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U. Glaeser

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FIGURE 27.14 Global hearing threshold L ptg for several simultaneous maskers as a function of frequency.<br />

FIGURE 27.15 Temporal masking effect.<br />

where zj corresponds to the smallest Lptg within the critical band of the jth masker. This is usually the<br />

left-hand side edge of this critical band.<br />

Masking also occurs when the signal either precedes or follows the masker (Fig. 27.15). This is the<br />

already mentioned phenomenon of temporal masking. In backward masking (pre-masking) the signal<br />

precedes the masker, while in forward masking (post-masking) the signal follows the masker. The premasking<br />

effect appears in 10–20 ms before the masker, while the post-masking effect is by one order of<br />

magnitude longer, i.e., in the order of 50–200 ms after the masker ends. In order to take the post-masking<br />

into consideration, the signal power Pj 10 occurring in time t should be seemingly increased<br />

according to equation<br />

Lpj /10<br />

=<br />

in which the post-masking coefficient c j(∆t) is given by<br />

© 2002 by CRC Press LLC<br />

Masking [dB]<br />

100<br />

90<br />

80<br />

70<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

0<br />

[dB]<br />

SPL , p L<br />

120<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

10 -2<br />

-20<br />

Pre-masking<br />

10 -1<br />

Frequency, f<br />

Simultaneous masking<br />

Masker<br />

10 0<br />

[kHz]<br />

(27.24)<br />

(27.25)<br />

but τj is a time constant depending on the jth critical band. In result, the simultaneous masking threshold<br />

rises according to the increased level Lpsj =<br />

10 log P .<br />

10 1<br />

Post-masking<br />

-20 0 0 20 40 ... 180 200<br />

Time [ms]<br />

Psj() t = Pj() t + cj( ∆t)Pj(<br />

t – ∆t)<br />

cj( ∆t)<br />

= exp( – ∆t/τ j)<br />

10 sj

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