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. 21 Anthony et al. Review of diver noise exposure 100 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 22 Vol 29, No 1, 2010 160 µ 140 120 100 80 60 40 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.

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