AUDIOMETER

Typical Number in Hospital: 4

Cost Bands: 2,3,4

References: 3

Treatment for deafness can vary from the fitting of an electronic hearing aid to major surgery, depending on the origin of the problem. In almost every case an audiometer is used to discover the magnitude of the hearing loss and to assist in the diagnosis of the type of deafness. The commonest type of audiometer produces pure tones at set frequencies and known amplitudes through headphones worn by the patient who is required to indicate (by pressing a button) which of the tones he can hear. Variations exist which do not use headphones (free-field or bone conduction audiometers) and the tones may be replaced by words which the patient is required to recognize and repeat. A further category of audiometer exists, which does not require the co- operation of the patient, and can even be performed under general anaesthetic. These are electric response audiometers (ERA) which detect whether an auditory stimulus (tone or click) has been received at the cochlea (electrocochleograph - ECochG), the brain stem, the cortex (slow vertex response), or in auditory reflex arcs as may be detected from the electromyogram of the post auricular muscle (PAM), sometimes called the crossed acoustic response.

A very simple audiometer might have only a small number of tone frequencies, with rather large steps between the tone amplitude settings and may be for free field use only. Such a device would probably be used mainly outside the hospital for screening large numbers of people for hearing defects quickly (e.g. school children). Such audiometers may produce a warble tone instead of a steady tone to eliminate the problem of standing waves in the testing room. At the other end of the scale a single apparatus might be capable of producing the signals required (including masking noises) and recording the responses involved in several of the tests mentioned above.

Hearing testing in the hospital is performed almost exclusively in the audiology department where they also fit hearing aids and take the impressions necessary for the manufacture of ear moulds.

Tympanometry, by which the quality of sound transmission across the middle ear is assessed, is sometimes called middle ear audiometry, and thus the instrument used may be called an audiometer. However, in this case the principle of operation is completely different. An audible tone is fed into the ear but it is not important whether this is heard by the patient or not. For further details see Acoustic impedance bridge.