On the Audibility of Comb Filter Distortions
Superpositions of delayed and undelayed versions of the same signal can occur at different stages of the audio transmission chain. Often it is a result of multiple microphone signals, sound reflections on walls or latencies in digital signal processing leading to comb-filter-shaped, linear distortions. The measurement of a hearing threshold for this type of distortion with its dependance on reflection delay, relative level and the type of audio content can be the basis for boundaries in everyday recording practice below which undesired timbral distortions can be neglected. Therefore, listening tests were conducted to determine the just noticeable difference for three stimulus categories and different time delays between direct and delayed signal from 0.1 ms to 15 ms, equivalent to 0.03 - 5.15 m of sound path difference, and to reveal the underlying sound qualities. The results show that comb-filter distortions can still be audible if the level of the first reflection is more than 20 dB lower than the level of the direct sound.
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