Audiometric Practices

A typical audiometer for this use consists of an audio-frequency source with amplifier, attenuator, and headset (air-conduction earphone, perhaps also a bone-conduction unit). The following are the standard test frequencies: 62, 135, 250, 500,1000, 2000, 3000, 4000, 6000 and 8000 Hz. Not all of them are available on all audiometers. The sound output is adjusted so that at each frequency the level at the ear represents the hearing norm. Suitable controls are provided; a graphic recording device may also be used. To speed up group testing, more than one set of earphones can be provided.

The basic procedure is simple; for each ear and test frequency, the sound level is slowly raised until the subject hears the tone; the level reached is the threshold and is so recorded. Typical forms are shown in Figure 6.1.8; the lower form is graphic and shows the response of both ears superimposed. The upper is an abbreviated tabular form convenient for keeping permanent records of employees.

In routine testing in industrial locations, regular tests are made only at frequencies of 500, 1000, 2000, 4000, and 6000 Hz, with occasional tests over the entire range.1 This abbreviated test takes less time than the comprehensive one; in addition, testing at the upper and lower extremes of frequency is difficult and subject to error. At the highest frequencies, problems of coupling between earphone and eardrum often occur. Differences or scatter of 5 dB in audiograms is not uncommon; it occurs except under the best laboratory conditions.

While an audiometric testing laboratory for the best types of clinical work is an elaborate installation, a facility for routine tests can be set up in an industrial plant occupying less than 100 square feet. It can be located in a first-aid station or even in a personnel office using a commercially available isolating booth for the audiometer and the subject.

Systematic differences in threshold levels have been found when one audiometric technique is changed to an other; these differences have been as large as 10 dB. Though some of these differences cannot be entirely explained, these points should be remembered: changes disclosed in a continuing series of audiograms are more likely to be reliable than any single audiogram, and uniformity and consistency in technique are essential.

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