I like the PI audio diffusers (the more fractal ones) quite a bit. I think they are some of the best bang for the buck. Palomino still has mine..I'll probably have to buy more for the new house.
Anyways, a few general rules to keep in mind when picking your diffusers:
You'll need *lots* for a proper effect. I've made surprising and noticeable changes with only a pair of 48" x 48" sections, and in some rooms even that much didn't make a big enough improvement to make me happy. So move them around and see what sounds best in *your* room. -Acoustics books say you really need 25% coverage of your surfaces to have proper diffusion - but I'm not sure I've even gotten that much done in my rooms yet.
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Think of diffusers like speakers; you have different sized devices for different frequencies. Just like you wouldn't expect a tweeter to handle midrange and bass frequencies, don't expect small diffusers to affect mid and lower reflections. The frequency range a diffuser will treat is dependent on the width of the wells or slots (high or mid frequency), and the depth and width of the overall device (bass). Lower midrange on down will just wrap right around a 24" x 24" diffusers with only 3" of depth, like it's not even there.
That said, spatial cues come from higher frequencies; so diffusers like the Decware ones are a clever design because they are most likely to have an appreciable impact on your imaging. But the more you can diffuse down into the midrange (meaning bigger and deeper diffusers), the more clarity, definition, and timbre will be revealed.
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Diffusers come in different flavors. I personally prefer QRD types as they seem to have a good effect, and are very mathematically predictable. Many diffusers out there are just scattering devices, which is better than nothing, but may have unpredictable effects. Again, experiment and move them around.
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Lastly, for those of us running low wattage, or are in general low volume listeners, I prefer the 1D diffusers over the 2D. Meaning the diffusers that diffuse in only 1 direction, typically horizontal. The reason being that the 2D devices like the checkerboard and skyline devices reflect in a spherical pattern...the sound hits the devices and sends the reflections up to the ceiling and down into the carpet as well as horizontal on your listening plane. Where the tall vertical 1D devices reflect everything on the horizontal keeping all that sound energy where you're listening.
Obviously I really enjoy diffusers, and they have made an amazing impact on so many rooms. So I get excited when I see people experimenting with them. I say Go For it! In fact, double your order! You won't regret it!