Our brains are incredibly powerful, but as we all know, they can be fooled with illusions of any sense.  You’re likely more familiar with optical illusions, as they are frequently shared in social media circles.  But auditory illusions can also occur – and they’re just as weird.

One of the more famous auditory illusions is the Shepard Tone auditory illusion.

Listen to the following:

 

This sound is known as a Shepard tone, named after Roger Shepard, and 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.”

In other words, it sounds very much like a tone that is continuously going down… only, it isn’t.  It’s actually a much smaller loop of a sound that begins at a high point and then drops down, going lower and lower. These loops have been placed one after the other – or stitched together – much like a panoramic image you might take with your smartphone.

Unless if you carefully listen to the clip, you’ll never quite be able to find the exact point at which one loop ends and the next loop begins. Your brain will be fooled and will interpret it as a a sound that is continuously “going down”.  This is, of course, impossible, because after a short time, the sound would be so low that you wouldn’t be able to hear it anymore.  But with this illusion, you do.

The Shepard Tone Illusion:  Described with an image 

If we could visualize what was going on, it might look something like M.C Esher’s Penrose stairs:Endless Stairs Illusion

Here’s what a spectrum view of an ascending Shepard done looks like on a linear frequency scale:

A spectrum view of ascending Shepard tones on a linear frequency scale.

The Shepard Tone is used in film to convey a sense of growing intensity. Take, for example, Hans Zimmer’s score in Dunkirk:

Zimmer told Business Insider:

“There’s an audio illusion, if you will, in music called a ‘Shepard tone’ and with my composer David Julyan on The Prestige we explored that, and based a lot of the score around that.“It’s an illusion where there’s a continuing ascension of tone. It’s a corkscrew effect. It’s always going up and up and up but it never goes outside of its range.

“And I wrote the [Dunkirk] script according to that principle. I interwove the three timelines in such a way that there’s a continual feeling of intensity. Increasing intensity. I wanted to build the music on similar mathematical principals. So there’s a fusion of music and sound effects and picture that we’ve never been able to achieve before.”

What did you hear, and how did it make you feel?