So I've been doing some more research on polys. I've learned a few things, I think...
1. Polys (as well as QRDs) work better when not placed side by side IF the same exact panel/pattern is being repeated. i.e. one poly is better than 3 of the exact same poly side by side (same with QRDs) BUT if the size (width of chord) of the poly is varied by 10% or more, then it can be ok to place them side by side(?). I found this hard to believe but it was based on a measured test (I hope) by Glenn Kraus (post #18) here:
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/266361-sonotube-diffusor.html2. Poly's can work to varying degrees of reflection and absorption depending on the material density/depth used for the curved surface. I have seen these built from dense cardboard Sonotubes (used for forming cement cylinders), plywood of varying thickness, Masonite (a type of plywood??), and even this cardboard-like plastic called Coroplast (
https://www.coroplast.com/)...BUT WHAT I DON'T KNOW is how to calculate what frequencies will be reflected vs what will pass through into the insulation for absorption.
3. Placing the insulation against the back of the curved surface changes the low end absorption of the poly. John Brandt discusses that a bit here on his design:
. He says that when the insulation does not touch the curved surface, the poly can help with LF decay below 100Hz but if a more broadband LF absorption is desired, then go ahead and let it touch. (He also mentions keeping the upper and lower ends of the poly open and not sealed. Some others mentioned if you seal it that it's like a tuned hemholtz that attenuates a single frequency...yea?)
4. Polys --> better for spatial scattering, poor temporal diffusion
QRDs --> worst spatial scattering, nice temporal diffusion
BUT I have no real idea what that means for a room like mine.
EDIT: Stuarts post that I linked to below explains this very well. Spatial scattering has to do with sending sound waves in different directions whereas temporal diffusion changes the time between returning sound waves (the varying depths of a QRD for instance...if I am understanding correctly)...here is that quote from Stuart:
The incoming wave travels down all of the wells, and bounces off the bottom of each one, then exits again. But the part that hit a deep well takes a longer time to come back, thus it has been "temporally shifted": some parts of the wave came back sooner, others later. This is one form of diffusion. But since the waves coming back out of adjacent wells are shifted in time, they are also shifted in phase relative to each other, which means that they can partially cancel each other out, or reinforce each other, in varying patterns, depending on frequency, well depth, angle of incidence, etc. So phase shift is another form of diffusion. And finally, the interaction of the phase differences also causes some parts of the reflected wave to be steered in different directions form other waves. That's a third form of diffusion, called spatial diffusion. So the wave coming out is very complex, with different parts doing different things, and going in different directions at different times and with different phases. Thus, if you have your ear up close, within a few feet, you will hear different things in different places. Ditto for the mics! These differences do eventually even out, as the wave-front moves away form the diffuser, and at greater distances the wave-front is just plain "diffuse", and sounds roughly the same wherever you listen to it..... or wherever you place the mic in it.
(here is some type of study that proves the benefits of polys over other types:
)
Uhhhhh yea and probably some other things I've picked up to think about but I think my biggest question is the Sonotube vs plywood vs Coroplast subject. I'd like to know what to expect in terms of frequencies reflected vs. absorbed. That would help me make a decision (I think??)...
Here are some other threads I've skimmed (sorry Stuart for the gearslutz links!
...I have been searching this forum as well but haven't seen as much on detailed poly builds EDIT: I apologize Stuart, this is a FANTASTIC explanation you made of various forms of diffusion here on post #6:
https://www.digistar.cl/Forum/viewtopic.php?p=2816#p2816 that helped a lot...I am still searching/reading more)...Andre, Glenn, Ethan, and John Brandt seem to have some pretty productive cross talk on these.
https://www.gearslutz.com/board/studio-building-acoustics/559833-polycylindrical-diffuser-design-2.htmlhttps://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/469065-d-i-y-polys.htmlCheers
Trevor