I guess you have to be a real audio techie to read this post, but I think it’s interesting stuff.
Most sound sources produce energy at multiple frequencies and different sound sources often combine. When this happens it is the more complex, composite, waveform that we hear. This combination of different overtones and sound changes the resulting wave shape. The term interfere or superposition is used to describe what happens when waves pass through each other or are superimposed.
If the two sound waves that collide are exactly in phase with each other they add together and produce a sound wave that is twice as loud. If they are exactly (180°) out of phase, they cancel each other out and the result is silence. If they two waves differ in amplitude the new sound wave is different in amplitude from either of them.
Let’s consider a real world situation where phase cancellation can occur. Suppose you place the speakers on a P.A. system too close to a live sound source such as a set of drums. The speakers might cancel out some of the sound of the drums by overlapping the natural sound of the drums with the amplified sound. The illustration below shows what happens when sound waves that are exactly in phase, 180° out of phase, and of different amplitudes, collide. Sources: Real World Digital Audio by Peter Kirn (Peach Pit Press, 2006, and mediacollege.com.