Mark Ross, Ph.D.

Developmenmts in Research and Technology
by Mark Ross, Ph.D.

Effectiveness of a Wax Protection System

Earwax, a perfectly normal occurrence, can cause problems with hearing aids.
The best hearing aid in the world will not eliminate the production of cerumen (earwax) in the ear canal, nor is it supposed to. Earwax is a perfectly normal occurrence in ear canals. It is produced in the external portion (cartilaginous) of the canal and is normally moved out when a person talks or chews. Earwax is designed for purposes that have nothing to do with hearing aids (i.e., lubrication, keeping insects out, etc.).
Unfortunately, when someone inserts an earmold or hearing aid in the ear canal, this prevents the wax from exiting the canal and this “perfectly normal occurrence” can produce problems in hearing aids. Sometimes these problems will simply be an inconvenience, like having to clean the sound bore periodically (every night would be a good idea). But sometimes they can be quite severe, as when clogging of the hearing aid’s sound outlet causes the hearing aid to malfunction or actually damages the hearing aid receiver (the internal “loudspeaker” of the hearing aid).
I’ve written about this issue in previous columns, pointing out how earwax problems are possibly the most frequent reason why hearing aids are returned for repair. If such returns could be reduced, this would be beneficial to consumers, dispensers, and manufacturers alike, in terms of cost and convenience.
There are a number of ways that wax can be prevented from entering the sound bore. The “MicroWaxbuster,” distributed by Knowles Electronics is one of these ways. In a recent article in The Hearing Review, Sergei Kochkin shows how the inclusion of the Waxbuster (the generic terms for the Waxbuster and its smaller version, the MicroWaxbuster) can reduce the number of receivers returned for repair to the manufacturer.
This inexpensive device is easily screwed into the sound outlet of any in-the-ear instrument. A small collar in the center of the Waxbuster protrudes slightly from the tip of the earmold. When the collar is pressed inward against a surface, it retracts into the sound outlet. This pressure activates a spring-load piston that pushes the wax out of the hearing aid.
Its operation is simple and apparently effective. There is no need to pry the wax out of the sound bore or to cover it with protective material. However, because the device is inserted right into the sound outlet, there has been some concern that it could modify the acoustic response of the hearing aid. While the Waxbuster is supposed to be acoustically transparent, laboratory research has shown that some acoustic modifications do occur. However, the changes seem to be positive rather than negative. That is, its inclusion provides a moderate increase in the high frequency gain of the instrument, something that is usually desirable.
In his study, Kochkin first considered the number of receivers sold to various customers (over seven million in a three-year period), with and without the inclusion of the Waxbuster. As it happens, most of Knowles’s customers (61 percent) did not purchase any Waxbuster, while 39 percent did. For those that did, Kochkin then looked at the number of returned receivers as a function of the proportion of Waxbusters that they did purchase (this proportion varied across a wide range). What he found was very interesting. There was a very significant and very direct relationship between the proportion of receivers purchased with Waxbusters and the number of returned receivers. That is, those who included Waxbusters with their receiver purchases demonstrated a much lower return rate, while those who did not include this device showed a much higher return rate.
This is one study that hearing aid manufacturers should take immediately to heart. For a minimum cost, they can apparently significantly reduce the number of hearing aids that have to be returned because of a malfunctioning receiver. However, the ultimate test for the effectiveness of this device, as Kochkin points out, would be a study where the randomly assigned experiences of consumers could be compared, half getting the Waxbuster and half not. Insofar as consumers now are concerned, there is no reason to wait; the Waxbuster (and other wax protection devices) can be retrofitted into one’s present hearing aids.

Directional Microphones in Behind-the-Ear (BTE) and In-the-Ear (ITE) Hearing Aids

Do dual microphones work better in one or the other type of hearing aid? Does it make a difference what type of hearing aid a person wears in order to achieve the known benefits of directional microphones?
As I’ve already commented a number of times in the past, one undisputed advance in hearing aid technology in recent years has been the inclusion of directional microphones. This is now being accomplished by combining and controlling the input from two separate microphones in order to achieve the desired directional effect. From the point of view of the user, what happens is that sound signals arriving from the rear and sides are attenuated relative to the sound signals arriving from the front. This action serves to increase the loudness of the desired speech signals (assuming that the listener is facing a talker) relative to background sounds, the speech-to-noise (S/N) ratio. And, in spite of the many technological innovations made in recent years, the S/N ratio is still the most important factor underlying speech perception.
Originally, dual microphones could be only found on behind-the-ear (BTE) hearing aids. For the last few years, however, they have also been incorporated in in-the-ear (ITE) hearing aids. This, naturally, begs the question of relative effectiveness: i.e., do dual microphones work better in one or the other type of hearing aid? Does it make a difference what type of hearing aid a person wears in order to achieve the known benefits of directional microphones?
This was the research question in a study published in the Journal of the American Academy of Audiology by Pumford, Seewald, Scollie, and Jenstad. Using the same BTE and ITE hearing aids, the investigators compared speech perception scores obtained with omnidirectional and directional microphone conditions. In the performance measure they used, scores were expressed in terms of the S/N ratio required to understand 50 percent of the sentences. That is, the lower the S/N ratio to achieve a 50 percent score, the more advantageous the hearing aid and/or condition being examined. .
When they compared just the omnidirectional performance of ITE and BTE hearing aids, the results showed a slight advantage to the ITE hearing aids. This is because, when one wears an ITE hearing aid, the pinna (sometimes called the auricle) is able to make a positive acoustical contribution to speech perception. (by slightly “shadowing” high frequency sounds that arrive from the side and rear). Unlike BTE hearing aids, whose microphone is located on top of the ear, thus precluding any acoustical contribution by the pinna, the microphones of ITE hearing aids are situated at the entrance to the ear canal, permitting the pinna to shield the ear canal from some high frequency sounds.
Since it takes two microphones, spaced somewhat apart, to obtain the directional microphone advantage, it is not possible accomplish this with the smaller in-the-ear hearing aids. There is simply no room for two microphones on the hearing aid’s faceplate spaced the appropriate distance apart. This finding, then, is only applicable when comparing ITE and BTE hearing aids that do not include directional microphones (of course, there well may be other circuits and features requiring a BTE hearing aid).
When the researchers compared the S/N ratio it took to obtain a 50 percent score with the BTE omnidirectional to the directional microphone condition, they came up with about a 5 dB advantage. This can make quite a real-world difference, as this figure would translate into an improvement of somewhere between 30 percent to 40 percent in a conventional speech discrimination test. The comparable figure for the ITE hearing aid was about 3 dB (or about 18 percent to 24 percent on a conventional test). From a purely technical point of view, therefore, it seemed that the advantages were with the BTE. However, when the microphone location advantage (about 2 dB) of the ITE is added in, the results for both the BTE and ITE hearing aids are the same (about 5 dB).
In brief, as far as a user is concerned, this study showed that ITE directional microphone hearing aids offer the same advantage as a BTE directional microphone hearing aid.
This study demonstrates something else, and that is the need for human research to test our preconceptions. Until I read this study, I simply “knew” that directional microphone performance was better with BTE than ITE hearing aids. This had been demonstrated in “polar plots” which look at the performance of directional microphones in an anechoic, echo-free chamber under carefully controlled acoustical circumstances. But when tested on actual subjects, it turns out that this laboratory difference doesn’t quite hold up. Human beings do have a funny way of fouling things up!
One caveat is in order. These results represent the average performance of the 24 subjects participating in the study. They cannot be applied to every individual. Specifically in this study, the performance between the two types of aids was equivalent for only 15 of the subjects in the study. For the remaining nine subjects, five did statistically better with the ITE while four did better with the BTE.

From Hearing Loss, July/August, 2002.
Acknowledgement: This column is supported, in part, by GRANT #133E980010 from the U.S. Department of Education, NIDRR, to the Lexington Center.
Mention of products or companies by the author does not Indicate SHHH endorsement, nor should exclusion suggest disapproval. Since everyone's communication problems and needs vary, SHHH suggests consulting with your hearing health professional.