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to result in some hearing loss.
Uncomplicated tympanic perforations reduce hearing by about 10-15 dB. Some people with almost complete
loss of the tympanic membrane can still understand a loud whisper.
Hearing is divided into two separate functions: sound conduction in the external ear, the tympanic membrane
and the ossicles, and sound perception in the cochlea, the auditory nerve, its nuclei and the complex cerebral
connexions of the auditory pathway. Any condition causing interference with the conductive mechanism
would result in a conduction deafness. Similarly, a lesion of the perceptive mechanism would result in a
perceptive (often referred to as sensorineural) deafness. Lesions in both the conductive and perceptive systems
result in a mixed type of deafness. In conductive deafness, the hearing loss is more marked in the lower tones
but speech discrimination may be normal. In the sensorineural type of deafness, various types of hearing loss
may occur, some with reduced speech discrimination.
NOISE
Noise may be defined as unwanted sound. An exposure to high noise intensity will cause harmful effects, e.g.
hearing loss or even the rupture of the tympanic membrane. The effects will depend basically on noise
intensity level, its quality (frequency spectrum), and exposure time. For aviation personnel particularly, two
considerations need to be examined: the risk of temporary or permanent hearing damage, and interference with
speech communications. Temporary hearing loss may occur through exposure to noise above 80 dB.
High-frequency sounds produce greater impairment than low-frequency sounds, thus the noise spectrum
needs to be considered before deafening effects can be determined.
Noise-induced hearing loss of the sensorineural type occurs first as a temporary threshold shift (TTS) as
measured audiometrically. This is considered due to fatigue of cochlear cells. Noise-induced temporary
threshold shifts can become permanent. The medical examiner should be concerned with temporary and
permanent threshold shift in aviation personnel. The TTS duration and magnitude depends on noise intensity
and exposure time. With intermittent exposure, TTS is reduced. It is normally not produced below 78 dB.
After two hours' exposure, resulting in a TTS of 50 dB, recovery will be complete after about sixteen hours.
Complete recovery of a 60 dB shift will take several days and tends to be slowest in the 4 000 Hz range. TTS
is a criterion for the determination of permanent noise damage risks. The possibility of its effect upon
audiograms should be kept in mind when studying audiograms of applicants who have been examined without
a sufficient time lapse after being exposed to aircraft noise.
The interference by noise on speech and communications is basically a masking process. Background noise
increases the hearing threshold. The extent to which the hearing threshold is increased is called speech
interference level, expressed in decibels. It is the average of the sound pressure levels in dB, in the octave
bands 600-1 200, 1 200-2 400 and 2 400-4 800 Hz and indicates the degree of interference with the ability of
people to communicate and to understand speech. Intermittent noise often causes less interference as
interpolation may compensate for gaps in what is actually heard in partly masked speech. An accurate and
comprehensive method for expressing speech intelligibility in noise is the articulation index, which is
described in the section dealing with speech audiometry. Maximum speech interference levels have been laid
down for predicting to what degree understanding of speech and communications is possible under noise
conditions.
Aircraft noise originates principally from propellers (for piston and turbo-prop aircraft), the engines and
exhaust (with different characteristics for jet, turbo-fan, turbo- prop and piston engines), and aerodynamic
flow or slipstream (speed, take-off, landing). The intensity of sound (noise) decreases proportionally to the
ICAO Preliminary Unedited Version — October 2008 III-12-11
square of the distance.
The noise background for speech and communications is primarily the flight-deck noise. Communication
equipment might be an additional noise source, although static and radio beams, which used to be disturbing
for flight crews in the earlier days of air transport, have now practically disappeared with improved equipment.
Flight-deck noise intensities for a number of aircraft are given in Table 12-3 and its dependence on speed will
be noted.
Centre-cockpit SIL, in dB
Aircraft Cruise High-speed descent
DC-6 78 85
F-27J 78 78
B-707 80 82
B-720 74 74
B-727 78 82
DC-9 74 74
DC-10-30 67
MD-80 68
B-747-300 70
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Manual of Civil Aviation Medicine 2(77)
