曝光台 注意防骗
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by the average normal ear. Most audiometers show decibels in minus as well as plus values. When a person
can hear a given frequency at -10 dB, he can hear that frequency better than average person. Similarly, when
the threshold of an ear is no more than 15 dB above zero, the hearing is considered to be normal though not
quite as good as average. A threshold of 30 dB at a given frequency means that this tone must be made 30 dB
more intense than for the average normal person in order to be heard. This person is then said to have 30 dB
hearing loss at the test frequency.
The young unimpaired human ear can detect sounds from 20 Hz to 20 000 Hz. The most important range for
speech perception is between the frequencies of 500 and 3 000 Hz, and the hearing requirements of Annex 1
are confined to this range. It is, however, not sufficient to test for the 500 to 3000 Hz range only. For
diagnostic reasons, testing is done above and below these frequencies to more thoroughly map the ability of
the ear to perceive sound and to indicate minimal losses of which the examinee is unaware but which may be
early symptoms of inner ear disease.
Hearing in the human ear is most acute at about 1 000 Hz. After finding the threshold for 1 000 Hz, the higher
frequencies are tested in the same manner and in ascending order (2 000, 3 000, 4 000, 6 000 and 8 000 Hz).
The 1 000 Hz frequency may then be re-checked, followed by the low frequencies in descending order (500,
ICAO Preliminary Unedited Version — October 2008 III-12-16
250 and 125 Hz). Then the ear selector switch is turned to the opposite ear and the sequence is repeated.
Masking
While one ear is being tested, the opposite ear must be masked to exclude it from the test. Failure to mask the
good ear is a very common error which leads the examiner to believe that the signal is being perceived in the
poor ear (which is under test). Masking is especially important in taking bone conduction measurements, and
it should be used with both tuning fork and audiometer examinations. The greater the discrepancy in hearing
between the ears, the greater the need for masking the better ear. Audiometers are equipped with a masking
sound (a mixture of frequencies, sometimes called "white" noise). The intensity can be varied. Determining
the proper amount of masking to use presents a serious problem. Although numerous systems of determining
the proper level have been suggested, all require knowledge of how much the threshold for a particular pure
tone will be shifted by a given amount of the masking tone. The following simple method can be used.
In air conduction testing, 50-60 dB of effective masking has been found to be sufficient to rule out the better
ear without being loud enough to interfere with measurements on the poorer ear. Bone conduction testing is
accomplished in the same manner as air conduction testing, except that the tone is delivered through the bone
oscillator positioned behind the ear on the mastoid bone. Octave frequencies tested in this manner are 250 Hz
to 4 000 Hz.
Audiogram
The audiogram (Figure 12-2) is a graph having two dimensions, intensity along the ordinate and frequency
along the abscissa. The intensity generally ranges from -20 to +100 dB, and the frequency ranges from 125 to
8 000 Hz.
Since the intensity (hearing loss) scale refers to average normal hearing, the (0) indicates no deviation from
normal. Any postitive (plus) number (normally plotted from the zero line downwards) indicates a degree of
hearing loss - the farther down on the audiogram chart, the poorer the threshold and the greater the intensity
required to reach it.
The applicant's threshold at each frequency is tested and plotted on the audiogram for each ear separately at the
appropriate 5 dB steps, using different symbols for air and bone conduction. In addition, the threshold is drawn
in red standard symbols () for the right ear and in blue symbols (×) for the left ear.
Calibration of audiometers
The need for international standardization of audiometers has been generally recognized. An international
standard was agreed upon in 1964. As stated in Note 1 to 6.2.5, the reference zero for calibration of pure tone
audiometers can be found in the current edition of the Audiometric Test Methods, published by the
International Organization for Standardization (ISO).
Audiometers must be tested at regular intervals and the calibration corrected as necessary by the manufacturer.
When such checks create difficulties, the reliability of audiometric testing procedures can be verified on the
basis of the mean hearing threshold for the various frequencies of at least 20 ears of healthy young persons
with normal tympanic membranes and without past ear disease or known exposure to high noise intensity
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Manual of Civil Aviation Medicine 2(81)
