Advances in Hearing Aid Technology
In the pre-digital (analog) age, hearing aids were essentially passive, one-channel amplifiers of sound. Although some frequency-shaping and output limiting was possible, incoming sound was amplified essentially the same amount whether that sound was soft or loud, low-pitched or high-pitched, speech or noise.
The digital hearing aid changed all that. Early digital instruments divided sound into two channels (instead of one) and provided non-linear amplification. These new circuits could provide amplification that varied as a function of the intensity and frequency of the signal and the circuit program could be custom-designed for the individual user.
Thus, a hearing aid could be programmed to provide less amplification of low-pitched and more amplification of high-pitched sound. It could also be programmed to amplify soft sounds by, say, 28 decibels (dB), but amplify loud sounds only 12 dB (or not at all).
As microchips became smaller and faster, engineers, software programmers, hearing scientists and audiologists worked to take advantage of the increasingly powerful processing capabilities of digital processors. Some of these advances are refinements of older digital technologies, while others are entirely new approaches.
For example, one of the important improvements of the early non-linear hearing aids was the ability to divide the sound signal into two channels. This provided a closer match of the amount of amplification (gain) needed for the user’s hearing loss in that frequency range. Today, digital hearing aids use up to 14 channels to match gain to the user’s hearing loss pattern more precisely.
Compression is the technique of automatically reducing amplification as sound gets louder. This can be important because individuals with sensorineural hearing loss are often hypersensitive to loud sounds. The loudness hypersensitivity often varies with frequency.
Early digital hearing aids offered improved compression, but often the amount of compression was fixed. Today’s digital hearing aids provide variable compression that can be customized for each individual’s sensitivity to loud sounds. The newer digital hearing aids can be programmed to provide different amounts of compression for different frequency bands, providing a wider range of comfort to the user.
Directional microphones provide more amplification of sounds coming from the front and less amplification of sounds from the back. Hearing aids are now using digital technology to control directional effects more precisely. Clinical tests have yielded scores for word recognition tasks that are 10-15% better than with conventional microphones. These benefits can be especially helpful in noisy settings such as restaurants.
Noise suppression uses the microprocessor’s capabilities and software programming to detect incoming sound that is relatively constant (typical of noise) and then reduces the amplification in that frequency range. The result can be improved comfort and word understanding in noisy settings. It’s important to note that these noise suppression circuits, while effective, do not eliminate the presence of background noise entirely, and some noises-such as sudden impact noise-are not suppressed.
Speech recognition circuits use the power of digital technology to detect and then provide additional amplification when speech is present. Speech is not random. That is, many speech sounds (especially vowels) have a pattern. A detector can be programmed to respond to the presence of a speech pattern and amplify that frequency range. Initial investigations trials have shown positive results for this approach.
Feedback is the whistle that can be caused by sound leakage from the hearing aid. Feedback reduction programs use digital technology to detect the presence of feedback, as well as determine the frequency causing the feedback. The hearing aid can then reduce the intensity of that frequency, or introduce phase-shifting to cancel or reduce the feedback.
Because a digital hearing aid can filter frequencies so effectively, feedback can be controlled or virtually eliminated today, even for individuals with severe hearing loss.
If the 1990’s saw the introduction of a radical new technology, the 21st century has seen the development of new ways to apply this new technology. We can expect these advances to continue as scientists, engineers and audiologists learn more about what people with hearing loss need and what technology can do.
Courtesy of Deborah Touchette, Au.D., CCC-A, FAAA
Doctor of Audiology
Paradise Hearing & Balance Center