Binaural Beats

Binaural beats are a psychoacoustic phenomenon in which two tones of slightly different frequency, presented separately to each ear, create the sensation of a third tone pulsing at the difference between the two frequencies. For example, a 400 hertz tone in one ear and a 420 hertz tone in the other produces a perceived beat of about 20 hertz. The phenomenon was first described by physicist Heinrich Wilhelm Dove in 1839 and is generally perceptible when the two carrier frequencies are below roughly 1000 hertz and differ from each other by less than about 35 hertz.