This illusion was invented by Roger Shepard, a psychologist, in 1964. He used a computer to create a series of tones, or a scale, that seems to rise forever.
TIP: although you can use the speakers on your multimedia computer, a Make sure the volume isn't too loud!
Can you perceive the point where these tones jump down and repeat the cycle?
Make Your Own Barbershop Pole
The Shepard's Tone illusion does in sound what the old-fashioned barbershop pole does visually, e.g. seemingly rise forever. This activity uses a graph of the of the Shepard's Tones' frequencies to create the same effect as the barbershop pole. The graph is called a spectrogram. It shows the Shepard Tone's frequencies changing through time. Can you see where they drop down and start to rise again in a cycle?
Spectrogram of the Eight Shepard's Tones
What's the point?
The 'Make Your Own Barbershop Pole' activity provides a visual representation of what you hear, the ever-rising sound to the Shepard's tones.
The point here is that your brain uses two main features to perceive the effects of this illusion:
As the tones play, they rise in pitch, gradually getting higher until they rise to half of their original frequency. Then they jump back down to their original frequency and repeat the cycle. You'd think that you could perceive where this jump in frequency occurs but that's where the volume changes become a factor.
Each of the eight tones has a unique volume curve, getting gradually louder or softer. The key is that when any tone finishes rising in pitch, its volume matches that of the next tone beneath it. So when the lower tone reaches the end of the cycle, its volume matches that of the higher tone.
Since these changes occur gradually and over a wide spectrum of sound, the result is a seamless transition in the cycle, just like the "barbershop pole" you made from its spectrogram.
Follow this link for more information on how to create your own spectrograms with free software. Use the BACK button on the top left of your browser to return to this page.
Just for Fun!
What do you think would happen if the set of tones had only six of the eight tones playing? Would you be able to hear the volume changes?
What can you conclude about the effect of speed and volume changes on these tones?
What other visual illusions are similar to these tones? [ HINT: think M.C. Escher ]
What other experiments might you try?
More Shepard's Tones Examples
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"Certainty generally is illusion,
– Oliver Wendell Holmes, Jr.
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