Rick,
Thank you.
Actually, the rating system for diffusion is similar to rating absorption - from 0.0 to 1.0. - And polys are indeed diffusers of sound. Ideally you should be approximately an arc's distance from them before a decent diffusive field develops.
To quote from Acoustic Absorbers and Diffusors by Cox & D'Antonnio;
"Diffusion coefficient, (d), is a measure of the uniformity of the reflected sound.The purpose of this coefficient is to enable the design of diffusers, and to also allow acousticians to compare the performance of surfaces for room design and performance specifications.
Scattering coefficient, (s), is a ratio of sound energy scattered in a non-specular manner to the total reflected sound energy. The purpose of this coefficient is to characterize surface scattering for use in geometrical room modelling programs."
That said... The random slat widths used for scattering are simply that. Phillip Newell uses a 5 - 3- 4 sequence on his slat widths, but it really has no significance other than the percentage of absorption vs reflection/diffusion - which the slat sizes and gap sizes determine. As long as the slats are not covering a sealed box, they do not create a Helmholtz response and the LF acoustic wave does not even 'see' the slats & therefore continues into the trap where it is summarily 'dealt with'.
Here is another quote from the same book concerning testing done on poly-cylindrical diffusors (emphasis added);
"A value close to zero has been measured for a concave surface designed to focus sound on a single receiver. A value of 1 can be measured for
a small single semicylinder, but a single semicylinder on its own is not much use, because it cannot cover a wide area. As soon as more complex surfaces are introduced, such as a set of semicylinders, the diffusion coefficient is reduced, because of the lobing introduced. This lobing is unavoidable in extended structures, and so the diffusion coefficient is rarely close to 1 for usable and realistic diffusers and diffuser arrays. A single semicylinder may produce complete diffusion, but to cover a wall a set of semicylinders are needed. This is why it is important to measure application realistic samples, as the scattering from a single object is not representative of the response from a periodic or modulated array."
- Good stuff.
If anyone needs (or wants) further explanation, please ask.
Cheers,
John