Diffusion in Home Cinema Design - November 21st, 2008
As well as absorption it is essential that the sound be diffused when it strikes a surface, in high end home cinema installations. Ideally we want the acoustic equivalent of a matt surface. Unfortunately most surfaces in a home cinema room, including large areas covered with absorptive acoustic panels, act like acoustic mirrors, with varying shades of darkness. In order to have a matt surface one needs a ‘bumpy’ wall and many things can be used to provide this. Unfortunately the bumps need to be at least an eight, and preferably a quarter of a wavelength in size to be effective.
This results in the requirement for very large objects at low frequencies, 1.25-2.5 m at 34 Hz, and very small objects at higher frequencies, 1.25-2.5 cm at 3.4 kHz. If the objects are too small, that is, less than one eight of a wavelength, they will not diffuse properly. If they are too big, that is, greater than about a half a wavelength, they will behave as acoustic mirrors in their own right and so will not diffuse effectively.
Clearly effective diffusion is a difficult thing to achieve in home cinema installations, in an ad hoc manner. Curved and angled structures found in many home cinema designs can help at mid and high frequencies, and at very high frequencies, greater than about 4 kHz, the natural rough textures of materials such as brick and rough cut stone are effective.
Because of the need to achieve well-defined diffusion characteristics, diffusion structures based on patterns of wells whose depths are formally defined by an appropriate mathematical sequence have been proposed and used. The design of these structures is quite involved and will not be mentioned here. However, home cinema designers would need to take into consideration such structures, if a high end dedicated home cinema installation is the case.