Diffraction

A phenomenon in the propagation of waves where the direction of a wave front (either sound wave or electromagnetic [light] wave) is altered when passing by an object or through a small aperture in a large surface. At shorter wavelength relative to the obstacle, sound (and light) will tend to reflect off the surface more and bend around it less (which partially explains why you can hear, but not see at a concert when someone is standing in front of you). Waves will also bend to fill an opening behind a surface (which partly explains why you can hear someone talking in the next room through an open door even though you can’t see them).