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protection has little value. First, earplugs block the ear canals that can compromise the diver’s ability to equalize the pressure in the middle ear as they move up and down in the water. More importantly, if humans detect sound through direct inner ear stimulation, then blocking the traditional air conduction pathway is not an effective means for preventing noise-induced hearing damage.
Currently, the only effective method of hearing protection underwater is the aforementioned neoprene hood. Numerous studies (reviewed by Fothergill et al., 2018) have characterized the effectiveness of a neoprene hood at increasing hearing thresholds in divers. As mentioned in Underwater Hearing Thresholds, a hood is effective at attenuating frequencies at 500 Hz and above, with the amount of attenuation increas- ing with frequency (as much as 20-30 dB of attenuation), although some of these effects are reduced with increases in pressure (Fothergill et al., 2018).
Therefore, divers exposed to certain SONAR systems or any tool that produces a lot of high-frequency energy can be protected from hearing damage by wearing a neo- prene hood. However, just about all the noise sources that were mentioned create broadband signals with a lot of low-frequency energy below 500 Hz in addition to high frequencies. Thus, a hood provides little-to-no protection from much of the acoustic energy from many underwater tools, explosives, pile driving, and boat noise.
Moreover, in many operations with underwater tools, divers wear helmets so that they can have dry ears. The NSMRL recently completed a data collection to explore the energy transfer function of the Kirby Morgan dive helmets (Figure 4). The measurements show that fre- quencies down to at least 50 Hz are attenuated by the helmet. Again, attenuation increases with frequency, and in the case of the helmet, there is a dip in attenuation at the resonance frequency of the helmet. Although the helmet provides more attenuation than the hood, the sound is being delivered via the more sensitive airborne mechanism of hearing. Therefore, the net effect at most frequencies is that the recommended exposure limits with a helmet will be lower than with a hood. This is especially true at lower frequencies. Thus, divers have few options for underwater hearing protection. Safety guidance for divers exposed to underwater noise must therefore account for the limitations in personal protec- tion equipment.
The in-air community has a wealth of human and animal studies that determined the upper limit of exposures that would induce hearing damage, such as temporary thresh- old shifts (TTSs) and permanent threshold shifts (PTSs) in hearing (reviewed by Clark 1991; Melnick 1991). TTS is defined as a temporary loss of hearing sensitivity after exposure to sound. Hearing conservation standards for in-air noise consider the onset of TTS as defining the upper limit of safe noise exposure. PTS is a shift in hear- ing sensitivity at a frequency or range of frequencies that does not resolve with time.
Unfortunately, data for the underwater onset of the TTS are extremely sparse. Only a few studies have been conducted on this topic, and the results, although incredibly valuable, are challenging to interpret due to the small sample size, high variance among divers, and challenges associated with measuring hearing immediately postdiving (reviewed by Smith et al., 1988). Additional studies later conducted by investigators at the NSMRL and in the United Kingdom attempted to measure diver aversion to low frequencies up to 2,500 Hz (reviewed by Fothergill et al., 2002).
To establish an international hearing conservation limit for divers, the United Kingdom and NSMRL worked together in the early 2000s to merge the extensive United Kingdom hearing threshold data with the underwater TTS and aversion data. Most of these studies and the underwater noise guidance for divers are not publicly available so they cannot be discussed in any detail. How- ever, suffice it to say that this guidance is used consistently and has proven effective in protecting divers.
Inadditiontoprovidingguidancefornoiseexposurerelated to hearing concerns, the NSMRL also works with organiza- tions that are involved with underwater explosives (UNDEX).
These communities are typically concerned with injuries to the lungs and other air-filled structures. There is established safe standoff guidance for underwater blasts, and the NSMRL continues to explore how to improve on and expand the guidance. Obviously, investigators cannot knowingly expose divers to the UNDEX to establish injury data, so instead physical model simulants (Figure 5) have been developed to better understand the injury mechanisms associated with blast exposure. Another entire article could be written on underwater blast injuries and the research associated with the protection of divers so, for now, we direct readers to Cudahy and Parvin (2001) as an excellent primer on the topic.
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