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Dr. Ross on Hearing Loss

Digital Hearing Aids: Magic and Marketing?

by Mark Ross, Ph.D.
This article first appeared in the
Hearing Journal (March 2000)

It seems to me, as I examine the marketing literature and testimonials regarding digital hearing aids, that it's time for a bit of corrective skepticism. I keep reading and hearing about how people are just thrilled with their new "digital hearing aids", comments by dispensers how they've had "better results with X digital hearing aid" than any other since they've been dispensing, and claims by industry that this new technology offers unprecedented listening benefits. An increasing number of full-page ads in newspapers and magazines, as well as commercials on the radio and the TV, are currently extolling the superiority of digital hearing aids. People just don't buy "hearing aids" any more; they purchase "digital" hearing aids. To mix a few cliché's, the bandwagon is rolling and nobody is commenting that the Emperor may not be very well attired.

As applied to hearing aids, the term "digital" only refers to a way of coding the acoustic input information, i.e. digital signal processing (DSP). And, judging from an article that appeared in the March 2000 issue of The Hearing Journal, this may be the only factor that the many digital hearing aids have in common. In this article, Gus Mueller interviewed representatives from seven leading hearing aid companies, asking them about specific features that might be unique to their product. The responses of the representatives - all very competent and respected professionals - were very informative but also very different.

While all the hearing aids they describe are digital, this begs the question of where the presumed "digital" benefits lie. Are we, in other words, to assume that there is some magic inhering in the fact that all of these aids utilize DSP regardless of how they go about it? Does digital processing per se produce superior speech perception scores, independent of the diverse speech processing strategies and architectures that different digital hearing aids employ? If so, this would imply that all current digital hearing aids, regardless of possible performance differences between them, would yield results superior to that obtained by the best analog hearing aid. It also assumes that the specific speech processing strategy used to modify speech signals is basically irrelevant. Stated in this fashion, I do not think that many of us would accept these assumptions.

Where "digital" hearing aids are clearly superior are in the connotations of the term, in how beautifully it lends itself to current marketing appeals. There is no doubt that the term has resonance, associated as it is with modernity, the power of computers, all the impressive technical developments of the 21st century. People apply the term "digital" in a magical, almost reverential manner, believing that they are describing the cutting edge of current technology and, of course, they are: digital hearing aids are truly technological wonders. But the question still remains: Do all of these hearing aids actually produce superior listening performance only by virtue of the fact they employ DSP?

Presumably, it is in the creative ways that digital processing is able to modify and deliver speech signals that produce listening superiority. However, instead of clinical research evidence in support this assumption, superiority is simply assumed on the basic of the technical descriptions of the hearing aids. As one reads the descriptions of their circuitry, it is difficult not to be impressed with them, they all seem so logical and are described so appealingly. Below, I've quoted or paraphrased just some of these descriptions and terms applied to advertised digital hearing aids as found in just one recent issue of a trade journal.

  • Digital (or adaptive) feedback suppression
  • Digital feedback management algorithm
  • Noise reduction by differentiating between noise and speech
  • Cochlear dynamics sound processing with 14 overlapping bands
  • Digital cochlear dynamics
  • Speech intensification system
  • Speech enhancement algorithm
  • Enhanced sound stabilizer
  • Enhanced speech intelligibility through sensitive voice processing
  • Loudness and dynamic range correction
  • ConTrast, artificial intelligence system enhancing critical speech sounds
  • Elaborate (as quoted) input and output multi-channel compressions system
  • Adjustable kneepoints, ratios, and selectable time constants
  • Six (or seven) band, three channel
  • Three channel AGC-I with syllabic compression, with broadband AGC-O
  • Digital processing calculates and processes a virtual sound match for users
  • Microphone noise reduction
  • VoiceSync a comprehensive system that manages speech
  • Digital perception processing
  • Fine scale noise canceler
  • Adaptive digital audio zoom
  • Auto select multi-memory system
  • Lowest compression threshold
  • Dynamic speech re-coding

In listing these terms, I do not mean to disparage or minimize the very real technical accomplishments they represent. Clearly, some first-rate minds and intense research efforts have been engaged in their development. Some of the terms describe familiar concepts (e.g. bands, channels, feedback suppression, compression parameters, etc.), but others are less clear beyond the clear intention that the feature is supposed to somehow improve speech intelligibility. Actually, not being an electrical engineer, I don't expect to understand the specific design features of advanced hearing aid circuitry. Nor am I overly disturbed if I can't follow the detailed psychoacoustic rationale of how the system is supposed to work. What I am able to understand, however, are clinical research projects, studies that compare the actual listening performance of hearing-impaired people with and without specific features. In other words, clinical validation. This is what is sorely lacking.

Technical descriptions, no matter how impressive they sound (pun intended) are not a substitute for clinical studies. As far as I can see, there has been almost no research published in peer-reviewed journals on most of the features unique to digital hearing aids. Instead, what appears to be happening is that the description and complexity of the specific electroacoustic features are themselves being interpreted as evidence of their performance effectiveness. But technical sophistication and creative designs do not, by themselves, translate into improved performance. This has to be directly examined. Indeed, there is the very real possibility that hearing aids are being designed that exceed the capacity of an impaired auditory system to benefit from them. That the many "bells and whistles" now being incorporated in many digital hearing aids are simply adding to the cost, but do not result in improved listening performance.

I am not suggesting that we ignore the detailed testimonials from satisfied clients, or the accumulating clinical experiences of dispensers. These are clearly relevant and very important considerations. I am asserting, however, that these are not enough. Surely, dispensers also require a body of objective research, studies that can help them guide their specific choices regarding a speech processing strategy. Depending upon the subjective impressions of clients, while valuable and clearly necessary, is not in itself sufficient. Dispensers can hardly lay claim to being a scientifically based discipline if only subjective judgments were used in selecting a particular hearing aid or speech processing strategy.

My memory in this field goes sufficiently far back for me to recall the many testimonials made about earlier generations of hearing aids. Every new development has had its champions and successes compared to previous models or technology. How can one possibly compare superlatives? How can one, for example, accept the glowing recommendations of a particular product by one dispenser, while at the same time receiving similar glowing descriptions of a different product by another dispenser? Everybody seems to have their "favorite" fitting, the one they've had their "best luck" with, the one resulting in the "most satisfied" clients they've ever had. Clearly, everybody can't be right. If they're all right, then no one is, since this implies little or no difference between the results that can be obtained with all of the various favorites.

But basically, what I object to is the almost magical manner in which the term "digital" is invoked, as if this itself ensured superiority. We know, from the very creative "hype" studies conducted by Ruth Bentler and her colleagues at the University of Iowa, just how powerful the "power" of suggestion is. When clients were told that one aid was digital and other was analog, they almost always preferred the digital aid even though the exact same digital hearing aid was involved in both trials. The subjects, and even in at least one case report a spouse, reported that they heard better with the "digital" hearing aid. And perhaps they did for a while.

Modern medicine recognizes the powerful mind-body interactions that can occur and may even depend upon it under some circumstances. None of us dispensers and consumers alike are immune from the power of suggestion. If people believe that they are being "treated" with the most advanced "medicine" that modern technology has to offer, and if dispensers reinforce this with similar, sincere beliefs of their own, then positive judgments from consumers should be no surprise. But how far can the power of suggestion take someone? Will the reality of their objective hearing capabilities forever remain in the glow of the term "digital?" Will questions regarding the relative value of different DSP algorithms (and different digital hearing aids) forever remain muted?

I do not question the fact that digital hearing aids are quality hearing instruments. And I also do not doubt some types of amplification strategies are only possible with DSP and may indeed be beneficial. Where I have my personal questions and doubts is, first, the paucity of objective evidence regarding specific speech processing strategies and, second, how the term itself is invoked as a substitute for this evidence. Hearing aid dispensers should be selling demonstrably better hearing, and all this implies, and not just depend upon marketing "hype" to sell their hearing aids for them.

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