Named after Roger Shepard, a cognitive scientist who worked at Bell Labs and was also a professor at both Harvard and Stanford, Shepard Tones are a series of sine waves, an octave apart, typically in twelve chromatic or seven diatonic steps. When the volume of each note in the series is properly adjusted and the series of notes is looped, the illusion is that the notes continually rise (or descend) in pitch, ad infinitem.
This is an audio illusion that's analogous to an M. C. Escher print...or a barber pole, where the spinning helical stripes appear to be ever ascending.
In ascending Shepard tones the notes are arranged this way. In the first pair of the series, the highest note of the pair is the loudest. Toward the middle of the series both octaves are equally loud. Near the end of the series the lowest note of the octave pair is the loudest. The opposite is true if the tones are descending.
The Shepard-Risset Glissando is a variation of the Shepard Tone, where as opposed to discrete chromatic or diatonic steps; the notes continuously glide from one pitch to the next.
