hearing aid

hearing aid Artificial instruments to aid hearing have been in use for at least four centuries and may date back even longer. For example, the simple measure of cupping the hand behind the ear is referred to in ancient Roman medical documents. A variety of purely mechanical devices were used from the seventeenth to the nineteenth centuries to increase the size and capacity of the ear to conduct sound. In the twentieth century, these were surpassed by electric hearing aids, which also amplify and, in more recent devices, process sounds in an attempt to improve the perception and recognition of speech and other environmental signals.

Most non-electric aids to hearing were portable so as to avoid restricting movement and were either worn or held by the listener. These included horn-like ear trumpets, which were made from wood, various metals, or even adapted conch shells, whose effectiveness in transmitting sound depended on their length and shape. Speaking tubes with funnel-shaped endings for the talker's mouth worked by attenuating sounds less than would be the case in the free field. A variety of artificial ears, including larger versions of the shape of the normal pinna, were also used in an attempt to improve upon the natural acoustical properties of the external ear. In addition to devices that amplified airborne sounds, some instruments took advantage of the fact that sounds can reach the inner ear, albeit much less effectively, by bone conduction. This was useful when middle ear disease was present, and involved connecting the hearing aid to the listener's teeth. Acoustical chairs and tables from which amplified sounds were conducted to the ears were also used in the eighteenth and nineteenth centuries. The most effective mechanical hearing aids tended to be large, conspicuous instruments. However, as with modern electric hearing aids, there was pressure to make these devices as inconspicuous as possible by reducing their size or by hiding them in beards, hairstyles, walking sticks, and fans. This inevitably led to a reduction in the benefit they provided.

The first electric hearing aids were developed in the US around the beginning of the twentieth century. Initially using radio valves, and then transistors, they essentially comprise three components. Sound waves are converted by a microphone into electrical signals that vary with the pitch and intensity of the sound. An amplifier is used to increase the gain of the signal, which is then reconverted into sound energy by a receiver and transmitted into the ear canal by a fine tube held in place by an individually moulded ear piece. As with the earlier mechanical aids, most receivers conduct amplified airborne sound, although a few devices, which may be used if there is a completely closed ear canal or if the ear is discharging chronically, work on the basis of bone conduction. Electric hearing aids are typically worn behind the ear, in the ear canal itself, or on spectacle frames. Body-worn aids are also sometimes used in the case of severely impaired individuals.

Hearing aids provide a personal amplifying system for the hard of hearing. In the case of a conductive hearing loss, the problem is lack of amplification. However, this form of deafness is often treated satisfactorily by drugs or surgery, and most people for whom hearing aids are prescribed actually suffer from a form of sensorineural hearing loss, where the intention is to make maximum use of residual hearing by boosting the input for the range of frequencies that are still audible.

Early electric hearing aids often distorted and restricted the acoustic information available, and sometimes actually made it harder for the listener to hear. However, the design and performance of hearing aids has greatly improved in recent years as a result of advances in signal processing. Individuals with poor hearing thresholds may exhibit normal sensitivity to more intense sounds. If this condition, which is known as loudness recruitment, is present, the gain of the hearing aid is adjusted automatically so that quieter sounds are amplified more than the most intense sounds. Modern hearing aids also provide an improved frequency response, which can be adjusted to suit the needs of individual hearing-impaired listeners, and, by including directional microphones, are beginning to enhance the listener's ability to understand speech in noisy surroundings.

Individuals with profound sensorineural deafness cannot be helped by conventional hearing aids because there are no or very few sensory cells left in the cochlea. However, the discovery during the past century that electrical stimulation of the surgically exposed auditory nerve results in the sensation of hearing has led to the development of electronic devices known as cochlear implants. Sounds are converted to electrical impulses by a microphone and processed by a control unit that is typically worn in the clothing. These signals are then transmitted to a radio frequency receiver implanted under the skin behind the ear and then to one or more electrodes inserted into the cochlea. The electrodes bypass the damaged or missing sensory hair cells and activate the remaining auditory nerve fibres directly. Initially, the implants comprised a single electrode. By varying the frequency of electrical stimulation, some individuals with single-channel implants can detect changes in pitch and, as long as deafness occurs after language acquisition, can even recognize simple melodies. Cochlear implants now include more than 20 electrodes, which, by stimulating selective groups of auditory nerve fibres, can elicit different sensations and therefore carry more information to the brain. Current work, including the use of animal models, is directed toward improving the way in which signals delivered to the electrodes are processed so that the patterns of nerve impulses generated in auditory nerve fibres are as close as possible to those that would normally be generated by acoustic stimulation.

Although the effectiveness of cochlear implants varies, they do restore some useful hearing that can enhance lip-reading and sometimes provide a good level of speech understanding, sufficient for conversing by telephone. Implants can also facilitate the acquisition of spoken language in profoundly deaf children.

Andrew J. King

Bibliography

Killion, M. C. (1997). Hearing aids: past, present, future: moving toward normal conversations in noise. British Journal of Audiology, 31, 141–8.
Moore, B. C. J. (1997). An introduction to the psychology of hearing, (4th edn). Academic Press, London.
Stephens, S. D. G. and and Goodwin, J. C. (1984). Non-electric aids to hearing: a short history. Audiology, 23, 215–40.

See also deafness; ear, external; hearing.