Two Source Sound Interference
“Wave interference is a phenomenon which occurs when two waves meet while traveling along the same medium. Wave interference can be constructive or destructive. Constructive interference occurs at locations where the two interfering waves are displaced in the same direction - either both up or both down. When constructive interference occurs at a location along a medium, the resultant displacement of the medium at that location is larger than the displacement of either of the two individual waves. A new and larger wave is constructed. Destructive interference occurs at locations where the two interfering waves are displaced in the opposite direction - one wave is displaced up and the other displaced down. When destructive interference occurs at a location along the medium, the two individual waves combine to produce a new wave which has a resultant displacement which is smaller than the displacement of either wave at the location. That is, the two waves combine to either partially or completely destroy each other.
A popular Physics demonstration involves the interference of two sound waves from two speakers. The speakers are set approximately 1 meter apart and produced identical tones. The two sound waves traveled through the air in front of the speakers, spreading our through the room in spherical fashion. A snapshot in time of the appearance of these waves is shown in the diagram below. In the diagram, the compressions of a wavefront are represented by a thick line and the rarefactions are represented by thin lines. These two waves interfere in such a manner as to produce locations of some loud sounds and other locations of no sound. Of course the loud sounds are heard at locations where compressions meet compressions or rarefactions meet rarefactions and the “no sound” locations appear wherever the compressions of one of the waves meet the rarefactions of the other wave. If you were to plug one ear and turn the other ear towards the place of the speakers and then slowly walk across the room parallel to the plane of the speakers, then you would encounter an amazing phenomenon. You would alternatively hear loud sounds as you approached anti-nodal locations and virtually no sound as you approached nodal locations. (As would commonly be observed, the nodal locations are not true nodal locations due to reflections of sound waves off the walls. These reflections tend to fill the entire room with reflected sound. Even though the sound waves which reach the nodal locations directly from the speakers destructively interfere, other waves reflecting off the walls tend to reach that same location to produce a pressure disturbance.)”
Read more: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/ipl.html