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It
is assumed that our hearing system process sounds in relatively
narrow frequency bands. It has been discovered that the part of a
noise that is most effective in masking a test tone, is the part of
its spectrum lying near the tone. Masking is achieved, when the
power of the test tone and the power of that part of the noise
spectrum lying near the tone and producing the sensation effect, are
the same. The parts of the noise outside the spectrum near the test
tone do not contribute to masking. Characteristic frequency bands
defined in this way have a bandwidth that produces the same acoustic
power in the tone and in the noise spectrum within that band, when
the tone is just masked.
Data
from many subjects has been collected to produce a reasonable
estimation of the width of the critical band. Although the lowest
critical bandwidth in the audible frequency region may be very close to
80 Hz, it is attractive to add the inaudible range from 0 Hz to 20 Hz
to that critical band, and to assume that the lowest critical band
ranges from 0 Hz to 100 Hz. Using this approximation, figure 2 shows
the average between the quiet threshold and 90dB. There is a small
tendency for the critical band to increase somewhat for levels above 70
dB
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