Typical scattering diagram of a good diffusor
In room acoustics diffusion of the sound field within a room is frequently increased by means of special wall structures. Even in the very first concert halls balconies and walls were decorated with rich ornamentation, rosettes, and figurines to serve this purpose. With modern and plainer architectural styles came the necessity to design special diffusors. The use of diffusors plays an important part, not only in large halls, but also in small rooms such as recording studios or practice rooms for musicians.
The structure of a diffusor is designed to deflect as much of the reflected sound energy as possible from the direction of geometric reflection and distribute it into other directions. Furthermore care is taken to uniformly cover a wide angular range.
Basically, diffuse sound reflections can be achieved by two means, that is either by three-dimensional structures such as the rosettes mentioned above, or by surfaces with locally reacting wall impedance. Wall impedance is the ratio of sound pressure to sound velocity at the surface of the wall. Wherever two materials with a different acoustic effect adjoin, a part of the incident wave is scattered diffusely.
Since scattering is a wave phenomenon, the dimensions of three-dimensional structures must be within the order of magnitude of the wavelength of the sound to be scattered. Semi-cylinders, prisms, or similar forms can be used to construct special geometrically structured surfaces.
For surfaces with locally reacting wall impedance the rule is that the wall impedance must change sufficiently within the frequency range considered. Phase grids of Helmholtz resonators are a good example. These are surfaces with periodically adjoining Helmholtz resonators.
Maximum length diffusors and quadratic residue diffusors belong to the so-called Schroeder diffusors. Their design is based on special number sequences. Individual wells which completely reflect sound and whose depth varies according to the number sequences are used to form structures with very good scattering characteristics.
Furthermore there are micro-perforated diffusors where the rigid wells of the Schroeder diffusors are replaced by stripes of micro-perforated resonators.