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   Rehabilitation Engineering Research Center
   on Hearing Enhancement

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Dr. Ross on Hearing Loss

Hearing Aid Fitting: Getting it Right (and is that all there is?)

by Mark Ross, Ph.D.
This article first appeared in
Hearing Loss (Sep/Oct 2008)

Hearing aid dispensers are occasionally thought of as “miracle workers.” As soon as the hearing aid is turned on, some people who haven’t heard well for years are suddenly able to hear again. The difference can be dramatic and the user may volubly express his or her gratitude and pleasure. Dispensers love to get this reaction, and who wouldn’t? It helps to validate their efforts on behalf of the person with a hearing loss. Without dashing too much cold water on this reaction, however, we should realize that for some people with a long-standing hearing loss, any enhancement - from a speaking tube to the latest digital hearing aid - may initially evoke such a reaction; all it takes is for previously unheard or poorly heard speech to become clearly audible. I won’t minimize this initial response to amplification, but the essential question remains: We should be asking not if the person is getting “some” benefit from a hearing aid (any piece of junk can provide some degree of improvement in certain situations), but whether the person is getting the maximum amount of auditory assistance reasonably possible with a hearing aid. That should be the goal in fitting a hearing aid.

The first step in achieving this is to ensure that amplified speech sounds are delivered into the residual (dynamic) hearing range. This objective is the same, regardless of the size, appearance, cost, or complexity of the hearing aid. If speech sounds are not as clearly audible as possible, then everything else that follows will be compromised to some extent. The dynamic range extends from a person’s threshold of hearing to the loudness level where sounds become uncomfortably loud. For example, the dynamic range of someone with a threshold of 50 db at 1000 Hz and a loudness tolerance level of 90 dB at that frequency would be 40 dB. Any sounds softer than 50 dB would not be heard, and any sounds louder than 90 dB would not be tolerable. All the “bells and whistles” of modern hearing aids, many very helpful for some people in some situations, ultimately depend upon this very basic dimension in hearing aid fitting.

Packaging the amplified sounds into this area is not always easy to do, since hearing aid loudness tolerance thresholds will often vary at different frequencies, while the level of the input speech signals are always changing. Under these conditions, getting the signal packaged just right for all types of hearing losses is not easy to do. As a prerequisite, however, it is clear that a dispenser must know the both the person’s thresholds of hearing and loudness tolerance. Just knowing the hearing thresholds is not enough. It is also necessary to be aware of the prospective hearing aid user’s loudness discomfort levels, since this determines where the maximum output level of the hearing aid should be set.

One of the first things Audiology graduate students learn in their hearing aid classes is to ensure that a hearing aid does not deliver sounds that exceed a user’s tolerance levels. If it does, and this happens often enough, it will invariably lead to dissatisfaction and/or rejection of the hearing aid. Regrettably, this type of situation is not an infrequent occurrence. One recent survey indicates that about 40% of hearing aid users are dissatisfied with their “comfort with loud sounds,” and that about 20% of them complain that some input sounds are actually “painfully loud.” In order to preclude these situations, a dispenser must determine the person’s loudness discomfort levels, and then program the hearing aid so that the maximum output does not exceed these tolerance levels. If this is done correctly, even the loudest input sounds should not cause discomfort. This is not a revolutionary or onerous demand; as a concept it has been with us for many years. Indeed, the “best practices” recommendations outlined by both the American Speech-Language-Hearing Associations (ASHA) and the American Academy of Audiology (AAA) include this provision (e.g. determining frequency specific loudness discomfort thresholds).

So, then, in practice how do hearing aid dispensers determine the output limits of the hearing aids that they fit to people? They are well aware of the necessity of keeping the hearing aid output below a person’s tolerance level, but instead of measuring loudness discomfort levels themselves, many depend upon the hearing aid manufacturer to do it for them via the proprietary hearing aid fitting program. With this, all the audiologist needs to do is enter the prospective hearing aid user’s hearing thresholds into the program. Based on the questionable premise that loudness discomfort levels can invariably be predicted from thresholds (not true), the program then displays the recommended output limits for the specific hearing aid being fitted. These appear on the computer monitor and seem to be a demonstratively ”scientific” way of determining the appropriate hearing aid output limits for a particular hearing aid user. Since 70% of hearing aids are fit using a manufacturer’s computer program, the validity of this practice does need to be examined. Two nationally recognized authorities on hearing aid fitting (Ruth Bentler and Gus Mueller) recently did just this; indeed, it was their study, their latest on this topic in a 14 year span, which inspired this article.

What they did was enter the same hearing loss into the hearing aid selection program of six different hearing aid manufactures using six different hearing aids. The recommended output limits should be the same, or very close, for all the hearing aids. Instead, what they found was that these levels varied as much as 15 dB at different frequencies. There is no way of knowing which one, if any, is correct. Can this degree of output variation be a problem? The researchers thought so. In practical terms, this means that some hearing aid wearers would experience loudness discomfort on various occasions with some aids, while other users would not be receiving as much amplified sound as they were capable of benefiting from. Either situation should be avoided.

What, then, does it take to get the hearing aid output right, e.g. achieve an optimum relationship between a person’s loudness tolerance level and the maximum output of a hearing aid? The answer is, in brief, for a dispenser to first directly measure the client’s frequency specific loudness discomfort levels during the pre-fitting process, use this information (as well as hearing thresholds) to set the appropriate maximum output on the selected hearing aid, and then finally to verify the actual fit during the post-fitting process via real-ear measures. (Real-ear measures are currently the “state of the art” method of determining the actual sound pressure levels at the eardrum.) However, only about 20-30% of hearing aid dispensers now follow these guidelines. It’s not as if these were simply “academic” requirements with little or no saliency in real-life. On the contrary, getting the hearing aid output just right has been shown clinically to be a key factor in a successful hearing aid fitting.

So why, then, don’t more hearing aid dispensers follow these procedures? A number of reasons are usually invoked, from time constraints, simple inertia (the “rut” is a comfortable place to be), to the belief that it is not necessary. These dispensers would justify their clinical practices by stating that their clients “do just fine” and that there was therefore no need to follow the time-consuming ASHA and AAA guidelines. For them, the computer display of the manufacturer’s selection program appears to offer the necessary objectivity; as we have seen, however, manufacturers will often differ in what they recommend (for both maximum output and frequency response) for hearing aid users with exactly the same hearing loss. In my opinion, it is equally problematical for a dispenser to depend primarily on a person’s subjective reactions (While “how does that sound?” may be a necessary question, and the most frequently asked one during a hearing aid selection, it should not serve as the only basis for a hearing aid recommendation.)

In fact, I can’t help but wonder why a hearing aid dispenser is necessary at all if the most common fitting recommendations are based solely on a manufacturer’s program, with little or no real-ear post-fitting verification taking place. In such instances, I can well understand a prospective hearing aid user shopping on the internet or mail order for hearing aids where they can be purchased much less expensively. As it happens, these outlets now represent an increasing percentage of total hearing aid sales and, given the current cost structure of modern hearing aids, it is likely that this “distance” purchasing will continue to increase in the future. Some internet sites offer “online” hearing testing, and an earmold impression kit with which a person can make their own ear impressions (with the help of a family member or friend). While at the present time, studies show that the results obtained with online testing and home-based ear impressions do not compare favorably with those obtained in a hearing center, these are areas in which technical improvements occur rapidly. Projecting not too far in the future, it may soon be possible to accurately measure one’s hearing thresholds via the internet, and combined with the growth of open-canal fittings which do not require ear impressions, earmolds too are becoming less and less necessary. Under these conditions, where would this then leave the professional hearing aid dispenser?

The high retail cost of hearing aids is generally rationalized by invoking the time it takes to evaluate the hearing loss, fit the aids, and then provide the necessary follow-up services to ensure a successful fitting. When this is done according to best professional standards, – which implies including only those aids with necessary special features – the high cost of hearing aids may well be justified. The hearing sense is just too important to treat casually; we know that appropriately fit hearing aids can have an enormous impact on one’s quality of life. And, of course, beyond hearing aids, many people with hearing loss have many needs and questions that relate to the hearing loss that require addressing. All this takes time and the professional does need to be recompensed for his or her services. It is when this high level of service is not provided to the prospective hearing user that one can question the cost/benefit value of the clinic-centered service delivery model.

My own feeling is that what is going to “sell” hearing aids in the future is not the product itself - excellent hearing aids are available from many sources – but the total “management” package offered by the professional hearing dispenser. In this package, the device itself will be ancillary to the expertise and total range of services offered by hearing aid dispensers. My own hope is that these services would include a post-fitting, family centered, group hearing aid program. I fully agree that hearing aids are a vital tool is reducing the impact of a hearing loss, but still the focus of the professional-client relationship has to be the hearing problems experienced by their clients and not just the hearing aids.

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