What Can Hearing Aids Do?

What Can Hearing Aids Do?What can hearing aids do other than amplify sound?
By Harvey Dillon, Ph.D. Director of Research, National Acoustic Laboratories, Australian Hearing

Hearing aids are getting smarter. This article will describe most of the things that modern hearing aids can do to help people with hearing loss. Undoubtedly the most important thing a hearing aid does is still amplifying – making sounds stronger – but in a variety of ways, hearing aids are getting better at doing that.
Tone controls
Hearing-impaired people usually have more hearing loss at some frequencies (pitches) than at others. Hearing aids therefore have to amplify more at some frequencies than at others, in a way that is tailored for each person according to the hearing aid prescription. Just as in home hi-fi sets, tone controls are used by the clinician to achieve this tailored amplification. Modern hearing aids may have half a dozen tone controls, rather than the single tone control found in older hearing aids, and so can more accurately be adjusted to match the individual prescription.
Slow-acting automatic volume control
The loudest sounds around us every day may be 100 decibels stronger than the weakest sounds that people with normal hearing notice. Unfortunately, people with hearing loss are not able to hear such a big range of sounds. One way to increase the range of sounds that can be heard is by turning up the volume control for weak sounds and turning it down for strong sounds. Having to frequently change the volume control can be annoying, however, and it is more convenient if the hearing aid has an internal automatic volume control. Of course, the hearing aid does not always know how loud the aid wearer would like sound to be in each situation. Consequently, many hearing-impaired people report that they get the best results if they combine an automatic volume control with an ordinary manual control that they can use to fine-tune the settings selected by the hearing aid.
Very-fast acting automatic volume control
The slow-acting automatic volume controls mentioned in the previous section are designed to do much the same things that a person would: They turn up the volume in quiet environments and turn it down in loud environments. An alternative is to have an automatic volume control so fast that it turns the volume up for every weak sound, no matter how brief, and turns it down for every strong sound, no matter how brief. These fast-acting automatic controls can enable even more sounds to be heard without being too loud. They may, however, decrease the distinctiveness of some sounds by removing some of the loudness differences that let us tell one sound from another. Either the slow-acting, or the very fast-acting automatic volume controls are very beneficial, but neither has a marked advantage over the other. The technical name for both types of controls is wide-dynamic range compression.
Multi-channel compression
It is common for hearing-impaired people to have much more hearing loss for some frequencies than for others. Such people can benefit from wide-dynamic range compression for those frequencies at which they have a lot of hearing loss. For the other frequencies where their hearing is more normal, the disadvantages of compression may outweigh the advantages. The solution to this is for the hearing aid to use multi-channel compression, in which the sound is split into multiple parts – one for each of several frequency regions. The correct amount of compression (i.e. automatic volume control operation) can then be applied to each channel, after which the signals in each channel are recombined. Multi-channel compression is most beneficial for those people whose audiogram shows a marked variation of hearing loss across frequency.
Noise reduction by directional microphones
Directional microphones enable a hearing aid to provide more amplification to sounds arriving from the front than to sounds arriving from other directions. Provided the hearing aid wearer is facing the person talking, and the noise is coming from other directions, directional microphones can make speech clearer in background noise. The advantage is marked if there is little reverberation (such as outdoors) or if the hearing aid wearer is very close (with a metre or so) of the person talking.
Directional microphones have some disadvantages. First, they will make speech less clear if the person talking is not directly in front (such as when a car driver listens to someone talking in the back seat). Second, they are very affected by wind noise when worn outdoors on a windy, or even breezy day. Many hearing aids enable the hearing aid wearer to switch the microphone from directional to normal (technically called omni-directional). This switchable directional microphone enables the most appropriate microphone response to be selected for every situation.
For each directional microphone there is one direction (somewhere to the rear) for which the microphone is particularly insensitive. The electronics within advanced hearing aids can control the direction at which this reduction occurs. These hearing aids automatically select the direction so as to minimize the amount of noise picked up by the hearing aid. This steerable microphone pattern is beneficial when there is one dominant source of background noise, such as one other person talking, or a noisy machine. Furthermore, for the steerable microphone to be fully effective, the noise has to either be very close, or the aid wearer has to be outdoors, so that echoes and reverberation do not affect the operation of the microphone. Under these circumstances the steerable microphone greatly improves clarity. In many circumstances (when noise comes from several directions), the steerable microphone works no better or worse than an ordinary directional microphone.
Adaptive noise suppression
Some noises have acoustic characteristics very different from speech. Traffic noise, for example has strong low-frequency sound, whereas cutlery and crockery noise has strong high-frequency sound. The best tone control settings for a person depend on the type of background noise present. Hearing aids with adaptive noise suppression sense the characteristics of the background noise, and automatically change the tone controls to suit. In traffic noise, for example, the amplification for low-frequency sounds is decreased. The amplification given to the strongest parts of the noise is therefore decreased. Unfortunately, the amplification given to the same frequencies in the speech signal is also decreased by the same amount. Consequently, adaptive noise suppression usually does not make speech easier to understand, but it does makes listening more comfortable and less fatiguing. For some noises and some hearing aids with many channels of noise reduction, there may be a small improvement in speech clarity as well. Some hearing aids take this approach to its logical conclusion: amplification at all frequencies is decreased whenever the hearing aid detects that no one is talking.

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