Octave illusion
Encyclopedia
Discovered by Diana Deutsch
in 1973, the octave illusion is an auditory illusion
produced by simultaneously playing two sequences of two note
s that are spaced an octave
apart, high to low, and low to high, in separate stereo
channels over headphones
. The tones are sine wave
s of constant amplitude
, and switch between high and low four times a second, with no amplitude drops at the transitions. So the listener is presented with a single, continuous two-tone chord, with the ear of input of each component switching repeatedly. (Listen over headphones to the example linked below.)
This pattern is almost never heard correctly, and instead gives rise to a number of illusions. Most people hear a single tone that switches between left and right ears while its pitch simultaneously switches back and forth between high and low. Most right-handers hear the high tone as on the right and the low tone as on the left. When the earphones are reversed most right-handers hear the same thing – the tone that had appeared in the right ear still appears in the right ear and the tone that had appeared in the left ear still appears in the left ear.
Other people experience entirely different illusions; for example they may hear a single high tone in the left ear alternating with a single low tone in the right ear, or obtain complex perceptions that involve three or four tones of different pitch. Right-handers and left-handers vary statistically in how they perceive the octave illusion, with left-handers less likely to hear the high tone on the right, and more likely to obtain complex perceptions.
Diana Deutsch
Diana Deutsch is a British-American perceptual and cognitive psychologist, born in London, England. She is currently Professor of Psychology at the University of California, San Diego, and is one of the most prominent researchers on the psychology of music...
in 1973, the octave illusion is an auditory illusion
Auditory illusion
An auditory illusion is an illusion of hearing, the aural equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds...
produced by simultaneously playing two sequences of two note
Note
In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....
s that are spaced an octave
Octave
In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...
apart, high to low, and low to high, in separate stereo
Stereophonic sound
The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...
channels over headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
. The tones are sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
s of constant amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
, and switch between high and low four times a second, with no amplitude drops at the transitions. So the listener is presented with a single, continuous two-tone chord, with the ear of input of each component switching repeatedly. (Listen over headphones to the example linked below.)
This pattern is almost never heard correctly, and instead gives rise to a number of illusions. Most people hear a single tone that switches between left and right ears while its pitch simultaneously switches back and forth between high and low. Most right-handers hear the high tone as on the right and the low tone as on the left. When the earphones are reversed most right-handers hear the same thing – the tone that had appeared in the right ear still appears in the right ear and the tone that had appeared in the left ear still appears in the left ear.
Other people experience entirely different illusions; for example they may hear a single high tone in the left ear alternating with a single low tone in the right ear, or obtain complex perceptions that involve three or four tones of different pitch. Right-handers and left-handers vary statistically in how they perceive the octave illusion, with left-handers less likely to hear the high tone on the right, and more likely to obtain complex perceptions.