Geometric Diffusers in Home Cinema Installations - November 10th, 2008
Cylinders, pyramids and plane surfaces are common items in home cinema rooms, and in this article the performance and design of these geometric surfaces will be considered, as well as their importance in home cinema installations.
While most diffuser design is about breaking up wavefronts by surface roughness or impedance changes, it must be remembered that even a plane surface in a home cinema room can cause diffraction from its edges, provided the surface has a similar size to the acoustic wavelength or smaller. Triangles or pyramids can produce dispersion, redirection and specular reflection depending on the geometry used. Applied correctly in a home cinema design, triangles and pyramids can form notch diffusers, where the energy in certain directions is much reduced. Curved surfaces are most obviously diffusers and more universally used; indeed a simple sphere or cylinder is very effective at spatially spreading reflections in a hemisphere or a single plane respectively, but this is not the only ingredient to a good diffuser for a home cinema room.
Furthermore, a single sphere or cylinder used on its own in a home cinema installation is not very useful, so many spheres or cylinders next to each other are needed. Then the scattering is as much about how the objects are arranged, periodically or randomly, as about the scattering characteristic of a single item. Well-designed curved surfaces can be a less obvious acoustical treatment in hi-end home cinema installations, and so will often blend into modern architectural home cinema designs.
Understanding the reflection effects of finite-sized plane surfaces in home cinema systems is important. With no surface roughness, any scattering generated is by edge scattering. Whether by accident or design, plane surfaces are probably the most common architectural surface used in home cinema design.
As an example, consider a finite-sized plane surface, near a speaker and the home cinema seating area. The surface is assumed rigid, hard and non-absorbing. If the speaker and the listener are chosen so that the geometric reflection point (the point at which the angle of incidence equals the angle of reflection) lies on the panel, then the following phenomenon takes place: at very low frequencies, when the wavelength is very large compared to panel size, little or no sound is scattered from the surface. At very high frequencies, when the wavelength is small compared to the surface size, strong specular reflection results.
This gives home cinema designers the possibility to define an approximate frequency below which the panel most effectively scatters sound in all directions in the home cinema room, and above which the panel produces more specular-like reflections.