The auditory staircase illusion is an auditory illusion presented by the American cognitive scientist Roger Shepard in 1964. The computer-generated illusion creates a perception of a sound gradually falling in pitch, yet the pitch of the overall sound does not decrease. The effect is created by presenting complex tones whose pitches are continuously changing in relation to each other.
Source: Shepard, Roger N. (December 1964). Circularity in Judgements of Relative Pitch. Journal of the Acoustical Society of America. 36 (12): (pp.) 2346–53.