Constructive and Destructive Interference pattern
When two sound waves occur at the same time and are in the same phase, (i.e., when the compressions of the two coincide and hence their rarefactions also), the waves reinforce each other and the sound becomes louder.
This is known as constructive interference. On the other hand, two sound waves occurring simultaneously and having the same intensity neutralize each other if the rarefactions of the one coincide with the compressions of the other, (i.e., if they are of opposite phase). This canceling is known as destructive interference.
Alternate reinforcement and neutralization (or weakening) take place when two sound waves differing slightly in frequency are superimposed. The audible result is a series of pulsations or, as these pulsations are called commonly, beats, caused by the alternate coincidence of first a compression of the one wave with a compression of the other and then a compression with a rarefaction. The beat frequency is equal to the difference between the frequencies of the interfering sound waves.