The Shepard Tone Is A Sonic Barber Pole

Brilliance | Dec. 18, 2017

A Shepard tone is a sound that seems to go up or down in pitch forever. At least it seems to, but it's only an illusion.

Why It's Noteworthy

Take a listen to the Shepard Tone in the video below. Sounds like the pitch is creeping up and up and up and up, forever, doesn't it? In fact, the sound is simply an auditory illusion — one that has been described as a "sonic barber pole." In the same way that the classic red-and white-striped barber pole seems to perpetually spin upward when it's just staying still, Shepard tones seem to infinitely rise in pitch, when it fact they are rising, then dropping an octave and rising again, on infinite repeat. So why does it feel like the sounds is always getting higher? This tone is made of multiple sine waves that trick our brain. Even though the sounds in the tone reach a certain point and then hop back down an octave and start again, our brains don't hear that jump. Instead they hear the tones rising or falling forever.

Why It's Worth Sharing

For anyone who loves optical illusions, audio illusions are another great (but lesser known) discovery. And now that you know about them, you may start to notice Shepard tones in pop culture, since they are a great vehicle for setting a mood. Like in The Dark Knight and The Dark Knight Returns. As the films' sound designer Richard King told the Los Angeles Times: "The Batpod is slicker and more refined. I used the concept of the Shepard tone to make the sound appear to continually rise in pitch. The basic idea is to slightly overlap a sound with a distinct pitch (a large A/C electric motor, in this case) in different octaves. When played on a keyboard, it gives the illusion of greater and greater speed; the pod appears unstoppable."

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