New Device May Protect Ears from Listener Fatigue

May 13 2011 - 9:42am

Listener fatigue results from being subjected to a continuous barrage of sounds of varying frequency and volume. It can result in discomfort and pain for some people when using in-ear headphones, hearing aids, and other devices that seal the ear canal from external sound.

The prevention is not always to not use the device, especially if it’s due to a hearing aid. Turning down the volume is always recommended for in-ear headphones to help prevent injury to the ear and hearing, but some listeners will experience pain and discomfort from use of in-ear devices even at low sound volume.

Sound engineers may have found the cause of listener fatigue due to in-ear devices and a potential solution.

In two separate papers and a presentation at the 130th Audio Engineering Society convention in London on May 14th, 2011, Stephen Ambrose, Robert Schulein and Samuel Gido of Asius Technologies of Longmont, Colo., describe how sealing a speaker in the ear canal dramatically boosts sound pressures and how a modified ear-tip can help alleviate, or even eliminate, that effect.

Using physical and computational models, the researchers show that sound waves entering a sealed ear canal create an oscillating pressure chamber that can produce a potentially dramatic boost in sound pressure levels. This boost appears to trigger the acoustic reflex which is a defense mechanism in the ear.

The acoustic reflex can dampen the transfer of sound energy from the eardrum to the cochlea by as much as 50 decibels. The acoustic reflex does not protect the ear drum from the excessive shaking that can occur with the oscillating pressures.

"Paradoxically, the protective reflex makes loud volumes seem lower than they really are," adds Gido, "potentially prompting the listener to turn up the volume even more."

The resulting physical strain of the oscillations in the pressure chamber and the repeated activation of the tiny muscles involved in the acoustic reflex are what the researchers believe may lead to listener fatigue.

Ambrose and his colleagues developed a way to counter the oscillations by using a membrane outside the ear drum to take the brunt of all the pounding. This "sacrificial membrane" disrupts the excessive pressure waves, protecting the ear drum and preventing the triggering of the acoustic reflex, ultimately leading to lower, safer listening volumes.

The papers describe two techniques for introducing the new technology. The simplest involves a retrofit that can be applied to existing in-ear headphones. This retrofit builds upon earlier studies of hearing aids. Audiologists have, in years past, drilled small holes to alleviate the pressure. However, the holes also led to squealing feedback effects and diminished sound quality.

Ambrose discovered that stretching a thin film of medical-grade polymer over the pressure-alleviating hole reseals the environment, yet provides a sacrificial membrane to absorb the abusive pressures that impact the users of many headphones, hearing aids and other devices.

The second technique for greater sound pressure reduction and potentially improved sound quality involves a more advanced corrective device developed by Asius. This small, inflatable seal device, Ambrose Diaphonic Ear Lens (ADELTM), looks like a tiny ear-sealing balloon and uses a novel, miniaturized technology called an Asius Diaphonic PumpTM to inflate the polymer membrane.


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The pump converts the alternating, compression-expansion waves of sound into a direct-flowing stream of molecules, filling the membrane using only minimal energy from the headphone speaker. The pump has enough force to both inflate the ear lens and keep the device comfortably in the ear canal for as long as the device is worn.

"The lens maintains desirable audio fidelity, especially at bass frequencies, and prevents feedback," says Gido. "The flexible membrane vibrates with the oscillating sound pressure in the sealed ear canal and radiates excess sound energy out of the closed space in front of the ear drum. In a sense, the flexible polymer membrane behaves like a second ear drum, which is more compliant than the real ear drum, allowing it to direct excess sound energy away from the sensitive structures of the ear."

The pump takes advantage of a physical property called a synthetic jet, a column of fluid that erupts when an acoustic wave passes through a small hole.

"As sound waves pass through any given small hole, the alternating pulses emerge and retract through the orifice like a small air-piston, hitting and knocking the surrounding air molecules forward like billiard balls," says Ambrose. "Other molecules join in the stream from the sides due to the low pressure created by the flow. This results in a sustained jet of air."

By integrating an inward flowing jet into the side of the sound port, Asius transformed a standard synthetic jet into a real pump capable of harvesting and storing inflation and deflation pressures.


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