Interference of sound:
When two or more simple harmonic waves of equal (or nearly equal) amplitude and frequency pass trough the same region of a medium with the same speed in the same direction, the waves overlap. As a result the amplitude of the resultant wave vanishes at certain places of the region and at certain places the amplitude is maximum. This phenomenon is called the Interference of Sound.
When the wave-trains are superposed at the same phase, then the intensity of sound is increased, this is called constructive interference. Again, when the waves are superposed at the opposite phase then the intensity of the sound is decreased, this is called destructive interference.