Time Masking

Masking can occur between sounds that are not present simultaneously. Non-simultaneous masking is also known as time masking, and can occur as pre- or post-masking on a test tone.

Simultaneous masking describes the effect when the masked signal and the masking signal occur at the same time. Pre-masking is when the test tone occurs before the masking sound. Post-masking is when the test tone occurs after the masking sound. The following figure shows the time regions of pre-masking, simultaneous masking, and post-masking in relation to the masking signal.

Figure 62. Time Regions of a Masking Signal's Pre-Masking, Simultaneous Masking, and Post-Masking

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Post-masking is a pronounced phenomenon that corresponds to decay in the effect of the masking signal. Pre-masking is a more subtle effect caused by the fact that hearing does not occur instantaneously because sounds require some time to sense. As indicated in the figure above, researchers typically can measure pre-masking for only about 20 ms. Post-masking is the more dominant temporal effect and can be measured for 100 ms following the cessation of the masking sound. Both the threshold in quiet and the masked threshold depend on the duration of the test tone. Researchers must know these dependencies when investigating pre- and post-masking because they use short-duration test signals to perform these measurements.