Shepard tone

Spectrum view of ascending Shepard tones (linear frequency scale)

A Shepard tone, named after Roger Shepard (born 1929), is a sound consisting of a superposition of sine waves 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.^[1]

Construction

Figure 1: Shepard tones forming a Shepard scale, illustrated in a sequencer

Each square in the figure indicates a tone, with any set of squares in vertical alignment together making one Shepard tone. The color of each square indicates the loudness of the note, with purple being the quietest and green the loudest. Overlapping notes that play at the same time are exactly one octave apart, and each scale fades in and fades out so that hearing the beginning or end of any given scale is impossible. As a conceptual example of an ascending Shepard scale, the first tone could be an almost inaudible C(4) (middle C) and a loud C(5) (an octave higher). The next would be a slightly louder C♯(4) and a slightly quieter C♯(5); the next would be a still louder D(4) and a still quieter D(5). The two frequencies would be equally loud at the middle of the octave (F♯), and the eleventh tone would be a loud B(4) and an almost inaudible B(5) with the addition of an almost inaudible B(3). The twelfth tone would then be the same as the first, and the cycle could continue indefinitely. (In other words, each tone consists of two sine waves with frequencies separated by octaves; the intensity of each is a gaussian function of its separation in semitones from a peak frequency, which in the above example would be B(4).)

	A Shepard–Risset glissando
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The acoustical illusion can be constructed by creating a series of overlapping ascending or descending scales. Similar to a barber's pole, the basic concept is shown in Figure 1.

The scale as described, with discrete steps between each tone, is known as the discrete Shepard scale. The illusion is more convincing if there is a short time between successive notes (staccato or marcato instead of legato or portamento).

Jean-Claude Risset subsequently created a version of the scale where the tones glide continuously, and it is appropriately called the continuous Risset scale or Shepard–Risset glissando. When done correctly, the tone appears to rise (or descend) continuously in pitch, yet return to its starting note. Risset has also created a similar effect with rhythm in which tempo seems to increase or decrease endlessly.^[2]

The tritone paradox

A sequentially played pair of Shepard tones separated by an interval of a tritone (half an octave) produces the tritone paradox. In this auditory illusion, first reported by Diana Deutsch in 1986, the scales may be heard as either descending or ascending.^[3] Shepard had predicted that the two tones would constitute a bistable figure, the auditory equivalent of the Necker cube, that could be heard ascending or descending, but never both at the same time.^[1] Deutsch later found that perception of which tone was higher depended on the absolute frequencies involved, and that different listeners may perceive the same pattern as being either ascending or descending.^[4]

Examples

• In a film by Shepard and E. E. Zajac, a Shepard tone accompanies the ascent of an analogous Penrose stair.^[5]
• In his book Gödel, Escher, Bach: An Eternal Golden Braid, Douglas Hofstadter explains how Shepard scales can be used on the Canon a 2, per tonos in Bach's Musical Offering (called the Endlessly Rising Canon by Hofstadter^[6]) for making the modulation end in the same pitch instead of an octave higher.^[7]
• In Super Mario 64, a modified Shepard tone is incorporated into the music of the endless staircase.^[8] Much like a real Shepard tone, the staircase itself gives players the impression that they are constantly running upwards, when in reality the game has simply locked them in place, and turning around reveals that they were actually running in place halfway up the stairs.
• In the film The Dark Knight and its follow-up The Dark Knight Rises, a Shepard tone was used to create the sound of the Batpod, a motorcycle that the filmmakers didn't want to change gear and tone abruptly but to constantly accelerate.^[9]
• Used as a compositional technique for digital orchestral music on a piece by composer Renaldo Ramai entitled "The Journey".^[10]
• The Shepard tone was a key aspect in Stephin Merritt's song "A Million Faces", composed for NPR's "Project Song".^[11]
• The ending of the song Echoes from the album Meddle by Pink Floyd features a Shepard tone that fades out to a wind sound (actually a white noise processed through a tape echo unit).
• The Austrian composer Georg Friedrich Haas uses Shepard tone effect towards the end of his orchestral piece In Vain.

See also

• Chorus effect
• Flanging
• Phaser (effect)
• Pitch circularity
• Strange loop
• Wave interference

References

External links

• Shahrokh D. Yadegari (August 25, 1992). "The Shepard Tone (The partials of a Shepard tone)". Self-similar Synthesis: On the Border Between Sound and Music (Master thesis). Archived from the original on 2013-02-08. 
• BBC science show, Bang Goes the Theory, explains the Shepard Tone
• Demonstration of discrete Shepard tone (requires Macromedia Shockwave)
• Visualization of the Shepard Effect using Java
• A demonstration of a rising Shepard Scale as a ball bounces endlessly up a Penrose staircase (and down)
• Shepard tone Keyboard on CodePen