Improving the signal-to-noise ratio through more sophisticated hearing aid technology has been a major goal of hearing aid engineers for years. At the current state-of-the-art there are only two hearing aid features that can do this. One is the directional microphone and the other is a personal FM listening system.
The loudness level of speech relative to the noise level is probably the most important factor determining how anyone, with normal hearing or with a hearing loss, can understand speech under noisy conditions. This relationship is termed the speech-to-noise (S/N) ratio and its applicability is quite straightforward: if people can’t hear the speech in a background of noise, then it won’t be understood. Within limits, the more the speech signal exceeds the noise, the better it will be understood.
On one hand, for someone with normal hearing, a speech-to-noise ratio of about plus 6 dB (i.e. the speech signal 6 dB louder than the background noise) will provide adequate, if not perfect, understanding.
On the other hand, most people with a hearing loss would barely be able to understand at this level; for them, it would be necessary to raise the S/N ratio to about 15-20 dB (and the more, the better).
Improving the S/N ratio through more sophisticated hearing aid technology has been a major goal of hearing aid engineers for many years. A number of modem hearing aids do include noise rejection circuits. While these can make speech more comfortable to hear in noisy places, they do not actually improve objectively measured speech perception scores.
These circuits operate by reducing the amount of amplification (the “gain”) at points in the acoustic speech spectrum in which the greatest amount of noise is present. In this doing this, however, they reduce the gain for both the speech and the noise. No hearing aid noise suppression circuit is able to strip away only the noise from a speech signal, particularly if the noise consists of other people talking (as in a noisy party). In other words, these hearing aid circuits do not increase the actual speech-to-noise ratio at any specific frequency band. And without an increase in the speech-to-noise ratio, it is unlikely that any improvement in speech perception can occur.
At the current state of the art, there are only two hearing aid features that can improve the signal-to-noise ratio.
One is a directional microphone, which has been discussed at length in previous issues of this Journal. Basically, directional microphones operate by reducing the intensity of sounds that arrive at the hearing aid microphone from any direction other than directly in front. Since the hearing aid user is presumably facing the desired source of sound (another person talking), then if the hearing aid user’s back is to the undesired sounds, the net result would be an increase in the S/N ratio of the talker’s speech. Thus directional microphone hearing aids can and do significantly improve speech perception in noisy places. Indeed it is a hearing aid feature that has received much research support attesting to its effectiveness (with the proviso that it must be used in a way that maximizes its acoustic properties).
The other hearing aid feature that will significantly increase the S/N ratio is a personal FM (frequency modulated) listening system. Strictly speaking, of course, a personal FM system is more than a hearing aid feature, since it entails an external device (the FM microphone transmitter) that works in concert with personal hearing aids. Properly used, when the FM microphone is located close to the source of sound, say someone talking, these systems are capable of delivering a speech signal to a listener much louder than any competing background sounds. That is, they are capable of increasing the speech-to-noise ratio more than any other hearing aid feature now available. While other assistive listening devices can provide similarly positive S/N ratios, at present only a personal FM system is sufficiently portable and flexible for use in everyday applications.
Studies on Both Directional Microphones and Personal FM Systems
The availability of both directional microphone hearing aids and personal FM systems begs the question of their relative effectiveness. Both can improve speech perception, but will they do this equally? Other than a study on a few children completed about 20 years ago that used technology currently outmoded, there have been no recent investigations of this topic.
This gap was recently filled by a study reported at an international conference devoted to FM systems. It was a multi-site study in which investigators from the University of Florida in Gainesville and Washington University School of Medicine in St. Louis took part, using the same research protocol in both places.
All 45 subjects from both sites were fitted with top-of-the-line digital behind-the-ear (BTE) hearing aids. Each aid was fitted with an FM “boot” that snapped under the base of the hearing aid. A personal FM microphone/ transmitter was provided to each subject (the PhonakTX3 HandyMic). First, each subject was tested binaurally with the hearing aids altemately set in the omnidirectional and directional modes. Then, the subjects were tested with an FM input, first just to only one ear and then to both ears.
The investigators used a speech measure that may be unfamiliar to many consumers. We are all familiar with the use of percentages to describe speech discrimination scores (between zero and a hundred percent). Such tests are a component of routine audiological evaluations. In the test used by these examiners, a different metric is used (the S/N ratio). In this test the noise level is held constant while the speech sentence intensity level is varied up and down until the person achieves an intelligibility score of 50 percent (or getting half the sentences right). At this point, the resulting S/N ratio is the score the person achieves. Research has shown that this method is more reliable and valid than conventional methods of testing speech discrimination abilities (though more difficult to convey to consumers).
On this test the best scores would be those with smaller or even negative signal-to-noise ratios. For example, if in one test condition with directional hearing aids the score is minus 2 dB, this means that the person could understand 50 percent of the sentences when the speech was 2 dB weaker than the noise (or a S/N of- 2 dB). And if this same person now obtains a score of + 2 dB with omnidirectional hearing aids this means that the speech signal had to be 4 dB more intense than the previous condition in order for this person to achieve the same 50 percent sentence comprehension score.
Now this may not sound like too much of an advantage. I mean, how important can 4 dB be? However, when these dB figures are translated into percentages, the differences can be quite significant.
For every 1 dB increase in the S/N ratio immediately beyond this 50 percent point, sentence recognition scores will improve by about 8 percent (until a maximum score is reached for a particular person).
So if the results in one condition are 4 dB better than another then the difference in sentence recognition scores could be about 32 percent (8 times 4).
In this example, in the test condition that produced a 50 percent score at an S/N of minus 2 dB, the person would score about 82 percent in the plus 2 dB condition. And that is a very significant difference! It’s important to keep this concept in mind as we discuss the results of this study. Another thing to keep in mind is that these differences do vary with the kind of speech material used in the test. For monosyllabic word discrimination tests, the increase may only be about 4 percent to 6 percent per dB but of course, this too can be very important for an individual.
The results of the study that compared just the directional and omnidirectional microphones were in accord with the previous studies on this topic. Averaging the findings between the two sites show a directional microphone advantage of 2.3 dB or about 20 percent improvement with conventional sentence identification tests. Functionally, this can be pretty impressive; anyone wearing a hearing aid would know when one heating aid condition produces a 20 percent increase over another condition.
From my perspective, however. the most interesting comparisons are those that were made between directional microphones and FM systems. When the FM signal was directed to only one ear, with the other ear using the hearing aid alone in the omnidirectional mode, there was a major and significant reduction in the S/N ratio required to produce a 50 percent sentence intelligibility score.
These differences were much greater than that which occurred between the directional and omnidirectional microphones. There was a further, and significant, decrease in the S/N ratio when the signal was directed to both ears via the FM system (recall that lower S/N ratios mean better hearing in this test). Clearly, the results of the study demonstrated that a properly deployed FM system to both ears (i.e., when the microphone is placed close to the source of sound) produces the most favorable listening condition.
Comparing Apples to Oranges
Actually, however, as I was writing this section I began to feel a bit uncomfortable with the study (though it was very carefully done). The comparison between listening through an F.M system and through hearing aids that use directional microphones is a bit like comparing apples and oranges.
While both increase the S/N ratio, in terms of their basic operation, FM systems and directional microphone hearing aids serve different purposes and have advantages in different types of situations. A hearing aid has to stay stuck on one’s head. The sounds that a person listens to have to travel from the source, however distant, to the hearing aids located on or in the ear. And it’s not always easy bringing one’s head close to the source of a sound! (Imagine placing one’s head six inches from a speaker at some lecture!)
The situation is different with an FM microphone. In this case, one can bring just the “ear” (the portable FM microphone) to the source of sound.
And nothing increases the signal-to-noise ratio more than placing an ear or a microphone close to a sound source.
But there are times and situations where one cannot easily deploy an FM microphone. For example, it’s not possible to bring the FM microphone close to the lips of every talker at a large dining room table. At times like this, it makes more sense to use directional microphone hearing aids.
The point is that the use of one does not preclude using the other, for different purposes and in different situations. In other words, both of these devices serve somewhat complementary purposes and each does what it was designed to do.
Cost Factors
There was another aspect of this study that was not included in the printed version. The experimenters permitted the subjects to use the aids and personal FM systems for the seven months duration of the project (during which time the formal testing took place). After the completion of the project the subjects were offered an opportunity to purchase the hearing aids and FM systems at a reduced cost.
Only four participants took advantage of this offer This was a somewhat discouraging piece of news to hear. After looking into it, however, it does seem that the final purchase cost was still pretty high and undoubtedly played a role in the subjects’ decisions. They had to purchase not just the FM receivers and microphones, but the topof-the-line hearing aids they had been wearing as well. Even at the reduced price, the total cost came to over $7,000 (or $4,000 less than the regular price).
People with hearing loss certainly do want to improve their functional hearing abilities, and a personal FM system can certainly accomplish this, but there are limits to what many people can afford. I suspect that the situation would have been different if the subjects had had the opportunity to just purchase the personal FMs alone (assuming their own were compatible with the FM boots as many modern hearing aids are). In this way, the cost to the subject could have been in the $3,000 range. Still, this is a high figure. Hopefully, as more people purchase these systems, the economies of scale can enter in, and the costs could be significantly reduced.
Self-Assessment Studies
Another multi-site study at the International FM Conference also examined the responses to personal FM systems and hearing aids by people with hearing loss, but they took a somewhat different approach. Instead of focusing on sentence intelligibility scores, they depended upon the responses people made to standardized self-assessment scales, one of which required them to rate their communication difficulty in 18 different types of listening situations. In addition, the subjects were asked to identify specific types of situations that they personally found problematical. What the examiners were trying to do was first, to determine candidacy criteria for potential FM adult users and second, to assess changes in the self-assessment scores after FM usage. Since these people were already experienced hearing aid users, any positive change in their ratings would presumably reflect the contribution made by the FM system.
Determining candidacy criteria is important because some people who are hard of hearing do need the assistance that a personal FM system can offer more than others. Indeed, we may be discrediting the entire concept of personal FM systems when blanket recommendations for an FM system are made. While degree of hearing loss is an important factor in determining FM candidacy, a more important one is a person’s own perception of communication needs and their communication difficulty. This is what the standardized self-assessment scale is designed to elicit. What it does is quantify the personal and subjective responses of a particular individual to specific communicative and social situations.
For lots and lots of reasons, people with similar hearing losses often differ on their perception of listening difficulties in the same type of situation. Some people are intrinsically more social and have greater communication needs. Others may find themselves in uniquely challenging situations, either because of their work or the nature of their leisure activities. And still others may have some degree of central auditory processing problems and require especially high S/N ratios to function adequately. Whatever the reason, it is something that must be determined on an individual basis.
The subjects in this second study were initially selected for inclusion on the basis of the clinical judgment of the audiologists who evaluated them. These were people who reported dissatisfaction with their hearing aids in at least one situation in which an FM system could be beneficial. When their self-assessment scores were compared to a comparable group of adults with hearing loss, the FM candidate group experienced greater communication difficulty than expected on the basis of their hearing loss. These problems were particularly apparent in situations that were noisy or highly reverberant, in which the subjects found themselves some distance from the talker, and in which there were inadequate visual clues. So it does seem that a self-assessment scale can be predictive of FM candidacy.
In the next phase of the study, the examiners then fit all the subjects with personal FM systems and instructed them on their proper use. This, actually, was one of the key elements in the studvi We know from previous research, including the study reviewed above, that many people who appear to be appropriate candidates for FM amplification nevertheless choose not to acquire them.
This study differed from previous ones in that before the subjects actually used the FM system, they were provided a formal training program in how to use them effectively. Each one of the four training sessions (at two-week intervals) was devoted to one of the self-identified areas in which the subjects reported exceptional communicative difficulty. The goal was to improve a subject’s communication performance in each self identified area of difficulty.
This is the part of the study that I particularly like. The researchers didn’t simply assume that their subects would know how to effectively deploy a personal FM system. This is not at all obvious for many people. What the examiners did was provide oral, written and picture support that described how the subjects’ could achieve each of their personal goals. They were taught how to use the systems and the most effective manner to deploy them in the types of situations that they identified.
Throughout the process, the subjects received encouragement and support. In a sense, it is the same kind of follow-up program that people receive (or should receive) when they acquire hearing aids for the first time. What may appear obvious to an experienced hearing aid dispenser (i.e. placing the FM microphone as close to the source of sound as possible, or shutting off the environmental microphone in some situations) may not be obvious to someone using an FM system for the first time.
When the self assessment results obtained before and after using the FM system were compared, the positive impact of the FM system was obvious. In all the areas covered by the questionnaire (social activities, home, and work), the subjects reported much less communication difficulty when they used the FM systeni.
With their hearing aids, their average ratings indicated effective communication occurred only about half the time. With the FM system, this average rating of communication effectiveness increased to about three-quarters of the time. Insofar as their personally identified problem situations were concerned, well over 80 percent reported improvement in their ability to communicate in all of them when they used the FM system.
Another component of the study compared the ratings that the subject’s made in listening satisfaction to the normative ratings often used by the hearing aid industry (the “MarkTrak” surveys). Satisfaction ratings were compared for 12 different listening situations (i.e. one to one, small and large groups, restaurants, cars, etc.).
These results showed that, except for music, the subjects reported greater satisfaction while wearing the FM and their hearing aids than just the hearing aids alone.
Personal Reactions to Using FM Systems
Considering the results of both studies, there’s no question that a personal FM system can significantly increase speech perception scores and reduce the number and severity of communication problems. Still, people are reluctant to wear them. One factor that often arises is embarrassment. Some people feel reluctant to use such a visible device, feeling it may open them up for ridicule and rejection.
The subjects in the second study were explicitly asked about how often they felt “embarrassed, ridiculed, or rejected” because they wore an FM system. Over 90 percent indicated that they never felt this way. Of course they were asked this question after they were already routinely using the FM system and after they received much encouragement and detailed instructions.
Every one of the subjects in this study chose to keep the FM system after the study. However, as Veterans they were not required to pay for them. They were asked how much they would be willing to pay for the hearing help represented by the FM system and they came up with an average of about $2,300. (This should be compared to the $3,250 retail cost of the two receivers and transmitter used in the study). In reality, however, we really don’t know how many of the subjects would have been willing to pay the full retail price for the FM system.
When the expense of an FM system is added to the cost of two hearing aids, a bundle of money is involved, too much for many people to afford. This is really unfortunate, considering the proven value of having a “portable, remote” ear, one that can be moved close to sound sources.
Personally, I am confident that, if the cost of personal FM systems were to be significantly reduced, more and more people with hearing loss would begin to acquire them. And the more people there are who purchase them, the more likely it is that the cost could be further reduced. Someday, perhaps, the FM microphone/ transmitter will be seen as a hearing aid “accessory” one that can be pulled out of one’s pocket (or purse) to be used when necessary. This day would come a lot sooner, in my judgment, if the industry were to develop an FM microphone/transmitter about the size of a ballpoint pen, one that could “capture” a single voice across a large table. Its utility could be enhanced were it to serve multiple purposes, such as remote control for one’s hearing aids. Perhaps, considering the accelerated rate of technological developments, this day will come in the not too distant future.
In summary, it is clear that both FM systems and directional microphones can help. While FM systems will improve the S/N ratio more than directional microphones, both devices can significantly help people with hearing loss.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Dr. Ross’ article was first published in Hearing Loss and was supported in part by GRANT #H133E980010 from the U.S. Department of Education, NIDRR. The opinions expressed herein are those of the author and do not necessarily reflect those of the Department of Education.
Mark Ross, Ph.D., is an audiologist and associate at the Rehabilitation Engineering Research Center (REC.) at Gallaudet University. He dates his emergence into the field of audiology to the time he attended the Army Aural Rehabilitation program as a patient in 1952. He received his doctoral degree from Stanford University and taught at the University of Connecticut and worked as a clinical audiologist at the Newington Children’s Hospital. Dr. Ross is the former director of research and training at the League for the Hard of Hearing and has served on the boards of SHHH and the International Federation of Hard of Hearing People. He has written a regular column on developments in research and technology in Hearing Loss since 1990. He and his wife, Helen, live in Storrs, Connecticut.