The indirect reflections arrive at discrete times related to their travel paths. Because the intensity, temporal distribution and directionality of these indirect reflections determine how we perceive the actual sound source, control of room reflections is a central consideration in acoustical design. Reflections can be controlled be using three basic classes of acoustical materials. Absorption Porous materials absorb sound by converting the acoustical energy into heat. An ideal absorbing surface attenuates most of the incoming sound arriving from any direction.
Absorption is the most specified sound modifying acoustical material.
In fact, the term acoustical material has almost become synonymous with absorptive ceiling tiles, fabric-covered fibreglass or rockwool, foam, bonded wood fibers and so on. This is unfortunate because absorption, while an important ingredient, is not the only acoustical treatment and for many applications it is the wrong ingredient. Reflection Flat or convex reflective surfaces, which are hopefully stiff and massive to prevent diaphragmatic absorption, essentially redirect or reflect sound like light from a mirror. If the scattering surface is concave, as unfortunately is found in many worship spaces and auditoriums, the sound can actually be intensified or focused at certain positions in the room, while being absent or diminished elsewhere. Diffusion Ideal diffusive surfaces neither absorb nor reflect sound, but rather scatter sound into many directions, instead of just the specular direction.