OneTaskBy Tim King · Updated
Shepard Tone Generator
Endlessly Rising or Falling Pitch Illusion
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How the Shepard Tone Generator works
This Shepard tone generator plays the famous barber-pole illusion of sound: a pitch that seems to climb, or fall, for ever without getting anywhere. Press Play, pick a smooth glissando or a stepped scale, set the speed, and the graph shows ten octave-spaced tones sliding under a fixed bell-shaped loudness curve, which is the whole trick laid bare.
How the Shepard tone generator works
A Shepard tone is a stack of sine waves spaced exactly an octave apart, from 20 Hz up to about 20 kHz. Every component moves up (or down) together. Their loudness is not equal: a bell curve centred around 640 Hz makes the middle partials loud and the extreme ones nearly silent. As a partial climbs out of the top it fades to nothing, and at the same moment a new one fades in at the bottom. Because your ear tracks the loud middle of the stack, you hear constant upward motion and never notice the hand-off.
Glissando or steps
Roger Shepard's 1964 original moved in discrete semitone steps, like a scale that never ends. Jean-Claude Risset later made a continuous version, usually called the Risset glissando, where the pitch slides smoothly. Both are here. Steps are good for showing that pitch class and pitch height are separate things; the glissando gives the stronger unsettling sense of endless movement used in film scores and games.
Using it
Keep the volume moderate, especially on headphones, since the illusion works at low levels. Slow speeds such as 10 to 20 seconds per octave sound most convincing. The readout counts how many octaves you have apparently travelled, which makes a good demonstration that the actual sound has gone nowhere at all.
FAQ
Why does the Shepard tone never actually get higher?
Each sine wave does rise, but when it reaches the top of the range it has already faded to silence, and a new one has faded in at the bottom. The overall spectrum repeats every octave, so after one cycle the sound is identical to where it started.
Where is the Shepard tone used?
It shows up in film and game soundtracks to build tension that never resolves, such as the helicopter and engine sounds in Dunkirk, the endless staircase in Super Mario 64, and many trailers. Psychology courses also use it to teach how pitch perception works.
Does the illusion work on phone speakers?
Mostly, though small speakers cannot reproduce the low partials, so the hand-off at the bottom is hidden anyway. Headphones or decent speakers at a moderate volume give the most convincing effect, because you hear the full smooth bell of partials.
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