What's next in hearing instrument technology?

May 2009

Editor: There has been a lot of technological progress in digital hearing aids in the past few years, but the only development that has been demonstrated to improve speech recognition is the directional microphone. So what kinds of things are in the works for future hearing aids?

Thanks to bhNEWS for this article.

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Q: What's next in hearing instrument technology?

A: As one who has looked skeptically at the claims of digital hearing aids, I now have to admit that the battle is lost. Not that the superiority of these aids has become evident in objective research studies, only that as the industry retools, this type of hearing aid is soon likely to be the only one available. So now, perhaps, it is time to place our focus elsewhere, on specific features and algorithms possible only with digital signal processing (DSP) and not on comparisons with analog hearing aids.

We know that DSP is capable of manipulating acoustic signals in an almost infinite fashion. We can deliver prescribed real ear gain for a large number of independent or semi-independent bands (channels), while varying any one or all of a large number of compression characteristics. We can provide directional hearing with one, two or three microphones, with either fixed or adaptive responses. We can reduce feedback through various means, distinguish between speech-like stimuli and competing sounds, or compress the signal in the frequency domain if we desire. In short, hearing aid engineers have been having a field day, as they creatively play with the possibilities that modern technology offers them.

What is needed, but is rarely seen, is comparative research projects that directly assess the relative merits of various features and signal processing algorithms upon the listening performance of adults who are hard of hearing. For example, we know that directional microphones help, but is there a real-life difference between those with fixed and those with adaptive characteristics? How much gain can be provided before feedback occurs with different feedback reduction systems?

Is there a difference in speech perception scores between two, three, seven, nine, 13, 20 and 24 bands of amplification? Do people with high-frequency hearing losses perform better with an audibility as opposed to a frequency compression approach to a hearing aid fitting? Does a remote microphone (personal FM system) offer realistic listening advantages? And how would such comparisons be affected by a person's communication needs as well as the nature and extent of different hearing losses?

Trying to determine this kind of information is not hopeless. I suggest the example that cochlear implants have been giving us for years. From their inception, all new speech processing strategies introduced by any one company, as well as comparisons between companies, have been accompanied by careful, controlled and longitudinal research. We need to go beyond the term "digital" to explore the specific features and speech processing strategies that these hearing aids can offer us.

(This article was supported, in part, by grant No. H133I980010 from the U.S. Department of Education, National Institute on Disability and Rehabilitation Research, to the Lexington Center in Jackson Heights, NY.)

-Mark Ross

A: To effectively address this question, a brief overview of what was attempted in digital technology is important. The introduction of digital hearing aids was a significant advancement from an engineering and technology standpoint. The clinician's ability to control the hearing aid's features certainly improved with digital technology. However, emphasized features like multichannel wide dynamic range compression (WDRC) and other amplitude compression strategies, dual microphones, and feedback reduction were already available in one form or another in analog instruments. To date, it seems that digital has merely replaced analog.

The key potential of digital technology-the ability to radically manipulate the signal-has not been used effectively to address the complexity of hearing loss and revolutionize the hearing industry.

In the mid 1990s, parallel to the development of digital aids, leading researchers substantiated findings that should have revolutionized the way we select and fit hearing aids. Researchers realized that damage to the inner hair cells could result in "dead" regions in the cochlea, and these regions may be common among individuals with moderate to severe sensorineural hearing loss.1,2 However, amplifying high-frequency dead regions does not improve speech understanding but may impair it.3-5

Researchers also found that dead regions cannot be reliably diagnosed by the audiogram.1,2 Unfortunately, this critical information has not yet found its way into mainstream audiology, nor has it influenced the design philosophy of digital aids.

The only hearing aid technology that does not ignore these findings is proportional frequency compression. Though several proportional frequency compression algorithms have been presented in recent literature,6-8 the only commercially available product with proportional frequency compression is ImpaCt Dynamic Speech Re-Coding (DSR) hearing aids by AVR. The design philosophy behind this technology is driven by the fact that dead regions in the cochlea do not benefit from amplification. If high-frequency hair cells are dead or damaged, speech information critical for intelligibility becomes unusable, even if amplified. Using revolutionary real-time speech analysis and frequency compression signal processing algorithms, ImpaCt "moves" specific high-frequency speech sounds into regions where viable hair cells can recode the critical information.

If hearing instruments are going to truly address the needs of people who are hearing impaired, clinical findings on hair cell viability can no longer be ignored. Researchers like Brian Moore, Chris Halpin and others have defined the problem and proposed methods such as the Threshold-Equalizing Noise (TEN) Test1 to identify dead regions. Combining modern diagnostic tools such as the TEN Test with hearing instruments capable of performing proportional frequency compression stands to be the breakthrough that makes digital more than a mere replacement for analog.