HEARING AID

Typical Number in Hospital: 1000

Cost Bands: 1

References: 3,4

A device for amplifying and delivering sound to the ear, (usually the better hearing of the two ears). Before 1930 these were passive devices, being variations of the 'hearing trumpet'. Since then they have developed with the various generations of electronics. Modern hearing aids are either body-worn microphones and amplifiers with a lead to an earpiece (strangely the earpiece is called a receiver), ear-level aids which fit behind the ear, housing both microphone and receiver, or even smaller devices which fit within the ear canal.

The frequency response of hearing aids is not flat. Sounds below 500 Hz are sharply attenuated and the output is also deficient in frequencies above 5 kHz (which are not required for speech intelligibility). Gain is usually nonlinear with increasing output and the typical full-on gain may be 60 dB. The microphone is usually an Electret type and the 'receiver' is usually a fixed coil and moving magnet type. If there is a defect in the mechanism which transmits sound from the ear drum to the cochlea (conductive deafness) sound is sometimes applied as vibrations through the mastoid bone just behind the ear. In this case the patient wears a head band or clip which causes the transducer to press on to the skin above the mastoid bone.

If one wears a hearing aid for a few minutes, the shortcomings become obvious. They are monaural (although sometimes two are worn) and there may be considerable distortion, resulting in harsh sound, but more serious is the intermodulation distortion. This means that when more than one sound is present (e.g. a motor car and a voice) the result is a jumble of sound which may be very difficult to make sense of. The amplifier is unlikely to have the required dynamic range to handle low and high sound levels at the same time or in rapid succession. Thus, clipping may be employed to limit the peak amplitude, or in more complex devices there may be an automatic gain circuit. This results in less distortion of larger signals but may have other undesirable effects, such as total loss of output following a large impulse of sound.

An aid with the required performance characteristics for each patient is chosen following clinical assessment and an audiogram. The effectiveness in use depends to a large extent on the quality of fitting of the ear mould.