Philomel Records : Musical Illusions : Tritone Paradox
The tritone paradox was first described by Deutsch at a meeting of the Acoustical Society of America (Deutsch, 1986) and first published by Deutsch, Music Perception, 1986. The basic pattern that produces the this illusion consists of two computer- produced tones that are related by a half-octave. (This interval is called a tritone). When one tone of a pair is played, followed by the second, some people hear an ascending pattern. But other people, on listening to the identical pair of tones, hear a descending pattern instead.

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