"Directional microphones on hearing aids are still a hot topic in the professional literature, as one recent development for which, and for once, there is an accord between marketing hype and performance."

The results of several recent studies have added further corroboration of their effectiveness. While these results unequivocally support their listening advantages, there were some interesting twists and some relevant incidental findings in the studies that make them useful to review in a little detail.

In the first study, by Valente, Sweetow, Potts and Bingea in the March 1999 issue of the Journal of the American Academy of Audiology, the researchers compared their subject's personal analog hearing aids to two digital hearing aids, one with directional microphone capabilities and the other not. Except for the directional microphone, the digital aids were otherwise similar and fit exactly the same way.

In both clinical sites in which the study took place, the results showed that the sublects heard better with the digital hearing aid that included a directional microphone compared to their personal hearing aids or to the other digital hearing aid. This superiority was most apparent under the most difficult listening conditions, when the noise background was somewhat louder than the speech signal they were listening to. Of course this is exactly the type of noisy situation in which people with hearing losses have the most difficulty and in which they need the most help.

The improvement was quite marked and apparent to the subjects whose sublective impressions corroborated the objective results. Performance with the two aids which did not employ directional microphones were quite similar, with the digital aid showing some marginal superiority over the analog aid particularly in the subjective comparisons.

What I also found interesting about this study was the description of the fitting procedures and the wide variety of the subjects' original hearing aids. The experimenters used a sophisticated fitting technique, in which the presence of acoustic feedback was determined at the target gain (amplification) levels for the three different frequency bands (low, medium, and high) processed by the hearing aid.

If feedback occurred, then the target gain had to be reduced by the degree of feedback value (i.e., if the target gain was 60 dB, and the feedback value was -10 dB, then the gain at that particular frequency band had to be set at 50 dB). What the examiners at both sites found was that the presence of feedback necessitated the reduction in the 'prescribed' gain. This was particularly true at the high frequencies and it affected about three-quarters of the subjects in the study.

What this means is that the presence of acoustic feedback (an audible squeal required that the desired amount of amplification had to be reduced for that frequency band. Clearly, controlling feedback by decreasing the gain is not a good idea. What this does is minimize the potential listening benefits of the hearing aid, in this instance for the important high frequency sounds of speech. This is a problem that, it seems to me, digital signal processing should be uniquely able to resolve. It is possible, as at least one manufacturer showed some years ago, to reduce acoustic feedback through phase reversal techniques. I'm sure there must be other techniques out there as well.

While this study fairly decisively demonstrated the benefits of directional microphones, it also revealed at least one major weakness in current hearing aid design. With the advent of digital hearing aids, it's long past time for hearing aid manufacturers to design feedback reduction techniques that can permit people to perceive all the amplified sound they are capable of benefiting from.

The other "incidental" finding of this study that I found interesting was the description of the subjects' personal hearing aids. All of them were satisfied with their current analog aids, many of which were advanced models incorporating multi-band amplification patterns and various levels of automatic gain controls. All of the subjects had fairly similar moderate hearing losses, showing a typical gradually sloping high frequency hearing loss pattern. Yet, in spite of their similar hearing losses, these people were fitted with a wide range of different types of hearing aids and speech processing strategies. The sublects were clients of hearing centers staffed by audiologists with excellent national reputations. How is it then that people with such similar hearing losses could be fitted with such a wide variety of hearing aids? How does this relate to the question about the "best" hearing aid often asked by hearing aid users?

In my judgment, what this wide variation reveals is how much of an "art" hearing aid fitting still is rather than purely a "science." While some portion of the range of fitting can be explained in non-acoustic terms (i.e., cost, cosmetics, the inclusion of such additional features as telecoils, etc.), other reasons for the different choices are not so clear. Probably, many of these other reasons relate to personal differences in listening experiences that are not reflected in a simple audiogram. These personal judgments are what skilled audiologists respond to when working closely with their clients.

Another possibility is that it makes no difference: that as long a the person perceives the maximun amount of sound, it doesn't matter what kind of aid he or she has been fitted with. I don't accept this notion; the evidence seems clear that changes in speech processing strategies do make a difference for different individuals. It would be very nice, however, and maybe this could be construed as "the" audiological challenge for the new millennium, if audiologists could refine the "science" of hearing aid fitting a bit more and depend less on the seat of their pants when fitting hearing aids (but never ignoring the "art" of working sensitively with their clients).

In the second study on directional microphones, which appeared April 1999 issue of the Journal of American Academy of Audiology, the researchers also compared the speech recognition performance of people wearing both directional and omnidirectional hearing aids. They used three aids that could be switched to either position, two of which were digital aids and one an analog instrument. Their findings corroborated the results of the first study (and of all other studies that have investigated the benefits of directional microphones on hearing aids). Particularly in noisy situations, people do hear better when using directional microphones. It should be emphasized that what these microphones do is reduce the loudness of sounds that come from any direction other than from directly in front. As with the study reviewed above, the advantages of the directional microphones were apparent whether or not the subject wore an analog or a digital hearing aid.

What was a little different with this study is that they also compared a "normal" listening situation (some reverberation, with noise coming from many directions), with the results obtained when the tests were conducted in a sound-treated room. What they found was that the benefits of the directional microphones were slightly reduced in the "normal" situation compared to the results obtained in the sound-treated room. We know from other research over the years that the more reverberant the listening environment, the less the listening advantages of directional microphones. However, except in the most extreme situation, some benefits should still be apparent.

From Hearing Loss, July/August 1999