One of the most widely used auditory illusions is Shepard's (1964) demonstration of pitch circularity, which has come to be known as the "Shepard Scale" demonstration. The demonstration uses a cyclic set of complex tones, each composed of 10 partials separated by octave intervals. The tones are cosinusoidally filtered to produce the sound level distribution shown below, and the frequencies of the partials are shifted upward in steps corresponding to a musical semitone (approx. 6%). The result is an "ever-ascending" scale, which is a sort of auditory analog to the ever-ascending staircase visual illusion.
Several variations of the original demonstration have been described. J-C. Risset created a continuous version. Other variations are described by Burns (1981), Teranishi (1986), and Schroeder (1986).
Two examples of scales that illustrate circularity in pitch judgment are presented. The first is a discrete scale of Roger N. Shepard, the second a continuous scale of Jean-Claude Risset.