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Noise Reduction Hearing Aids: Why They're Needed, and How They Work

Editors Note: Dr. Mark Ross is a consultant for the Rehabilitation Engineering Research Center (RERC) on Hearing Enhancement at Gallaudet University, he is a Professor Emeritus of Audiology at the University of Connecticut, and has also served as the Vice President of the Hearing Loss Association of America (HLAA), formerly Self Help for the Hard of Hearing (SHHH). As a world renowned author and researcher in hearing rehabilitation and having worn hearing aids for almost 50 years, Dr. Ross is uniquely qualified to write on the topic of hearing loss and amplification and we are so pleased to feature some of his publications here on Healthy Hearing. This article is reprinted in cooperation with HLAA www.hearingloss.org the nations largest organization for people with hearing loss.

mark_ross.gifIntroduction

Some years ago on a visit to New York, my family and I decided to have brunch at one of the "in" places in Greenwich Village. As we entered, we were greeted by the happy chatter of other diners. While these sounds would be a problem for me, it was an expected one and one that I was prepared to face. The loud music coming from the many loudspeakers on the walls, however, was another matter. When I asked the manager if he could turn the level down, he nodded and I went back to the table. Nothing happened. Then my daughter, who can be more assertive on my behalf than I am, went to him and said "My father wears hearing aids and he can't hear when the music is so loud. Could you please turn the sound down just a bit? He ignored her and she asked again. Finally, in apparent exasperation, he exploded "The sound is part of our ambience and I will not turn it down." Needless to say, our visit to the "in" place ended quickly.

I'm sure that just about anybody wearing hearing aids can relate to this experience. It is certainly not unique to me. As a 1993 Hearing Instruments article put it, "Restaurateurs perceive noise as communicating the popularity and success of the restaurant to patrons. Noise, they contend, creates an image or ambience." The study found an average of 73 dBA in 27 San Francisco restaurants, with a range extending beyond 80 dBA. To put these figures in perspective, at three feet an average speech signal would measure about 45-50 dBA. While this is a 12-year-old study, the situation has, if anything, gotten worse since then. And therein lies the dilemma faced by hearing aid users. Most of us would classify noise (any sound that interferes with verbal communication or that we perceive as unpleasant) as a noxious stimulus, one that has been proven to cause a host of problems. However, for many people in our society, sounds - at least of certain types and in certain situations - are not regarded as "noise" but rather as a desirable part of everyday life. "Lively" restaurants are just one example.

Piped-in music can be found in just about every large public place (supermarkets, drug stores, department stores, etc.). My complaints to the management and personnel in these places have been met with a blank stare of incredulity: What on earth could I be talking about? They evidently believe that the background music is not only desirable, but necessary. On many city streets, the normal sounds of traffic can make conversation very difficult for the hearing aid user. And to the normal traffic noise, we can add the thunderous blare emanating from car stereos, which "treat" bystanders to the ultimate limits of advanced sound systems (as well as the driver's taste in "music"). The newest exhibitionist trend is the installation of train horns in cars that can deliver an unbelievable 150 dB blast of sound to nearby listeners (whose subsequent reactions are a source of amusement to the drivers!). In my neighborhood, the sounds of spring include the roar of noisy lawn mowers; autumn is greeted by the howl of leaf blowers; and the fall can boast the intense buzz of a chain saw. Winter has its snow blowers, but fortunately we're indoors at that time. Mufflers, I'm sure, can be made for all of these devices, but they appear to be an unknown entity.

The level of sound radiating from the loudspeakers at some musical performances is enough to cause the sternum to vibrate and the ears to ring. (Second-hand reporting only; I'm not about to experience this myself!) Discotheques, nightclubs and raves are notorious for the levels of sound they provide to their patrons (who evidently wouldn't have it any other way). Apparently, at least in public settings, we're uncomfortable with quiet. Everyday life, it seems, now requires that we be immersed in some type of background sound with the level apparently getting higher all the time. It is not unusual now for people to leave a movie house with ringing or a "stuffy" feeling in their ears. This is understandable, since sound levels as high as 118 dB have been reported in some of them.

Of course much depends on how the sound is perceived by listeners. For young people listening to the music they like, it may be next to impossible to overdo the loudness level. The higher the better. For people who don't like the music, however, then any level is too high (and I personally put piped-in music in this category). For me, it really doesn't matter what the musical selection is - not when I am an unwilling listener to what I perceive to be a noxious stimuli). I put this involuntary exposure to sound in the same category as second-hand smoke. But regardless of how positively or negatively the sounds may be regarded by a listener, there is an intensity level at which the ear will begin to sustain damage.

The long-term negative effects of excessive sound exposure have been known for many years. The most notable consequence is a noise-induced hearing loss (NIHL), but a prolonged exposure to loud sounds can cause other problems as well. There is a body of research that demonstrates that such exposure can raise blood pressure, affect the cardio-vascular system, disrupt sleep, cause breathing and digestive problems, and reduce efficiency in performing daily tasks. Children's learning can also be affected. Several studies have compared the test results of children in noisy schools to those in quieter schools. These have found superior performance by the children in the quieter schools.

An alarming number of studies have documented the presence of a noise-induced hearing loss (NIHL) among school-age children. There is no question but that children are often exposed to excessive levels of sound. Many children, particularly the older ones, make a practice of prolonged listening to earphones, whose measured sound output is often in excess of 115 dB. Just about every parent can testify to the high volume level on the TV or stereo preferred by their youngsters. Children (and people in general) do exhibit individual differences in their ability to tolerate loud sounds. While some may show little damage from continuous sound exposure, others are not so lucky. For them, it won't take too long to develop a permanent hearing loss given long-term exposures in excess of 115 dB. One researcher reports that excessive sound exposure is apparently the reason why up to 75 percent of high-school seniors he examined have the beginnings of a permanent hearing loss (at 4000 Hz, usually the first frequency to be affected). From what I've been able to determine, this was not the case 40 or 50 years ago.

For people with hearing loss, the most immediate and apparent effect of noise is its impact on interpersonal communication. There is no such thing as "desirable" sound for the hearing-impaired person at a time when they're trying to communicate. They are not immune to the other problems that may be caused by noise, but for them its interference with communication is something that occurs daily, every time they talk to somebody in a noisy place. In this instance, the nature of the noise doesn't matter, whether it is considered "desirable," i.e., music (and the selection is irrelevant), or undesirable sounds such as the racket produced by a pneumatic drill. People wearing hearing aids have to communicate under every acoustic circumstance.

There are a number of different ways for a hearing aid user to improve speech perception in the presence of competing sounds. One, as has been alluded to, is to attempt to get the offending sounds reduced. I know one young man who required that the musicians at his wedding reception sign a contract agreeing to keep the music level below 70 dB. Any more than that and they would be penalized. It was wonderful; it was actually possible to talk to the other people at the table. A second technique is to use strategic seating in an unavoidably noisy location by trying to find the quietest spot in the room. But this is not possible in many places. A third method would be to employ a close-talking mike, such as a personal FM system. This is probably the most effective technique of all, since it directly boosts the speech-to-noise ratio. A fourth option is to use hearing aids that include directional microphones. These really do work, but also require strategic seating for maximum effectiveness. And a fifth solution, and the topic of this paper, is to use hearing aids equipped with a digital noise reduction (DNR) circuit. A very informative article on DNR hearing aids by Gus Mueller and Todd Ricketts recently appeared in The Hearing Journal.

In an ideal DNR system, the hearing aid will reduce only the undesired noise while leaving desired speech signals intact. In this ideal situation, the hearing aid is supposed to "know" what sounds it is expected to "hear" and pass on to the listener and what sounds it should reject. To reach this goal, engineers and scientists have focused on the acoustic differences between speech and noise and devised sensors that respond to these differences. While noise can vary in an almost infinite number of ways, we know that the nature of speech is syllabic, with relatively intense vowels followed by weaker consonants and pauses. In normal speech, there are about four to six such syllables (or "modulations") per second.

In the most common type of DNR implementation, speech is differentiated from noise by an analysis of the modulations. In a quiet environment, the intensity range of these modulations is at a maximum. In this instance, the hearing aid "knows" that the signal is purely speech. However, when other sounds are present, the situation changes. These sounds can consist of other people talking, music, or just everyday background sounds. The presence of these other sounds tends to flatten the modulations; that is, the competing sounds fill the gaps between syllables or even override the vowel peaks. The full range of the modulated speech signal is no longer observed.

When the extent of the modulations is reduced, the DNR circuit "knows" that noise is present. When this occurs, the hearing aid automatically turns itself down. The amount of gain (amplification) reduction usually depends upon how much the noise affects the modulations. Usually, the greater the effect upon the modulations, the more the gain is reduced. We should understand that while this gain reduction reduces the amplification of the competing noises, it also decreases the energy in the speech signals. In other words, DNR circuits affect both speech and noise in exactly the same way. There is no way that I know of that a DNR circuit can eliminate only the noise from a simultaneous speech and noise signal. (Noise reduction earphones can do this but they require a predictable noise source in order to operate, unlike the normal acoustical environment of a hearing aid user.)

Fortunately, modern hearing aids are "multi-band" and since the DNR circuit works within each separate band, it is possible to reduce the amplification in the noisiest channels only. The amount of gain reduction depends upon a number of acoustical factors, with different manufacturers using their own decision rules. Primarily this would depend upon how much noise is present, but other factors, such as the overall levels of the speech and noise, may also be considered. It is a common practice for a DNR circuit to vary the amount of gain reduction depending upon a band's frequency location. Frequencies more important for understanding speech are not reduced as much as frequency bands less important for the understanding of speech.

All DNR aids, then, are not alike. Some hearing aids provide 16 dB or more in gain reduction, while others will reduce the gain by only 5 dB at the same frequencies. They also differ in how rapidly they increase and decrease the gain and how they affect different types of sound. For example, Ruth Bentler compared two DNR aids in the degree to which they reduced speech, noise, and music at different frequencies. The two hearing aids differed not only from each other but also in the extent to which they reduced the gain of speech, music and noise across frequency. In other words, all hearing aids that contain a DNR system are not alike in this respect (nor in many others as well). It is not known how these two aids, and by implication other hearing aids with DNR systems, would compare in respect to objective testing and subjective preferences in a real-life situation.

There has been a fair amount of research performed on DNR systems in hearing aids. From what I can see a consensus appears to be emerging: people do prefer listening in a noisy situation while wearing a hearing aid that includes a DNR circuit. Hearing aid users report that the background noise does appear to recede and that incoming speech does "sound" better with the DNR system. The entire listening experience is not quite as stressful or as fatiguing as it ordinarily would be. These subjective preferences have been found when people wear DNR hearing aids under both high and low-noise conditions.

In view of these findings, it may be surprising to note that these subjective preferences are rarely found when speech perception scores with and without a DNR system are compared. One study (conducted by Jason Galster from Vanderbilt University) did show a slight preference for the DNR condition, but this was attributed to the noise condition (a steady-state white-like noise unlike a normally complex and changing acoustical environment). A follow-up study now being conducted, with a more realistic competing noise situation, is not demonstrating the same improvement found earlier. Generally, therefore, it can be concluded that short-term speech perception studies have not found higher scores with a DNR system than without one.

It has been speculated that speech perception scores would demonstrate an improvement after long-term listening, since there would be less fatigue when listening with a DNR system in a noisy situation. More relaxed people do "listen" better than people under stress. While this comment is only speculative, it may explain why many people wearing such systems report an improvement in their ability to understand speech. Perhaps, after a while they really do hear better. But this is not the same as the promise inherent in the label "digital noise reduction," that is, the technical capability of some miracle hearing aid to virtually eliminate all competing sounds when listening to speech in a noisy situation. That's a dream still to be realized.

Mark Ross, Ph.D., is an audiologist and associate at the Rehabilitation Engineering Research Center (RERC) at Gallaudet University. He dates his emergence into the field of audiology to the time he attended the Army Aural Rehabilitation Program as a participant in 1952. He received his doctoral degree from Stanford University and taught at the University of Connecticut and worked as a clinical audiologist at Newington Childrens 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.

This article first appeared in the September/October 2005 issue of Hearing Loss the bi-monthly publication of Hearing Loss Association of America (formerly SHHH) www.hearingloss.org. This article is supported in part by Grant #H133EO30006 from the U.S. Department of Education to Gallaudet University. The opinions expressed herein are those of the author and do not necessarily reflect those of the Department of Education.

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