Beat frequencies   

When two periodic sounds are less than 30 Hertz apart in frequency and mixed together, the amplitude of this combined signal will fluctuate as the two sounds alternatelyreinforce and cancel each other.  These amplitude fluctuations are called beats.
 As the difference in frequency approaches 30 Hz (or so) the frequency of the beats moves from subaudio rates to audio rates and the distinct and aurally discrete undulations become a "roughness" or continuous component of the overall sound. This is a result of phase cancellation and reinforcement.
Beats are audible in most musical instruments and are very useful in tuning - the beating disappears when two sounds have the same frequency. Beats are a physical phenomenon (they occur in air when sounds are mixed), an electrical phenomenon and occaisionally a subjective phenomenon