A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the bass 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.
Construction
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 oneoctave 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 C4 and a loud C5. The next would be a slightly louder C4 and a slightly quieter C5; the next would be a still louder D4 and a still quieter D5. The two frequencies would be equally loud at the middle of the octave, and the twelfth tone would be a loud B4 and an almost inaudible B5 with the addition of an almost inaudible B3. The thirteenth tone would then be the same as the first, and the cycle could continue indefinitely. almost any smooth distribution that tapers off to subthreshold levels at low and high frequencies would have done as well as the cosine curve actually employed." The theory behind the illusion was demonstrated during an episode of the BBC's show Bang Goes the Theory, where the effect was described as "a musical barber's pole." 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.
Variants
Shepard-Risset glissando
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 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.
Tritone paradox
A sequentially played pair of Shepard tones separated by an interval of a tritone produces the tritone paradox. 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. In 1986, Diana Deutsch discovered the paradoxical auditory illusion where scales may be heard as either descending or ascending. 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.
Examples
In The Police song, King of Pain, during the bridge before the 3rd chorus, portions of a Shepard Tone akin to the Shepard-Risset glissando can be heard.
In a film by Shepard and E. E. Zajac, a Shepard tone accompanies the ascent of an analogous Penrose stair.
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'sMusical Offering for making the modulation end in the same pitch instead of an octave higher.
In Super Mario 64, a modified Shepard tone is incorporated into the music of the endless staircase, the staircase to the penultimate room in the castle. 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.
The Shepard tone was a key aspect in Stephin Merritt's song "Man of a Million Faces", composed for NPR's "Project Song".
The ending of the song "Echoes" from the album Meddle by Pink Floyd features a Shepard tone that fades out to a wind sound.
The song "Slow Moving Trains" from Godspeed You! Black Emperor's album F♯ A♯ ∞ begins with the sound of a Shepard tone.
In the film Dunkirk, a Shepard tone is used to create the illusion of an ever increasing moment of intensity across intertwined storylines.
The Shepard tone is sometimes used to build tension in electronic dance music. As the so-called 'build-up' in progressive dance music is a very important aspect of the musical structure, the Shepard tone can be used as an effect to raise the song's energy to a certain level. Examples of modern-day dance music where a Shepard Tone has been used as rising effect are 'Leave the World Behind', and 'Born To Rage'
The track "Endlessly Downwards" by Beatsystem from the 1995 album uses a descending Shepard tone throughout its 3:25 running time.
The track "Always Ascending" by Franz Ferdinand from the 2018 album of the same name features a rising Shepard tone throughout the song. The video for the song echoes the effect, with the camera apparently rising continually throughout.
