When you listen to two sound waves of similar frequency you will hear the intensity of the resultant sound steadily rising and falling. This is known as beats.

Let's say that at one point in time the waves are exactly in phase with one another. Constructive interference would produce a loud sound.

If the waves have different frequencies, as time passes they will become more and more out of phase until they are exactly half a wavelength out of phase. Completely destructive interference takes place and you hear nothing. The phase difference continues to grow and, the closer it gets to a full wavelength shift, the higher the intensity.

This continues, with the intensity of the sound oscillating from maximum to zero and back again continually. If the two sounds have very similar frequencies it takes quite a while for them to get out of phase. The further apart they are in frequency the faster the cycle of rising and falling intensity.

This phenomenon is known as beats, and the frequency at which the amplitude of the resultant wave rises and falls is known as the beat frequency.

The beat frequency equals the difference in frequency between the two waves.