ROSSLYN, Va., Sept. 11, 1998 - Imagine an invisible hearing aid that never squeals with feedback and digitally enhances speech while silencing background noise.
Such a device is under development and has been tested in animals with encouraging results. Jonathan Spindel, Ph.D., a biomedical engineer and assistant professor at the University of Virginia's Department of Otolaryngology, is preparing to take the next step toward developing a fully implanted prototype for humans. "Our tests to date have shown that the signals produced with our magnetic hearing device are very nearly those of natural acoustic sound," said Spindel.
The unique device would capture sounds with a miniature microphone implanted in the ear. After passing through a small processing unit and an electromagnetic coil, both also implanted, amplified vibrations would be sent to the inner ear via a tiny magnet attached to the inner ear's round window, a thin membrane at one end of the cochlea.
About as large as a pencil point, the tiny magnet would send vibrations through the cochlea, the fluid-filled organ shaped like a snail shell, and stimulate its thousands of hair cells used in normal hearing. The new device is the first to use an electromagnet to stimulate the inner ear via the round window.
A major feature of Spindel's approach is that the device doesn't obstruct the normal hearing process. "Leaving the middle ear system intact and establishing a second independent input pathway to the inner ear opens the possibility for using the normal acoustic pathway and round window electromagnet simultaneously to establish constructive and destructive sound patterns in the inner ear," said Spindel.
The device could enhance the sound of a person's voice, for example, by generating sound waves matching those of the voice as it reaches the ear. What ultimately reaches the brain and what the user actually hears is the net effect of combining the natural sound patterns with those generated by the magnetic hearing device. In this way, the sound waves from the device amplify the desired sound.
To reduce background noise, however, the device simply generates a sound pattern that mirrors the pattern of the undesirable sound. In this case, when coupled with the natural sound pattern, the net effect the user actually hears is little or no sound at all; each sound wave is the opposite of the other and they cancel each other out.
Spindel said he envisions a human prototype with a separate external control device used to tune in the desired frequencies and tune out unwanted sounds and noise. Changes in the settings would be relayed to the fully implanted hearing device remotely. Further development depends on future funding, said Spindel.
Another advantage of the new magnetic hearing device is eliminating acoustic feedback, or the high-pitched "squeal." "Conventional hearing aids are essentially a microphone, an amplifier and a speaker, very similar to the sound systems at concerts or a public speech," said Spindel. And like those systems, he added, acoustic hearing aids are susceptible to feedback. With feedback, the microphone picks up much of the sound produced from the speaker and feeds it back through the system in a repeating loop, causing a high-pitched squeal. But hearing aids are much more susceptible "because the components of the system are so incredibly close together," said Spindel.
To overcome this, modern acoustic hearing aids rely on a very tight fit to ensure that the sounds produced on one end don't reach the microphone on the other end. But even modern, custom-fitted acoustic hearing aids can loosen with activity and become susceptible to the "squeal", said Spindel.
Because the new hearing device uses magnetic rather than acoustic vibrations, feedback is virtually eliminated.
Materials provided by The Whitaker Foundation. Note: Content may be edited for style and length.
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