10 Mind Melting Illusions (And Exactly How They Work)

8. The Shepard Tone

Not all illusions are a trick of the light, the Shepard Tone is a bit of mind-melting weirdness for the ears instead.

The tone sounds as though it is continually getting higher and higher in pitch, yet it never seems to disappear to a supersonic squeak, a bit like those ringtones that only teenagers are supposed to be able to hear.

The illusion is created by modulating the volume of the individual tones as they ascend, forcing the brain to focus on different notes at different times. Overlapping tones that play simultaneously are timed so that they are exactly one octave apart, fading in and out to make it impossibly to determine the beginning and end of the scale.

The illusion has been referred to as the "sonic barber's pole" as a reference to those seemingly endlessly ascending red and white spiralling poles outside barber shops.