Critical band level of the masking noise, L
G
, masking index, a
v
The critical band level, L
G
, of the masking noise is given by equation (C.9):
where
LS is the mean narrow-band level;
Δf
c
is the width of the critical band about the tone frequency, f
T
, in Hertz (Hz);
Δf is the line spacing (frequency resolution), in Hertz (Hz).
The masking index, a
v
, is given by equation (C.10):
where
f is the central frequency of the critical band.
NOTE1. Masking is the raising of the audibility threshold for a sound as a result of the influence of another
sound.
The masking threshold, L′
T
, is that sound pressure level of a sinusoidal test tone that is required for it to be just
perceivable in the presence of a masking noise (critical band level, L
G
). The masking threshold is determined
in repeated tests in which a group of subjects with normal hearing can just perceive the tone in 50% of the
cases.
The masking index, a
v
= L’
T
- L
G
, is the difference between the level of the test tone, L′
T
, and the critical band
level, L
G
. At low frequencies, the masking index has a value of approximately −2 dB. Above a transition range
between 0.2 kHz and 1 kHz, it falls at a constant logarithmic rate to −6 dB at 20 kHz.
Determination of the decisive audibility of a narrow-band spectrum, ΔL
i
Every tone, that is distinguishable in terms of this standard, has the assigned audibility value, that is calculated
as shown in the equation (C.11):
where
j – index of the next tone under investigation.
Since the all components of the above formula are found according to the previous sections, the decisive
audibility of the individual narrow-band spectra can be calculated.
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