PEITTOILMIÖ

  

Masking plays a very important role in everyday life. For a conversation on the sidewalk of a quiet street, little power is necessary for the speakers to understand each other. However, if a loud truck passes by, the conversation is disturbed. The speakers can no longer hear each other if speech power is kept constant.

Figure 3 Schematic drawing to illustrate and characterize the regions within which premasking, simultaneous masking and post masking occur. Note that postmasking uses a different time origin than premasking and simultaneous masking [ZWFA90].

 

Figure 4 Level of test tone just masked by critical-band wide noise with center frequency of 1 kHz and different levels as a function of the frequency of the test tone [ZWFA90].

 
Masking effects can be measured not only when masker and test tone are present simultaneously, but also when they are not. In the latter case, the test sound has to be a short burst or sound impulse that can be present before the masker stimulus is switched on. The masking effect produced under these conditions is called pre-stimulus masking, (figure 3). This effect is not very strong, but if the test sound is present after the masker is switched off, then a quite pronounced effect occurs. Because the test sound is present after the termination of the masker, the effect is called post-stimulus masking, or postmasking. Figure 4 shows the dependence of masked threshold on the level of a narrow band noise centered at 1 kHz. Narrow band noise means a noise with a bandwidth equal to or smaller than the critical bandwidth (about 100 Hz below and 0.2 f above 500 Hz).

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