of around 10%. These external pods, which contain electric motors and drive shafts, are positionable, which means that ships equipped with such pods (suspended omnidirectional propulsion system) no longer need rudder blades. The Queen Mary II¸ the largest cruise ship in the world which recently emerged from Chantiers de l’Atlantique, has six of these pods.
Noise emitted by a propeller is linked to turbulence created by the phenomenon known as “cavitation” (because of the air bubbles that form on the propeller blades) and by the characteristics of the blades themselves (number, type, surface). Propellers are one of the main sources of noise emitted by the ship, and the noise is particularly obvious and can enable the ship to be identified (Husson6).
On board high-speed ships, water jets created by gas turbines replace propellers, which means that a considerable amount of weight is saved and levels of noise pollution fall.
Noise produced by a ventilation system mainly comes from the ventilators and their drive motors and shafts, and is caused by their shape and circulation speed, and air intake and discharge vents.
Influence of noise sources
Noise generated by engines and ancillary devices tends to spread throughout the ship.
The level of noise in engine rooms mainly comes from the various engines that are housed there. The overall noise level in one location is the sum of the acoustic intensities at that location, caused by each engine in the location, and to which is added any influence of sound reverberation on the walls. In a generally reverberant engine room, as a first approximation it is reasonable to consider that the noise level is the same throughout the room, unless one is next to a particularly noisy engine (less than 2 metres away).
In common areas, most noise is transmitted via partitions, floors and ceilings. Ventilation systems and doors, furniture and partitions that are subject to deformation can have an influence over the level of noise in a particular place by generating parasitic noise. Noise that is transmitted by partitions, floors and ceilings mainly originates from vibration energy produced by the propulsion system and propeller, but also comes from impact and movement of the ship caused by sea conditions. Appliances on tables or fixed to walls are also sources of disruptive noise.
Noise transmitted by the structure in question is reduced in proportion to the distance from the source of excitation and in inverse proportion to the size and transmission coefficient of the surface.
Apart from noise transmitted by the structure, there can also be airborne noise caused by exhaust systems of motors, ventilators and appliances such as hydraulic generators, steam valves etc.
The noise level inside a gangway is often higher than the level measured inside the accommodation. This is generally due to airborne noise from internal combustion engine exhausts, ventilation systems and some ancillary systems such as hydraulic cargo systems, lift machinery and the wind. Some equipment that is located inside gangways (e.g. VHF, BLU) is also a source of noise. In terms of noise from gas exhausts, the position of the upper part of the funnel with respect to the gangway determines the level of noise in the gangway. The sound spectrum of exhaust noise is mostly low-frequency, so glass partitions in the wheelhouse should not be relied upon to provide sound isolation that is sufficient to reduce noise levels noticeably. If ventilation system casings are nearby, this is another very troublesome source of external noise. Ventilator noise is loud and can sometimes reach 120 dB(A). This noise is transmitted directly to the outside via slats, and these slats can cause troublesome noise if air travels through them at high speeds. In addition, on some ships, some ancillary systems such as air conditioning units are found near the gangway. Finally, if the wind is high and reaches speeds of approximately 60 km/h (force 8) with respect to the ship, there can be whistling in the handrails and hoist halyards. In addition, because of the high drag coefficient of the wheelhouse, the wind’s effects on the wheelhouse lead to significant levels of background noise.