FROM 2^ND EDITION BY DOMINIQUE JÉGADEN. REVIEWED AND UPDATED BY KAIA IRGENS.

E.6.1 Introduction

Noise is a significant stressor on board ships. Increases in engine power, the emergence of significant vibration and the fact that living facilities are located above the propulsion mechanism all mean that noise reduction has become a matter of onboard comfort as well as  of crew health..

E.6.2 Main noise sources on board ships

Engines

The vast majority of ships are propelled by diesel internal combustion engines. Based on the revolutions per 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. The latter are more powerful than other types of engine but which create more noise.    

At equal power levels, the airborne noise produced by these engines is proportional to the speed of rotation and the 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 a high-frequency noise created by turbo blowers. In addition to noise created by the engine itself, we should take into account the noise transmitted via combustion gas exhaust pipes or funnels.

Apart from noise generated by the main engine there is also noise from secondary engines, such as electricity generators, reducers and ancillary machinery such as winches and 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 transmitted to the ship’s structure and by extension, the noise from the acoustic radiation produced in this way.

Some of the largest ships, with a gross tonnage greater than 60,000, particularly oil and gas 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.

Electric propulsion systems

In the future, there will be more and more electric motors, which cause considerably less noise than any other type of propulsion system. The idea for the 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 completely, 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 and installation and use, for example the 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 the removal of various fluid systems that are associated with conventional types of architecture. The principle of electrification has already been widely adopted in civil shipbuilding, particularly of passenger ships, because of the increased comfort it provides with low noise levels and no vibration. A relevant example is the Star Princess, a cruise ship carrying 1700 passengers built by Chantiers de l’Atlantique in 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 in a suspended omnidirectional propulsion system, no longer need rudder blades. The Queen Mary II¸ also built by Chantiers de l’Atlantique, has six of these pods.

Propellers

Noise emitted by a propeller is linked to turbulence created by the phenomenon known as “cavitation”, due to the bubbles that form on the propeller blades, and by the characteristics of the blades themselves, such as number, type and surface. Propellers are one of the main sources of noise emitted by the ship. The noise is particularly obvious and can enable the ship to be identified.

On board high-speed ships, water jets created by turbines replace propellers, which saves a considerable amount of weight and levels of noise pollution fall.

Ventilation

Noise produced by a ventilation system mainly comes from the ventilators and their drive motors and shafts. Noise is caused by their shape and circulation speed, and their air intake and discharge vents.

E.6.3 Transmission of noise on board

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 housed there. The overall noise level in one location is the sum of the acoustic intensities there, 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 less than 2 metres from a particularly noisy engine.

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 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 reduces 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, some ancillary systems such as hydraulic cargo systems, lift machinery and the wind. Some equipment that is located inside gangways, for example, VHF and 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.

E.6.4 Noise levels on board ships

Merchant vessels

As mentioned above, the main source of noise is the propulsion mechanism, and therefore the highest levels of noise are found in its vicinity. In most ships, the noise in machinery spaces is greater than 100 dB(A), and can sometimes be as high as 110 dB(A). Table 1 provides mean noise levels in various types of engine room on merchant ships.

Table 1

In other locations, however, noise levels are generally between 60 and 75 dB(A). Technological progress has ensured that on passenger ships, particularly cruise ships, cabin noise levels are around 40 dB(A).

Fishing vessels

Fishing vessels are generally smaller than merchant ships, and fishers spend much longer on board over the course of a year than merchant seafarers do. These vessels pose noise problems that are more difficult to remedy. The above noise sources on merchant ships are obviously also present on fishing vessels but in addition, there is the noise of the winches used for launch and for lifting fishing equipment. The table below reproduces the noise levels found on board various fishing vessels of the same tonnage:

Table 2

Noise in the engine room exceeds 105 dB and is perceptually equivalent to levels found on board merchant ships of any size. The difference between merchant ships and fishing vessels is that there is not always a soundproofed engine control room on smaller fishing vessels. Noise levels in sleeping quarters on fishing vessels less than 30 m in length are very high because the crew quarters and engine room are so close together. There is also a high level of noise on the fishing deck. Modern fishing trawlers have covered fishing decks, which provide greater safety and comfort than outside work, but which increase noise levels as they act as resonance chambers.  

Some authors^[1] have measured median noise levels for ships - it is best not to use the term “mean”, as decibels are logarithmic values that cannot be added arithmetically. For a common type of 24-metre trawler, median noise levels were 76 dB on the gangway, 80 dB in the wardroom, 86 dB in the steerage compartment, 84 dB on the fishing deck, 82 dB in the crew quarters and 106 dB in the engine room.

The most relevant exposure measurement in relation to the risk to hearing is the equivalent continuous level (Leq) to which the seafarers are subjected during one day and during one trip. Data recorded in situ show that in 55-60 m trawlers, there are equivalent average noise levels of around 85 dB over a 14-day trip.

As fishers are on board for 24 hours a day over several days, levels that would be considered to be disease-causing in workers on land cannot easily be applied to them. Much also depends on the job an individual carries out on board. Table 3 shows average equivalent dosimeter levels for a whole trip for crews of four semi-industrial 34-metre fishing trawlers. 

From Andro & Dorval                                 Table 3

The only fisher on board who is not exposed to a level of noise that causes trauma to the auditory system is the skipper. All others are. The recognised threshold above which there is a risk of hearing loss is 80 dB(A), 8 hours per day. This is derived from onshore studies, where the working day is normally around 8 hours. However, if the formula for calculating equivalent continuous levels is used and applied to exposures over the the full 24 hour day , we notice that a Leq_24h of 82 dB(A) corresponds to a Leq_8h of 95 dB(A), according to energy equivalence laws. This means that a seafarer who is exposed to a noise level of 82 dB(A) over 24 hours is exposed to a stressor and a risk to hearing that is equivalent to that of a worker who is exposed to 95 dB(A) for 8 hours a day. In this type of fishing, engineers are subject to highly traumatic levels of noise.  If we study the noisiest times of the workday, we see that length of stay on deck, handling fishing equipment, mending nets, processing the catch etc., means that fishers are exposed to 85 dB for an average of 13 hours per day. Then there is time spent in the crew quarters, during sleeping periods, around 5-6 hours per day, exposed to 83 dB, and meals in the wardroom, 81 dB for three hours, and gangway watch, 73 dB, for 2 hours. For a conventional trawler of between 15 and 25 metres in length, the equivalent continuous noise level over 24 hours has been calculated as Leq_24h = 83.6 dB(A).

Therefore, we have to consider that fishers, apart from skippers of large industrial trawlers, are at high risk of hearing damage due to noise, whatever job they do on board.

E.6.5 Effects on seafarers’ hearing

Several international studies have been published regarding the effects of ship noise on seafarers’ hearing^{[2] [3] [4] [5] [6]}.

Merchant vessels

We should remember that if a person is exposed to noise greater than 80 dB(A) for 8 hours per day or more, this can harm the inner ear, bilaterally and more or less symmetrically. This damage will worsen as the period of exposure lengthens and will affect higher frequencies, primarily above 4000 Hz. This is permanent endocochlear perception hearing loss in the context of chronic auditory fatigue.

As is shown, it is the engineers on board merchant ships who are exposed to the most noise, with equivalent average levels generally being above 85 dB(A). An audiometry study carried out in 1983^[7] concluded that there was a small region of hearing loss at 4000 Hz in merchant ship engineers, which was most noticeable in those over 40 years of age. Seafarers involved in other occupations were not affected.

Median values, combined results from both ears.

At similar ages, onboard engineers had noticeably less hearing loss than did subjects who worked in noise levels of 95 dB or 100 dB 8 hours per day.

A 1998 study^[8] found hearing loss in 26.8% of engineers, compared with 16% of seafarers in the deck department and 9.9% of supervisors. These differences were statistically significant.

The moderate levels of hearing loss observed in engineers may be explained by the fact that their exposure to engine noise is tempered by soundproofing of the engine control roomsin merchant ship engine spaces, which means that exposure to significant noise is confined to routine patrols and maintenance tasks, and the fact that they wear ear protection when carrying out such tasks. If the noise is constant or only slightly fluctuating, this can also moderate its effects. If work is distributed throughout the year in the form of 2-3 months of work followed by a holiday of the same length, this is a significant factor responsible for the moderate nature of hearing loss.

Fishing vessels

The situation on board fishing boats, however, is very different. As already noted, Andro and Dorval have shown that seafarers on board high-sea fishing vessels are subjected to constant noise levels 24 hours a day. Taking a ‘average’ vessel, seafarers are exposed to 84-86 dB when working on deck, 76 dB when on gangway watch and 82 dB when resting in the crew quarters. In parallel with this audiometric study, a similar study was carried out on 113 fishers on board the same type of vessel^[9]. The results showed that there was noise-related hearing loss with a window of hearing loss at 4000 Hz, which worsened with age and length of service. The hearing deficits were compared with the French standard NF S 31-013. This contains estimations from international epidemiological data of hearing in workers over 40 years old who had been exposed to quasi-stable levels of industrial noise, at 90, 95 and 100 dB, for 20 years. The hearing deficit for fishers with an average age of 40 and with an average of 23 years of exposure, lay between the levels of deficit for standardised subjects exposed to 90 dB and those exposed to 95 dB in terms of high frequencies, and were still greater for low frequencies.

A continuous noise level of 85 dB 24 hours a day, as experienced by fishers, is calculated as equivalent to a continuous noise level of 90 dB over 8 hours. In other words, a seafarer who experiences 85 dB 24 hours a day has hearing loss equivalent to that of a worker exposed to 90-95 dB of factory noise 8 hours a day. These results clearly show that high-sea fishing is an occupation that carries a risk of hearing loss due to noise.

This risk was specifically recognised in a report by the European Parliament on safety and accidents in sea fishing, dated 12 March 2001^[10] that states ‘Incessant noise creates an aggressive climate on board and means that fishermen sleep little and badly, making it difficult for them to obtain the rest they need...’ The results of a 2006 study^[11] involving 18,000 audiography tests on French seafarers confirmed the findings of the previous studies. This study confirmed that fishers are at greater risk than commercial seafarers. In maritime transport, seafarers on board oil tankers and cargo ships are observed to be at the greatest risk.

The problem with noise on fishing vessels is one of individual protection; as we have seen, in current standard practice, vessels do not have specific soundproofing. Individual protection could be effective^[12], but the fishers would have to wear it 24 hours a day, which is not practicable, although it could potentially be possible to constantly wear custom-made earplugs. For all vessels, the only valid improvement would be soundproofing of quarters when the vessel is built.

E.6.6 Non-hearing effects of noise on seafarers

Introduction

All signals picked up by the auditory system are transported via the nervous system either :

• directly, via specific pathways, which link the inner ear with the auditory cortex that takes in the signal and recognises its significance;
• or via an indirect route, a non-specific pathway. These are collaterals of the direct pathways that lead to the reticular activating system that regulates arousal, and is in turn connected to the limbic system and other parts of the brain, to the autonomic nervous system and the neuroendocrine system that play crucial roles in regulation of physiological functions in attention and behaviour.

These non-specific pathways explain why an irritant noise, even if it is of low intensity, generally in excess of 60 dB, introduce a subjective dimension and can cause psychological harm and other problems, such as a stress reaction, not directly or solely linked to the physical properties of the noise. The level of harm to an individual does not fully correlate with the level of noise, however, there is a correlation between harm to an entire population and noise levels.         Noise therefore belongs in the category of environmental stressors. This kind of stress is often more severe than expected because the subject has little or no control over the source of the stressor.

Sleep and alertness

On board non-soundproofed ships, the greatest risk not related to hearing is that sleep is disturbed by noise. A seafarer lives 24 hours a day in the confined environment of the ship, and should experience the good-quality sleep that is essential if the body is to recover from fatigue and maintain proper biological functions.

• Slow-wave sleep is involved in repair of tissues that are involved in physical effort.
• Rapid eye movement (REM) sleep restores the higher functions of the nervous system (alertness, learning, memory, adaptiveness and intent).

However, noise above 60 dB causes sleep problems in the form of

• reduced total amount of sleep,
• reduced duration of REM sleep,
• increased occurrence of night-time waking.

This sleep disturbance leads to increased fatigue and irritability. The problems are cumulative, and a vicious circle can develop whereby serious sleep dysfunction can arise, leading to physical exhaustion and overwork. Such noise is frequently found on all types of ships. It is therefore reasonable to think that seafarers in general suffer from sleep problems that worsen general fatigue.

Tamura et al.^[13] studied the sleep patterns in three subjects exposed to 65 dB noise from a ship’s diesel engine for five nights. Their sleep in such conditions was compared to their sleep in a quiet environment. They found that the number of episodes of REM sleep, and the duration of these episodes, were reduced, and that the time between these episodes of REM sleep was increased. They also reported a reduction in subjective sleep quality and difficulties in falling asleep. At noise levels of around 60 dB, the same authors^[14] in 2002, observed that, although seafarers got used to such noise levels in terms of subjective sleep parameters, there were still disturbances in the physiological parameters.

Rabat et al.^[15] carried out a sleep study on rats exposed to a recording of warship noise for 9 days, and compared the results to those of rats sleeping in a quiet environment. They confirmed that normal sleep structure was distorted, leading to a ten-hour debt of slow-wave sleep. The number of episodes of deep sleep was increased, but the duration was shorter than normal. Also, like Tamura et al., they found a six-hour debt of REM sleep - the number of episodes of REM sleep was reduced, as was their duration. The consequences of such sleep disturbance were significant, and appeared after the noise stopped. Such consequences involved the ability to commit information to long-term memory and the extent of the memory problems positively correlated to the extent of the debt of slow-wave sleep. They also demonstrated two types of sleep-related behaviour. One group of rats was resistant, and rats in this group rapidly recovered their ability to memorise, and one group of rats was vulnerable, and had significant problems. These results strengthen the theory that there are differences between individuals in terms of noise sensitivity.

Tirilly^[16] studied sleep patterns in coastal sea fishers and demonstrated the importance of sleep at night, which can be of short duration but which must occur at the same time each day for a given individual. This is known as ‘anchor sleep’ and maintains biological rhythms. In this study, the mean level of alertness in seafarers fell very soon after leaving port and the sleep deficit was between 60-90 minutes/24 hours, caused by fragmentation of sleep. On average six episodes of sleep were observed, with a total daily sleep period of between 5.5 and 6.5 hours. 

Alertness may be defined as maintaining attention during activities requiring prolonged periods on watch, particularly gangway watch. Alertness is reduced in proportion to the intensity of the noise, and this can result in attention problems. Noise also increases the risk of human error^[17] and, particularly in fishing, this can be a non-negligible cause of accidents.

Intellectual performance, in terms of psychomotor ability, reasoning and capacity to commit to memory, seems to be reduced if noise is above 85 dB and, as discussed, this is linked to sleep problems. There may also be effects in intellectual capacity at sound levels above 80 dB, but this depends on the frequency of the noise, whether it is intermittent or not, how long it lasts and its significance. Poulton^[18] observed that people working in constant noise initially performed better than those in quiet environments, but that there was gradual deterioration in performance if the noise persisted. Poulton thought that the physical intensity of the noise masked the signals produced by the machines used by the operator as a guide to performance when the environment was quiet. When the signals were masked, performance levels worsened.  When the noise began, the stressor seemed to cause a sudden burst of physiological and behavioural stimulation that overcame the harmful masking effects of the noise. This effect, however, gradually loses its impact, and subsequently there is an inescapable loss of performance. Another explanation might be that processing of noise using cortical filtering might impose an additional workload on this central brain structure. The capacity devoted to this task would be unavailable for other tasks, which would cause a reduction in the ability to reason and process information.

These problems could cause errors of judgement on board ship, which in some cases could have dramatic consequences including failure to properly understand orders when undertaking difficult manoeuvres, a risk of damage to machinery through negligence caused by reduced judgement or abnormal levels of fatigue.    

Noise-induced cardiovascular problems

It is generally agreed that noise causes generalised vasoconstriction. This vasoconstriction persists as long as the noise exposure continues^[19].

This phenomenon has been discussed frequently^{[20] [21]}, and the problem of the link between noise and blood pressure problems has been the subject of many studies. Despite the fact that the methodology of many of these studies has been criticised, 80% of the studies suggest the existence of such a link.

Blood pressure increases in those exposed to noisy conditions and the duration of the increase correlates to the length of exposure to the stressor. The increase depends not only on the level of sound but also on many other factors in the work environment such as the type of work and the category of staff^{[22] [23] [24]}. In addition, it has been demonstrated that employees with work-related hearing loss have significantly higher diastolic blood pressure than a control population with no hearing problems^{[25] [26]}

Link between noise and hypertension

The link between noise and hypertension was first suspected after it was noted that use of anti-hypertensive drugs was greater in areas near airports than it was in quieter areas^[27]. Many subsequent studies have confirmed this link^{[28] [29] [30] [31] [32] [33] [34]} and there have been several studies in shipping, with similar results^[35][36]. It is shown that levels of hypertension are significantly higher among engineers aged over 40 on board merchant ships (18.90%, N=164) than they are among non-engineer personnel of the same age (N=291) of whom 11.68% were hypertensive. This difference is not found in younger subjects. Levels of hypertension in the engineer group were independent of other risk factors such as a family history of hypertension, obesity and alcoholism. The relative risk of hypertension due to noise has been calculated at 1.62 and this is similar to the results of other studies^[37]. Roodenko et al.^[38] also found increased levels of hypertension in engineers when compared to deck crew and catering personnel. Noise can also be responsible for myocardial infarction^[39].

The difference between engineers and non-engineers aged between 40 and 55 is significant (p=0.05) (D Jégaden, C Le Pluart, Y Marie, B Piquemal 1986)

Effects on vision

Subjects who are regularly exposed to noise experience a reduction in nocturnal visual acuity and difficulties with depth perception, associated with a narrowing of the visual field. This narrowing can be as much as 10° at the red end of the spectrum. These abnormalities can be very troublesome when on gangway watch at night, when night vision is needed, and when the ambient lighting is red, but usually only occur if noise levels are above 100 dB. Noise-induced stress seems to reduce dopamine synthesis and dopamine is a neurotransmitter that is used by the retina.

Effects on endocrine system

Stress caused by noise of 60 dB or above causes the same type of endocrine change that is seen in all types of stress, that is, the release of catecholamines and cortisol. One study^[40] showed that three days of noise exposure appears to cause significant increases in corticosteroid and adrenaline levels. An effect on immune functions has also been observed, with an increase in oxidative stress.

Indirect noise-related effects

Noise limits people’s ability to communicate, and contributes to isolation, which is already a significant phenomenon on board merchant ships. Intelligibility of a conversation reduces in proportion with the increase in background noise and the distance between the participants. At a distance of one metre, communication is only possible if the noise level is lower than 75 dB.

A high level of noise may mask a warning or alarm indicating danger, or may lead to incorrect interpretation of instructions. Noise may be a direct cause of accidents. In 1955, Sir Lionel Heald confirmed that ‘men who worked on the flight decks and were exposed to tremendous noise from aircraft and ventilating machinery became extremely careless, got into the way of the planes, fell over things and got themselves injured.’^[41] According to Poulton (1979^[42], 1981^[43]), noise has a masking effect on non-intentional auditory signals and on the ‘internal monologue’ that each individual uses to overcome deficiencies in short-term memory. More generally, it is possible that noise, by masking a whole range of auditory signals that are characteristic of an environment, creates an impression of isolation, which leads to lack of attention and negligence. It is therefore essential, when choosing alarm equipment, to check that the acoustic power and frequency of the signal (low-frequency sounds mask high-frequency sounds) are adequate for the planned area of use. In addition, it has been demonstrated^[44][45] that ambient noise increases the risk of accidents, particularly in subjects with hearing loss.

E.6.7 The problem of multiple stressors

Noise is just one of many stressors that affect seafarers on board ships. Among others, there is also vibration, and heat in some cases. The question arises as to whether these stressors interact with each other. This area is very complex, and little is known about it. In the maritime field, which is greatly affected by this problem, the scientific literature is particularly sparse.

Noise and vibration

Some studies^[46][47][48] suggest that whole-body vibrations play a role in the aetiology of noise-induced hearing problems. Effects on low frequencies seem to be increased if there is combined exposure to noise and vibration (Pinter 1973)and some studies consider that while vibrations play a role in noise-induced hearing problems, this would only be of the order of 5 dB at temporary threshold shift (TTS)² that is, auditory fatigue measured 2 minutes after exposure. ^[49][50] Pekkarinen (1995^[51]) indicates that whole-body vibrations of between 2 and 10 Hz at 10 ms^-2 seem to increase auditory fatigue (TTS) when noise levels are above 90 dB_A.

Noise and heat

Several authors have attempted to show that there is an antagonist interaction between noise and heat.  However, it seems as though the effects of associated noise and heat may be synergistic, antagonistic or negligible, depending on the intensity of the stressors, type of work and length of exposure. According to Pekkarinen^[52], high temperatures increase auditory fatigue

Noise and chemical agents

It has also been established that there is a synergistic effect between exposure to noise and exposure to various solvents, toluene, styrene, xylene and trichloroethylene in particular, as well as carbon monoxide, which increases ototoxicity and therefore also hearing loss^[53][54][55].  (Sass-Kortsak et al. 1995)

Smoking can also increase the ototoxicity of noise^[56] (Molvaer and Lehmann 1985^[57]). However, Bur^[58]finds that smoking is an independent risk factor for incidence of hearing loss.

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