Page 28 - Spring2022

UNDERWATER HEARING IN HUMANS
Figure 4. NSMRL head simulant (Ned Stark; left) used for testing sound transmission in the Kirby Morgan 37 dive helmet (right).
is on average around three times poorer than comparable studies conducted in air.
More directional hearing studies were conducted by a French team led primarily by Sophie Savel (Savel and Drake, 2014). They found that lower frequency sounds and white noise were easier to discriminate than higher frequencies. Divers were able to identify angles to the left and right successfully but had severe challenges with front/back discrimination. They did find that divers in general were more successful at all localization studies with experience, something that Feinstein (1973) also noticed. This included experience and training with the experiment cues as well as general diving experience (i.e., total number of career dives).
Interestingly, in one of their studies, Savel et al. (2009) had divers wear neoprene hoods with holes cut around the ears. They also plugged the ear canal with homemade neoprene ear plugs. When the ears were plugged with neoprene, the divers’ ability to localize sound dropped significantly, suggesting that the ear conduction pathway could play a role in sound localization underwater. The authors postulated that a phase difference at the cochlea between the arriving direct inner ear stimulated sound and ear conducted sound could provide some directional cue. This hypothesis needs further investigation, but Savel et al. are not the first to notice a drop in localization capabilities when the ear canals are blocked by neoprene (Norman et al., 1971).
Underwater Noise Exposure and
Hearing Conservation
There are many sources of underwater biological sounds ranging from marine mammals to fishes and inverte- brates, although there has been no record of any of these sounds being of obvious concern to human hearing. Rather, anthropogenic or human-made sounds under- water are the primary sources of concern.
There is one kind of noise that divers cannot avoid: the sound of their own breathing. The bubbles produced during respiration in Scuba and surface-supplied air are quite noisy. Indeed, this is the reason why divers are required to breath-hold during the hearing tests. The bubble noise is not dangerous to humans, but it is not quiet. This was one of the reasons for the development of the rebreathing system.
Breathing noise is also a concern in helmeted divers. The NSMRL and others in the US Navy (Curley and Knafelc, 1987) have documented that the sound levels measured during inhalation and exhalation in the standard Kirby Morgan dive helmets (Figure 4) often exceed the tradi- tional 85 dBA hearing safety limits. These helmets are the current standard for working divers. Divers also use a valve to blow air into the helmet to defog the faceplate, which greatly exceeds the limits. An additive effect is cre- ated by the communication system within the helmet. Divers must often turn the sound level up to effectively hear and communicate with people on the surface.
Looking beyond diver-produced noise, there are many external anthropogenic sources of sound that could potentially impact divers (e.g., underwater explosions, tool noise, pile driving, sonar, or boat noise). Any of these sources could generate high levels of acoustic energy. Knowing that hearing underwater is different than in air, how does one determine what is safe or unsafe in terms of human exposure? This is where the problem gets challenging!
We now assume for the sake of a discussion that the divers have wet ears (i.e., nonhelmeted). The two primary challenges for providing safety guidance underwater are the lack of personal protective equipment and the dif- ferences in underwater hearing abilities compared with in-air hearing.
Let’s start by talking about the types of protection that exist. Wearing earplugs or any kind of over-ear sound
28 Acoustics Today • Spring 2022