What is a scale illusion?

A binaural scale illusion occurs when two different sequences of notes are played in each ear, and when they are heard together the listener hears an unbroken scale.

Here is a recording of the illusion, with each individual sequence of notes played separately, then combined. This is best experienced through stereo headphones. (from Diana Deutch)

What is the Shepard’s Tone?

The Shepard’s Tone is an audio illusion in which we seem to hear a constantly rising tone. Much like how the lines on a barbershop pole seem to constantly rise or fall, the tones in the Shepard’s tone seem to rise forever. In reality, there are new tones introduced in the lower regions as the tones in the higher regions extend out of audible range. The average pitch stays the same, but we perceive the scale to be constantly rising into infinity.

Example of a shepard’s tone:

Here is the spectrogram of that audio recording (both are from University of Wisconsin Eau Claire)


What is the McGurk Effect?

The McGurk Effect is a perceptual illusion in which what you see affects what you hear. When what you see conflicts with what you hear, you may hear a third phoneme that is mixture of the other two phonemes. If the video shows a person saying “ga” and the audio plays “ba” you may hear a third, intermediate phoneme, “da”. This perceived intermediate phoneme is the brain’s way of satisfying the conflicting audio and visual inputs.

This illusion is known to persist even once one learns about it. While in some visual illusions one can “see through” the illusion and the illusion is broken down, one will still perceive the third phoneme if presented with conflicting audio and visual inputs.

Musical precursors to the Shepard’s Tone

There are many examples of the shepard’s tone used in musical compositions before it was even discovered or named. It can create a sense of rising excitement and energy in the music, but can also create an eerie and unsettling tone.

Tchaikovsky passage: (from University of Wisconsin-Eau Claire)

In this musical excerpt from Tchaikovsky’s Symphony No. 6 in B Minor (Pathetique), we can hear an example of the shepard’s tone illusion before it was recognized as such. In the score we can see that only one octave is used in the scale, but when we listen to the music we can hear the illusion that the notes seem to continue above an octave.

Pathetique_Score_Clip_1 Pathetique_Score_Clip_2

(from Leonard Bernstein’s The Infinite Variety of Music)

Passage from Alban Berg’s 1925 opera Wozzeck

“In his 1925 opera Wozzeck, Berg employed a continuously rising scale that was orchestrated in such a way that the upper instruments faded out at the top of their range while the lower instruments faded in at the low end.” (Diana Deutch)