13.
Hearing.
a. Hearing is the perception of sound by the brain. It involves the response of the ear to sound waves, the
transmission of impulses through nerves to the brain, and perception by the brain of the transmitted intelligence. There is
a measurable variation among individuals in the ability to hear, since hearing for a given person depends on the loudness
and pitch of the sound. An approximate determination of hearing ability in terms of loudness only can be made by
measuring the maximum distance at which the ticking of a watch can be heard. A more complete and accurate method
involves the use of a device called an audiometer. The audiometer enables an experienced operator to construct a
scientific graph of the hearing ability of an individual. This graph then may be compared to what generally is accepted as
normal hearing ability. The audiometer consists of a calibrated audio oscillator, the frequency and amplitude of which
may be varied, and a telephone receiver for the reproduction of sound waves. The frequency can be varied from 0 cycle to
about 25,000 hertz per second, and the amplitude can be adjusted to make the intensity of the sound (loudness) vary
through a wide range.
b. In conducting a test with the audiometer, the instrument first is adjusted to any chosen
frequency--for example, 1,000 hertz--then, at that frequency, adjusted to an amplitude so low that the sound from
the receiver is inaudible. The amplitude then is increased gradually until a point is reached where the sound becomes
just perceptible to the ear of the person being tested. This point is called the threshold of audibility for that
frequency. For any given frequency it is the lowest intensity at which sound is audible. In the normal ear, the
threshold of audibility varies with the frequency of sound, so that its ability to hear some frequencies is greater than the
ability to hear others. In addition to this variation, the threshold of audibility for different frequencies is different for
different individuals. For these reasons, a number of frequencies are tested in the measurement of hearing with an
audiometer. The lower curve of figure 7 shows how the average, or normal, threshold of audibility varies with sound
frequency. The dip in the lower curve indicates that the average ear is most sensitive to frequencies in the vicinity
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