The Shepard tone is an auditory illusion: A succession of overlapping scales are played, each ascending, and each scale fades out as its successor fades in an octave lower. The resulting impression is of a climbing pitch that never “arrives” anywhere, a rising note that never gets higher.
Among many other applications, this sound was used for the Batpod in Christopher Nolan’s films The Dark Knight and The Dark Knight Rises — the vehicle seems constantly to accelerate without ever changing gear. “When played on a keyboard,” wrote sound designer Richard King, “it gives the illusion of greater and greater speed; the pod appears unstoppable.”
01/31/2022 UPDATE: Similarly, the Risset Rhythm seems to speed up: