PURE TONE AUDIOMETER

Typical Number in Hospital: 5

Cost Bands: 4

References: 3,8

Although a true test of hearing ability would have to include a test of comprehension of the spoken word, a simpler test to find the patient's threshold of hearing at several standard frequencies is more practical and usually sufficient to direct the course of treatment. Thus most audiometric tests involve the presentation of tones of increasing or decreasing intensity to establish the level at which the patient first becomes aware of the sounds (hearing threshold). A pure tone audiometer generates tones throughout the audible range (usually at each octave between 125 Hz and 8 kHz) at amplitudes related to the expected thresholds for a normally hearing person. The difference between the patient threshold and the normal (i.e. the hearing loss) is expressed in decibels (dBHL) and recorded on an audiogram chart.

The tones are usually presented to the patient through headphones, but a loudspeaker can be used, although this is less precise. If there is a large difference between the hearing loss occurring at each ear (greater than 40 dB) then a masking tone must be introduced to the better ear while the test tones are presented to the other. Having established the hearing loss using the air-conduction methods as above, the procedure is usually repeated using bone conduction to determine whether the same loss is recorded when the inner ear is stimulated directly.

A diagnostic audiometer consists of a signal generator producing tones and masking noises, with calibrated scales of frequency and amplitude, and capable of driving headphones or bone conduction transducers. A simpler (screening) type of audiometer is often used in schools and health centres. This does not normally have a bone conduction facility, but may be fitted with special noise-excluding headphones.