Page 24 - Spring2022
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UNDERWATER HEARING IN HUMANS
they adapted to the terrestrial environment. Let’s “dive” deeper into what is going on between these two different environments and their effects on these auditory systems.
The most important differences between air and water in this context are their relative density and compressibility that, when combined, define the acoustic impedance of these two fluids. The acoustic impedance of the human head is very similar to that of water, which is unsurpris- ing because most human soft tissues are close to 80% water. When surrounded by air, the high acoustic imped- ance of our heads reflects most sound energy, whereas underwater, sound travels through our heads instead of being reflected off them. Unfortunately, this removes the ability of the outer ear to “catch” and focus sound onto the tympanic membrane (eardrum).
Furthermore, the tympanic membrane and ossicles (middle ear bones) normally match the acoustic impedance of air-conducted sound and transmit the vibration to the fluid-filled cochlea. When stimulated via this sound path, the ossicular vibration produces a displacement wave in the fluid of the cochlea. Underwater, this traditional pathway is ineffective because sound energy transmission would have to travel from water (ear canal) to air (middle ear) and back to fluid (inner ear). Instead, sound energy is conducted through the skull directly to the ossicles and cochlea.
Like the human head underwater, the minnow’s body is also “acoustically transparent.” Fish ears have dense oto- liths in contact with the sensory hair cells of the auditory region of the ear. As sound travels through the minnow’s body, there is a relative lag between the motion of the dense otolith and surrounding tissues. This results in the ciliary bundles of the sensory cells being “bent” and there- fore stimulated, allowing the minnow to hear the sound.
Humans do not have otoliths. Without the sound energy being transmitted through the traditional lower imped- ance pathway, the displacements produced in the cochlea of the human inner ear are much smaller than the sen- sory organ had evolved to detect. Smaller displacements mean less stimulation of the sensory hair cells and reduced hearing sensitivity, as discussed in Underwater Hearing Thresholds.
Preliminary evidence for this underwater acoustic pathway came from studies by Wainwright (1958) who had divers
plug up their ears with their fingers. The divers were still able to detect sounds, although it was later pointed out that the bones in the fingers could still be transmitting the sound to the cochlea and the tissue of the finger would also be acoustically transparent (Smith, 1969).
Later, Hollien and Brandt (1969) had divers wear ear plugs underwater. Interestingly, the investigators had the divers put the ear plugs in prior to submersion, thereby trapping the air within the ear canal. In theory, this would eliminate the impedance mismatch around the tympanic membrane, which it did, but it ultimately just moved the mismatch of the air/water interface to the location of the ear plugs. Regardless, hearing thresholds were no different between tests with and without ear plugs, supporting the direct inner ear stimulation hypothesis.
Further evidence for direct inner ear stimulation comes from a study by Smith (1969). Smith tested underwater hearing thresholds in divers with known impaired in-air hearing but normal in-air bone conduction thresholds. In-air bone conduction hearing bypasses the outer and middle ear, so Smith was comparing whether air-con- ducted or bone-conducted thresholds better predicted the divers’ underwater thresholds. The results from the underwater testing revealed no evidence of raised under- water hearing thresholds regardless of the divers’ in-air hearing thresholds.
Hollien and Feinstein (1975) then tested diver hearing with three scenarios: (1) bare headed, (2) wearing a neo- prene dive hood, and (3) wearing a neoprene hood with rubber tubes inserted into the ear canal through holes in the hood. As discussed in Underwater Noise Expo- sure and Hearing Conservation, neoprene is an effective blocker of sound transmission, especially at frequencies above 500 Hz. In the Hollien and Feinstein study, the divers’ hearing thresholds were significantly higher in scenarios 2 and 3 where the hood reduced direct inner ear stimulation and tubes to the ear canal had no effect on hearing thresholds.
In summary, humans can hear underwater but not through the traditional in-air hearing pathway. There is evidence that sound transmission underwater to the cochlea is occurring directly through the skull, but what kind of impact does this have on human hearing sensitivity?
24 Acoustics Today • Spring 2022






















































































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