When two sound waves of different (but close) frequencies are played together, the alternating constructive and destructive interference causes an oscillation of the resulting waveform
frequency 1 – frequency 2 = Beat frequency
502 Hz – 500 Hz = 2 beats per second
An exaggerated version of beating occurs when creating a ‘reece bassline’ (common in drum’n’bass and dubstep). In this instance two sawtooth waves are dialed in on a synthesizer, and one of the sawtooth waves is detuned a few cents. This causes a beat frequency between the fundamental frequency of the two oscillators, a beat frequency between the two 2nd harmonics, a beat frequency between the two 3rd harmonics and so on. The result is an evolving filter pattern that travels through the sound.