Radio interference off of a building?
same thing happens with sound waves.
if you want to try it get two subwoofers and hook them up right next to each other.
now play a nice low note.
now reverse the polarity on one sub and play the same note.
it will be a whole lot quieter due to the reversed sound waves canceling each other out.
also it is possible to bounce sound off a wall and get this effect buy harder to predict and "tune" .