The human ear has very strange abilities. An example of this are so-called binaural beats (in German: with two ears perceived strokes).
The unusual thing is that there are actually no beats at all (at least as long as you listen with headphones).
The left and right ear are played slightly different tone frequencies. From the difference of the frequencies (in Hz) results then the frequency of the beats. For example, if the left ear hears a 440 Hz tone and the right ear 439 Hz, the difference results in a binaural beat per second (1 Hz). At 438 Hz on the right ear, the frequency doubles to 2 Hz.
The effect works similarly when the sounds are output through speakers. However, physical overlays and interferences – ie beats – are actually present here. The amazing thing about binaural beats with headphones is that there is not really that superposition of sounds. Each ear hears a continuous sound at a constant frequency.
The beats seem to arise only during processing in the brain.