Shepard tone

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.

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.

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.

[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).

A spectrogram of ascending Shepard tones on a linear frequency scale
Figure 1: Shepard tones forming a Shepard scale, illustrated in a sequencer
Shepard tone as of the root note A (A 4 = 440 Hz)
Shepard scale, diatonic in C Major , repeated 5 times
Moving audio and video visualization of a rising Shepard–Risset glissando. See and hear the higher tones as they fade out.
An example of Risset's accelerating rhythm effect using a breakbeat loop
Sequence of Shepard tones producing the tritone paradox
An example of an ascendent perpetual melody