Sound waves generated in a classroom must through an open doorway in order to propagate into the hallway.

diffract through an open doorway in order to propagate into the hallway.

Diffract

Explanation:

The diffraction of sound waves is the description of how sound waves bend, change direction, as they travel around the edges of obstacles. The diffraction occurs as a result of the bending of sound waves as they find the path of least resistance to the travel path. The amount and magnitude of the diffraction occurs in proportion to the opening in the obstacle pathway. The diffraction is a property of sound that helps it bend around the corners and obstacles.