Digital Hearing Aids From the Perspective of One Consumer/Audiologist

Abstract

Recent developments in hearing aids are reviewed in the context of the author's personal experience as an audiologist and as a hearing aid wearer. The need for evidence of benefit specific to digital signal processing in hearing aids is stressed, as well as addressing cost-benefit ratios in view of the high cost of digital hearing aids.

The Marketing of Digital Hearing Aids

Over the past 50 years that I can personally recall, there have been ambitious and hyperbolic marketing appeals about new developments in hearing aids. Even as a professional audiologist, presumably able to separate the hype from the facts, it was, and is, hard for me not to be influenced by the plausible nature of many of these marketing claims. The nature of our economic system demands that all products put their best foot forward in an effort to attract customers. Hearing aids are no exception to this practice. Any and every hearing aid feature is grist for the marketer's mill. Exaggeration is expected; the notion of “caveat emptor” usually excuses marketers from all but the most egregious claims. The fact that hearing aids are designed to alleviate problems in a crucial sensory avenue does not make the rules of commerce any less applicable to hearing aids than to any other product.

There is no doubt that potential hearing aid purchasers respond to these often very convincing appeals. People with a hearing loss desperately want to hear better; most, regardless of how advanced or new their hearing aid is, can relate occasions when they are having specific communication problems. In short, these marketing appeals do not fall on “deaf” ears. On the contrary, they are heard (and responded to) time and time again. We see this pattern with every new hearing aid development, from the monopack (in which the battery and amplifier are located in a single body-worn case) hearing aids, to the introduction of transistors, to the first ear-level hearing aids, to completely-in-the-canal aids, and to a host of technological and cosmetic advances in between. But no appeal, no slogan, no new development has caused the impact that was felt when the first behind-the-ear (BTE) digital hearing aids were introduced.

At first, only 2 companies marketed the all-digital BTE hearing aid (a body version had previously been available for a short time; see Levitt in this issue¹). The response to this new development dwarfed any reaction to any of the previous generations of analog hearing aids. Suddenly, it appeared that hearing aids were being catapulted into the 21st century, into the computer age. It was like comparing a manual typewriter to a modern computer. Now it seemed that hearing aids could also benefit from the almost exponential growth of advances in digital signal processing (DSP), advances that currently affect every facet of our lives. Now it seemed that people with hearing losses could also be the beneficiaries of the scientific revolution characterizing our age. Even if the reactions to their advent were sometimes exaggerated, it was still undeniable that these aids were indeed an impressive technical achievement.

The introduction of digital hearing aids did not go unnoticed by the mass media (always looking for human interest stories). In 1997, the New York Times and the Wall Street Journal were among the major papers that featured articles on this new development designed for people with hearing loss. Case examples were included in which the use of these aids had an almost transforming effect on the users’ lives. These stories were picked up and amplified by many other media outlets as well. The buzz at every meeting of hard-of-hearing folks that I attended during this period focused on this marvelous new development. Stories were told and examples given of the almost miraculous difference these new digital hearing aids made for some people. Reports included testimonials from people who claimed that they were hearing better than they had in years, that now they could understand speech in noise (something they could not do previously), and that the resulting sound quality was unsurpassed.

In sum, there seemed to be a general feeling among consumers that the hearing aid industry had finally created a state-of-the-art hearing aid, one that incorporated the progress made in the scientific revolution taking place in recent years. Performance expectations soared; people could not wait to try one of these miraculous devices. But they just did not buy hearing aids any more: they purchased digital hearing aids. The term itself was employed in a magical, almost reverential manner. People believed— rightly—that they were buying into the cutting edge of the latest technology available. They assumed that these hearing aids would greatly diminish, if not eliminate, their hearing problems.

But it wasn't just consumers whose expectations soared; the professionals who served them by and large shared the same high expectations. It was as if the technical feat alone, the use of DSP in hearing aids, was a sufficient demonstration of their functional superiority. The professional community did not appear to be unduly alarmed that there were only a few published studies comparing digital to well-fit analog hearing aids; this didn't seem to be a high priority at the time. The bandwagon rolled, and everyone—professionals and consumers alike— clambered aboard, without pausing to reflect whether clinical evidence was available to support the high expectations.

I had difficulty understanding at the time what particular dimension(s) of a digital hearing aid's output was supposed to produce the listening superiority that such hearing aids were presumed to confer. This was made very clear to me in a trade journal discussion (The Hearing Journal, March 2000) monitored by Gus Mueller. In the article, he asked representatives of 7 leading hearing aid manufacturers to recount what features were unique in their company's new digital hearing aids. Every one of the hearing aids discussed in the article employed different speech-processing strategies (number of bands, time constants, compression characteristics, etc). Each one, however, was presented as the optimal choice for potential consumers, even though they were all different. This hardly seemed to be a logical proposition; could they all be right? What it seemed to come down to was that these hearing aids were all fully digital, which is about all they seemed to have in common.

We were being asked, in other words, to assume that there was some magic inherent in the fact that all of these hearing aids used DSP, as if this in itself was evidence of superiority. If so, then it follows that the specific speech-processing algorithms used were basically irrelevant. This is not a conclusion that an audiologist can easily accept. Of course, all of these aids could be programmed to ensure maximum aided audibility at various input levels, but most hearing aids, analog or digital, can do this. Could it be that all the preceding digital processing was basically irrelevant, that all that was necessary to maximize speech perception was optimum aided audibility with minimum distortion?² Is all the rest just overkill? Could it be that the resonance and connotations of the word digital, and not actual speech perception differences, were the major difference between digital and advanced analog hearing aids?

The importance of labeling, of the power of suggestion, was examined in the classic study by Bentler et al.³ This was done when there were only 2 BTE digital aids on the market. Two groups of 20 subjects participated in the study. Outcome measures included a number of behavioral speech perception tests and self-report scales. In group A, the subjects alternately wore first one and then the other digital aid for a month. But—and this is the critical point— for 1 of the trial periods, they were informed that they were using a state-of-the-art digital hearing aid, while for the second month, they were told that they were using a conventional hearing aid. In reality, the subjects used only 1 of the 2 digital aids for both months (rotating between aids). Test results showed no differences in speech perception scores between the 2 conditions; labeling did not affect objective test performance. Labeling, however, did significantly influence the scores on a subjective measure of benefit, the reverberation and background noise subscales of the Abbreviated Profiles of Hearing Aid Benefit,⁴ and very clearly influenced the subjects’ overall hearing aid preferences. When asked which hearing aid they preferred, they overwhelmingly preferred the digital aid to the presumably conventional one (in reality, also a digital hearing aid).

The influence of the term digital is even more apparent for group B. This group wore exactly the same hearing aid, with exactly the same fitting parameters, for both months. But for one of the months, they were informed that the aid was a modern digital aid, while for the other month, they were told that the aid they were wearing was a conventional one (identifying marks were obscured). That is, the aids would ostensibly be switched after a month but in reality were “switched” right back to the same hearing aid. Again, the subjects overwhelmingly preferred the hearing aid labeled digital as opposed to the one labeled conventional. Again, no differences were observed in behavioral speech perception measures.

This is about the clearest evidence of the power of suggestion that I've seen in any research on hearing aids. Unlike group A, which used 2 different digital hearing aids, group B was fitted with the exact same hearing aid for both conditions. The only difference was the different label applied to the same hearing aids. This was a double-blind study, and the subjects were not aware that they were in reality using a digital aid for the entire 2 months. In a double-blind study, the experimenters are also not supposed to be aware of the test condition. But this was a practical impossibility in this instance. As it happens, when they analyzed the data, the experimenters did find an experimental-expectancy bias in that scoring, and data entry errors were “made in a systematic way that biased results in the experimenter's expectations of the study outcomes,”³ that is, in favor of the digital rather than the analog labeling condition. After the correction of these errors (the data being reanalyzed by an uninvolved party), no significant labeling effects were observed. Thus, the results do reflect a true double-blind design.

From a consumer's perspective, what I find particularly interesting about this study are the reasons the subjects gave for preferring the digital to the “analog” hearing aid. Their comments are the kind that consumers make all the time, to their clinicians as well as to other consumers. People reported that with the digital aid, they “heard better in groups,” that “the sound was clearer and more natural, crisper, more fine-tuned, better high pitches,” and so forth. These are exactly the kinds of remarks that professionals hope to hear when they fit new hearing aids, as well as exactly the kind of listening experiences that consumers hope to report. But, as the study showed, the preferences for the digital aid were actually based on the expectations evoked by the label rather than the actual listening experiences. What are we to make of this?

Are we now supposed to dismiss a user's personal judgment as flawed, too influenced by the hype that accompanies the sale of hearing aids (any aid, not just digital aids)? This would be impossible to do, even if it were desirable. Maybe we are just talking about a placebo effect, but this phenomenon has long taken its place among respectable professional nostrums. Insofar as hearing aids are concerned, expectations have always played a role, albeit more powerfully now with the advent of DSP processing in hearing aids. But how long can the glow generated by positive expectations last? I do think that at some point, reality will rear its ugly head. People who have hearing loss requiring amplification are going to experience some communication difficulties on occasion. They won't hear normally or “like a bird” (an expression I heard old-time hearing aid dealers employ years ago in describing their body-worn hearing aids). Hopefully, they will hear better than they did before being fit with the new digital hearing aids. But we really have no way of knowing if they would have done the same with advanced and well-fit analog hearing aids, or, for that matter, with different adjustments on the same digital hearing aid.

In other words, I think the term digital itself will take us only so far. There is going to be a time when a person's actual experiences listening through the hearing aids will determine his or her satisfaction. Until relatively recently, there did not seem to be any features included in digital hearing aids that had unique real-life applications. For example, the aided audibility curves of digital and analog hearing aids appeared to be quite comparable. In terms of this dimension of performance, the preceding digital processing was not a very relevant consideration; what seemed to count most was the extent of the sound that people actually heard.² To be just a bit facetious, it seems the key to hearing better is being able to hear more.

In my opinion, consumers have been unnecessarily paying a lot of extra money for digital hearing aids. What they were receiving was undeniably the latest and most advanced technology but not demonstrably superior hearing performance. There is a tendency to equate the technical descriptions of function with improved listening performance. It's hard for anyone, much less an unsophisticated consumer, not to be impressed with terms used to describe speech-processing strategies (eg, artificial intelligence, cochlear dynamic sound processing, speech intensification system, speech enhancement algorithm, and many, many others). All one has to do is examine recent copies of the trade journals or the Web sites of the various companies to see the impressive range of technical/marketing terms used to describe the capabilities of the newest digital hearing aids. They offer an allure, and an implicit (often explicit) promise, that is hard to resist.

Does Digital Processing Increase User Benefit/Satisfaction?

I suspect that professionals reading these technical descriptions would be even more entranced than a consumer would be. Audiologists know enough to understand the stated intention of some particular development and to appreciate just how sophisticated the stated objectives are, without necessarily having to comprehend all the technical details. It's hard not to be impressed with the awesome range of engineering advances that are described. For example, hearing aid designs based on cochlear dynamics would have an intrinsic appeal difficult for audiologists to ignore. Surely, many professionals must feel that it is self-evident that such features would be of demonstrable benefit to those wearing hearing aids.

It does not follow, however, that because an aid is capable of performing the most sophisticated speech-processing algorithm it will also improve a hearing aid user's speech perception performance. I don't question the fact that new hearing aid developments modify the way the speech signal is processed. These can easily be demonstrated in a laboratory analysis. Still, the basic question remains: Do these modifications actually help people to hear better? In brief, neither an impressive technical accomplishment nor the most heartfelt testimonial by a satisfied hearing aid user can be construed as behavioral evidence for superior listening performance.

Only fairly recently has the power inherent in DSP been harnessed to create processing algorithms that can be demonstrated to increase speech perception abilities and listening comfort. The 3 algorithms that I think are most significant, and that could not have been achieved without digital processing, are noise reduction schemes (increasing comfort rather than speech perception per se), adaptive directional microphones, and feedback cancellation systems.^5,6 The latter, in my opinion, has the most far-reaching implications insofar as consumers are concerned.

The newest type of feedback cancellation systems do not modify the frequency response when they act to increase the gain available before the onset of feedback, unlike previous generations, which notched out or decreased the audibility of the high frequencies. One recent study found that the gain before feedback could be increased by 10 dB or more in 4 of the 6 hearing aids evaluated.⁷ This feature alone can be tremendously beneficial to many hearing aid users. Recently, I began using a power aid with this kind of feedback cancellation system. For the first time in years, I have been able to achieve my desired output without being limited or disturbed by feedback. And my experiences need not be unique; people whose amplification benefits have been limited because of perennial acoustic feedback, despite repeated trials with different ear molds, now have an opportunity to experience feedback-free amplification.

It is not only people with the most severe hearing losses who can benefit from this new development. The newest buzz word in hearing aids appears to be open-ear hearing aid fittings. In the last issue of Hearing Products, I counted 14 advertisements for this type of hearing aid. Consumers with relatively good low-frequency hearing can now be fit with hearing aids without experiencing the occlusion effect, a heretofore limiting and unpleasant side effect caused by inserting a foreign body into one's ear canal. I have no doubt that this development has significant real-life positive implications for many hearing aid users. This development would have been impossible without an effective feedback cancellation system employing DSP. Furthermore, we can safely predict that these systems will become even more effective over time.

My problem with open-ear fittings lies in their use of thin-wall, narrow-diameter (0.8 m) tubing to convey the signal from the amplifier to the ear canal. The marketing appeal by the manufacturers lays great stress on their “invisibility,” a blatant cosmetic appeal that is disturbing on 2 counts. The first concerns the acoustic effect of the narrow (almost invisible) tubing. It has long been recognized that there is a loss of high-frequency transmission when a narrow tube is used to deliver an acoustic signal. Hearing aid engineers are aware of this effect and do include a response correction in the programmer when narrow tubing is entered as one of the fitting parameters. Still, questions do remain since it is unclear how effective these corrections are.

Basically, what we don't know is the overall acoustic effect of the narrow tubing, the direct reception of the low frequencies into the ear canal, plus the reestablishment of the normal resonance of the external ear canal. Nothing I could find in the literature describing the open-ear fitting presents the relative effect of narrow versus more conventional tubing in the real-ear terms. (I did request such information from the representatives of several companies who market these aids, but no luck.) We should know if it is necessary to compromise target output goals when a narrow tube is used. Even more important, the consumer should be made aware if he or she has to pay an acoustic price to realize a cosmetic preference. With these aids, one could use a #13 tubing and still realize the benefits of an open-ear fitting with perhaps a “teeny” bit more visibility (horrors!).

Another aspect of the blatant cosmetic appeal is the message it sends to people with hearing loss. As we well know, this is an old problem.⁸ Yes, people respond to these appeals, and yes, some people will use hearing aids only if they are completely invisible. This is the reality of the world in which we operate. The message of such marketing appeals, however, is that (1) people somehow look better without a visible hearing aid (with its corollary, ie, that the less visible the aid, the better they look) and (2) that a hearing loss is a shameful condition that must be hidden from other people's awareness. I suspect that as many people are deterred from acquiring hearing aids because of these subliminal messages as obtain them because the visible evidence of their hearing loss can now be hidden. But of course, we really don't know; clearly, my personal biases are showing. What we do know, however, is that the advent of digital hearing aids (with feedback cancellation) has made invisibility more of an option for more people, along with more comfortable and more effective hearing aid fittings.

It does seem to me that the “magic” of the digital label is beginning to fade. Already, it seems, most hearing aids—and not just the most expensive ones—are now digital instruments. A full-page ad in the May 4, 2006, New York Times advertised a fully digital instrument for less than $35.00 ($60 for 2). DSP is now commonplace; it is used in just about every electrical toy, appliance, or device that is currently being made. The issue about whether digital aids per se are superior to analog is now moot. Pretty soon, if not already, just about all new hearing aids will be digital. We've reached the point at which it is now necessary to rephrase the question of superiority in terms of specific features and algorithms, rather than digital per se. In other words, does a client display a particular communicative need that can be met by only a specific feature? And do the predicted (and measured) benefits justify the cost? From what I can see, audiologists only rarely become involved in assessing the cost-benefit ratio of hearing aids, although techniques are now available to assess this issue.⁹ Perhaps we should do more of this sort of assessment.

At the least, consumers simply want to hear better with hearing aids. They believe, and have been led to believe, that they can best achieve this goal with the latest (and most expensive) digital hearing aid. But this does not necessarily follow; actual (as opposed to the placebo effect) listening superiority requires its own demonstration. Recently, an entire issue of the Journal of the American Academy of Audiology was devoted to just this issue, that is, “Evidence-Based Audiology” (volume 16, number 7, 2005).¹⁰ When one or another particular hearing aid or feature is recommended for me, what I'd like to hear from the audiologist is a reference to the evidence supporting this decision. A commitment to an evidence-based approach would give me this assurance. While I respect comments by audiologists prefaced by the phrase “in my experience,” this is not my preference, certainly not if convincing research evidence can be brought to bear. And if not, why not? And if not now, when?

Think about the dilemma facing consumers who have been asked to purchase 2 hearing aids, costing about $5000. The audiologist advises them that in his or her experience, these are the best aids they can purchase, ones that will fully meet their hearing needs. But then they see ads in the newspaper or on the Internet advertising hearing aids for half or even less than half that cost. They do a little comparison shopping and hear how a different audiologist has “had great luck” with yet a different hearing aid, one perhaps costing even more or perhaps much less. (There does not seem to be any linear relationship between cost and claims!) Who are they supposed to trust? If they visited 10 audiologists, they might get 5 or 6 completely different hearing aid recommendations, each incorporating somewhat different electroacoustic dimensions or speech-processing stratagems.

It seems to me that the profession is faced with a dilemma. On one hand, it insists that doctoral-level training is necessary to successfully accomplish a full range of responsibilities (which includes the selection of a particular hearing aid from among a large number of possibilities). On the other hand, there still appears to be a dearth of behavioral evidence that can relate the unique characteristics of a hearing loss to a specific set of hearing aid recommendations. Even with all the programming options now available on a digital hearing aid, it still seems to come down to the question we asked 50 years ago: “How does it sound to you?” It's not as if this question is irrelevant or unimportant—on the contrary, it's an absolute necessity—but it should be preceded by selecting options based on rules of evidence. Calling some hearing aid “a top of the line digital” has great sales appeal, but it doesn't help people hear better. I'd say the mystique of the label digital is fading and that shortly, these aids will no longer be seen as panaceas. And, as in the past, audiologists must still help their clients rein in their expectations and deal with the listening reality underlying the overblown marketing rhetoric.

A Hearing Aid Is Not Always Enough

No discussion about hearing aids, particularly one focused on a consumer's perspective, would be complete without a concluding statement about the possible need for other types of hearing-assistive technologies, as well as the inclusion of a routine hearing aid orientation (preferably group) program. Any and every hearing aid, digital or analog, is basically a way of trying to reduce the problems caused by a hearing loss. When the focus is not on hearing aids per se but on the amelioration of the functional consequences of a hearing loss, then we see that people often need and can benefit from hearing-assistive technologies in addition to hearing aids.^11–13 This determination can best be made within the structure of a group hearing aid orientation program, one that also offers an opportunity for hearing aid users to ask their questions, share their experiences, learn various communication strategies, and become more effective hearing aid users. More than 30 years ago, when the profession insisted on the need to dispense hearing aids, it justified this demand as a necessary prerequisite in providing total aural rehabilitation services to its clients. I'd say it's time for the profession to put up (but not shut up!).