Harvey Dillon

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.

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.

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.

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.

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.

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.

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.

As mentioned earlier, different internal settings of the hearing aid are best for different listening situations. Multi-memory hearing aids (also called multi-program hearing aids) give the aid wearer some control over the type of amplification provided. The clinician adjusts each of two or more programs within the hearing aid to be best for different situations. In daily use, the hearing aid wearer then selects, possibly using a remote control, the program that best suits each listening situation. The hearing aid wearer may switch between programs on the basis of the type of background noise, or depending on whether the person wishes to maximise speech clarity or overall comfort.

As an alternative to the aid wearer switching between programs, some hearing aids can automatically switch between programs. They decide which program to select by analysing the acoustics of the sound being picked up by the hearing aid. For example, auto program switching hearing aids may switch between one program that is best for quiet environments and one that is best for use in noisy places. The program for noisy places may have been set up with a directional microphone and decreased amplification of low-frequency sounds.

A commonly encountered problem with hearing aids is that they whistle. This effect, technically called feedback oscillation, occurs when enough amplified sound leaks back from the ear canal to the hearing aid microphone. This can arise when the earmould or hearing aid is old or improperly positioned, when there is too much wax in the ear, or simply when the hearing aid is turned up too much. One solution is to ensure that the hearing aid is never turned up too much, either by the clinician, the aid wearer, or automatically by the hearing aid itself. Feedback management systems in hearing aids measure (during hearing aid fitting) the amount of amplification that can be provided at each frequency without whistling, and then ensure that the amplification can never be increased enough to cause whistling. The advantage is that little or no whistling occurs; the disadvantage is that the amount of amplification provided to high frequency sounds may be inadequate.

A more sophisticated solution to the whistling problem is for the hearing aid to measure the amount of signal leaking back to the microphone, and automatically cancel this sound by adding some equal but opposite sound. As a result, hearing aids with feedback cancellation can provide about 10 decibels more amplification without whistling than can hearing aids without this feature.

All of the features described so far work on the signal once it reaches the hearing aid. There are two ways by which a much, much cleaner signal can be delivered to the hearing aid. The first of these sends a magnetic signal from a loop to a telecoil inside the hearing aid. The loop can be inside a telephone, or can surround many seats within a room, such as in a movie theatre or church.

The second technique sends a radio signal from a transmitter worn by a talker or positioned near a TV speaker across the room to a radio receiver built into, or connected to, a hearing aid. Either of these solutions makes speech much clearer than can any of the other features described in this article. Wireless systems and telecoils achieve such stunning results by not allowing noise or reverberation to mix with the signal they carry.

Sometimes, a person can have so much hearing loss at some frequencies that no useful amplification can be achieved. The person then loses all the information that is present in speech at these frequencies. Typically, it is the high-frequency (consonant) information that is lost. Some (few) hearing aids use frequency transposition to shift the information at these frequencies down to lower frequencies where it can be better heard. The evidence for the benefit of this feature is unclear at this stage.

Although all frequencies that we can hear are useful, for each sound of speech there are just a few frequencies that convey the essence of that sound. Some hearing aids automatically detect these frequencies within each sound of speech and provide extra amplification to these key frequencies while that sound is present. This process is called spectral enhancement. The evidence for the benefit of this feature is unclear at this stage.

Just as spectral enhancement emphasises the key frequencies in each sound, transient enhancement emphasises the parts of sound that change rapidly in time. Most notably, these are the plosive sounds: p, b, t, d, k, g, but there are others. As with spectral enhancement, the evidence for the benefit of this feature is unclear at this stage.

Although we associate surgical implantation with cochlear implants, there are now several implantable middle-ear hearing aids. These devices put sound into the ear by vibrating the bones of the middle ear rather than the usual method of vibrating the eardrum with sound waves. The result is similar: amplified vibrations are passed to the inner ear. Potential advantages include increased comfort from not blocking the ear canal, and avoiding whistling.

You will notice that until now the D-word "digital" has not been used. That’s because it’s more useful to talk about what hearing aids do, rather than what they are made of (i.e. digital versus analog technology). There is no doubt, however, that many of the features talked about in this article would simply not be possible without digital technology. Features that would not be feasible in an analog hearing aid include Steerable directional microphones, Adaptive noise suppression, Automatic program switching, Feedback cancellation, Spectral enhancement, and Transient enhancement. The combination of features now available makes hearing aids work better than even a few years ago, and further advances are being made every year. Unfortunately, even the best hearing aids do not restore hearing to normal, so there is no substitute for protecting hearing by avoiding prolonged exposure to excessive levels of sound.