We know enough by now to understand that sound must be distributed throughout the space as equally and uniformly as possible. An uneven concentration of sound creates an unbalanced energy field which changes the listener's perception depending on where he/she is positioned. Just think how utterly unacceptable such a situation would be in a theatre: some spectators would hear the sound in a certain way, whilst others would perceive the sound differently. That's why appropriate diffusers are used to reflect the sound in all directions within the space.
One sound diffusion technique consists in covering a reflecting wall with a highly uneven surface. The following diagram describes how to create such a surface:
These panels are made up of various layers whose role is to chop up the waves of a certain frequency band. The first of these layers has a wider surface and acts on the diffusion of low frequencies. On top of it we have the second layer, whose surface is smaller than the first, and which acts on the middle frequencies. Further layers can be added, smaller in surface-area, to act on the higher frequencies. The reflection of sound on such a surface is diffused in all directions, producing a more evenly distributed sound field.
Convex surfaces also are quite often used to reflect sound in all directions, especially in large environments. The following figure illustrates a ceiling covered with this kind of diffusion panels:
This solution is adopted by various auditoriums around the world. The following pictures shows the biggest hall (2400 seats) in the Rome's Auditorium. You can easily spot the diffusion panels on the ceiling and another diffusing layer right above the stage: