Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. When two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave. The new wave is the sum of all the different waves. Wave interaction is called interference.
A sound wave is the pattern of disturbance caused by the movement of energy traveling through a medium (such as air, water, or any other liquid or solid matter) as it propagates away from the source of the sound. The source is some object that causes a vibration, such as a ringing telephone, or a person’s vocal chords. The vibration disturbs the particles in the surrounding medium; those particles disturb those next to them, and so on. The pattern of the disturbance creates outward movement in a wave pattern, like waves of seawater on the ocean. The wave carries the sound energy through the medium, usually in all directions and less intensely as it moves farther from the source.
If the compressions and the rarefactions of the two waves line up, they strengthen each other and create a wave with a higher intensity. This type of interference is known as constructive.
When the compressions and rarefactions are out of phase, their interaction creates a wave with a dampened or lower intensity. This is destructive interference. When waves are interfering with each other destructively, the sound is louder in some places and softer in others. As a result, we hear pulses or beats in the sound.