Review of Current Hearing Aid Research

This article is part of an ongoing series on the 2002 SHHH Convention in Seattle, WA. This article covers Mark Ross' Workshop entitled "Review of Current Hearing Aid Research".

Mark Ross, Ph.D., from the Rehabilitation Engineering Research Center on Hearing Enhancement (RERC), presented a workshop on the current state of hearing aid research. For those who don't recognize the name, Mark is a recognized authority on hearing loss and hearing aids. As an audiologist and hearing aid user, he brings a unique perspective to subject of hearing aids. The bottom line of his presentation is that he is impressed with recent technological developments, but he's somewhat skeptical of some of the manufacturer's advertising claims. He started his presentation by outlining some of the hearing aid-related truths, as he sees them:

- The ONLY purpose of a hearing aid is to aid hearing. Sure, it's nice if a hearing aid is small, cheap, etc., but if it doesn't aid hearing, it has no value.

- The only purpose of a new hearing aid or a new feature is to BETTER aid hearing.

- Consumers and professionals are inundated with sophisticated marketing and technical descriptions by manufacturers. It seems that the greatest advances have perhaps been in advertising!

- Superior aided performance is not demonstrated by personal testimony, marketing appeals, or technical descriptions.

- Superior aided performance can only be demonstrated by controlled clinical research on human beings. The research must focus on comparing aided and unaided performance and on comparing one hearing aid feature or speech processing strategy with another. It is preferable that the research be blinded (even the people conducting the research don't know which feature is being tested on a particular subject at a particular time) and be performed by independent researchers.

Ross cautioned that marketing is very appealing (by design), but also very misleading because it does NOT report performance results.

An example of good research into performance results was a clinical trial of an Adaptive Feedback Canceller (AFC). The research determined how much amplification hearing aid users were able to get with and without the use of AFC. The results showed that the average user was able to get an additional 10 db of gain using AFC. The clear conclusion is that AFC can be a good feature. [Editor: The maximum allowable amplification in this experiment was limited by the presence of feedback. By enabling AFC, some of the feedback was cancelled, allowing the user to select additional amplification.]

Another interesting study was conducted to determine the effect of people's (both hearing aid users' and audiologists') expectations on perceived performance. The study consisted of two groups of hearing aid users, each of which used two different hearing aids for a month each. Group A used two different digital aids (Digital X and Digital Y). Group B used the same digital aid (Digital Z) for both months, but was told that they were using a digital aid for one month and an analog aid for the other month.

Objective measures of the people in Group B showed equivalent performance with the two "different" hearing aids. However, the users rated their performance with the digital aid as superior to their performance with the analog aid. People preferred the digital aid by 16-2 in one study and 17-1 in another. The clear implication is that people's expectations are that digital aids are "better" than analog aids.

Even more revealing is the behavior of the audiologists. When writing their subjective evaluations of the performance of the people in Group B, the audiologists rated the performance higher with the "digital" aid than with the "analog" aid. This is true despite equivalent performance on the objective measures. Furthermore, subsequent subjective evaluations by other audiologists based on the same objective data, but without information regarding the type of hearing aids involved, evaluated the two aids as equivalent.

Mark was careful to point out that there is no indication that the audiologists are intentionally rating the "digital" aid higher than the "analog" aid; rather, the results are a byproduct of the audiologists' unconscious bias towards digital aids.

There is no question that digital processing enables the addition of many features to a hearing aid, and some of these features may be of benefit to the user. But excluding all the bells and whistles that are possible with digital aids, the performance of digital and analog aids is equivalent.

Another factor to consider is the convenience of using a particular feature. Several years ago we investigated the use of "body pack" FM systems. People who used them did better than people who used their hearing aids alone. But people weren't generally willing to buy the FM systems, because of their inconvenience.

We recently had a group of hearing aid users try the Microlink [Editor: Miniaturized FM System whose tiny receiver attaches directly to the hearing aid.] in nine different environments, and found that people did better with the Microlink than with their hearing aids alone in all nine environments. Moreover, most participants were willing to buy the Microlink with their own money. The reason is because it's more convenient to use than the older "body pack" systems.

Mark next evaluated another study that attempted to determine the effect of various hearing equipment on people's quality of life. Compared to people who used no equipment to assist their hearing, people who used a hearing aid with an omnidirectional microphone reported a quality of life improvement; people who used a hearing aid with a directional microphone reported a larger improvement; but, surprisingly, people who used a hearing aid and assistive listening devices (ALDs) reported NO improvement. Upon investigation, Mark discovered that the ALD was a Pockettalker and that it was used on average only a half hour a day. He suspects that the lack of quality of life improvement was due to the limited time that the Pockettalker was used. But the publicized study results are that ALDs don't contribute to improved quality of life.

The best way to improve the performance of hearing aids is to improve the signal to noise ratio (SNR) by reducing the noise. Recent research has focused on two methods of noise reduction. One is to break the sound spectrum into several channels and to process each channel independently. Channels with little or no noise present are amplified more than channels with a lot of noise present. When the channels are recombined, the resulting SNR is increased. A second method relies upon the consistency of some noise sources. A noise source that is constant can be subtracted from the incoming sound, leading to a much-improved SNR.

A couple of observations before questions:

- In a recent study of 1600 people who had hearing aids between six and 12 months, 71% wore them at least four hours a day, but 7% had not worn them at all in the past two weeks.

- While lab studies indicate that people have improved performance with more advanced technologies, studies of people in the field do not support this finding. Mark questions these results.

Q. Is it true that the only really effective way to find the hearing aid that is best for you is to try many different hearing aids?
A. All of today's quality aids have a nearly infinite range of adjustments, so many of them are "best" for an individual. It's really not practical or necessary to try a huge number of aids; it's probably more important to get the aid properly adjusted.

Q. How can we get independent hearing aid research?
A. The Rehabilitation Engineering Research Center on Hearing Enhancement (RERC) is one organization conducting independent research. But the research is difficult, because there are so many factors that can be adjusted.

Q. You haven't discussed cost today. Has there been any research on benefits vs. cost? I'm thinking of third party providers and whether they provide a hearing aid that's "good enough".
A. I really haven't seen anything that compares the performance of devices based on price.

Q. What do you think about the trend for current hearing aids to provide the user less control?
A. The manufacturers seem to be substituting their judgment for the user's judgment. I don't think that strategy accounts for the real world. I'd prefer to retain the control myself.

Q. Most audiologists seem to feature the aids of a few manufacturers. What if I want an aid that audiologists don't know much about?
A. 95% of all people can be fit with just a few aids. Only people with unusual hearing loss really need to pursue some of the more unusual aids.

Q. Amplification is important, but isn't discrimination also important?
A. We really need to take a holistic approach. Communication strategies are important, as are assistive devices, and hearing aid tune-ups. I think we sometimes focus too much on the hearing aid and too little on everything else.

Q. I've had to try up to seven aids before finding the right hearing aid for me. Is that unusual?
A. I think your time might be better spent finding the right person to fit your aids, rather than the right aid. Then you need to trust the person you select.

Q. Do you think we'll ever be able to fit hearing aids ourselves?
A. You really can't expect to replace the six years of studying required to become an audiologist.