Audio Illusions – The Shepard Tone

Most people are familiar with the concept of optical illusions, but it’s also possible to create auditory illusions. One such example is the Shepard Tone…

Keeping Up With The Shepards

The Shepard Tone is a famous audio illusion named after Roger Shepard, an American cognitive scientist born in 1929. Described as a sort of auditory barber’s pole, it creates the impression of a sound continuously rising or falling in pitch forever (even though it is of course not actually doing so).

Essentially it is created by superimposing sine waves on top of one another, with the base pitch either rising or falling, and overlapping notes (those that play at the same time) being set an octave apart. As you progress through the sequence, the volume of the notes in one progression becomes gradually lower, just as the volume of the notes the next progression begins and becomes louder, such that it is impossible to identify the start/end points of any progression.

Just as we have visual illusions where objects may seem to spin in one direction or another depending on the viewer’s perception (or indeed the classic two faces/one candlestick illusion), it’s also possible to create a Shepard tone that may appear to be either ascending or descending. A sequentially played pair of Shepard tones separated by half an octave can create this illusion, and this is known as the tritone paradox.