Shepard tone

A Shepard tone is a sound consisting of a superposition of tones separated by octaves. When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.{{ref|shepard}} This can be constructed by creating a series of overlapping ascending or descending scales. Similar to the Penrose stairs optical illusion (as in M.C. Escher's Ascending and Descending) or a barber's pole, the basic concept is shown in this illustration:

Related Topics:
Sound - Octave - Auditory illusion - Penrose stairs - Optical illusion - M.C. Escher - Barber's pole

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Shepard tone is named after Roger Shepard.

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Each square indicates a tone, any set of squares in vertical alignment together making one Shepard tone. The color of each square indicates the velocity (volume) of the note, with purple being the quietest. Overlapping notes that play at the same time should be exactly one octave apart, and each scale should fade in and fade out, so that it is impossible to hear the beginning or end of any given scale. The scale as described, with discrete steps between each tone, is known as the discrete Shepard scale, but, after Shepard, Jean-Claude Risset created a version of the scale where the steps between each tone are continuous, and it is appropriately called the continuous Risset scale or Shepard-Risset glissando. He has also created a similar effect with rhythm in which tempo seems to increase or decrease endlessly.{{ref|tempo}}

Related Topics:
Velocity - Volume - Octave - Jean-Claude Risset - Rhythm

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To make the illusion more convincing, it helps to have some amount of space between each note (staccato or marcato instead of legato or portamento). When done correctly, the tone appears to rise (or descend) continuously in pitch, yet return to its starting note.

Related Topics:
Staccato - Marcato - Legato - Portamento

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