Hi!
The Book is worth it.
Regarding the repetition thing. Think it's best understood on a synthesesis basis. A small bit of the sequence is not random in itself, it does not have a flat fourier tranform. Only the total sequence works as intended at the lowest frequencies. Starting and stopping the sequence (waves striking the ends of the diffuser) makes a bit of a mess with the sound waves, those that strike both within and outside the diffuser. For the larger lower frequencies, diffusion only takes place with an "optimal effect in the middle" and broken up edges of the waveform on all other places. Repetition solves this problem. Since the sequence can be phase shifted in any direction, starting or stopping at some arbitary place in a repeating sequence is as good as having a long sequence. A repeating array will give this effect. But it introduces lobing problems in the far field scattering response. Breaking up the pattern with an inverted diffuser is good. Though I have a feeling using a different prime number for the modulation is better.
The diffusion coefficient graphs for QRD's looks like harmonics of instruments. Having different tunings seems like a good thing to me, making it more random across a larger range of frequencies. Or perhaps it makes it sound weird? Perhaps one need to tune the diffusers to each other, like musical instruments? Perhaps it would make sense to tune them to musically relevant frequencies? Hmmm... Anyone?
If the above was too confusing: the jist of it is that a single period only "works in the middle", a repeating pattern avoids this but it have lobing problems, with a modulated array being the best choice. Most of this can be found in the google book extract from The Book.
Though I still can't see why a say a modulated array of 4 prime 11 and 1 prime 13 diffusers would be any better than say a sequence based on prime 59. Anyone?
Quote:
Originally Posted by terry j  Diffusers are not effective below the (take your pic: Davis, Shroeder #1, Schroeder #2, 3 modes/Hz) cutoff frequency of a room.[/I][/B]
It was avares, and here is the original post Sonotube Diffusor
post 12 |
Hmm.. I got a hunch it's because its unpractical to build a large enough diffuser. Haven't tried to calculate the size of these things, but by the time you get to seriously affect the room modes, the diffuser may be so large as to be impractical? Andre? Are you there?
Quote:
Originally Posted by terry j Ok then, if you are after a certain goal (like a drop in RT at the high end), HOW do you achieve that? |
A drop in high end is easy. Absorption is most effective at higher frequencies and less so at lower. By the time you've got enough to seriously affect low end, high end is usually too dead. Adding back some high end response can be done by adding thin reflectors (paper, cardboard, wood etc depending on frequency) to the absorbers.