Huygens-Fresnel theory

Any disturbance or displacement of particles in a medium will produce a hemispherical wave of energy and each neighbouring particle is excited as the wave propagates and, in turn, becomes the source of a new hemispherical wave of energy. The way the wave spreads depends on how the individual waves are superimposed on each other. It is important in loudspeaker technology that the superimposition of one wave over another is effected not only on the basis of their amplitude but also in phase with each other. This explains how interference occurs.