Acceptable Noise Levels (ANL)

The most frequent complaint by people who wear hearing aids concerns their difficulties in tolerating background sounds.  Noise is the enemy of the hearing aid user; it affects not only speech comprehension, but the very willingness to wear hearing aids. One of the first comments often made by new hearing aid users relates to the high, and disturbing, levels of noise they can now hear. Indeed, we all know people who have rejected hearing aids because the noise was overwhelming; they were simply not prepared for the noisy onslaught to which they were now being exposed. Until recently this was simply an anecdotal impression; it had not been systematically investigated.  However, about 16 years ago, Dr. Anna Nabelek of the University of Tennessee felt that this observation should be looked into further, and more objectively. She began investigating whether a person’s willingness to tolerate background sounds was indicative of successful or unsuccessful hearing aid usage.  The theory was that people who could tolerate louder levels of background noise would have more success using hearing aids than those who could not, and that this effect was quantifiable and reliable.  She has been studying this concept ever since, and just recently (October 2006) edited an entire issue of the Journal of the American Academy of Audiology (JAAA) devoted to this topic.  She and her colleagues labeled the concept “Acceptable Noise Levels” or ANL.

Specifically, ANL refers to the maximum level of background noise that a person is willing to “accept, or put up with” without becoming tense or tired. To measure it, the person first sets the loudness of a speech signal at a comfortable loudness level (by telling the audiologist “more” or “less” or by personally adjusting a control). Next, while the speech signal is still playing, a background noise level is introduced and varied in loudness until the person says the limits of tolerance have been reached.  The difference between the intensity level of this background noise and the level of the comfortable speech is the ANL. For example, if a 40 dB noise level is all a person can tolerate, and if the intensity of the comfortable speech is 55 dB, then the ANL is 15 dB (55 dB minus 40).  The more noise a person can tolerate the smaller the ANL. If this same subject could tolerate 50 dB of noise rather than 40 dB, then the ANL would be 5 dB (55 dB minus 50 dB). The background noise is almost always less than the comfortable speech level, according to Dr. Nabelek, as practically nobody is willing to tolerate noise levels equal to or greater than the speech signal (excepting what happens at various rock concerts of course!). The test itself takes only a few minutes and can be done while wearing hearing aids, with earphones, or through a loudspeaker prior to the actual selection of a hearing aid. The results are essentially the same.

What researchers have found in studies dating back to 1991 is that the more background noise a person can tolerate, the more likely that the person will use hearing aids successfully.  For the latest 2006 studies (the ones reported on here),  the investigators  developed a  questionnaire to determine who used hearing-aids “successfully” on a full-time basis, who used them part time, and who became (after trying)  non-hearing aid users.  “Success” was defined as the willingness to wear hearing aids whenever needed.  Part-time users wore them only occasionally, while non-users were those who completely rejected hearing aids.  Using these classifications, only 36% of the 191 subjects in the study could be considered “successful” hearing aid users. This agrees well with surveys that show that only 31% of hearing aid users find their use satisfactory in noisy places. The percentage of part-time users was also 36%, with the remainder of the subjects being classified as non-users (tried but rejected).

These classifications were then compared to the ANL scores obtained by the subjects. What this analysis revealed is that people with low ANLs (7 dB or less) have approximately an 85% chance of being successful hearing aid users (i.e., wearing hearing aids whenever needed).  This percentage is about as high as one gets in Audiology with predictive tests of any kind; it is an impressive figure. Those whose ANL scores exceed 13 dB ( poor noise tolerance) are unlikely to be successful hearing aid users (i.e., a probability of about 10%), while those with ANL scores between 7 dB and 13 dB fall in between, with a probability of successful use somewhere around 50%. This is the group that may provide the best candidates for auditory training exercises, or hearing aids that incorporate noise reduction circuits or directional microphones (more on this below).

In addition to measuring the acceptable noise level, the researchers also tested the subjects’ ability to perceive speech in noisy environments. Intuitively, one would predict that people with high scores on the speech perception test in noise would also be classified as successful (full-time) hearing aid users. Surprisingly, this was not the case. There was no relation between hearing aid acceptance and the scores the people achieved on the speech perception test. The scores for all three groups of subjects (full-time, part-time and non-users) were similar. The question was then asked whether these three groups of subjects differed in other respects besides their speech perception scores. As it happens, they did.

In one of the studies reported in this issue of JAAA, researchers reported some differences between the brain scan responses of people with high versus low ANL scores. The findings of this physiological study suggest that the ability to accept the presence of background sounds may be an inherent ability that some people have and others don’t.  The point emphasized in this study, and in other studies cited by the researchers, is that tolerance of noise and speech perception in noise are evidently two different skills.  Since they are independent, and both extremely pertinent – one in predicting whether a hearing aid will be worn at all (the ANL test) and the other (speech perception test) in indicating how much benefit a person will  receive from the aid –  it was recommended that both types of measures be administered  during the hearing aid selection process. What this result implies is that a focus  primarily or exclusively on improving speech perception during the hearing aid selection process may be a bit short-sighted; it is also necessary to determine that a person will accept amplification at all once exposed to our noisy world.

We should note that these results are fully applicable only to people with hearing losses similar to those of the subjects in this study.  The degree of hearing loss of those people who rejected hearing aid usage or wore them only occasionally averaged about 10 dB less than that of the group who became full-time users.  Perhaps as the degree of hearing loss becomes more severe, full-time usage becomes less an option and more a necessity.  For example, for someone who has a hearing loss in excess of 60 or 70 dB hearing aid usage is essential regardless of the person’s willingness to accept background noises.  He or she must either learn to deal with the noise or restrict hearing aid usage to quiet situations. Our task with such a person is to employ the best technology and training available to reduce the impact of noise. The part-time and non-users in this study, on the other hand, were evidently able to function adequately (by their estimation at least!) without hearing aids; their hearing loss was not that severe.

One of the papers in this issue of JAAA compared monaural and binaural amplification.  Although, as expected, speech perception scores were higher with binaural amplification, on the average there was no difference in ANL scores between the two aided (monaural and binaural) conditions.  However, a significant minority of the subjects in this study (13 out of 39) did have better ANL scores (i.e., could withstand more noise) in the monaural than in the binaural condition. This outcome can provide information regarding which ear to amplify in instances when a monaural fitting is necessary for some reason; it may also help to explain why some people seem to prefer monaural to binaural amplification (which should, nevertheless, remain the norm). It would be easy enough for an audiologist to test ANL in the monaural and binaural conditions and take any major differences into consideration while fitting the aids.

There appears to be no question about the reality of the ANL concept: People do differ in their willingness to tolerate background sounds, and this difference is reflected in the willingness to accept hearing aids. Clearly there is a need to either reduce the apparent noise level, or to somehow increase a person’s noise tolerance.  There are three known ways to reduce the perception of background noise using current hearing aid technology. It is also possible that with auditory training we can increase a person’s tolerance of background noises. Each of these will be discussed.

Personal FM System 

This is the best technique I know of for increasing the speech-to-noise ratio (the level of speech relative to the level of noise), thus reducing the unpleasant and interfering effect of background sounds. While the noise is still there, the desired sound is significantly louder and the background sounds are therefore less disturbing. With a personal FM system, a sound signal is picked up at the source (a person talking, TV, etc.) and transmitted via an FM radio wave directly to either an FM receiver incorporated inside a hearing aid or to an FM receiver module  plugged into the base of a BTE hearing aid. Unfortunately, an FM system is also the least favored solution among consumers, in part because many remain unaware of this option while others are reluctant to use it. It does take a moderately assertive individual – one who accepts the reality of his or her hearing loss and is willing to publicly display the FM microphone/transmitter – to routinely use such a system in the effort to hear better. FM systems are also expensive (though their price is falling fairly steadily) and we should not ignore that factor as a deterrent to their usage. I look forward to the day when a small FM microphone/transmitter (about the size of a ballpoint pen) is considered a routine accessory that accompanies most hearing aid purchases and can called upon whenever necessary. But the more realistic view, and the one that prevails now, is the inclusion of noise reduction features within wearable hearing aids.

Noise Management in Hearing Aids

Many current hearing aids include what is described as a “Noise Management” or “Noise Reduction” feature. Although accomplished differently, all of them have the goal of reducing the unpleasant impact of background sounds. For people whose ANL scores fall in the mid-range (about 7 to 13 dB) and whose chance of a successful hearing aid fitting is 50% (according to the Nabelek study),  this type of aid should noticeably increase their chances of becoming  full-time hearing aid users. Hearing aids that include such a feature can and do decrease the perception and annoyance of background sounds.  Because, however, desired speech signals will often occur at the same time as interfering noises (likely to be other people speaking), when a noise management program is activated, the intensity of both the desired and the undesired sounds will be reduced. In other words, these aids are not able to just “peel away” the interfering speech noises from the desired speech signals. But a noise management feature can reduce the overall perception of noise, thus making hearing aid listening more pleasant.  So while they do not directly increase speech perception scores through the hearing aid, they can increase the likelihood that someone with a mid-range ANL score will accept the need for a hearing aid and become a full-time user.

Directional Microphones

Directional microphones, on the other hand, will directly decrease the intensity level of background sounds – those emanating from the sides and rear of a listener. By not also reducing the intensity of the speech signals arriving from directly in front of the listener, hearing aids with these microphones effectively increase the speech-to-noise ratio. In the directional mode, therefore, directional microphone hearing aids can decrease the perception, and thus the possible annoyance, of background sounds. As with the noise suppression feature, this effect can increase the chances of people with mid-range ANL scores accepting hearing aids and becoming full-time users.

Although we know that hearing aids with directional microphones can increase speech perception scores, a finding that has been demonstrated repeatedly in laboratory research, a few caveats are in order. In the real world, it is necessary for users to place themselves in a position where the speech source of interest is directly in front of them with the undesired sound signals to their rear and sides. This is not always easy or possible. Fortunately, some of the newer two- and three- microphone adaptive directional microphone arrays can make many of these decisions for users. These microphones will automatically modify their directional characteristics depending upon the source of the noise and whether the noise is stationary or moving.  Some include provisions for dealing with background noises that arrive simultaneously from several different directions.  However, directional microphones will not work well in highly reverberant rooms and do require that the listener be relatively close to a talker (within the so-called “direct-field”). How close depends upon the amount of reverberant noise (with more reverberant situations requiring closer positioning). But even with all these caveats the directional microphone is a desirable hearing aid feature – one that I do recommend.

Auditory Training

Non-technical solutions to hearing aid usage and benefit are not much in vogue and haven’t been emphasized for years. Still, for people with severe communication difficulties (as reported by either themselves or a loved one) who reject or are unable to adapt to hearing aids because of intolerable background noise, a regime of auditory training may be very beneficial.  If technical solutions are inadequate or not helpful, then such training may well bridge the gap between hearing aid acceptance and non-acceptance. The training need not be involved or elaborate. Based on my own experiences as a professional and as a long-time consumer, I have little doubt but that with perseverance most people can learn to adapt to slightly higher levels of noise. They may not like it (which I can understand), but they also may be able to derive sufficient benefits from hearing aid amplification to make the effort worthwhile. I would suggest regularly scheduled increasingly loud sound exposures while wearing well-adjusted hearing aids, the goal being not to improve comprehension (which would be a nice bonus) but to increase the person’s tolerance of loud sounds in the environment.  Before engaging in this activity, I would strongly recommend consultation with a professional audiologist; sometimes, tolerance problems also relate to the nature of the amplified sounds being delivered by the hearing aid.

In summary, the Acceptable Noise Level (ANL) is an easily understood phenomenon which can help explain why some people seem to show an inordinate degree of difficulty in accepting and using hearing aids. However, it should be regarded merely as a challenge – not as an insuperable barrier to benefiting from hearing aid amplification.  Technical and non-technical help is available for those people with low tolerance for background noises.