Shepard tone

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 seems to continually ascend or descend in pitch, yet which ultimately gets no higher or lower.[1]

A spectrum view of ascending Shepard tones on a linear frequency scale.

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 C4 (middle C) and a loud C5 (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 D4 and a still quieter D5. The two frequencies would be equally loud at the middle of the octave (F♯4 and F♯5), 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. (In other words, each tone consists of two sine waves with frequencies separated by octaves; the intensity of each is e.g. a raised cosine function of its separation in semitones from a peak frequency, which in the above example would be B4. According to Shepard, "(...) 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."[1]

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."[2]

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 rather than legato or portamento).

Variants

Shepard-Risset glissando

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.[3] When done correctly, the tone appears to rise (or fall) 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.[4]

Tritone paradox

A sequentially played pair of Shepard tones separated by an interval of a tritone (half an octave) 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.[1]

In 1986, Diana Deutsch discovered that the perception of which tone was higher depended on the absolute frequencies involved, and that an individual would usually hear the same pitch as the highest (this is determined by the absolute pitch of the notes).[5] Interestingly, different listeners may perceive the same pattern as being either ascending or descending, depending on the language or dialect of the listener (Deutsch, Henthorn and Dolson found that native speakers of Vietnamese, a tonal language, heard the tritone paradox differently from Californians who were native speakers of English).[6][7]

Perpetual melody

Pedro Praticio observed in 2012 that by using a Shepard tone as a sound source and applying it to a melody he could reproduce the illusion of a continuously ascending or descending movement characteristic of the Shepard Scale. Regardless of the tempo and the envelope of the notes, the auditory illusion is effectively maintained. The uncertainty of the scale the Shepard tones pertain allows composers to experiment with deceiving and disconcerting melodies.[8]

Examples
  • In a 1967 film by Shepard and E. E. Zajac, a Shepard tone accompanies the ascent of an analogous Penrose stair.[9]
  • The ending of the 1971 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).[10]
  • Queen's album A Day at the Races begins and ends with a Shepard tone produced by a harmonium. (Album liner notes)
  • In his 1980 book Gödel, Escher, Bach: An Eternal Golden Braid, Douglas Hofstadter explained how Shepard scales could be used on the Canon a 2, per tonos in Bach's Musical Offering (called the Endlessly Rising Canon by Hofstadter[11]:10) for making the modulation end in the same pitch instead of an octave higher.[11]:717–719
  • In 1995, Ira Braus argued the last notes of the 1885 piano work Bagatelle sans tonalité, composed by Franz Liszt could be continued to produce a Shepard scale using Hofstadter’s technique.[12]
  • In the video game Super Mario 64 for the Nintendo 64 console (1996) there is a musical piece that incorporates a slightly modified Shepard scale played in the background when the player tries to climb the never-ending stairs located in the penultimate room of Peach's Castle. This auditory illusion complements the spacial loop effect seemingly giving the impression that the stairs never end.[13]
  • In Godspeed You! Black Emperor's 1997 debut F♯ A♯ ∞, the opening song The Dead Flag Blues has a section mainly consisting of tornado sirens using a Shephard tone to create its unique apocalyptic atmosphere.
  • Austrian composer Georg Friedrich Haas includes the Shepard scale (at the end, among other moments) in his experimental orchestral piece in vain (2000/02).[14]
  • Christopher Nolan said in an interview that the soundtrack of his film The Prestige (composed by David Julyan) explores the potential of Shepard tones as a fundamental basis for compositions.[15]
  • In Stephin Merritt's 2007 song "Man of a Million Faces", composed for NPR's "Project Song", the Shepard tone is a key aspect.[16]
  • In the 2008 film The Dark Knight and its 2012 follow-up The Dark Knight Rises, a Shepard tone was used to create the sound of the Batpod, a motorcycle that the filmmakers did not want to change gear and tone abruptly but to accelerate constantly.[17]
  • In the 2017 film Dunkirk, a Shepard tone is used to create the illusion of an ever increasing moment of intensity across intertwined storylines.[18]
  • In Lucrecia Martel’s feature film Zama (2017) we find an extensive use of the Shepard tone creating a "loud and shreechy soundscape, in order to achieve closeness to the viewer", according to the director.[19]
  • The 2018 track "Always Ascending" by Franz Ferdinand from the 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.[20]

See also

References
  1. Shepard, Roger N. (December 1964). "Circularity in Judgements of Relative Pitch". Journal of the Acoustical Society of America. 36 (12): 2346–53. doi:10.1121/1.1919362.
  2. "Clip from Series 4, Episode 6". Bang Goes the Theory. 18 April 2011. BBC. It's like a barber's pole of sound.
  3. "Jean-Claude Risset, who reimagined digital synthesis, has died - CDM Create Digital Music". CDM Create Digital Music. 22 November 2016. Retrieved 30 December 2019. The sound for which Risset is best known is perhaps the most emblematic of his contributions. Creating a sonic illusion much like M.C. Escher’s optical ones, the Shepherd-Risset glissando / Risset scale, in its present form invented by the French composer, seems to ascend forever.
  4. Risset rhythm
  5. Deutsch, Diana (1986). "A musical paradox" (PDF). Music Perception. 3 (3): 275–280. doi:10.2307/40285337. JSTOR 40285337.
  6. Deutsch, D. (1992). "Some New Pitch Paradoxes and their Implications". Philosophical Transactions of the Royal Society B: Biological Sciences. 336 (1278): 391–397. doi:10.1098/rstb.1992.0073. PMID 1354379.
  7. DEUTSCH, DIANA; HENTHORN, TREVOR; DOLSON, MARK (2004). "Speech Patterns Heard Early in Life Influence Later Perception of the Tritone Paradox". Music Perception. 21 (3): 357–372. doi:10.1525/mp.2004.21.3.357. ISSN 0730-7829.
  8. Patricio, Pedro. From the Shepard tone to the perpetual melody auditory illusion. Proceedings of the 9th Sound and Music Computing Conference, SMC 2012. 5-10, 2012.
  9. Shepard, Roger N.; Zajac, Edward E. (1967). A Pair of Paradoxes. AT&T Bell Laboratories.
  10. Hurskiy, S. (2018). "Auditory illusions in modern film industry and music" (PDF). Zhytomyr State University Library. Retrieved 25 September 2020.
  11. Hofstadter, Douglas (1980). Gödel, Escher, Bach: An Eternal Golden Braid (1st ed.). Penguin Books. ISBN 0-14-005579-7.
  12. Braus, I. (1995). "Retracing one's steps: An overview of pitch circularity and Shepard tones in European music, 1550–1990". Music Perception. 12: 323–351. doi:10.2307/40286187. JSTOR 40286187.
  13. Phillips, Winifred (14 February 2014). A Composer's Guide to Game Music. MIT Press. ISBN 978-0-262-02664-2.
  14. Hutchinson, Mark (April 2019). "Stairways in the Dark: Sound, Syntax and the Sublime in Haas's in Vain". Tempo. 73 (288): 7–25. doi:10.1017/S0040298218000943. ISSN 0040-2982.
  15. Guerrasio, Jason. "Christopher Nolan explains the biggest challenges in making his latest movie 'Dunkirk' into an 'intimate epic'". Business Insider. Retrieved 14 November 2020.
  16. Stephin Merritt: Two Days, 'A Million Faces'. NPR (video). 4 November 2007. Retrieved 9 October 2015. 'It turns out I was thinking about a Shepard tone, the illusion of ever-ascending pitches.'
  17. King, Richard (4 February 2009). "'The Dark Knight' sound effects". Los Angeles Times.
  18. Haubursin, Christopher (26 July 2017). "The sound illusion that makes Dunkirk so intense". Vox.
  19. Gemünden, Gerd; Spitta, Silvia (1 June 2018). "'I Was Never Afraid': An Interview with Lucrecia Martel". Film Quarterly. Vol. 71 no. 4. pp. 33–40. doi:10.1525/fq.2018.71.4.33. ISSN 0015-1386.
  20. Template:Ci web
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