Civil Engineering Reference
In-Depth Information
exposure. These changes are usually localized and involve only part of the organ of Corti,
corresponding to certain (high) frequencies. The destruction of the hair cells in the organ
of Corti is an irreversible process, and the resultant hearing loss is permanent. However,
if the noise exposure time is short there may be only temporary swelling of the organ,
which is reversible and causes only temporary hearing impairment, called temporary
threshold shift (TTS) (Ward, 1976).
A person with auditory nerve damage first loses hearing of the higher frequencies at
around 4000 Hz (Loeb, 1986). It is then difficult to hear a woman's voice but it is easier
to hear a man's lower pitch. The person affected soon begins to speak louder and in a
monotone voice, since the modulating effect of hearing is impaired. Because low tones
are heard better than higher ones, it becomes difficult to understand words and sentences.
Low pitch noise seems unduly loud and conversation becomes difficult in a noisy
environment. Amplification of the sound through a hearing aid may not solve any
problems, since high frequencies will still not be heard.
In contrast, a person with conductive deafness will complain that others in
conversations do not speak loud enough. Understanding is not impaired if the sound level
is sufficiently high, and such a person can benefit from the use of a hearing aid.
As mentioned, the first and most notable damage caused by excessive noise is to
hearing, in particular frequencies at about 4000 Hz. However, there is extreme variability
in the individual reactions to noise. Similar loss of hearing may also occur because of
aging (presbycusis).
Loss of hearing may also be caused by ear infections, several diseases (mumps,
measles, scarlet fever), and by common colds. Helander (1992) suggested that
presbycusis may actually be caused by the cumulative effect of common colds over a
lifetime. These viral infections can destroy auditory nerve cells.
13.4
HEARING PROTECTORS
There are two types of hearing protectors that are commonly used in industry: ear plugs
and ear muffs (Berger and Casali, 1997). The plugs are designed to occlude the ear canal
and are available in many types of material. Cotton has traditionally been used, but
unfortunately, and contrary to popular belief, it affords no protection. Ear plugs made out
of rubber, neoprene, glass down, and plastics offer good protection. Custom-molded ear
plugs are also available (Casali and Park, 1990). They are made individually to fit the ear
canal and offer excellent protection. Ear muffs are designed to cover the entire external
ear. They consist of ear cushions made of soft spongy material or specially filled pads to
ensure a snug fit.
Ear plugs provide a sound attenuation of between 15 dB for low frequency sounds and
35 dB for higher frequencies. At frequencies above 1000 Hz, muffs provide about the
same protection as plugs. At frequencies below 1000 Hz, certain muffs provide more
protection than plugs. Ear plugs and ear muffs may be worn together in intense noise
situations. This combination provides an additional attenuation of approximately 5 dB.
Workers who regularly wear ear protection report that they actually hear conversations
better. Cutting down the noise level that reaches the ear helps to decrease the distortion in
the ear so that speech and warning signals are actually heard more clearly. An analogy
