When you listen to two sound waves of similar frequency you will hear beats - the intensity of the sound rising and falling.

When the waves are exactly in phase with one another constructive interference produces a loud sound.

For waves of different frequencies, as time passes they drift out of phase until 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. The closer the waves are in frequency the slower the cycle of rising and falling intensity.

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.