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Dr. Ross on Hearing Loss

Audibility and Useful Hearing Aid Features

by Mark Ross, Ph.D.
This article first appeared in
Hearing Loss (Jan/Feb 2003)

In preparing this column, I ordinarily consult the trade and professional literature for information on significant developments in hearing assistive technologies (hearing aids and other devices). There is usually no shortage of research studies to review and other developments to report. In recent years, in particular, there has been a virtual banquet of technical developments and speech processing strategies, thanks in large part to the flexibility that digital signal processing offers. Indeed, there is such an abundance of information that drawing any kind of conclusion is often very difficult. In this paper, I'd like to do two things. The first is to review the concept of audibility as it applies to hearing aid amplification; this is a basic principle in hearing aid amplification that applies to all hearing aids and speech processing strategies. The second purpose is to take stock of where we are now in terms of hearing aid features that have been corroborated by clinical research.

Audibility

This is going to sound simplistic, but I'm going to say it anyway: The purpose of hearing aids is to amplify sounds sufficiently so that they can be heard by a listener. Audibility is the key concept underlying speech perception; generally speaking, the more audible a speech signal is, the easier it can be understood. This rather obvious statement says that the more speech sounds we hear, the better we understand. As we look closer into this concept, however, we'll see that it is not quite as easy to provide as it is to describe.

By definition, a person with a hearing loss either does not hear incoming speech or hears only some portion it. Since people usually have different degrees of hearing loss at the different frequencies (ranging from low to high pitch sounds), the portion they hear, or do not hear, is going to differ depending upon the configuration of their hearing loss (their audiogram). It is, therefore, necessary to individualize a hearing aid fitting so as to provide somewhat more amplification at certain frequencies than at others. So far so good. However, the picture gets a bit more involved at this point.

Just because a person has a hearing loss does not mean that he or she can tolerate louder sounds than someone with normal hearing. Usually, the tolerance level for someone with a hearing loss is about the same as that of people with normal hearing. Thus, the hearing-impaired person experiences a reduction in the dynamic range of hearing, i.e., the number of decibels between the threshold of hearing and the level where sound is just too loud.

Let's say a person's threshold at a particular frequency is 60 db and his or her loudness tolerance level is 90 dB. This results in a dynamic range of 30 dB but only at that frequency. Other frequencies may have the same or different dynamic ranges. The challenge of the hearing aid dispenser is to adjust the hearing aid amplification pattern so as to package all input speech levels into the different (and reduced) dynamic ranges across frequency. Furthermore, this has to be done in such a way as to ensure that no matter how loud the input level, the output level does not exceed a person's loudness tolerance (in this example, 90 dB).

At this stage, other decisions enter into the fitting process. The intensity level of speech that people are exposed to varies from soft whispers to loud shouts. We would like the hearing aid to provide appropriate audibility across frequency for all input sound levels. The fitting goal is for the person to be able to hear soft speech softly, but to definitely hear it (without a hearing aid the person would not hear it at all) and loud speech loudly, but tolerably so. Input levels between these two extremes should be heard at intermediate levels. Actually, what I'm describing is a wide dynamic range compression (WDRC) hearing aid (a feature discussed in earlier issues of this journal).

What a WDRC (or automatic gain control) circuit does is vary the degree of amplification, depending upon the intensity level of the input. In the case of a single-band conventional hearing aid, it is the loudest input sound that controls the automatic gain control circuit. This can be a problem in certain cases. Assume that the input is an intense, relatively low frequency signal. When this happens, the hearing aid amplification is reduced for all the sounds being processed by the hearing aid, not just the low but the high frequency sounds as well. Since the higher frequencies are somewhat more important for speech perception than the lower ones, this action of the automatic gain control can actually be detrimental to speech perception. That is, the amplification of the higher frequencies is reduced when they should not be. However, modern technology has provided an answer to this: multi-band hearing aids.

In a multi-band hearing aid, input speech sounds are divided into a number of separate bands, each of which is separately programmed and controlled. In a two band system, for example, the dividing point may be at a frequency of 1500 Hz, with the low band below that point and the high frequency band above it. Three band systems would divide the frequency spectrum into three portions, and so on. Intense loud input signals in one band will have only a minimal effect upon the amplification pattern in other bands since each is separately controlled. Thus, in the example of an intense low frequency input signal, audibility can be preserved for the higher bands in spite of the operation of an automatic gain control circuit in the low band. Multi-band hearing aids also permit the audiogram to be fit a bit more precisely than possible with a single band hearing aid. However, while multi-band hearing aids do appear to make a lot of sense, there is little evidence to indicate that speech perception improves beyond about four or five bands.

Even though audibility is the key ingredient underlying speech perception, and one that applies across all types of hearing aids, it is not the only ingredient. For example, while two hearing aids may provide exactly the same audibility, a person might not hear exactly the same with both of them. There are a number of other factors that come into play, such as time constants (how quickly the hearing aid decreases and increases the amplification), various kinds of distortion factors, and so on. Nevertheless, in spite of the existence of these other factors, working to maximize amplified audibility is a good initial goal in any hearing aid fitting.

Useful hearing aid features

In the discussion above, I commented on two hearing aid features that have proven to be useful to many hearing aid wearers: Wide Dynamic Range Compression (WDRC) and multi-band hearing aids. I will not, therefore, repeat myself here except for one point relevant to WDRC aids.

A volume control is often considered unnecessary in a WDRC aid. Since the aid automatically and rapidly makes the gain adjustments, some feel that there is no need for user control over volume. This draws too simple a picture. While it is true that eliminating the volume control offers definite advantages for many people (for example, those who have difficulty manipulating the tiny volume wheel on the hearing aid as well as those who just don't want to bother), other people prefer to have more personal control over their listening experiences.

The loudness programming that takes place during the hearing aid fitting is fixed and cannot be changed unless the aid is reprogrammed. In real life, however, a hearing aid user's loudness preferences may vary depending upon the environmental circumstances and the person talking. There is no way that any programming in a clinic can predict every listening situation that a user will experience. Some of us (and I include myself) prefer to make our listening decisions specific to each such circumstance. For these hearing aid users, the benefits of WDRC aids are reduced when they are given access to the volume control (but it does take UFA unfettered finger access!).

Directional microphone hearing aids

I have discussed this feature on a number of recent occasions and will only make some summary comments here. Directional microphones are a hearing aid feature with significant research support. Because directional microphones suppress sounds that arrive from the side and the rear, their operation, in effect, increases the speech to noise ratio (or, to return to an earlier topic, it increases the audibility of the desired speech signal relative to other sounds).

To achieve the full benefit of a hearing aid with directional microphones, a user must be sensitive to the surrounding acoustic environment. If the user were to talk to someone whose back was to a noisy sound source (such as the band at a social function), the directional microphones would do little good. In such a case, both the desired sound signals (the other person talking) and the undesired sounds (the music) would be coming from the same frontal direction. The position of the two people would have to be reversed in order for the directional microphones to be effective.

Directional microphones also do not work too well in highly reverberant conditions. In such a situation, the sound signals are bouncing off the surfaces of the room and impinging upon the microphone from every direction. Thus both undesired and desired signals are arriving from the frontal position and the hearing aids cannot distinguish them. Nevertheless, in spite of these limitations, I do think that directional microphones can offer significant help in many situations and are a very useful hearing aid feature.

Feedback cancellation

Acoustic squeal with hearing aids occurs when some portion of the amplified sounds (usually the higher frequencies) escapes from the ear canal, reaches the microphone of the hearing aid, and gets re-amplified. This action begins the feedback cycle of amplification and re-amplification of the same signals, resulting in the acoustic squeal. Over the years, a number of ways to control acoustic feedback have been suggested.

The traditional solution has been to focus on the earmold, to try to seal the amplified sound in the ear canal with tighter and tighter earmolds. However, there are limits to our ability to seal the amplified sound within the ear canal. We need another way to reduce acoustic feedback. Electronic means have been recommended for years, but were not really feasible until the advent of digital hearing aids. Recently, several hearing aid companies have introduced hearing aids that incorporate realistic electronic means of reducing acoustic feedback. In user terms, this means that a person can obtain more gain from a hearing aid before the onset of acoustic squeal.

Electronic solutions have included reduction in the high frequency gain of the hearing aid when feedback is sensed. This may indeed minimize acoustic feedback but, at the same time, audibility for speech signals at the high frequencies is also reduced. A variation on this method is the use of a notch filter. Such a system requires that the hearing aid include some kind of sensor that can detect and measure the frequency of the squeal and then reduce the gain in a narrow band just around the offending frequency. However, this method also requires modifying the hearing aid's frequency response when feedback occurs. While such modifications may be minimal, audibility is still reduced somewhat. Still, this method of controlling feedback is likely to have much less of a negative effect than the signal distortions caused by acoustic feedback.

The optimal method is one in which feedback can be reduced electronically without any modifications in the basic response of the hearing aid. This method also depends upon a sensor circuit that can continually detect and monitor the occurrence of acoustic feedback. However, rather than using a notch filter to reduce the feedback, a signal is created within the hearing aid which is of opposite phase to the feedback frequencies. Essentially what this does is cancel the feedback. It is as if two people of equal strength were simultaneously pushing on opposite sides of a swinging door. The door would remain in the central position as each person's effort cancels the other's.

There are now at least four hearing aid companies that incorporate some variation of this feedback cancellation method in their hearing aids. Studies have shown that most people can achieve 10 dB more gain without feedback, with some obtaining even better results. This means that aided audibility need no longer be restricted by the onset of acoustic feedback. Not only that, but this result can be achieved while wearing a looser, more comfortable earmold. This is one development that I'm eager to try myself. It is certainly a feature that I would recommend for people who have been bothered by acoustic squeal in their hearing aids.

Personal FM systems

In my opinion, a personal FM system may be the most flexible and effective recent hearing aid development. While classroom FM systems have been widely used for well over 30 years, it is only fairly recently that FM receivers (actually, miniature FM "radios") have been incorporated in behind-the-ear (BTE) hearing aids and are thus feasible for use by adults. With a modern personal FM system, the FM microphone-transmitter (about the length of an eyeglass case and half its width) easily becomes a "remote" ear. By this I mean that it can be placed closer to a sound source than is possible with our actual ears (or hearing aids). The sound signals are picked up by the FM microphone and transmitted to the FM receivers in the hearing aids. Let me give a few examples.

At any noisy gathering, the user can hold the microphone in his or her hand and place it closer to a talker's lips than is physically possible with hearing aids. When in a restaurant with just one other person, the user can place the microphone next to that person (or it can be worn around the speaker's neck). If there is more than one person at the table, the microphone can be placed at its center, or closer to the people who are particularly difficult to hear. Other uses are: talking with someone on a noisy street or in a moving vehicle, listening to lectures or in classroom settings, and picking up the sound from a TV set when away from home. (I'm assuming that most SHHH members use some sort of TV listening device at home.) The uses are endless and they all have one principle in common: The closer we can locate an "ear" to a sound source, the greater the speech to noise ratio (or audibility). No other hearing aid development, no matter how positive it may be in other respects, can increase the signal to noise ratio to the extent possible with a personal FM system.

So why aren't these systems more widely used by people with hearing loss? For two reasons, the first being the cost. Existing FM microphone-transmitters, plus FM boots for each BTE hearing aid, may add several thousand dollars to the price of the aids. That can be quite a disincentive, even to motivated people.

Just recently, however, a low-cost personal FM system called the "Conversor" has been introduced to the market. It comes in two parts: the FM microphone-transmitter, which is about the size of a thick eyeglass case; and a body worn FM receiver that is suspended around the neck. The suspension cord doubles as a neckloop responsible for transmitting an electromagnetic signal to the telecoils ("audiocoils") in the hearing aids. While the technical description seems very impressive, I have no information as of yet on its real-life performance. Still, if this system works as advertised, it should help reduce the financial objections to using a personal FM system. (The system is marketed by the Hal-Hen company in New York.)

But I think the more important reason relatively few adults use personal FM system is their visibility. It is not possible to utilize such a system without "advertising" the presence of a hearing loss (and the low-cost "Conversor" is even more visible than existing models). The listening advantages are there and they are undeniable. Unfortunately, however, some people choose to forgo these advantages rather than display visible evidence of their hearing loss and hearing needs. Overcoming this reluctance is, of course, what membership in an SHHH chapter can help people do, that is, help people accept and acknowledge their hearing loss and be more proactive in asserting their own communicative needs. Once this is done, the use of a personal FM system can be accepted as a logical and effective way for hearing-impaired people to improve their functional hearing capabilities.

Telecoils (or "Audiocoils")

No discussion of "useful hearing aid features" would be complete without mentioning telecoils. Telecoils were originally designed to pick up the magnetic field around telephones. For many people with hearing loss, particularly those with more severe hearing losses, a telecoil is the most effective way to communicate on a telephone. What I'd like to comment on here, however, is the role that telecoils can play as an assistive listening device "receiver." Right now, in the U.S., this potentially valuable function of telecoils is sadly underutilized.

There are three assistive listening systems that can be employed in a variety of venues, such as theaters, auditoriums, and houses of worship: FM radio, Infra-Red (IR) and Induction Loop (IL). At the present time, FM and IR are the most commonly used assistive listening systems in the U.S. They both require the use of a special receiver to pick up the FM and IR signals. Facilities that use these systems must organize some method of checking the receivers in and out, instructing patrons how they should be used, and ensuring that the receivers are working properly. Judging from the complaints I receive, and from my own experiences, the logistics of managing receivers are the major source of problems in ensuring auditory access in large area listening situations. It seems that almost anything that can go wrong does, at one time or another. (Murphy's Law seems to work overtime when it comes to assistive listening systems!). Of course, when everything is working as it should, the advantages of either of these two systems are many and marvelous. But, still, it does seem that receiver problems will always be a pain in the neck.

With an induction loop (IL) system, on the other hand, no special "receiver" is required, as long as a person's hearing aids include telecoils. An IL system generates an electromagnetic field in the listening area, a signal that can be picked up by telecoils in the same way they pick up telephones signals. All that is required is for a person to switch on the "T" coils and, presto, auditory access is achieved. This is an enormous advantage for hearing aid users. There is no need to check out special receivers. Not only is this more convenient, but it is especially conducive to use by those people who are reluctant to wear a visible assistive listening device. Furthermore, and most important, hearing aid users can be assured that their "receivers" are functioning well and that any individualized hearing aid programming is still operative.

We seem to be in a "chicken and egg" situation insofar as IL systems are concerned. There are relatively few telecoils included in hearing aids because only their role with telephones has been stressed. And there have been relatively few IL installations, in large part because too few hearing aids contain telecoils. The easiest way to break this cycle, in my judgment, is to increase the number of telecoils in hearing aids. That this can be done effectively has been conclusively demonstrated by Dr. David Myers, an SHHH member in Michigan, who has initiated a project called "Let's Loop America". In the course of this effort, he has succeeded in convincing most hearing aid dispensers in his area to routinely include telecoils in the aids they dispense. In addition, he has managed to convince several large hearing aid companies to increase the proportion of hearing aids with telecoils that they produce. Who says an individual activist cannot make a big difference!

I think that part of the reason that relatively few telecoils are being used is precisely because they are called "telecoils," a label which appears to restrict their function to telephone use. Perhaps if we relabeled these little coils of wire (technically, "induction coils") and called them "audiocoils," the totality of their potential contributions could be more easily realized.

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