This Audio Illusion Will Easily Fool You
If you’re an audio fan, chances are you’ve heard of the Shepard tone, a sound that uses concurrent sine waves, separated by octave, to create the illusion of a single sound endlessly rising in pitch.
While this effect seems quite illusive at first, it’s not as confusing as you think. Basically, when one of the sine waves reaches a peak in pitch, a new sine wave in a lower octave subtly fades in. If you listen closely, you can hear the lower pitches being introduced periodically.
Hear the Shepard-Risset glissando effect, a variation of the Shepard tone that descends.