On the Audibility of Comb Filter Distortions

Abstract

Superpositions of delayed and undelayed versions of the same signal can occur at different stages of the audio transmission chain. Sometimes it is a deliberate measure to provide audio material with certain spatial or timbral qualities. 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, a listening test was conducted to determine the just noticeable difference for three stimulus categories (speech, a snare drum roll and a piano phrase) 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. 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.