A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves.

When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.

It has been described as a “sonic barber’s pole”.

Below is ten hours of seeming auditory descent:

Unsurprisingly, a number of artists have used this effect in music, games, and movies. Some cool examples of this from the Wikipedia article of the phenomenon:

• In Super Mario 64, a modified Shepard tone is incorporated into the music of the endless staircase.
• In the film The Dark Knight and its follow-up The Dark Knight Rises, a Shepard tone was used to create the sound of the Batpod, a motorcycle that the filmmakers didn’t want to change gear and tone abruptly but to constantly accelerate.
• The end of the Beatles song “I Am the Walrus“.
• The end of the Pink Floyd song “Echoes“

Via Wikipedia.