This is called interference.
There are three types are interferences
1. Constructive Interference
Constructive interference occurs when two waves “strengthen each other and create a wave with a higher intensity” (NDT Resource Center) when compressions and the rarefaction of the two waves line up.
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm
2. Destructive Interference
Destructive Interference occurs when the compressions and rarefactions are out of phase, their interaction “creates a wave with a dampened or lower intensity” (NDT Resource Center). When waves interfere 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.
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm
3. Dead Spot
Dead spots are places where no sound at all can be heard. Dead spot occur when the compressions of one wave line up with the rarefactions from another wave and cancel each other. When destructive interferences are overly destructive, the waves can produce dead spots.
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm
“The shape of the building or stage and the materials used to build it are chosen on interference patterns. They want every member of the audience to hear loud, clear sounds” (NDT Resource Center).
