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FIG. 6.1.7 Normal presbycusis curves. Statistical analysis of audiograms from many people show normal losses in hearing acuity with age. Data for men are represented by solid lines; those for women by dotted lines.

Group surveys—of young men at college entrance examinations, for example—show increasing percentages of individuals whose audiograms look like those of men many years older. This indication is almost invariably of noise-induced hearing loss. If the audiogram shows losses not conforming to this pattern (conductive losses), more careful checking is indicated; such an audiogram suggests a congenital or organic disorder, an injury, or perhaps nervous damage. Group surveys are valuable in locating individuals who are experiencing hearing damage without realizing it; it is often not recognized until the subject begins to have difficulty in conversation. By this time irremediable damage occurred. Such tests are easily made using a simple type of audiometer.

As a part of a hearing-conservation program—either a public health or an industrial program—regular audio-

metric checks are essential. For this purpose, checking only threshold shift at several frequencies is common. The greatest value of these tests is that they are conducted at regular intervals of a few months (and at the beginning and the termination of employment) and can show the onset of hearing impairment before the individual realizes it.

A valuable use of the screening audiometric test is to determine temporary threshold shifts (TTS). Such a check, made at the end of a work period, can show a loss of hearing acuity; a similar test made at the beginning of the next work period can show if the recovery is complete. The amount and duration of TTS is somewhat proportional to the permanent threshold shift (PTS) which must be expected. Certainly if the next exposure to noise occurs before the ear has recovered from the last, the eventual result is permanent hearing impairment.

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