Audiometry
The quantitative assessment of individual hearing, either normal or defective. Three types of audiometric tests are used: pure tone, speech, and bone conduction tests. Such tests may serve various purposes, such as investigation of auditory fatigue under noise conditions, human engineering study of hearing aids and communication devices, screening of individuals with defective hearing, and diagnosis and treatment of defective hearing. In all of these situations, individual hearing is measured relative to defined standards of normal hearing.
The pure-tone audiometer is the instrument used most widely in individual hearing measurement. It is composed of an oscillator, an amplifier, and an attenuator to control sound intensity. For speech tests of hearing, word lists called articulation tests are reproduced on records or tape recorders. Measurements of detectability or intelligibility can be made by adjusting the intensity of the test words. To make bone conduction tests, sound vibrations from the audiometer activate a vibrator located on the forehead or mastoid bone.
Scientific advance in audiometry demands careful control of all environmental sound. Two types of rooms especially constructed for research and measurement of hearing are the random diffusion, or reverberation, chamber and the anechoic room. In the reverberation chamber, sounds are randomly reflected from heavy nonparallel walls, floor, and ceiling surfaces. In the anechoic room, the fiber glass wedges absorb all but a small percent of the sound.
The measurement of hearing loss for pure tones in defective hearing is represented by the audiogram (see illustration). Sounds of different frequencies are presented separately to each ear of the individual, and the intensity levels of the absolute thresholds for each frequency are determined. The absolute threshold is the lowest intensity which can be detected by the individual who is being tested.
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In clinical audiometry the status of hearing is expressed in terms of hearing loss at each of the different frequency levels. In the audiogram the normal audibility curve, representing absolute thresholds at all frequencies for the normal ear, is represented as a straight line of zero decibels. Amount of hearing loss is then designated as a decibel value below normal audibility. The audiogram in the illustration reveals a hearing loss for tones above 500 Hz. Automatic audiometers are now in use which enable individuals to plot an audiogram for themselves.
Articulation tests are speech perception or speech hearing tests used to assess hearing and loss of hearing for speech. The threshold of intelligibility for speech is defined as the intensity level at which 50% of the words, nonsense syllables, or sentences used in the articulation test are correctly identified. The hearing loss for speech is determined by computing the difference in decibels between the individual intelligibility threshold and the normal threshold for that particular speech test. Discrimination loss for speech represents the difference between the maximum articulation score at a high intensity level (100 dB), expressed in percent of units identified, and a score of 100%. The measure of discrimination loss is important in distinguishing between conduction loss and nerve deafness.
Bone conduction audiograms are compared with air conduction audiograms in order to analyze the nature of deafness. Losses in bone conduction hearing generally give evidence of nerve deafness, as contrasted to middle-ear or conduction deafness. See Ear (vertebrate), Hearing impairment
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