National Geographic News
A diver at Ginnie Springs, Florida.
Divers can hear better than people on land (file picture).

Photograph by Wes C. Skiles, National Geographic

Ker Than

for National Geographic News

Published May 16, 2011

People may one day be able to hear what are now inaudible sounds, scientists say.

New experiments suggest that just vibrating the ear bones could create shortcuts for sounds to enter the brain, thus boosting hearing.

Most people can hear sounds in the range of about 20 hertz (Hz) at the low end to about 20 kilohertz (kHz) at the high end.

Twenty kHz would sound like a very high-pitched mosquito buzz, and 20 Hz would be what you'd hear if "you were at an R&B concert and you just stood next to the bass," explained Michael Qin, a senior research scientist at the Naval Submarine Medical Research Laboratory in Connecticut.

"It would be the thing that's moving your pants leg."

(See "Ultrasonic Frog Tunes Its Ears Like a Radio Dial.")

Under certain circumstances, humans can also hear frequencies outside of this normal range. For instance, divers underwater can detect sounds of up to a hundred kHz, according to Qin's recent experiments.

It's unclear why the divers have enhanced hearing underwater, but it may be because the sounds travel directly through the bones to the brain, he said.

Vibrating Ear Bones Boost Hearing

In normal hearing, sound waves traveling through the air or water enter our ear canals and strike our eardrums, causing them to vibrate. Our eardrums are connected to three tiny, connected bones called the malleus, incus, and stapes—popularly known as the hammer, anvil, and stirrup, due to their shapes.

(Explore the human body.)

As the stapes bone rocks back and forth, it pushes against a fluid-filled structure called the cochlea in the inner ear. Resembling a tiny snail, the cochlea contains tiny hair-like structures that translate the pressure waves in the jostling fluid into nerve signals that are sent to the brain and interpreted as sounds.

"If you think of the hearing system as one long chain of events, there are multiple places in which bone conduction or underwater hearing can bypass that chain," he said.

For example, bone conduction occurs when very high-frequency sounds directly stimulate the ear bones, sending signals to the brain without activating the eardrums. This is how some species of whales hear underwater.

(Also see "Fossils Show How Whales Evolved to Hear Underwater.")

"The core of our work is trying to understand underwater hearing and bone-conduction hearing, and to determine if they share the same underlying mechanism," he said.

Alternatively, certain ultrasonic frequencies might stimulate the fluid in the cochlea.

"It could be like hitting a wrench against a water tank," Qin explained. "The fluid itself could go into oscillation."

Superhuman Hearing Devices on the Horizon?

Qin and his team are now exploring which bones are most likely to be most sensitive to ultrasonic vibrations.

Could such research lead to devices that give us superhuman hearing or improved hearing aids? Qin is mum for now.

"That's the great thing about basic science, right? It lets you know how things work, and you can bend it to many applications."

Qin and his team will present their research at the annual Acoustical Society of America meeting in Seattle in late May.

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