Usuario:OboeCrack/Zona de pruebas

Zona de Pruebas

Vista espectral de tonos Shepard ascendentes (escala de frecuencia lineal).

Un tono Shepard, nombrado en honor a Roger Shepard, es un sonido que sonsiste en una superposición de ondas sinusidales separadas por octavas. Cuando se tocan con la altura base del tono moviéndose hacia arriba o hacia abajo, reciben el nombre deescala de Shepard. Esto crea la ilusión auditiva de un tono que continuamente asciende o desciende en altura, aunque finalmente fa la impresión de que ni sube ni baja.​ Ha sido descrito como una «poste de barbero sónico».​

Construcción de una escala de Shepard

La ilusión acústica puede construirse creando una serie de escalas solapadas ascendentes o descendentes. Al igual que la ilusión óptica conocida como escalera de Penrose (también en la litografía de M. C. Escher Ascending and Descending) o un poste de barbero, el concepto básico se muestra en la figura 1.

Cada recuadro de la figura indica un tono, cualquier serie de cuadrados alineados verticalmente juntos forman un sólo tono Shepard. El color de cada cuadro indica f each square indicates thevolumen de la nota, siendo el púrpura el más silencioso y verde el más intenso. Las notas solapadas que se tocan a la vez están exactamente a una octava y cada escala se desvanece y se hace aparecer de manera gradual, así escuchar el comienzo y final de una escala dada es imposible. Un ejemplo conceptual de una escala de Shepard ascendete sería empezar como primer tono con do₃ casi inaudible (do central) y un do₄ fuerte (una octava más agudo). El siguiente sería un do₃ ♯ 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 ten sine waves with frequencies separated by octaves; the intensity of each is agaussian function of its separation in semitones from a peak frequency, which in the above example would be B(4).)

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

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). As a more concrete example, consider a brass trio consisting of a trumpet, a horn, and a tuba. They all start to play a repeating C scale (C–D–E–F–G–A–B–C) in their respective ranges, i.e. they all start playing Cs, but their notes are all in different octaves. When they reach the G of the scale, the trumpet drops down an octave, but the horn and tuba continue climbing. They're all still playing the same pitch class, but at different octaves. When they reach the B, the horn similarly drops down an octave, but the trumpet and tuba continue to climb, and when they get to what would be the second D of the scale, the tuba drops down to repeat the last seven notes of the scale. So no instrument ever exceeds an octave range, and essentially keeps playing exactly the same seven notes over and over again. But because two of the instruments are always "covering" the one that drops down an octave, it seems that the scale never stops rising.

Jean-Claude Risset subsequently created a version of the scale where the steps between each tone are continuous, and it is appropriately called the continuous Risset scale orShepard–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.​

Shepard scales in music

• 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​) for making the modulation end in the same pitch instead of an octave higher.​
• Pink Floyd's 1970 composition "Echoes" ends with a rising Shephard-Risset glissando.
• Songwriter Stephin Merritt makes use of a Shepard scale in his 2007 composition "A Man of a Million Faces," written for NPR Music's Project Song.​​
• The Shepard tone has also been used in the 2008 film The Dark Knight to give the Batpod its transmissionless sound.​
• Canadian post-rock band Godspeed You! Black Emperor also used the Shepard tone at various points in their 1997 album F♯ A♯ ∞.
• Marcus Schmickler's 2010 computer music album Palace of Marvels (Queered Pitch) includes several musical explorations of Shephard scales.​
• Wilco's song "Born Alone" from their 2011 album "The Whole Love" ends with a descending Shepard glissando, as described by lead singer Jeff Tweedy:

  "So I came up with the idea that we would end the song with a Shepard tone, which is a series of chords that when repeated continuously sounds like its descending or ascending. It's kind of a musical trick—it sounds like it's endlessly going deeper and deeper into the abyss."​

Véase también

• en:Barberpole illusion
• en:Pitch circularity
• Bucle extraño

Enlaces externos

• The partials of a Shepard tone
• BBC science show, Bang Goes the Theory, explains the Shepard Tone
• Demonstrating an audio example of a continuous "endlessly descending" tone
• Demonstration of discrete Shepard tone (requires Macromedia Shockwave)
• Visualizació del efecto Shepard empleando Java
• Demostración de la Escala Shepard en la escalera infinita del videojuego Super Mario 64
• El tono Shepard de Wikipedia en un formato más fácil de acceder