Review of Diver Noise Exposure

doi:10.3723/ut.29.021 International Journal of the Society for Underwater Technology, Vol 29, No 1, pp 21–39, 2010

Review of diver noise exposure

TG Anthony, NA Wright and MA Evans QinetiQ Ltd, Hampshire, UK Technical Paper

Abstract • Assess the risk to all employees, including divers, Divers are exposed to high noise levels from a variety from noise at work of sources both above and below water. The noise • Take action to reduce the noise exposure that exposure should comply with ‘The Control of Noise produces these risks at Work Regulations 2005’ (CoNaWR05, 2005). A • Provide hearing protection if the noise risk detailed review of diver noise exposure is presented, cannot be reduced sufficiently by other methods encompassing diver hearing, noise sources, exposure • Ensure legal limits on noise exposure are not levels and control measures. Divers are routinely exceeded exposed to a range of noise sources of sufficiently high • Provide employees with information, instruction intensity to cause auditory damage, and audiometric and training studies indicate that diver hearing is impaired by • Conduct health surveillance where there is a risk exposure to factors associated with diving. Human to health. hearing under water, in cases where the diver’s ear is The CoNaWR05 requires employers to take wet, is less sensitive than in air and should be assessed specific action at certain noise action values. These using an underwater weighting scale. Manufacturers of relate to the levels of exposure to noise of divers diving equipment and employers of divers have a joint averaged over a working day or week and the responsibility to ensure compliance with the exposure maximum noise (peak sound pressure) to which values in the CoNaWR05, although noise is only one they may be exposed. These values have been hazard to a diver, and a balanced risk assessment reduced by 5dB from previous regulations and are: must be applied to the whole diving operation. A diver noise reduction strategy is proposed, and a health • Lower exposure action values: daily or weekly surveillance programme involving audiometric tests for exposure of 80dB(A)1 re. 20µPa; peak sound divers should be established. pressure of 135dB(C) re. 20µPa • Upper exposure action values: daily or weekly Keywords: diver, hazard, noise, diving helmet, sound exposure of 85dB(A) re. 20µPa; peak sound pressure of 137dB(C) re. 20µPa 1. Introduction • Exposure limit values: daily or weekly exposure of 87dB(A) re. 20µPa; peak sound pressure of Divers are routinely exposed to high levels of 140dB(C) re. 20µPa. noise arising from a variety of sources, including self-generated breathing noise, communications, Although an employee may work or be exposed underwater tools and dive site noise above to noise for a range of working times during and below water. Audiometric studies with the day or week, the average noise exposures are divers have identified concerns in respect of normalised to the equivalent of a nominal 8-hour accelerated/excessive hearing loss (Zannini et al., working day and five working days per week. 1976; Edmonds and Freeman, 1985). Where action can be taken to reduce noise risk, To meet the requirements of the European Eco- then this should be done relative to the level of risk. nomic Community (EEC) Directive 2003/10/EC This is the principle of reducing risk to a level ‘as (EEC, 2003), new noise exposure regulations were low as reasonably practical’ (ALARP). introduced in April 2006. These were implemented There is a growing body of evidence that divers in the UK by ‘The Control of Noise at Work are exposed to noise levels that put them at risk Regulations 2005’ (CoNaWR05), Statutory Instru- of hearing damage (Parvin et al., 2001; Evans ment 2005, No. 1643 (Health and Safety Executive et al., 2007; Wolgemuth et al., 2008). Compliance [HSE], 2005). with the CoNaWR05 requires a complete noise These require employers to prevent or reduce 1The A-weighted scale, dB(A), takes into account the sensitivity of risks to health and safety from exposure to noise at the human ear in air and indicates the way in which airborne work and to: noise is related to sound perception.

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risk assessment of a diver’s working environment µ underwater and may require changes to diving practices and 80 equipment, as well as the wearing of hearing protection. Design modifications of diving equipment to 60 produce less breathing noise, use noise cancellation 40 techniques to reduce incident noise at the ear air and provide hearing protection for divers are all 20 technically feasible. 0

–20 2. Diver hearing under water and at pressure Sound pressure level (dB(A) re. 20 Pa 10100 1000 10000 100000 2.1. Human hearing in air and under water Frequency (Hz) In air, the human ear responds to sound frequen- Fig 1: The threshold of hearing in air and in water cies in the range 20–20kHz. Sound waves entering (Parvin et al., 1994) the outer ear reach the eardrum (tympanic membrane), causing it to vibrate in synchrony, which results in the transmission of sound to the cochlea the ears are ‘dry’. In this case, determining the within the liquid-filled inner ear. This mechanism noise hazard is achieved using the same method as of hearing is known as tympanic sound conduction. for occupational noise hazard assessment on land, A further mechanism for producing hearing is which is based on the A-weighted scale for sound. bone conduction, where sound is conducted to All that is required is knowledge of the incident the inner ear through the bones of the skull, noise at the diver’s ear. bypassing the outer and middle ears. For airborne Therefore, the type of breathing apparatus worn sound reception, the overall contribution of bone by a diver, i.e. diving helmet (dry ear) or hood (wet conduction to hearing is rather small. ear), is important in determining the noise hazard. Hearing under water differs from hearing in As hearing is more sensitive in air than in water air as the acoustic properties of water and air (Parvin et al., 1994), it is assumed that a given noise are different. Unlike sound in air, where much level is more damaging to the ‘dry’ ear than the of the incident sound energy is reflected by the ‘wet’ ear. skull, sound in water can propagate relatively freely through the human body, as the acoustic properties 2.3. Underwater auditory thresholds and of human tissue and water are similar. As sound frequency sensitivity goes through the skull, it excites the cochlear, Early studies of underwater auditory thresholds producing sound independent of the outer ear produced results that tended to show a large degree and eardrum. This so-called bone conduction route of variability in threshold levels (e.g. Hamilton, was, for many years, considered to be the only 1957; Montague and Strickland, 1961; Brandt and way in which sound could be heard under water Hollien, 1969). These findings are likely to have (Montague and Strickland, 1961; Norman et al., resulted from a failure to control the relatively 1971; Hollien and Feinstein, 1975). high background noise levels in water, along with It has, however, now been established that tympanic conduction is also involved in hearing inaccuracies in measuring the sound intensity under water (Smith, 1969, 1985; Parvin and at the subject’s head and failure to establish Nedwell, 1993), although sound is coupled less whether subjects had normal tympanic and bone effectively. Consequently, hearing under water is conduction hearing (Parvin and Nedwell, 1993). much less sensitive than in air. The underwater auditory threshold curve has been determined by Parvin et al. (1994; see Fig 1). 2.2. ‘Wet’ ear/‘dry’ ear effect Both the underwater and airborne curves are When using a self-contained underwater breathing displayed for comparison. apparatus (SCUBA) or a band-mask, a diver’s head Comparison of the air and underwater auditory is surrounded by water and the ears are ‘wet’ threshold curves (Fig 1) shows the following: (i.e. there is water in the auditory canal and in contact with the tympanic membrane). In this • The human auditory system is most sensitive to situation, estimating the noise hazard requires use waterborne sound at frequencies from 400Hz of an underwater weighting scale that adjusts for to 1kHz, with a peak at approximately 800Hz. decreased hearing sensitivity under water. Hence, these frequencies have the greatest For an enclosed helmet breathing apparatus, potential for damage however, the diver’s head is surrounded by air, or • Within this frequency band, underwater hearing an alternative gas such as nitrox or heliox, and is 35–40dB less sensitive than in air

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20 Sound pressure level (dB re. 20 Pa) 0 10 100 1000 10000 100000 Frequency (Hz)

External hood noise Internal hood noise UW-weighted noise inside hood

Fig 2: Sound levels inside and outside a diving band-mask hood while operating a rock drill, together with the UW-weighted noise exposure (Parvin et al., 1994)

• For airborne sound, hearing is most sensitive An example of the underwater noise hazard of a between 2 and 6kHz, with a maximum sensitivity small, compressed-air rock drill operated by a diver at approximately 4kHz. However, underwater wearing a hood is shown in Fig 2. The underwater hearing is less sensitive at these frequencies, and output noise from the drill (labelled ‘external hood so the noise hazard is reduced noise’) is broadband, being high across the entire • Above 6kHz, there is again reduced hearing un- spectrum and varying between 120 and 140dB. The der water compared with air, although hearing is level at a diver’s ear is reduced at frequencies above still possible at frequencies as high as 16kHz 200Hz due to attenuation by the neoprene of the • Below 400Hz, the hearing threshold drops off diver’s band-mask (labelled ‘internal hood noise’). at a rate of approximately 35dB per decade to Using the UW-weighting scale, the noise hazard 40Hz. This is not as rapid as for hearing in air and of the rock drill is calculated for a diver wearing suggests that sound at frequencies below 100Hz a diving hood (wet ear) (Parvin et al., 1994) by contributes to underwater sound perception to applying the UW-weighting scale to the noise level a far higher degree than in air, and so may be a at the diver’s ear (i.e. values given by the ‘internal greater hazard hood noise’ curve). From Fig 2, it can be seen that most of the noise hazard under water (shown by the • For relatively high frequencies, a higher level curve labelled ‘UW-weighted noise inside hood’) of noise would be permissible under water than lies between 30 and 1000Hz. For a diver with a would be in air as a result of the reduced wet ear, the noise hazard is reduced considerably, sensitivity of the ear under water. with a maximum level of less than 80dB, due to the 2.4. Underwater noise weighting scale reduction in hearing sensitivity under water. The total noise hazard of the rock drill, obtained The exposure action values of 80 and 85dB(A) in by summing the UW-weighted noise values over the CoNaWR05 are applicable to the air environ- frequency, is 83.8dB(UW) re. 20µPa. The dB(UW) ment rather than the underwater environment, in noise level of this rock tool will contribute to a which the ear is ‘wet’. In order to assess the noise daily noise exposure that may require action to hazard under water, it is necessary to re-assess these be undertaken. values to take into account the reduced sensitivity of the human ear under water. A method for achieving 2.5. Hearing in the hyperbaric environment and this was developed by (Parvin and Nedwell, 1995), the effect of different gases who defined an ‘underwater noise weighting scale’ Breathing gases used in diving other than air, measured on the dB(UW) scale, by analogy to the i.e. oxygen in nitrogen mixtures (nitrox, for use to A-weighted scale, dB(A). a depth in the order of 40m), oxygen in helium The scale defines the relationship between water- mixtures (heliox, used for greater depths), along borne sound incident on a water-filled external ear with oxygen in nitrogen and helium (trimix), may and the resultant auditory perception. Sound levels also affect hearing sensitivity. Heliox and, to a lesser on the dB(UW) scale are obtained by calculating extent, trimix are used in saturation diving, where the difference between the auditory threshold in air divers live and work at depths greater than 40m and the threshold in water at each frequency, and for long periods of time, several days or weeks then applying the A-weighted scale. being possible.

23 Anthony et al. Review of diver noise exposure

Hyperbaric environments for saturation diving permanent threshold shift (PTS) may occur. This have increased pressures and gas densities e.g. rang- mechanism is the basis of noise induced hearing ing from 200kPa (density 0.6kg·m−3) at 10m, to loss (NIHL), and thus there is the requirement to 2100kPa (density 3.6kg·m−3) at 200m (densities are control noise exposure as a means of reducing the typical values for heliox mixtures). In this envi- risk of long-term damage. A study conducted by ronment, many aspects of physiological function Curley and Knafelc (1987) identified moderate TTS are affected, such as respiration, cardiovascular in divers using surface-supplied diving apparatus for function and vision (Flook, 1987). As the ear con- dives of 120min duration. They also reported that, tains gas-filled cavities, changes in pressure and gas with the exception of one diver, hearing returned density associated with the hyperbaric environment to pre-dive levels within 24 hours of surfacing. It is, might also be expected to affect hearing, either therefore, possible that divers experiencing a TTS transiently or long-term. may suffer long-term hearing loss. Early studies suggested that hearing is impaired The review identified 15 studies that have in- by saturation diving. Fluur and Adolfson (1966) vestigated hearing loss in divers and/or conducted investigated the effects of hyperbaric air on hearing audiometric surveys. Most of the studies identified function and found hearing loss at around 500Hz used a combination of audiometric testing, medical and at 3–5kHz, with losses increasing with depth. examination for ear pathology and questionnaires Thomas et al. (1974, 1979) reported a similar to identify diving experience, history of barotrau- pattern of hearing loss with divers breathing heliox mas and noise exposure. at 100–300m, along with increases in hearing deficit The majority are retrospective studies, i.e. com- with depth. There was also an increase in hearing paring divers and non-divers’ hearing at the time sensitivity at 2 and 6kHz when breathing heliox. of the study and looking backwards in time at their However, more recent studies have not con- diving history. Some are prospective studies, which firmed these adverse effects on hearing. O’Reilly are a more powerful experimental design because et al. (1977) reported no changes in diver hearing they identify a group of divers, assess their hearing following a saturation dive to 186m (1960kPa) and then re-test after a period of time, looking at lasting 24 days. Mendel et al. (2000) investigated before and after effects. Their records of diving US Navy divers during saturation deep dives to activities, noise exposure and other relevant events 300m (1000 feet of sea water) and found that are also likely to be more accurate. hearing function was similar under hyperbaric pressure and in heliox to hearing on the surface; 3.1. Studies showing no differences in hearing in fact they found hearing sensitivity improved at 6 loss between divers and controls and 8kHz. Some early studies were unable to establish that Studies have also investigated the effects of there was impairment in divers’ hearing. Brady breathing different gases at normal air pressure, et al. (1976) investigated 97 US Navy divers and i.e. not in a saturation environment, and found age-matched controls, taking into account diving that hearing sensitivity was unaffected, for example, experience, incidents involving barotraumas, type while divers breathed a mixture of 20% oxygen and of diving equipment and prior noise exposure. 80% helium (Waterman and Smith, 1970). Although there was a significant relationship Overall, recent audiometric studies have shown between noise exposure and hearing, the noise that hearing is unaffected by the increased pressure exposure was not always associated with diving and gas density of hyperbaric environments, with or occupational noise. There were no significant the exception of hearing at high frequencies differences in hearing acuity between divers and which may be improved slightly. Furthermore, non-divers, and the study concluded that the factors breathing different gases (heliox and nitrox) in the investigated had only minimal effects on auditory absence of increased pressure also does not affect sensitivity. hearing. It therefore seems appropriate to apply These findings were consistent with those of the CoNaWR05 regulations directly to hyperbaric Shilling and Everley (1942), who examined divers environments without modification when assessing and submarine personnel and found no differences diver noise exposure. between the hearing of divers and non-divers, after taking age into account. There was, however, significant hearing loss in those who showed 3. Diver audiometric surveys evidence of ear disease, infection or barotrauma, Following exposure to elevated noise levels, a and this group’s hearing was worse than the hearing temporary impairment of hearing may occur, of divers without ear trauma or infection. known as a temporary threshold shift (TTS). Other evidence from Coles and Knight (1961), With repeated exposure to these noise levels, a who investigated 62 divers and submarine-escape

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instructors, suggested that minor occurrences of divers deteriorated faster than in non-divers, with barotraumas associated with diving procedures did a mean hearing deterioration of 6.6dB (significant not result in permanent hearing loss. at specific frequencies). The authors hypothesised Among the problems with some of these retro- that the hearing deterioration was due to hair spective studies is that separating the effects of age cell damage associated with repeated, long-term and experience on hearing loss, which are usually compression/decompression cycles in divers. confounded, has not always been considered. Also Further evidence for an increased rate of hearing the groups selected as both non-diving controls deterioration in divers is seen in a study by and divers may have been exposed to other noisy Molvaer and Albrektsen (1990). They investigated environments, and as a result it is not possible 116 professional divers before and after an interval to discriminating hearing impairments due solely of approximately 6 years and found that although to diving. the hearing thresholds of younger divers (less than 35) were lower than in unscreened (i.e. not 3.2. Evidence for hearing loss in divers – from visually or by other means, e.g. acoustic impedance, retrospective studies checked for damage to the auditory system) normal In contrast with the studies described earlier, population at comparable age, the gap closed there is a substantial body of evidence indicating with increasing age. See Fig 3, where the line that the hearing of divers is impaired by diving with squares is higher than, but getting closer noise. Edmonds and Freeman (1985) investigated to or lower than, the line with diamonds and 28 professional abalone divers, who averaged six is always lower than the line with triangles. The years of diving, and concluded that more than divers had higher hearing thresholds than screened 70% had high-frequency hearing loss to an extent non-divers of the same age, as illustrated in Fig 3, that was eligible for compensation. Ear pathology where divers less than 30 years old had a hearing and barotraumas were excluded as a cause, and level midway between the non-diver screened and the group was not exposed to other occupational unscreened groups. However, for the three older noise. Similarly, Zannini et al. (1976) investigated age groups, the divers were more similar to 123 professional divers and found that 76.9% of the unscreened non-diver population for higher divers had impaired hearing, a percentage that was frequencies of hearing. This suggests that diver significantly higher than non-divers. hearing deteriorates faster than non-diver hearing. Although Skogstad et al. (1999) were unable to Skogstad et al. (2000, 2005) conducted two establish significant differences between the hear- prospective studies and suggested that the number ing thresholds of construction divers compared of years of diving is significantly related to hearing with age-matched controls (group size of 26 in both loss, in a dose-response fashion. The study in 2000 cases), the divers showed reduced hearing in the investigated 54 young occupational divers divided left ear compared with the right ear from 3 to into high-exposure and low-exposure groups, and 8kHz. Both divers and controls in the study were tested at start of the study and after three years of occupationally exposed to relatively high levels of diving. At the start of testing, the hearing of the noise, and so both were likely to have impaired high-exposure group was reduced compared to that hearing. A study of Royal Navy (RN) divers’ of the low-exposure group. During follow-up after audiograms, conducted by the Institute of Naval three years, the combined groups showed reduced Medicine (INM), compared the hearing thresholds hearing ability at 4kHz in the left ear only. There with data for an otologically normal population and was an association between auditory function at identified a reduced percentile of normal hearing 4kHz and total number of years diving, suggesting at 500Hz (Johnston and Pethybridge, 1994). a dose-response relationship. The second study by these authors in 2005, involving a six-year follow-up 3.3. Evidence for hearing loss in divers – from of 47 divers, indicated that the divers’ hearing prospective studies was reduced at 4 and 8kHz. The study concluded Evidence for the presence of hearing impairment that mild hearing impairment can occur in young in divers has been strengthened by a number of professional divers, although divers’ hearing acuity prospective studies, which looked at groups of was better than that of the general population. divers before and after a period of time of the order of several years. Haraguchi et al. (1999) 3.4. Evidence for increased rate of hearing loss conducted a prospective study with 18 professional with age in divers fishery divers over five years. At the start, they had Several studies have suggested that although divers normal hearing or some hearing loss, and changes initially seem to have better hearing than the were determined after eliminating the effect of age. general population, their hearing deteriorates The investigation concluded that the hearing of faster than non-divers. Selection procedures mean

25 Anthony et al. Review of diver noise exposure

Age: <30 yrs Age: 30 – 35 yrs 10 10 0 0 –10 –10 –20 –20 –30 –30 –40 –40 Hearing level (dB) Hearing level (dB) –50 –50 01234567 01234567 Frequency (kHz) Frequency (kHz)

Age: 35 – 40 yrs Age: >40 yrs 10 10 0 0 –10 –10 –20 –20 –30 –30 –40 –40 Hearing level (dB) Hearing level (dB) –50 –50 01234567 01234567 Frequency (kHz) Frequency (kHz)

divers non-divers - screened population non-divers - unscreened population

Fig 3: Increasing deterioration in hearing with age of divers compared to non-divers (Molvaer and Albrektsen, 1990)

that the hearing of young divers at the start of their ‘examination of the long-term health impact’ careers is better than that of the general population, (ELTHI) of diving study reported by Ross et al. which includes individuals with ear diseases and (2007). This involved a questionnaire study of occupational damage. lifestyle, occupation and health status on behalf of Further evidence for the increased rate of the UK HSE. The postal survey included a large hearing deterioration in divers has come from two number of HSE-registered divers (2958) and a additional studies. Molvaer and Lehmann (1985) similar number of controls who were non-divers investigated 160 professional divers and compared working in the diving industry. The divers were them with a standard population of non-divers, all further divided into offshore (OSD) and non- of whom were grouped by age according to decades offshore (NOSD). The OSDs reported a higher between 20 and 60 years. As expected, hearing sen- incidence of hearing impairment (17%) compared sitivity decreased with age/diving experience, and with NOSDs (11%) and controls (11%). Hence the also with smoking and subjectively assessed noise ex- study lends support to the objective investigations posure. The study indicated that while hearing acu- described earlier using audiometry, where divers’ ity in younger divers was better than an age-matched hearing was impaired. general population, hearing in the older age groups Overall, the majority of studies reviewed indicate was the same as the non-diving population. that diver hearing is impaired by exposure to factors A further investigation by Zulkaflay et al. (1996), associated with diving. Of these, several studies involving 120 Malaysian Navy divers and 166 also suggest that divers’ hearing deteriorated faster non-diver naval personnel, indicated that divers than non-divers’ hearing, i.e. increased the age- older than 30 years showed greater hearing loss related deficit. All the prospective studies provided at 4, 6 and 8kHz than non-divers. The hearing evidence that the hearing of divers is impaired. damage was considered to be due to the combined The hearing deficits are likely to be due to the effects of a high noise environment, decompression combined effects of noise and pressure, including illness (DCI) and minor residual damage due to barotraumas and DCI, although it is difficult to inner ear barotraumas. The study again concluded separate out the influences of these individual that divers’ hearing deteriorated faster than non- factors. In particular, it is difficult to establish from divers, with the effects seen at higher frequencies the audiometric data that the hearing impairments (4–8kHz). are due to noise per se. This is partly because the effects are likely to be due to combinations of 3.5. Evidence for hearing loss in divers from a factors and because the noise levels they have been qualitative survey exposed to are essentially unknown. Noise exposure Finally, evidence for hearing loss in divers from was only assessed subjectively and could have also an HSE-sponsored survey was obtained from the been very variable within the diving group, thus the

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80 µ Average Source Level(Leq) = 100.0dB re. 20 Pa. 60 Internal Helmet Noise µ Average Level (Leq) = 103.0dB(A) re. 20 Pa. µ Noise Dose (Lep,d) = 94.1dB(A) re. 20 Pa. 40 Rest Period µ Average Level (Leq) = 99.6dB(A) re. 20 Pa. Sound Pressure Level (dB re. 20 20 Internal helmet noise (Linear) Internal helmet noise dB(A) Source level (Linear) 0 0 500 1000 1500 2000 2500 3000 3500 Time (sec)

Fig 4: The breathing noise measured from an open-circuit demand diving helmet (Parvin et al., 1994) link between noise and hearing loss was difficult to press compressor producing 100dB(A) re. 20µPa establish in these investigations. at 1–2 feet from the source and located near the operator. This required personnel to wear hearing 4. Sources of noise protection that reduced the noise to 97dB(A) re. 20µPa. Measurements obtained for a life 4.1. Ambient underwater noise levels support buoy diesel generator and compressor were Naturally occurring ambient noise in the ocean 105.6 and 94.6dB(A) re. 20µPa, respectively. Again, arises from turbulence and pressure fluctuations, personnel were required to wear ear-muff hearing as well as from wind-dependent noise such as protection, providing attenuation of 20–25dB. bubbles, waves and spray from surface agitation. For comparison, a home living room may be These phenomena generate a background noise 40dB(A), typical office environment 65dB(A), busy level of around 100–140dB re. 1µPa at 1m over street noise 80dB(A) and a road drill 100dB(A). the frequency range 10Hz up to 20kHz (Wenz, 1962)2. Man-made noise sources, such as those 4.3. Self-generated breathing noise and from shipping and offshore oil exploration and helmet noise production, are widespread and therefore are Divers produce a high level of breathing noise effectively ambient. The noise associated with these generated by airflow through the regulator demand operations can be very high; for example, seismic valve during inhalation. Bubble noise is also survey air-gun source levels are typically up to 198dB produced during exhalation and speech by air re. 1µPa at 1m (Goold and Fish, 1998) and oil rig released from the regulator, leading to significant operations are around 159–189dB re. 1µPa at 1m noise. For a diver wearing a free-flow helmet, there (McCauley, 1998). is also the noise from the airflow. Typical examples of breathing noise, under 4.2. Ambient dive site noise levels comparable subsurface conditions, associated with Diving sites are typically very noisy above water, an open-circuit demand diving helmet are shown so divers are exposed to high levels of noise in Fig 4 and with an open-circuit band-mask in Fig 5 throughout the working day. Wolgemuth et al. (Parvin et al., 1994), demonstrating the dry ear/wet (2008) estimated the likely total noise dose received ear effect. A comparison between the lines that are by a diver over 24 hours by combining in-air and the middle shade of grey gives a clear indication of in-water noise sources at a dive site. The diving the comparative noise hazard for divers wearing a operation was conducted from a salvage barge band-mask compared with a diving helmet. which accommodated equipment used to power Fig 4 gives the average internal helmet noise underwater tools. These include a compressor level (Leq) at the diver’s ear as 103dB(A) re. 20µPa, producing in-air noise levels of 99.4dB(A) re. 20µPa and associated daily noise dose (Lep,d), for a at 10 feet from the source, and a hydraulic drill 1-hour exposure and no other exposure being considered, of 94.1dB(A) re. 20µPa, which exceeds 2 Noise under water is very different than in air and, by convention, the CoNaWR05 exposure limit value. Conversely, underwater noise measurements are referenced to a pressure of 1µPa at a distance of 1m from the source (Urick, 1983). Underwater Fig 5 gives the average noise at the diver’s ear in a sound is therefore written as dB re. 1µPa at 1m. band-mask (Leq), i.e. a wet ear with the underwater

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Internal hood noise (linear) Internal hood noise (dB(UW)) Source Level (linear) 140 Pa.)

µ µ 120

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Sound Pressure Level (dB re. 20 Internal Hood Noise 20 µ Average Level (Leq) = 60.3dB(UW) re. 20 Pa. µ Noise Dose (Lep,d) = 56.1dB(UW) re. 20 Pa. 0 0 500 1000 1500 2000 2500 3000 Time (sec)

Fig 5: The breathing noise measured from an open-circuit demand band-mask hood breathing apparatus (Parvin et al., 1994)

weighting scale applied, as 60.3dB(UW) re. 20µPa, return line systems being used for saturation diving. and associate daily noise dose (Lep,d), for a 1-hour The data from Evans et al. (2007) indicated that the exposure and no other exposure being considered, primary source of noise for the diving helmets is of 56.1dB(UW) re. 20µPa, which is comfortably less exhaust bubbles formed on exhalation. The study than the CoNaWR05 lower exposure action value3. also measured the noise levels during communi- The noise levels of various types of diving helmets cations and demisting for each of the helmets, differ depending on the design characteristics, demonstrating increases in each case and indicating such as positioning of exhaust valves and supply that use of demisting and communications should hoses, and the conditions under which the noise both be kept to a minimum. testing has been carried out, e.g. whether the The noise levels from the study by Evans et al. tests were manned or unmanned testing and the (2007) are presented in Fig 6. It is apparent that: type of environment (anechoic chamber versus open water). • Noise levels in the helmets increased with Evans et al. (2007) conducted manned trials increased diver ventilation rate in an anechoic chamber to measure the noise • Helmets producing exhaust bubble had higher levels in three different types of diving helmet: noise levels than those that did not the Diving System International (DSI) SuperLite • Communications (and flushing through) create (SL) 17B, Kirby Morgan SL-17K and Divex ‘Dirty a high noise level Harry’. During normal breathing, the internal • Communications require a noise level in the helmet noise levels at the divers’ ear were 78.7, 88.0 order of 15dB above background for the commu- and 91.1dB(A) re. 20µPa for Dirty Harry, SL-17K nications to be audible. and SL-17B, respectively, increasing as ventilation Other studies also indicate that the high noise rates rose (see Fig 6). The differing noise levels levels described earlier are typical of those seen were due to various design aspects: the Dirty Harry with many diving helmets. As an example, levels helmet system is part of a gas return line system that of 100.0 and 104.1dB(A) re. 20µPa (at rest and does not produce bubbles on exhalation, while the during exercise, respectively) were recorded inside other two have different internal volumes and valve the AH3 free-flow diving helmet at simulated depths configurations. of 5, 30 and 50m (Anthony et al., 1994). A further It is worth noting that the Dirty Harry system, study by Reimers and Summitt (1973) recorded and the associated noise levels, are comparable with 106 and 116dB(A) re. 20µPa inside USN Mark V 3Noise assessment terms are defined as: equivalent continuous helmets for air and heliox, respectively. sound level (LAeq). LAeq is the A-weighted energy mean of the noise level averaged over the measurement period. It can be It is clear from these studies that the noise considered as the continuous steady-state noise level that would have intensities (with the exception of the Dirty Harry the same total A-weighted acoustic energy as the real fluctuating helmet with the diver breathing normally) are noise measured over the same period. Noise dose (Lep,d): The an appreciable noise hazard to divers. If used CoNaWR05 is based on the ‘noise dose’ incurred by A-weighted noise energy received that day. The noise dose is the sum of the for typical working dive durations, the daily noise total noise energy averaged over an 8-hour working day. dose is likely to exceed the upper exposure action

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(dB) 94 Dirty Harry aeq

L 91 88 85 L , Dirty Harry, Right 82 Maximum aeq L , Dirty Harry, Left Voluntary aeq 79 L , KMB17K, Right Ventilation aeq L , KMB17K, Left 76 aeq Raised Laeq , KMB17B, Right 73 L , KMB17B, Left Ventilation aeq 70 0 20Normal 40 60 80 100 120 140 160 Breathing Ventilation (l.min-1 (STPD))

Fig 6: The effect of diver ventilation rate, communications and demisting on helmet noise levels (Evans et al., 2007) value, 85dB(A) respectively, of the CoNaWR05. It is, 180dB re. 1µPa at 1m for a rock breaker (Parvin therefore, appropriate to implement noise control et al., 2001) and 211.4dB re. 1µPa at 1m for an measures to restrict the noise dose to divers. underwater stud gun (Sterba, 1987). If the control measure is to restrict permissible As with self-generated helmet noise, it is clear dive durations, in some instances this will result in that for the divers wearing helmets with a dry unrealistic dive times. It is not the intent of the ear, the noise intensities are above the allowable CoNaWR05 to restrict dive durations to unaccept- levels for normalised 8-hour working day exposures, able levels, but to use a range of control measures as they exceed the upper exposure action value, of which dive duration may be one. However, it is 85dB(A), of the CoNaWR05. It is, therefore, appro- an unqualified duty for employers to reduce noise priate that control measures are implemented to exposure to levels that comply with the CoNaWR05. restrict the noise dose to divers. For divers wearing a band-mask with a wet ear, 4.4. Tool noise the levels comply with the requirements of the The operation of underwater tools by divers CoNaWR05. However, it should be noted that a generates extremely high levels of noise. Al- band-mask does not offer physical head protection though the divers’ breathing apparatus can provide for divers conducting underwater engineering and some protection from external waterborne noise, has an increased a risk of divers developing the total noise dose from tools, combined with ear infections. helmet and self-generated breathing noise, can be considerable. 4.5. Compression chamber noise Parvin et al. (2001) measured the noise exposure Noise during the compression and decompres- from several underwater tools for divers wearing sion cycles of a compression chamber reaches an open-circuit demand diving helmet and an sufficiently high intensities to present an auditory open-circuit, band-mask-hood breathing apparatus. hazard. Measurement of internal chamber noise Example noise levels are shown in Fig 7. For the to obtain a hyperbaric noise dose estimate is open-circuit demand diving helmet, levels were required to determine whether the levels exceed 83dB(A) re. 20µPa for background noise and up the CoNaWR05 for personnel inside. to 112dB(A) re. 20µPa during tool operation, Measurement of external chamber noise is also indicating a noise hazard both with and without tool required to assess the noise exposure of chamber operation. For the band-mask breathing apparatus, operators and support personnel. Noise levels levels were 60dB(UW) re. 20µPa for background typical of hyperbaric chambers during a dive ascent noise and up to 71dB(UW) re. 20µPa during tool and descent are 108 and 112dB(A) re. 20µPa, operation, therefore a much lower noise hazard. respectively (Murry, 1972). Further examples are Noise levels generated by underwater tools were shown in Table 1. measured in a number of other studies that further In summary, from the studies of noise sources demonstrated the potential for auditory damage in reviewed, there are many examples where noise many cases. Examples of typical source levels are intensities within the diving environment exceed

29 Anthony et al. Review of diver noise exposure

160

140 Source Level Pa.)

µ µ 120

100

80 Internal Helmet Noise

40 Internal Helmet Noise 105.9dB(A) 20 Sound Pressure Level (dB re. 20 0 1 10 100 1000 10000 100000 Frequency (Hz)

Fig 7: Noise levels generated by a hand drill for a diver wearing an open-circuit demand diving helmet (Parvin et al., 2001)

Table 1: Example noise doses for sources commonly encountered by divers

Noise source LAeq (average over 1-hr noise dose Permissible exposure # time recorded) (calculated from LAeq) duration (time to dB(A) re. 20µPa dB(A) re. 20µPa CoNaWR05 Lep,d) Diver workplace noise Example noise sources on deck Hydroblaster compressor1 99.4 90.4 5min Hydraulic drill press compressor1 100.0 91.0 4min

Diver subsurface noise Example diving helmet noise Dirty Harry diving helmet – normal 78.7 69.7 10.5h breathing2 Dirty Harry diving helmet – maximum 93.4 84.4 21min ventilation2 Dirty Harry diving helmet – during 105.8 96.8 1min 25s communication2 US Navy Mk V3 113.0 104.0 14s SuperLite 17K diving helmet 110.4 101.4 25s with surface-supplied diving equipment (SSDE)4

Example underwater tool noise Stud gun5 185.4 176.4 <2s Hydroblaster1 152.2 143.2 <2s Hydraulic drill press1 128.9 119.9 <2s

Compression chamber noise Example chamber noise Type 1 non-TUP chamber6 145.3 136.3 <2s Admiralty Mk1 chamber7 110.5 101.5 25s Duocom Holders Variant chamber8 108.0 99.0 45s # Permissible exposure duration without hearing protection or other control measures to remain lower than CoNaWR05 lower exposure action value 1: Wolgemuth et al., 2008; 2: Evans et al., 2007; 3: Summitt and Reimers, 1971; 4: Samways and Parvin, 2005; 5: Sterba, 1987; 6: Searle and Parvin, 1995a; 7: Searle and Parvin, 1995b; 8: Searle and Parvin, 1996

the allowable daily noise dose; consequently, they 5. Noise exposure values and guidance are sufficiently high to produce hearing loss in 5.1. Noise exposure values in air and under water divers. In particular, self-generated breathing and UK noise exposure legislation is based on the helmet noise, along with the noise from underwater EEC Directive 2003/10/EC, which is implemented tools and compression chambers, all produce by CoNaWR05 (EEC, 2003; HSE, 2005). These exposures capable of causing auditory damage. regulations define the exposure values at which

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employers must take action to reduce the noise using the noise exposure values specified by the hazard to their employees. The legislation is based CoNaWR05. All that is required is measurement on the average daily noise ‘dose’ of the ‘A-weighted’ of the sound level incident at the ears within the noise energy received normalised to an 8-hour diving helmet. The dry ear scenario is worst case in working day, five days a week. terms of noise hazard due to the greater sensitivity The following factors should be considered in of hearing in air than under water. However, when regard to the implementation of these regulations. the ears are wet (e.g. SCUBA divers), the exposure values specified in CoNaWR05 must be adjusted for 5.1.1. Noise energy and time dependence the reduced hazard; for example, with the method Currently, the upper-exposure action-value noise suggested by Parvin et al. (1994), an UW-weighting level is that which is incurred during continuous scale should be used. exposure of 85dB(A) re. 20µPa for a normalised Hooded divers: For divers with a wet ear wearing 8-hour period, each working day. By limiting the diving hoods (e.g. foam neoprene), hearing is exposure time, a trade-off may be achieved for further protected by the attenuation factor of exposure time against noise level, such that for each the hood, which is found to be around 5–15dB, halving of exposure time, a doubling (3dB) of the depending on the frequency of the noise (Parvin sound energy is permissible. Therefore, under cur- and Cudahy, 2003). It should also be noted that due rent legislation, 88dB(A) re. 20µPa is permissible to the compressible nature of foam neoprene, the for 4 hours, 91dB(A) re. 20µPa for 2 hours and attenuation reduces appreciably with depth. When so on. The noise energy and potential for hearing assessing underwater noise exposure, the noise level damage of each of these exposures are the same. within the hood and incident on the ear should The maximum peak level is 137dB(C) re. 20µPa. be recorded at the appropriate depth, and the It should be recognised that the principle of the UW-weighting scale applied. CoNaWR05 is to reduce noise at the source to a level Hyperbaric pressure and gases other than air: that may be considered ALARP and, thereafter, Research has shown that hearing during saturation to apply further control measures. Purely limiting diving, where the ears are at hyperbaric pressure noise dose by exposure time does not follow and exposed to gas mixtures other than air, is this principle and relies on no further noise similar to hearing in air at surface pressure (Mendel exposure occurring for the remainder of an 8-hour et al., 2000). Hence, the noise hazard can be working day. As illustrated, diving sites are noisy determined by applying the CoNaWR05 values with environments so this is not necessarily practical. no adjustment. Therefore, alternative noise control measures are Saturation diving: The CoNaWR05 identifies also required. exposure values averaged and normalised for an 5.1.2. Noise exposure under water 8-hour working day or weekly exposure values for The CoNaWR05 dB(A) values apply to noise in an nominally five working days per week. In saturation air environment rather than under water. It has diving, the divers are exposed continuously, which been shown that the human ear is less sensitive to has been recognised for other occupational expo- sound in water (i.e. when the auditory canal is filled sure limits. In EH75/2, the HSE has promulgated with water) than in air (Parvin et al., 1994). Due to techniques for assessing continuous hyperbaric the reduced hearing sensitivity of the ear immersed chemical exposure (HSE , 2000). in water, the CoNaWR05 exposure values should On the assumption that noise exposure should use an appropriate dB weighting to determine the also be assessed for continuous exposure – com- noise dose that divers are exposed to under water. bined with the principle such that for each doubling − Parvin et al. (1994) have proposed a method to of exposure time, a halving ( 3dB) of the sound translate the criteria for assessing noise exposure energy is permissible – then if a 40-hour working in air to exposure under water, using a knowledge week 80dB value is extrapolated to a full 168-hour of hearing sensitivity under water to calculate an week, it would require the average exposure to underwater weighting (UW-weighting) scale. The be 6.2dB(A), i.e. 10 log(168/40), less than the method assumes that any reduction in hearing normalised value. sensitivity equates to an equivalent increase in For saturation exposure, and considering that allowable noise dose. Accordingly, since hearing is no additional factors are required for pressure or less sensitive under water, a higher level of noise gas mixture, an average continuous noise level of is tolerable. 73dB(A) re. 20µPa would provide the diver with a noise dose at the lower exposure action value of the 5.1.3. ‘Wet’ ear/‘dry’ ear effect CoNaWR05. However, the levels in the NORSOK Where the diver is wearing a helmet and the ears are U-100 standard (NORSOK, 2007) would provide a dry, the noise hazard may be determined directly more comfortable living environment.

31 Anthony et al. Review of diver noise exposure

5.2. Comparison of expected noise exposure with undertaken using the HSE online system available current noise regulations on the HSE website4 and illustrated in Fig 8. 5.2.1. Noise doses from various sources in the Examples of the total noise exposure of divers diving environment are completion of a surface-orientated air dive and From the discussion of noise sources, it is evident a saturation dive. Noise dose estimates of divers are that divers may often be exposed to noise intensities for a nominal 12-hour working day and a dive with exceeding the CoNaWR05 values, although the self-generated helmet noise. actual hazard will depend upon the duration of Example 1: Surface-orientated dive total noise exposure and the type of diving apparatus worn. dose calculation. Table 2 shows the overall noise Table 1 shows the noise dose received in 1 hour dose from a dive to 40m to be 83dB, and so by a diver for various sources, along with the it exceeds the CoNaWR05 lower exposure action permissible exposure duration to comply with the value (Simpson et al., 2000). requirements of the CoNaWR05 (lower action Example 2: Saturation dive total noise dose values). Noise control measures are required in estimation. Total exposure includes noise during many cases. compression in the living habitat and during diving The noise sources include: (Nedwell and Needham, 1995); see Table 3. The total estimated noise dose is 88.4dB(A) re. 20µPa, • Diving workplace noise, where generators and thus exceeding the CoNaWR05 upper action value compressors associated with underwater tools are and the proposed continuous exposure action value located on diving support vessels of 73dB(A) re. 20µPa. • Subsurface noise, including self-generated breath- Example 3: Total noise dose over a 12-hour ing noise, the operation of underwater tools working day. The total noise exposure for a and communications equipment; diving with conceptual working day lasting 12 hours (see enclosed helmets is the prime contributor to Table 4) gives a total exposure of 89dB(A), underwater noise exposure, as for SCUBA and exceeding the upper exposure and limit values. band-mask divers the wet ear provides some Example 4: Noise dose attributable to helmet mitigation against the noise hazard noise. In order to comply with the requirements • Compression chamber noise during saturation of the CoNaWR05, if time of exposure is the diving or surface decompression, where noise only control measure implemented many current levels are typically very high. diving helmets place severe restrictions on permissible dive durations, particularly when the physical 5.2.2. Estimation of divers’ total daily/weekly workload is high and with appreciable use of noise exposure communications. For example, the Dirty Harry The total noise exposure averaged over an 8-hour helmet, the quietest tested by Evans et al. (2007), day is required to be lower than the exposure value. produced acceptable noise when breathing rates Estimating this requires the identification of all were normal at 78.7dB(A) re. 20µPa. High venti- noise sources to which the diver is exposed, rather lation rates increased the noise output to 93.4dB, than only during diving. A complete daily exposure and use of communications produced a further could comprise: increase to 105.8dB, requiring time limitations to • Noise during transit to the dive site by boat or be considered for these activities. A dive lasting helicopter 1 hour and comprising more than approximately • Ambient noise at the dive site 15 min of high physical workload and 3 min of • Subsurface noise during dive (subsurface am- communication time would exceed the permissible bient noise, self-generated breathing noise and daily noise dose. tool noise) In summary, these four examples demonstrate • Noise exposure in a compression chamber. that divers may frequently be exposed to noise levels exceeding the CoNaWR05 values, and hence noise Furthermore, the work patterns of divers are control measures are required. often highly variable, so taking account of noise exposure both above and below water on an 6. Control of noise exposure appropriate time frame is important. Estimating 6.1. Responsibilities arising from the CoNaWR05 weekly noise dose may be a more appropriate measure than daily. 6.1.1. Diver hearing and CoNaWR05 noise exposure The noise dose received by a diver over a given The evidence from audiological studies indicates time frame may be estimated from the average noise that diver hearing is impaired by exposure to level (LAeq) for each component of an exposure 4Please see: http://www.hse.gov.uk/noise/calculator.htm (accessed and the duration of that exposure. This may be in January 2010).

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Fig 8: Daily noise exposure calculator

Table 2: Noise dose for a dive to 40m (Simpson et al., 2000) Depth 40msw Dive time 40mins Equipment SuperLite helmet Noise sources and typical sound pressure levels (SPLs) at 4kHz (dB) Dive activity Duration of Background noise UBA Comms Tools Sum (dB) exposure (mins) Dressing 15 80 80 Descent 2 85 70 85 On bottom 5 85 70 85 Working with tool 30 — 70 90 88 On bottom 3 85 70 85 Ascent and decompression 37 85 70 85 On surface 5 85 70 85 De-kit 15 80 80

1.87 Lep,d 83dB Notes: (i) Dive times are based on US Navy tables (ii) Background noise level onboard a diving support vessel derived from Nedwell and Needham (1995) and Hicks (1993) (iii) Underwater breathing apparatus (UBA) noise derived from Nedwell and Needham (1995) (iv) Tool noise received through the helmet at the diver’s ear while using a water jet, derived from Molvaer and Gjestland (1981) (v) Communication noise estimated

Table 3: Noise dose for a typical two-week tour of duty for a North Sea diver (Nedwell and Needham, 1995) Level in No. of Percentage Percentage dB(A) hours of hours of total dose Compressing 95 1 0.3 3.0 Living chamber, active 80 112 33.3 1.0 Living chamber, asleep 70 112 33.3 10.5 Bell duty 90 30 8.9 28.3 Diving 90 60 17.9 56.5 Bell checks etc 75 21 6.2 0.6 Totals 336 100 100.0 Total equivalent level 88.4dB(A) re. 20µPa

33 Anthony et al. Review of diver noise exposure

Table 4: Noise dose for a conceptual 12-hour working day

Task/activity Noise level LAeq dB(A) Exposure duration (h) Dose Lep,d (dB(A)) for task Onshore travel 70 0.5 58 Fast boat transit 90 1.5 83 Dressing 80 0.25 65 Dive 1 89 2 83 Surface interval (4h) 80 4 77 Dive 2 89 2 83 Fast boat transit 90 1.5 83 Onshore travel 70 0.5 58 Duration of working day: 12h 15min

Daily noise exposure (Lep,d): 89 dB(A)

factors associated with diving. It is also apparent Thus the requirement to determine and supply from the discussion of noise levels in the diving data on noise levels applies to the provision environment that divers are routinely exposed to of equipment for all aspects of diving opera- levels exceeding those defined by the CoNaWR05. It tions, including surface machinery (e.g. com- therefore seems appropriate that control measures pressors), diving apparatus (e.g. diving helmets), are implemented to reduce divers’ noise exposure, diver tools and hyperbaric facilities (e.g. compres- and thereby reduce the risk of long-term hear- sion chambers). ing deficit. Once it has been established that a noise hazard 6.1.3. Employers’ responsibilities exists, manufacturers and employers have a joint Employers are responsible for ensuring that divers responsibility to reduce noise so that divers are and all other employees are not exposed to not exposed to intensities above the exposure noise that exceeds the values identified in the values. In reducing the noise levels, employers are CoNaWR05. Assessment of diver noise exposure required to demonstrate that the risk of noise must take into account all relevant factors, includ- hazard is ALARP, in line with the requirements ing working practices and equipment, plant and of CoNaWR05. other sources of noise not directly related to the diver’s immediate task. 6.1.2. Manufacturers’ responsibilities There is a link with the manufacturers of equipment, in that employers have a responsibility Designers and manufacturers of diving equipment under CoNaWR05 to include consideration of the are responsible for ensuring that noise levels of choice of appropriate work equipment emitting the diving equipment are ALARP technically. This is least possible noise5. embodied within UK law in that the Supply of If assessment indicates that the exposure values Machinery (Safety) Regulations 1992 (Statutory are likely to be exceeded, employers are responsible Instrument 1992 No. 3073), or SM(S)R92 (HSE, for eliminating or reducing noise at source to 1992). The SM(S)R92 requires that ‘machinery ALARP. If this is impractical, hearing protection must be so designed and constructed that risks must be provided. resulting from the emission of airborne noise are reduced to the lowest level taking account of 6.2. Guidance for reducing diver noise exposure: technical progress and the availability of means of principles of noise control reducing noise, in particular at source.’ An outcome of this review has been to highlight Manufacturers are also required to supply that the risk to divers’ hearing is not simply technical data specifying the noise output level due to the act of diving and using underwater of equipment and noise characteristics (frequency breathing apparatus. The hazards are multifaceted spectra), as this data is essential for assessing the and embrace all aspects of a diver’s working life total noise dose of divers. and environment. Thus the solution and control Diving breathing apparatus is defined as personal principles invoked must also be multifaceted. protective equipment (PPE) and, as such, does not There are three fundamental approaches to fall under the SM(S)R92. However, the European reducing noise exposure: Norm for umbilical supplied diving apparatus – BS EN 15333-01 (British Standards Institute • Elimination or reduction of noise at source [BSI], 2008) and BS EN 15333-02 (BSI, 2009)– • Reduction of environmental noise at the ear requires a manufacturer to identify the noise levels • Wearing hearing protection. within diving helmets and provide the information 5Please see: http://www.hse.gov.uk/noise/goodpractice/ to the user. lownoisemachines.htm (accessed in January 2010).

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These are listed in hierarchical order, with the 6.3.2. Dive site noise control of noise at source being the optimum Reduction of diving site noise produced by com- solution, and using hearing protection the least pressors, power generators and other equipment on desirable. a dive site via engineering solutions is a viable ap- However, diving – and particularly commercial proach to reducing total diver noise exposure. The diving – where underwater engineering is being principles of noise reduction are relatively well- undertaken is by its nature a hazardous process. established for these sources and include: Noise is but one hazard of many that a diver faces. A balanced risk assessment must be applied to the • Fitting exhaust mufflers on internal combustion whole operation, as fully mitigating against one risk engines may exacerbate risk from another source. • Fitting silencers to compressed air exhausts A significant example outlined in this review is • Isolating (using rubber mounts and flexible the balance between the reduction in noise dose connections) a vibrating noise source to separate from a diver having a wet ear compared to a dry ear it from the surface upon which it is mounted and the physical head protection offered by a dry • Fixing damping materials (such as rubber), or helmet diving system compared to a wet SCUBA or stiffening materials to panels to reduce their band-mask system. tendency to vibrate Whilst the ideal scenario is both to reduce • noise dose to be fully compliant with CoNaWR05 Building enclosures or sound proof covers and to provide full physical head protection, it is around noise sources not achievable with equipment that is currently • Fitting sound absorbing materials to hard, available in the diving industry, yet it should be a reflective surfaces to reduce noise. prime objective to achieve this. It is also not possible here to identify the balance An additional approach is to provide ‘quiet’ between these two risks, as it is highly dependent areas on work and dive sites where divers may be upon the nature of the task being undertaken. Thus protected from ambient noise during periods when the current principle for noise control must be they are not involved in a diving operation. a balanced risk assessment considering all factors linked with longer term action to reduce all risks, 6.3.3. Tool noise including noise exposure. Many of the previously mentioned engineering solutions may also be applicable to the reduction 6.3. Control of noise at source of tool noise, such as fitting exhaust mufflers and The control of noise at source entails eliminating silencers. As with diving helmets, any pneumatically or reducing the noise output for each item of driven tool that is exhausting bubble will be equipment that contributes to the noise dose. The emitting a high noise level. Using tools that various noise sources can be addressed as follows. are hydraulically rather than pneumatically driven will reduce noise levels, as will moving exhaust 6.3.1. Self-generated breathing noise and bubbles away from the diver. The duty cycle of helmet noise the tool may also be adjusted during use, as A significant contribution to a diver’s noise expo- this will reduce the average noise exposure of sure arises from diving helmet, specifically from the operator. exhaust bubbles generated during exhalation. Re- ducing or eliminating this source is likely to reduce self-generated breathing noise substantially. By 6.3.4. Compression chamber noise addressing the noise from exhaust bubbles during A substantial amount of the noise during pressuri- exhalation (e.g. by eliminating exhaust bubbles or sation come from turbulence arising from high- moving exhaust bubbles away from the helmet), pressure gas merging with still air. Commercial diving breathing apparatus manufacturers may be silencers are available to reduce this noise (Simpson able to design helmets that have an appreciably et al., 2000) and are reported to achieve noise reduced self-generated noise levels. attenuation of around 10dB at low frequencies, It has been recognised that audio communica- rising to 20dB at high frequencies. A further tions are also a major contributor to a diver’s noise method to reduce noise inside chambers is to fit dose. As communications require a sound level in acoustic cladding to reflective surfaces. Chamber the order of 15dB greater than the background, isolation to reduce the mechanical coupling of the any reduction in internal helmet noise will have steel structure of the chamber to the supporting a proportional reduction in the noise dose from surface is a further tried and tested method of communications. reducing noise.

35 Anthony et al. Review of diver noise exposure

6.4. Reduction of environmental noise at the ear Given the existing noise exposure levels, and consequently the reduced allowable exposure (dive) 6.4.1. Helmet soundproofing times, this would also not be a practical approach Helmet noise may be further reduced by incor- because the work (dive) time would be too short. porating acoustic insulation in and around the However, should noise levels be better controlled at diving helmet shell to provide soundproofing. This source, then this may become a more viable option approach is currently being investigated by some in some circumstances. manufacturers and may significantly reduce noise levels at the diver’s ear. 6.6. Hearing protection Hearing protection should only be considered 6.4.2. Noise attenuation of diving hoods when all other noise control measures have been Neoprene diving hoods that either stand alone unable to reduce noise to an acceptable exposure. If or as part of a band-mask provide protection when all other control measures have been applied by attenuating noise levels at the divers’ ear the noise dose still exceeds 85dB(A) re. 20µPa, (Curley and Downs, 1986; Parvin and Cudahy, 2003; then the CoNaWR05 mandates the use of hearing Fothergill et al., 2004). They also reduce noise protection. If the noise level is between 80 and intensities by approximately 5–15dB, depending on 85dB(A) re. 20µPa, employers are required by the the thickness of the neoprene. As the thickness CoNaWR05 to inform any persons exposed to these of the neoprene is reduced with depth, the levels and make hearing protection available upon attenuation decreases with increasing depth. request. As discussed previously, divers with a wet ear Although ear-muff-type hearing protectors are and diving hood have some protection from noise routinely used throughout industry, they are only hazards due to the reduced sensitivity of the suitable for use on the surface on a diving site, or in ear under water, as well as noise protection by compression chambers if they have been drilled (a their diving hood. However, also as previously small nominally 2–3mm-diameter hole in the centre indicated, this may not be a realistic single of the earmuff shell). Drilling the ear muff allows noise control option. Helmets are often preferred gas to move freely between the inside and outside of the ear muff, preventing any pressure differential for many diving tasks as they provide physical and the associated risk of barotrauma. head protection. Conventional ear-muff hearing protectors cannot be used within current diving helmets, as they 6.4.3. Active noise reduction (ANR) simply will not fit within the space available. During ANR can be used in communications system ear- the helmet noise trial conducted by Evans et al. pieces to reduce the background noise transmitted (2007), earplug hearing protection was successfully by diver communication systems, in order to reduce used and worn within diving helmets; the system overall noise levels generated during use of com- also allowed viable audio communication. The munications. This method has been successfully im- Emtec hearing protectors used (Fig 9) provided plemented to improve the intelligibility and noise attenuation ranging from 13.9dB at 63Hz, to 41.1dB reduction of aircrew communications headsets at 4kHz. (Dunlop et al., 1987; Powell et al., 2003). Placing ANR inside diving helmets to reduce environmental 6.7. Health surveillance programme for diver noise has been suggested to avoid the need for noise exposure earplugs, an approach that has also been adopted As part of the CoNaWR05, if a risk assessment in prototype helmet-integrated ANR systems for indicates there is a risk to the health of employees aircrew (Williams et al., 1990). The application of exposed to noise, the employer shall ensure that ANR within vehicle cabins could also be applied to such employees are placed under suitable health diving environments and transport to reduce diver surveillance. Given the potentially high levels of noise exposure. noise to which divers are exposed, management of noise exposure risk for divers should include es- 6.5. Reducing time of exposure to noise tablishing a comprehensive health surveillance pro- When all practical measures have been undertaken gramme. This involves the following (HSE, 2008): to reduce the source of the noise, noise exposure • Providing regular hearing checks in controlled may be controlled by administrative means, such as conditions limiting the time of exposure. This principle may • Telling employees about the results of their also be applied to voice communications, e.g. lim- hearing checks iting communications with the diver. However, as • Keeping health records communications are an essential safety and work • Ensuring divers are examined by a doctor when function requirement, this is not a viable option. hearing damage has been identified.

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Fig 9: Emtec earplug hearing protectors fitted to a cutaway anatomical model and a human ear

7. Summary helmets) are ‘worst case’, requiring use of the The audiometric studies reviewed indicate that A-weighted scale to assess the hazard. diver hearing is impaired by exposure to factors Hearing sensitivity in hyperbaric environments associated with diving. Furthermore, several studies and gases other than air is similar to hearing in also suggest that divers’ hearing deteriorates faster air at normobaric pressure. Thus the noise hazard than that of non-divers, i.e. the age-related deficit is associated with hyperbaric and mixed-gas diving increased. The hearing deficits in divers are likely to may be calculated using the A-weighted scale and be due to the combined effects of noise, ear disease the exposure levels in the CoNaWR05 applied and pressure, including barotraumas and DCI. It without modification. However, for continuous is, however, difficult to separate out the individual hyperbaric noise exposure (i.e. saturation diving), influences of these factors, thus attributing the an average noise level of 73dB(A) re. 20µPa will hearing deficits to noise exposure on the basis of provide a noise dose at the lower exposure action the audiometric studies is not possible. value of the CoNaWR05. Commercial divers are routinely exposed to Compliance with CoNaWR05 requires the cal- a range of noise sources of sufficiently high culation of divers’ total daily or weekly dose to intensity to cause auditory damage. These sources take into account all activities above and below include dive site noise, self-generated breathing water. Manufacturers of diving equipment and noise, underwater tool noise and compression employers of divers have a joint responsibility to chamber noise. Self-generated breathing noise and ensure compliance with the exposure values in the communications are major contributors to divers’ CoNaWR05. Manufacturers should supply technical noise exposure when wearing diving helmets. data specifying the noise output of their equipment. In relating the noise exposure of divers to any In reducing the noise levels of their equipment, hearing loss, it is necessary to consider the nature employers are required to demonstrate that the of human hearing under water. The sensitivity of risk of noise hazard is ALARP, in line with the hearing is reduced in water compared with air, requirements of CoNaWR05. However, noise is and so underwater sound will produce less hearing only one hazard to a diver, and a balanced risk damage than airborne sound. Assessing the noise assessment must be applied to the whole diving hazard under water requires an adjustment to operation, as fully mitigating against one risk may the exposure values specified by the CoNaWR05, exacerbate others. and this review has recommended the use of an Current noise control measures for divers are underwater weighting (UW) scale to achieve this. inadequate; additional control measures are re- It should be noted that the reduction in hearing quired to reduce the noise hazard to within sensitivity under water only applies to divers with the occupational exposure values. A diver noise a ‘wet’ ear (i.e. with water in contact with the reduction strategy should employ the following head and in the auditory canal), such as SCUBA hierarchy: divers and band-mask divers. Divers wearing hoods are also further protected by the sound-absorptive • Eliminate or reduce noise at source, e.g. by properties of neoprene (approximately 5–15dB at redesigning the equipment generating noise the surface, reducing with depth). In terms of • Provide noise attenuation at the diver’s head/ear, hazard, divers with a ‘dry’ ear (i.e. wearing diving e.g. by noise insulating materials or ANR

37 Anthony et al. Review of diver noise exposure

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