Dr. Ross on Hearing Loss
Digital Hearing Aids: Still More Comments
by Mark Ross, Ph.D.
The newest and most exciting hearing aid development in recent years has been the advent of the digital hearing aid. We are witnessing an increasing interest in these devices by all segments of the industry, with the different hearing aid manufacturers attempting to differentiate themselves from their competition by extolling the unique advantages of their own systems. Consumers are being inundated with claims and counter claims and have great difficulty in sorting through them. One recent effort to organize the topic in a coherent fashion was the November l998 issue of "The Hearing Journal", which was entirely devoted to digital signal processing systems.
The extent of new developments in this area can be seen in a five-page table, which lists both the currently available digital hearing aids as well as those soon to be introduced. Included in the table is a brief summary of the special features and unique aspects of each of the hearing aids. Sixteen different hearing aid companies are represented, with several offering more than one model digital hearing aid.
The new aids come in all sizes and styles, ranging from behind-the-ear models (BTE) to completely-in-the-canal (CIC). All of them are capable of being modified electroacoustically in any number of creative ways. The degree of amplification can be varied as desired across frequency, while different types of automatic gain control can be separately programmed at selected bands of frequencies within the audible range.
Just some of the special features listed in the table include: multiple memories, multi-band noise reduction, multi-microphones, low battery warnings, feedback reduction, and digital cell phone compatibility. Given the flexibility that digital signal processing permits, we can expect to see additional features periodically introduced. This represents quite a technological feat and one which hearing aid companies may justly take pride in.
As I read through the articles in this issue, however, I could sense a bit of unease from the authors about the paucity of clinical studies attesting to and supporting the objective superiority of digital hearing aids. Nobody doubts their technical sophistication, or the fact that they are first-rate instruments. Nobody currently using a digital hearing aid need fear that they have been fit with an inferior instrument. Moreover, all can be reprogrammed and "tuned-up" to try to ensure optimal satisfaction. What makes some people uneasy (and I'm one of them) is the "hype" associated with their introduction and marketing. There is an implication of universal applicability and general superiority that has not yet been convincingly demonstrated in careful, comparative research.
This is addressed in one of the articles, by David Fabry an audiological researcher from the Mayo Clinic, who discusses the "facts, myths, and leaps of faith" associated with digital hearing aids. After commenting that the "facts" regarding speech perception superiority are equivocal, and exposing some of the "myths" (e.g., no need for a volume control, aids will last longer, cost not an important factor), he concludes with his "leap of faith" (with which I concur) that digital aids will eventually improve speech perception more than is now possible with even the best analog hearing aid.
Dr. Fabry makes another important point: It is not only hearing aids that are going to be affected in our digital future, but hearing aid fitting procedures as well. Digital technology will permit selection procedures to be automated and directly linked to the hearing aid while a person is wearing it. The program would then automatically select the appropriate amplification strategy depending upon the results of the tests. When machines can do all of this, what we have left is the human factor, the one-on-one interactions between the audiologist and the consumer. This, in my judgment, more than any other factor, will determine not only how successfully a person will adapt to hearing aids, but also how well this person can deal with the wider ramifications of a hearing loss. In this orientation, technology is the crucial first step, but not the end of the (aural rehabilitation) process.
In another article in this same issue, Ruth Bentler from the University of Iowa describes ongoing studies that are designed to investigate the influence of marketing "hype" upon a person's hearing aid preferences. These studies seemed so provocative that I contacted her directly to make sure that I had my facts right. What she and her colleagues did was to alternately try two different digital aids, adjusted according to the manufacturer's specifications, on their subjects for a month each. For the purposes of the study, however, one of the aids was labeled "digital" and the other "analog". At the conclusion of the trial, they were asked which one they preferred.
When fitting the "digital" aid on the people, she recited the manufacturer's marketing appeals (CD sound quality, microprocessor, etc.). With just a few exceptions, the subjects preferred the sound quality of the aid that was labeled "digital" (as indeed it was) to the aid labeled "analog" (which was actually another digital aid). Since the aids were rotated among the subjects, both aids took turns being labeled analog and digital. Clearly, expectations and belief influenced the judgment of the subjects.
This is seen even more clearly in another section of the study. Twenty of the subjects took turns wearing a presumed "digital" and presumed "analog" hearing aid. In reality, the subjects wore the exact same digital instrument for both conditions. That is, they would ostensibly switch aids after a month of wear, but in reality they would be "switched" right back to the original aid. Again, except for a few exceptions, the subject's overwhelmingly preferred the "digital" hearing aid to the "analog" aid. The only difference in these two conditions was the subject's belief that one of the aids represented the cutting edge of modern hearing aid technology. It is interesting to note that speech perception scores under the two conditions were the same. Evidently, the subjective impressions and expectations did not translate into improved comprehension of speech.
What are we to make of this? This research was not conducted to discredit a person's personal judgment about performance with a hearing aid. When somebody relates specific instances in which one type of aid performs noticeably better than another does, he or she must be believed. All of us who wear hearing aids make these kinds of judgments all the time. They cannot simply be dismissed. If we could not trust the evidence of our own ears, we could hardly make any kind of comparative judgment regarding hearing aids. In other words, we have no choice but to trust ourselves to make choices that are correct for us.
On the other hand, as these studies do show, we can be swayed by our expectations and by clever marketing appeals. This is hardly surprising; every time we read a paper or magazine, watch TV and listen to the radio, we are inundated with messages that affect what we purchase, who we vote for, and how we think. When these messages concern something so central to our lives as our functional hearing ability, no wonder we find ourselves influenced. We really do want to hear better. Logically it makes sense to expect that the advent of digital signal processing systems, with all the flexibility and all the possibilities that these imply, should offer advantages that more traditional hearing aids do not. For many current digital hearing aid users, I believe they do.
Ultimately, however, what we are searching for is not expectations or belief, but the identification of specific electroacoustic factors, for the wide variety of hearing losses that exist, that will produce superior speech perception and positive subjective reactions. "Belief", I think, can carry us only so far; finally, in the long run, the hearing aids we wear will have to be objectively superior in the listening conditions that are most important to us.
What makes the situation so problematical for consumers is the fact that some of the biggest "believers" are those professionals who actually dispense these hearing aids. They are now spending a great deal of their time and energy trying to master this impressive new technology. After learning all (or something) about "digital signal processing" and how to program these aids on people, it is understandable that some audiologists would be, at least unconsciously biased, in their favor. They also begin to see what they want and hope to see.
Confounding this positive predisposition regarding their potential performance, is the fact that these aids are much more expensive than advanced analog aids with a consequently much higher profit margin. We hate to think that this factor influences hearing aid recommendations, but it would be naive to dismiss the possibility that economics do play a role. It is difficult for a consumer or an audiologist to be completely objective about such a high-tech, expensive instrument, one that draws so heavily on the language and expectations of modern science.
We both need to believe: the consumer that he or she is hearing better, the audiologist that he or she is providing the best hearing aid services available. This is the reason why objective evidence is so important, not to challenge our hopes and expectations, but to support them. Because even the most knowledgeable hearing aid dispenser may still be in a "learning curve" in fitting digital hearing aids, people trying these aids should be prepared, and not hesitate, to return often for "tune-ups". These instruments are capable of an awesome variety of electroacoustic adjustments, but these cannot be made unless hearing aid users work closely with their audiologists. Whatever kind of hearing aid is used, the "partnership" relationship between the user and the dispensing audiologists is still the pre-eminent consideration in ensuring optimal hearing aid usage.