Dr. Ross on Hearing Loss
Environmentally Adaptive Hearing Aids
by Mark Ross, Ph.D.
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).
The newest generation of digital hearing aids are simply loaded with environmentally adaptive features. In addition to controlling loudness, they operate on many other dimensions of .a hearing aid’s performance. In point of fact, it is the number of these features and the extent that listening decisions are automatically selected that define a “premium” hearing aid. The ultimate premium aid is one that is presumed to work optimally and automatically in every changing environment. (“Look Ma, no hands.”). At a minimum, these aids would include the following features: dynamic feedback suppression, adaptive directional microphones, Auto-T coils, multiple channels (up to 20), multiple memories, and various noise reduction or speech enhancement programs. Since understanding speech in noise is the most frequent complaint voiced by hearing aid users, manufacturers have devoted most of their research and engineering efforts in premium hearing aids to address this issue.
The signal processing algorithms employed by many hearing aids are designed to classify incoming sounds as speech in quiet, speech in noise, noise alone, mechanical sounds, impulse sounds, wind noise, and music (not necessarily all in any one hearing aid). Each of these noise conditions requires a special signal processing strategy intended to maximize speech comprehension in a particular environment. At the premium level, the hearing aid is presumed to automatically shift into the appropriate speech processing strategy, as the nature of the noise background changes. At less than the premium level (e.g., entry, economy, low, mid, or mid-high level aids), a user may be required to manually select a desired program, if it is available (e.g., speech in noise, directionality, music, etc.). Whether the convenience of automatic shifting is worth the extra cost of a premium level hearing aid is a question that people must answer for themselves, it being understood that the necessary comparative information is being supplied by the hearing aid dispenser.
Speech enhancement programs are another interesting environmentally adaptive feature found in many current advanced hearing aids. Through the use of special detectors, this program analyzes the speech to noise ratio that exists in each channel of the hearing aid. When the speech to noise ratio is positive (speech louder than the noise), the degree of amplification (gain) is automatically increased in that particular channel. When the noise signal dominates, the gain in the channel is reduced (perhaps via another adaptive noise program). Hearing aid manufacturers make a persuasive theoretical case regarding the merits of such automatic features; however, it would be reassuring to hearing aid users if the theoretical merits could be validated by research conducted on human beings. Actually, this comment applies to just about all of the special features found in advanced hearing aids, not just speech enhancement or noise suppression programs.
Automatic telephone coils (Auto T coils) are another adaptive feature that can be found in many premium hearing aids. All a user has to do is answer the phone normally, and the T coil in the hearing aid will automatically be switched on. After a stumbling start (the first Auto T coil did not work with induction loop systems), most hearing aid companies now apparently make provision for both the Auto T coil and for a way to manually access the T coil for other than telephone usage (as an assistive listening device receiver). However, while hearing instruments may include this capability, the aids do have to be appropriately programmed in order for the capability to be realized. Since most new hearing aid users probably never heard of a T coil, it is up to the dispensing professional to inform his or her clients what it is, how it should be used with both telephones and with induction loop systems, and what the clients have to do to access it on their hearing aids (e.g., toggling to the right memory).
This is where the human factor comes in; too often, from what I keep hearing, the T coil is often the forgotten element in the hearing aid selection and fitting process. It may not be as glamorous as the other features in an environmentally adaptive hearing aid, but it does directly impact upon the basic purpose of a hearing aid, and that is for the person to hear better in many types of situations. As one personal example, I use the T coil in my hearing aid and cochlear implant while talking on the telephone (a neckloop permits me to use both ears); at religious services, I plug a neckloop into the provided FM receiver; and, finally, I listen to the TV using a small floor loop connected to the TV. It is the T coil that makes all these functions possible and I can’t imagine being without it.
During a post-fitting visit, the Audiologist may look at the hearing aid use data. If the data shows only minimal use of a hearing aid, then the question is “Why?” If the log shows that the person is in quiet 90% of the time but has the directional microphones switched on most of the time then, again, the question is “Why?” Does the person understand how to use this feature? In one real-life example, a client complained that the VC did not work. Checking the data log, the Audiologist noted that the volume control was never changed. It turns out that the client was pressing on the battery compartment when trying to make volume control changes! Before we laugh at this user’s ineptness, we should reflect how often we’ve all done things like this ourselves.
Hearing aids have come a long way since the days that a user had to constantly “fiddle” with the volume control in order to get it right. And there’s no doubt that a great deal of creative engineering has made life easier for many hearing aid users in recent years. But I would add the following caveat: There is still a need, in my opinion, for the human factor. Audiologists still have to “listen” to their clients, judge their listening needs and capabilities, and make their recommendations accordingly. And that may include reducing or eliminating some of the “automatic” control for certain operations, such as partial volume control and manual selection of the T coil.