A Shepard tone, devised by Roger N. Shepard in the 1950s, is a sonic paradox that has been described as "a sonic barber pole," or as a sonic equivalent of Escher's endless staircase. Basically, it is a series of notes that is perceived by the listener to continually either rise or fall in pitch.
James Tenney's For 12 Strings (Rising) produces the effect of a rising Shepard tone by beginning with a low note on one of the contrabasses, then having it glissando up an octave. When it completes the first octave, it begins again; another bass enters on the same note and does the same thing while at the same time, a violoncello enters on a lower note and glissando up an octave. It repeats again, another cello enters, and the violas come in on a higher note. Then the second violins enter the same way, and finally the first violins. The entire sequence is repeated about 400 times, and the steady rising pattern produces a Shepard tone effect.
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