Tritone Paradox

The tritone paradox is an auditory illusion in which a pair of Shepard tones separated by a tritone, half an octave apart, is heard by some listeners as ascending in pitch and by other listeners as descending. Roger Shepard argued in 1963 that such tone pairs should sound genuinely ambiguous, but psychologist of music Diana Deutsch found in 1986 that individual listeners actually judge the pairs consistently, with the direction each person hears depending on where the tones sit on the chromatic circle, an effect that also varies with the listener own language or dialect.