The question of the "best" hearing aid keeps rearing its head, and I keep avoiding a direct answer. Mainly because I don't know (and neither really, does anybody else!). Why I don't know is amply illustrated in an article that appeared in the March 2000 issue of The Hearing Journal.

In this article, Dr Gus Mueller interviewed representatives from seven leading hearing aid companies (GN Resound, Oticon, Phonak, Siemens, Sonic Innovations, Unitron, and Widex), 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. The difficulty, for consumers and professionals alike, is that while the rationale for the variations in signal processing strategies offered by these seven companies all seem very logical, they are also somewhat contradictory.

All the hearing aids reviewed include more than one channel of amplification, with the actual number varying from three to 20. Some of the aids permit fine adjustments in specific channels in one dimension (i.e., amplification), while tying together other channels for other types of signal processing (different types of automatic volume control). The nature of the automatic volume control (AVC) circuit varied considerably among the seven hearing aids, with some circuits controlling the loudness level at the input, some at the output, and others at both the output and the input. Some of the aids employ fast time constants in modifying sound levels, while others do it more slowly. A few aids include dual time constants, depending upon the nature of the input sounds. The methods and rationale the seven different companies employ for noise and feedback control also differ considerably.

If this is all getting somewhat confusing, join the crowd. Clearly, they all can't be right. In fairness, I should point out that all of these hearing aids permit individualized adjustments depending upon a user's listening preferences. Even so, major differences in "built-in" speech processing strategies remain. (One feature, not explored in this article, for which there is unanimity regarding potential effectiveness, is directional microphones.) Perhaps, as was implied in the discussion re the disposable hearing aid, there are limits to the technical refinements that impaired ears can benefit from.

What is necessary is clinical research in which the various strategies are tested on people who display a range of hearing losses. Of course, such research should be based on sound theoretical principles, but theory alone is not enough. A great deal more clinical research is necessary. We need studies that compare, on the same people, the different speech strategies that current hearing aids are capable of delivering. Further, such research should be "blinded" to the extent possible; that is, neither the subject nor the examiner should know which is the experimental and which is the control condition. Otherwise, there is the possibility that unconscious bias, either on the part of the experimenter or the subject, will influence the results. (In studies completed by Dr Ruth Bentler, she found that people consistently preferred a "digital" hearing aid over an "analog" aid, in spite of the fact that in reality either both aids were digital or exactly the same digital hearing aid was tried in both cases).

What we don't have with the hearing sense is some sort of functional "gold standard" comparable to 20/20 with the visual sense. No currently used psychoacoustic or audiological measure is able to predict the potential speech perception skills of impaired ears in a variety of environmental circumstances. We don't have an absolute speech perception "target" to aim at when testing people with hearing aids. We don't know, in other words. the maximum capacities of a particular impaired ear to perceive speech in a variety of circumstances. What we do have are relative comparisons - relative to a person's previous or other hearing aids, to various adjustments on the one being tried, or to internalized predictions by the audiologist on what performance this person should obtain. Are these relative comparisons useful? Of course they are; they're very useful. But they don't answer the question that many people have - e.g. "Is there some other hearing aid or other speech processing strategy than can help me hear better?" Certainly, there are other dimensions of hearing aid usage that we are concerned with, such as the quality of sounds and the comfort of listening. Still, the most significant dimension of all, the reason why people wear hearing aids in the first place, is the improvement of speech comprehension, and this is the one that still virtually remains in the "trial and error" stage. By now, we should be able to do better.

Originally published in Hearing Loss - September/October 2000