Repetition pitch
Repetition pitch is an unexpected sensation of tonality or pitch that often occurs in nature when a sound is reflected against a sound-reflecting surface, and both the original and the reflected sound arrive at the ear of an observer, but with a short time delay between the two arrivals.
Pitch and delay time
When the delay of the repeated sound is too large, the observer perceives an echo; when the delay of the repeated sound is generally smaller than 30 ms, he perceives the original sound only, but with a tonality superimposed. Therefore, this perceptual phenomenon has been named Repetition Pitch. In general, the perceived RP is equal to the reciprocal value of the delay time between the original and the repeated sound, or in formula: RP = 1/T. RP is most salient when the original sound is wide-band in frequency content and does not produce pitch itself.
Probably the first written report of the phenomenon dates from 1693 when Christiaan Huygens noted such a pitch in the sound from a fountain repeatedly reflected against the steps of a large stone staircase in the garden of the castle of Chantilly in France. In an open field, one might be able to hear a gliding RP when a plane flies over. In music, the phenomenon is sometimes deliberately created by electronic means to superimpose a pitch or coloration effect on the original music. In room acoustics and sound recording, however, the phenomenon often causes an unwanted coloration of the original sound. See for various sound demos.
Uses and studies
Blind people might use RP to locate obstacles by clicking the street surface with their cane, thus producing a wide-band impulsive sound that is reflected against the obstacle. RP has been subject of various studies, both psychophysical, electrophysiological and behavioural.