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B-737-500 72
B-737-600/800 77
A-320 74
B-737-436 74 76
B-757-236 71 73
B-767-336 70 72
BAe ATP (advanced turboprop) 72 74
Concorde 74 78
Table 12-3. Cockpit noise levels of representative older and current airline aircraft in terms of
speech interference levels (SIL)
PERSONAL EAR PROTECTION
Without control and protection, the hearing mechanism can become impaired permanently and/or speech
communication affected adversely. In many instances, use of ear protectors is the most practical means of
shielding man from these noise effects. The main function of an ear protector is to reduce the sound pressure
level of the noise reaching the sense organ by serving as an acoustical barrier between the source of the noise
and the inner ear.
Ear protectors
There are two basic types of ear protectors - the insert type and the muff type. The efficiency of a protector is
usually expressed in terms of the amount of noise reduction provided through 300 to 3 000 Hz, a frequency
range critical for the hearing of speech. Generally ear protectors are more effective for the higher frequencies
of this range.
The insert-type protector reduces the noise level reaching the inner ear by plugging the external ear canal. It
ICAO Preliminary Unedited Version — October 2008 III-12-12
may be made of rubber, neoprene, plastic, silastic or cotton impregnated with wax. The effect of commercially
available plugs of differing materials and shapes varies little, except for user acceptance. New features are
being introduced regularly. Polyethylene tubes through the longitudinal dimension of the ear plug have been
used. Slit valves and other modifications are being tried for pressure equalization between the ambient air and
the parcel of air contained between the ear plug and the tympanic membrane. The best ear inserts are those
which are flexible enough to conform to the variations in the shape of the ear canal. However, these ear inserts
can be dislodged by jaw movements which occur in talking, chewing or yawning and require readjustment to
assure an air-tight seal. If the ventilation channel is blocked by for instance earwax, significant
pressure-induced ear pain can occur, especially if the pressure change occurs fast. Therefore, it is very
important that the ventilation channel in insert-type ear protectors is checked by the pilot before every flight
and that the pilot’s ear canal is frequently inspected for cerumen and skin irritation (external otitis).
The ear-muff type reduces the noise level by enclosing the auricle of the ear, providing an acoustical seal
against the head. This may be prefabricated or custom fitted from such materials as rubber or soft plastic. The
external ear is covered completely. The muffs, mounted on an adjustable headband or on a protective helmet,
consist of rigid cups with cushions of soft sealing material placed around their rims. There are those who feel
that muffs usually provide more protection (attenuation) than insert devices. The average attenuation for muffs
is 35 dB. Although the ear muffs are generally easier to fit, care must be taken to ensure that a seal is made
between the side of the head and the muff cushion. Modifying the muff for reasons such as for wires to ear
phones, a pressure relief, or for ventilation, impairs its efficiency to reduce sound. Efficiency can be reduced
also when wearing glasses which create a leak where the stems of the glasses pass under the ear cushion. This
can be rectified by wrapping a piece of foam rubber around the stems where they go under the muff.
The simultaneous use of an ear insert and a muff offers more reduction of noise than either one alone and
provides the potential for the maximum reduction of sound transmitted through the external ear before sound
transmission by bone takes place. The perfect protector cannot provide more than about 55 dB of noise
reduction, for above this level sound begins to reach the inner ear through the vibration of the bones of the
skull. Whatever the type of ear protector, insert or muff, its effectiveness depends on its ability to obtain and
maintain an air-tight seal, and accompanying directions should be followed to assure a proper fit and acoustic
seal.
In recent years Active Noise Reduction (ANR) has become widespread. The mechanism is, in short, that noise
is removed by emission of a sound wave of the exact same frequency as that of the incoming sound wave but
in the opposite phase, thus eliminating the noise. The technique is limited to lower frequencies (up to 1200 Hz),
it is therefore important to use additional passive noise protection.
Generally, ear protectors have no adverse effects on understanding speech in noise, provided the voice is
raised above background noise level, either in face-to-face communication, loudspeaker communication or
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Manual of Civil Aviation Medicine 2(78)
