Environmentally Adaptive Hearing Aids - Part One

By Mark Ross

Editor: Hearing aids are smarter all the time, and are now clever enough to adapt to the environment in which a person is trying to hear. Here's Mark Ross with his thoughts on these devices.

This article was first published in the May/June issue of Hearing Loss magazine, a publication of HLAA, and is reprinted with the author's kind permission.

This is part one of two parts.

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May 2009

The only sound dimension that was "adaptive" with my first hearing aid was the loudness. I could choose to turn the volume control up or down, and that was it. True, the aid also contained an accessible tone control with three choices (normal, high-tone, and low-tone), but for the most part, I (and most hearing aid users) left that one alone. Evidently, I was one of those volume control "fiddlers" that the hearing aid dispensers at the time inveighed against. "Don't fiddle with the volume wheel," they would counsel their clients. "Normal hearing people can't keep adjusting the volume of the sound, and neither should you." But they really didn't know what they were talking about. For example, a 10 dB increase in sound input may double the loudness for a normal hearing person, while for a hearing-aid user this same 10 dB increase may cause an intolerably loud auditory sensation (because of the phenomenon known as "recruitment"). Conversely, while for a normal hearing person, a 10 dB decrease in sound input may reduce the loudness sensation by about half, for a hearing aid user this same reduction may make the sound totally or virtually inaudible.

So to get the loudness sensation right, or as close to it as possible, most hearing aid users at the time employed what I facetiously termed AFC, or automatic finger control (audiologists just love acronyms). We constantly fiddled with the volume control. For many of us, it became an almost instinctive action, one that scarcely occupied our attention. But there were also many people who, for a number of very legitimate reasons, found the necessity of constantly adjusting the volume control to be a difficult or inconvenient task. Years ago, the hearing aid industry responded to this by including an automatic gain control circuit (AGC) in their hearing aids. This was, I believe, the first automatic, or environmentally adaptive, feature to be included in hearing aids. And it is probably still the most important. The modern version of an AGC, present in just about all digital hearing aids, is called "Wide Dynamic Range Compression" or WDRC (another acronym, sorry about that). Basically, what this feature does is adjust the degree of amplification of the entire range of loudness input levels, while at the same time ensuring that no input sound is amplified to the point where it is uncomfortably loud. In a way, this feature does the "fiddling" for the hearing aid user. Soft sounds are boosted in loudness, while loud inputs are toned down somewhat. The goal is to ensure comfortable sound sensations across the entire range of frequencies amplified by the hearing aid, regardless of the changing input sound levels.

The specific amounts of amplification used in hearing aids possessing this feature are set during the fitting process. The hearing aid user makes judgments as to the loudness of input sounds (too quiet, just right, too loud, etc.) in quiet and under various simulated environmental noise conditions, and this information is then programmed into the hearing aid. And there it remains until and unless the hearing aid is later reprogrammed. But does this initial programming get it right, all or most of the time? This was one of the questions asked in a recent study published in the Hearing Journal.

The investigator (Dr. Gitte Keidser) asked 28 experienced hearing aid users to keep a structured listening diary for two weeks. At specific intervals during the day, they would log the specific category of background sound they were then encountering (speech in quiet, speech in noise, music, mostly quiet, or mostly noisy). They then recorded their judgment of the overall loudness level of the sound, as either just right, louder or softer than preferred, or much louder or much softer than preferred. All the subjects were individually fitted with an "environmentally adaptive hearing aid." The programming goal was to provide a comfortable loudness sensation in the various sound backgrounds. The question asked by the study was: How often did the aid get it right in each of the listening backgrounds? As I consider the results, the answer seems to vary from "not bad" to "not good enough."

In listening to speech in quiet, only 55% of the participants rated the loudness as "just right," with the others feeling the speech was either somewhat louder or softer than preferred. "Just right" judgments declined when listening to speech in noise, with only about one-third of the subjects feeling satisfied with the loudness; the remainder of the time, they felt that the sound was either softer or louder than preferred. In a noisy situation, with nobody talking, over half the subjects felt that the aid provided a loudness sensation louder than they would prefer. The highest loudness comfort scores were obtained when listening to the radio or TV (58%). This makes sense if we consider that here the participants did have access to a volume control, but on the TV set and not on the hearing aid itself.

The study also noted a certain inconsistency in the ratings. Over the course of the study, the participants often found themselves in the same type of listening situation (e.g., speech in noise), either during the same day or on different days. However, they did not necessarily rate the loudness sensation the same each time they encountered a similar type of listening situation. And this should not surprise us. Recall, that during the hearing aid programming process, the loudness level was adjusted to provide a comfortable listening experience - but only for that day, with that specific speech stimulus and noise background. In short, the loudness programming was a snapshot of reality but not reality itself. For that, one has to get out of the clinic and into real life. In real life, little about a listening situation is static; the composition of the noise, the person talking, the hearing aid user's interest in the message, and the physical surroundings are always changing. A one-time hearing aid programming decision cannot, therefore, completely replicate a person's everyday listening experiences.

Not surprisingly, the study concludes that some individual manipulation of the volume control is desirable, even with an environmentally adaptive hearing aid. The findings indicate "that at least two-thirds of the participants would, at some point, want gain adjusted when in a given class of environment." In other words, there is still a need for the tried-and-true volume control. Fortunately, it is possible to have it both ways. The hearing aid can be programmed to provide reasonable "ball-park" loudness in various situations, while an external volume control can be included for finer loudness tuning when desired (for example, manual control of plus or minus 5 dB around the automatic values).