The Tritone Paradox
A sequentially played pair of Shepard tones separated by an interval of a tritone (half an octave) produces the tritone paradox. In this auditory illusion, first reported by Diana Deutsch in 1986, the scales may be heard as either descending or ascending. 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. Deutsch later found that perception of which tone was higher depended on the absolute frequencies involved, and that different listeners may perceive the same pattern as being either ascending or descending.
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