In music theory, a tritone is the musical interval of three whole tones, equivalent to a diminished fifth (also called a half-octave). The tritone paradox is an auditory illusion discovered by Dr. Diana Deutsch in 1986 . Some people hear the pattern going up in pitch and some hear it going down in pitch. Some studies have shown that people from different geographical regions perceive the tones differently. (The study compared listeners in the UK to listeners in California.) Furthermore, Dr. Deutsch also found that children and their mothers tend to perceive the tone in the same way. This suggests that dialect and language one hears in infancy could play a role in how one hears the tone and, possibly, music.[more]
The user can select the frequency of the first tone and its duration. The "version" setter determines whether the second tone is a tritone higher ("up") or lower ("down") or a chord with both ("paradox").[less]
 D. Deutsch, "Tritone Paradox." (Jan 29, 2016) deutsch.ucsd.edu/psychology/pages.php?i=206.