Diffraction arises because air-pressure waves cannot abruptly end at an edge

Evidence

Diffraction occurs because the variations in air pressure, due to the compressions and rarefactions in the sound wave, cannot go abruptly to zero after passing the edge of an object. This is because there is interaction between the adjacent molecules that are propagating the wave. In order to allow the compressions and rarefactions to die out gracefully in the boundary between the wave and the shadow, there must be a region in which part of the propagating wave changes direction, and it is this bent part of the wave that forms the diffracted component. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 422

Categorization

• DEFINE: Explains the mechanism behind why sound bends instead of stopping sharply at boundaries.