A SHEPARD TONE is named after Roger Shepard (born 1929).  Technically, it is a sound created by a superposition of sine waves separated by octaves and where the bass tone either moves up or down. What you hear is an AUDIO ILLUSION of a tone that continually ascends or descends but that ultimately never seems to go higher or lower!!! Click on the image below to hear the SHEPARD TONE EFFECT in ascending form. (There is also a descending form.)

The SHEPARD TONE EFFECT is explained in the following video clip.