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however, be expected, when investigations are carried out on persons in other
age groups, but also the 18-25 years group can
show
a considerable dis-
placement of the curves. This is due to a more or less pronounced hearing
loss, displacing primarily the MAF-curve (minimum audible field) towards
higher sound pressure levels.
The rise of hearing loss can be due to the age of a person whose hearing
organs become less sensitive as the person grows older and is attributed to
the normal process of
aging
(presbycusis). This decrease in sensitivity is
particularly
appreciable
at high frequencies. Hearing loss will however, also
arise if the ear is exposed to sounds of a certain duration having a sound
pressure level which exceeds
a
certain limit. In this case the hearing loss is
caused by the environments (e.g. noisy machinery,
airport
noise etc.),
and
regardless of the frequencies of annoyance, the loss seems to be most pro-
nounced at high frequencies.
Hearing loss may be either temporary or permanent depending on the strength
and
duration of the action of the noise. The Temporary Threshold Shift (TTS)
that can be found immediately after a vigorous sound exposure will disappear
during relatively short time, while a Permanent Threshold Shift (PTS) is
characterized by its lasting effect.
Hearing Aid Measurements.
The use of hearing
aids
is obviously the only way to
equalize
hearing loss,
and
many types of hearing aids have been developed. They
can
all be
characterized as acoustic amplifiers, each consisting of microphone, electronic
amplifier and
earphone
built together in different ways.
The Hearing Aid
Test
Box is designed for the purpose of comparing the
properties of hearing aids. It
should
be noted that the results obtained will
not indicate
exactly
the
performance-o
f hearing
aids
in actual use. The
basic
set-up
for the testing of a hearing
aid'
is
shown
in the block diagram,
Fig. 0.3.
6
Test Box
(Anechoic enclosure)
Meter
165137
Fig. 0.3. Block diagram of the basic set-up in hearing aid measurements.
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