Diffusion is the process of spreading or dispersing radiated energy so it is less direct or coherent. A Diffuser is a device that does this. The plastic covers over fluorescent lights in many office environments are diffusers. They make the light spread out in a more randomized way so it is less harsh. In audio, diffusion is a characteristic of any enclosed (or partially enclosed) space. It is caused by sound waves reflecting off of many complex surfaces. For example, a flat concrete wall produces a pretty distinct echo when sound reflects off of it. However a brick wall, while still pretty reflective, tends to diffuse the sound reflections and produces a much less distinct echo. This is due to both the surface of the brick itself and the mortar between the bricks (more specifically the edge diffraction of the joint between the two). All surfaces will of course differ and it is usually a variety of surfaces that create the most randomized diffusion of sound. Diffusion is a very important consideration in acoustics because it minimizes coherent reflections that cause problems. It also tends to make an enclosed space sound larger than it is. Diffusion is an excellent alternative or complement to absorption in acoustic treatment because it doesn’t really remove much energy, which means it can be used to effectively reduce reflections while still leaving an ambient or live sounding space.