Quality Standards for Adult Hearing Rehabilitation Services

3. Hearing Aids, Hearing Aid Styles and the Rehabilitative Context for Hearing Aids

3.1 Hearing Aids

Middle ear problems leading to conductive hearing loss are potentially managed by surgery. At present there are no surgical or medical interventions for sensorineural hearing loss, and the only effective management available is the provision of amplification via hearing aids. Some conductive hearing losses are not suitable for surgery and also require management via hearing aids. Hearing aids require an appropriate rehabilitative context to be effective.

Defects in the middle ear lead to a conductive hearing loss which is a simple attenuation (quietening) of sound which often varies only relatively little as a function of frequency. However, the vast majority of hearing losses (particularly in the elderly) are sensorineural in origin and result from damage to the hair cells in the inner ear which convert sound into nerve impulses. Sensorineural hearing losses are usually more severe at high frequencies than at low frequencies. Vowels in speech have predominantly low frequency energy, while consonants are predominantly high frequencies. Thus speech can be audible though not intelligible.

In addition to simple attenuation of sounds, sensorineural hearing loss results in a number of other distortions. This results in listeners with sensorineural hearing loss being much more susceptible to the effects of background noise than their normally-hearing counterparts. Simple amplification (making sounds louder) will not necessarily remove all of the difficulties that such a listener experiences. Furthermore, while people with sensorineural hearing loss experience impaired auditory sensitivity (inability to hear quiet sounds), more intense sounds are perceived as just as loud by such individuals as they are by people with normal hearing. In particular thresholds of uncomfortable listening are not elevated in the same way as hearing thresholds. Thus listeners have a reduced range of hearing (dynamic range) between the threshold of hearing and the threshold of uncomfortable listening.

A hearing aid is required to take a signal that a listener wishes to hear and to amplify it so that its components are above threshold but not uncomfortably loud. This means that higher frequency sounds have to be amplified by more than lower frequency sounds. Hence a hearing aid has to have the capability to shape the way it amplifies sounds according to the profile of a listener's hearing loss. Hearing aids which have greater degrees of flexibility in this regard will be more effective.

Given that listeners have reduced dynamic ranges, hearing aids are required to amplify low intensity sounds by more than they will be required to amplify high level sounds. This differential amplification as a function of level will vary with frequency, because the dynamic range between thresholds of hearing and threshold of uncomfortable listening varies between low and high frequencies. This form of hearing aid processing is termed "amplitude compression", because it attempts to squeeze, or compress, the wide range of input signals into the reduced range of hearing.

Because listeners with sensorineural hearing loss experience more difficulty in noise than normal hearing listeners, hearing aids attempt to amplify the signal by more than any noise. One option is a directional microphone. The hearing aid is more sensitive when it is pointing towards a sound source and is less sensitive to sound sources which are off to the side or behind the listener. This is effective given that people usually orientate themselves so that they are facing a sound source that they want to hear. Thus a microphone with a directional pattern can help to improve the relative levels of the signal and the noise.

Any amplifier is prone to whistling or feedback and hearing aids are no exception. If the sound delivered to a hearing impaired listener's ear is able to leak back to the microphone such feedback can occur, even in the presence of well fitting ear-moulds. Hearing aids can attempt to identify when feedback is likely to occur and to either try and cancel it or to turn down the gain in a particular frequency region so that feedback is avoided. The next section gives a short and simplified list of the sorts of processing and fitting features that are potentially available in hearing aids.

3.2 Hearing Aid Styles and Implementation

Hearing aids can be classified by the physical type and size, as well as the ways in which the processing features are achieved. The majority of hearing aids used in the NHS in Scotland are behind-the-ear ( BTE), which is sometimes called postaural. More miniature devices (in-the-ear ( ITE) or completely-in-the-canal ( CIC)) offer greater cosmetic acceptability to listeners, though sometimes at the expense of their ability to provide the processing that is required. These are often chosen when the option is available.

Until the 1990's all hearing aids achieved their processing by analogue electronics (i.e. amplifiers and filters were employed in exactly the same way, though on a miniaturised scale, as the technology in domestic hi-fi systems). When a control required to be adjusted, this was achieved by a small screwdriver-driven potentiometer, similar to the base or treble control on a domestic music system. These are analogue hearing aids. One development was the ability to control these analogue hearing aids using digital computer systems, leading to digitally programmable hearing aids. In these hearing aids the underlying processing was still achieved by analogue technology, but was now controlled from a computer, removing the need for a series of miniaturised controls on the hearing aid.

More recently has been the development of fully digital devices where, in a similar manner to developments in music systems, the electrical signal is represented in digital format and the mathematical and signal processing is achieved using this form of technology. Potential advantages of digital processing are increased ability to shape the frequency response to match a hearing loss, greater flexibility in compression characteristics, and greater capabilities to manage feedback.

Hitherto digital hearing aids have only been available at the "top end of the market", and have been comparable in price to the most expensive analogue devices. However, as manufacturers devote more and more of their research, development and manufacturing capability to digital implementations, the relative cost penalties of digital versus analogue devices have inevitably narrowed and nearly all new hearing aid models brought to market are now digital.

3.3 The Rehabilitative Context for Hearing Aid Fittings

Digital hearing aids can be programmed so that they are tailored to match the acoustical characteristics of an appropriate target derived from the listener's hearing loss; the fitting can also be verified using real-ear measures to ensure that a hearing aid is indeed delivering an appropriate acoustical signal. The patient's listening needs can also be considered whilst programming the aid, particularly in setting up a number of different programmes for use in different situations.

There is a need for appropriate patient contact time both in fitting and follow-up to ensure an understanding of the mechanical competence with the hearing aid system (which, if not adequately performed, will undermine hearing aid use and acceptance). Fine tuning of the hearing aid can also be important, especially if based on comments after the patient has tried the aid for several weeks in different listening situations.