The sound that is heard in most environments is a combination of the direct sound straight from the source or sources and the indirect reflections from surfaces and other objects. For instance, in room acoustics, both the direct sound and the reflections from the walls, ceiling, and floor are key in determining the quality of the acoustic. To take another example, outdoors, the reflection from the ground can significantly reduce noise at certain low frequencies. Hence, one of the central topics in acoustics is how to manipulate these reflections that alter the way the sound behaves, and is ultimately perceived.