Humans possess the ability to hear far higher pitched sounds when underwater than they can while on terra firma. How can they do it? By "hearing" with their bones rather than through the normal pathways of hearing, US Navy researchers report.
The way in which humans hear above or below water differs; that difference means they only hear between 20 and 20,000 hertz through the air, while they can catch sounds all the way up to 200,000 hertz when submerged.
On land, humans hear through air conduction. Sound pressure waves cause tiny disturbances in the air that travel into the ear canal and vibrate the ear drum, which is connected to the three smallest bones in the body, the ossicles of the middle ear.
The ossicles are connected to the cochlea, which is filled with fluid and contains "hair cells," or tiny protrusions that also move, stimulating the auditory nerve, which sends signals to the brain.
How humans hear
"Human hearing is magnificent," said Bill Martin, a hearing scientist at the Oregon Hearing Research Center in Portland. "The ear drum only has to move less than the diameter of a hydrogen atom for us to perceive sound."
The cochlea, too, is complex. Similar to a rolled up piano keyboard, portions of the cochlea are responsible for hearing specific frequencies that help humans differentiate between, say, a bird tweeting and a garbage truck.
But underwater, humans don't hear using the normal channels. Instead, the study found that humans hear through bone conduction, which bypasses the outer ear and the ossicles of the middle ear.
Instead, sound comes through the mastoid, or the bone you can feel if you put your fingers behind the ear.
"By using bone conduction, the human ear can actually receive sounds at frequencies way higher than most people would have expected," Martin explained.
While experts aren't sure yet how that might translate into practical use, scientists say the finding provides a tantalizing glimpse into human's evolutionary past, as well as clues that may help in the development of new types of hearing aids.
In the study, researchers from the Naval Submarine Medical Research Lab in Groton, Conneticut, tested the range of hearing for submerged participants in a very quiet pond on a military facility.
"Frequencies that are too high to hear are called ultrasounds," said Michael Qin, principal investigator and a senior research scientist. "What made this study very interesting was that in the underwater case, divers could hear frequencies beyond 20 kilohertz (20,000 hertz) and into the range traditionally called ultrasound, which is outside what was thought to be the range of human hearing."
This residual, typically unused ability to hear higher frequencies probably has something to do with evolution, Martin explained.
Humans hear best between about 500 to 3,000 hertz, which just so happens to correspond to the range of human speech. For perspective: middle C on a piano is 256 hertz, while the highest key is about 4,000 hertz.
"The middle ear, in fact, works as a filters to block out frequencies so that we don't hear the things that moles and elephants hear, and it blocks out very, very high frequencies so we don't hear what bats and dogs hear," Martin said. "We are in the middle, which is absolutely perfectly matched to the sounds our vocal tract produces."
Findings give hope for future
Qin and his colleagues are also studying underwater hearing and low frequencies. The research could have implications for Navy divers, including occupational health issues such as safety when using underwater equipment.
"It also opens up all sorts of things for aided listening devices," he said.
A conventional hearing aid uses air conduction to amplify the sound that travels through the ear canal and the ear drum. There are also hearing aids that use bone conduction, which are sometimes used in people who can't use a normal hearing aid because of abnormalities of the outer or middle ear, Martin said.
Translational hearing aids are another type of device that attempt to harness some of the residual function of the cochlea to hear higher frequencies, Martin said. (Prior research has shown that people have some hearing ability to 40,000 and even 100,000 hertz, but this is the first study to show people may be able to hear all the way up to 200,000 hertz underwater, he added.)
"The cochlea is much more capable than we ever use it for, and there may be a whole realm of prosthetic devices that could come out of this," he said.
Because this study was presented at a meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
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