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In Reply to: RE: Diffusor w/ Fiberglass for absorbtion? - Will this work? posted by Ethan Winer on November 20, 2007 at 14:10:27
"That's not really a diffusor, but it's better than a bare wall."
Well, Everest observes that they are in use in some top-flight recording studios 'today'—1994 for my 3rd edition—and that by using polycylindrical absorbers/diffusors "it is acoustically possible to achieve a good diffuse fieldd along with liveness and brilliance, factors tending to oppose each other in rooms with flat surfaces". He also notes that one of the problems associated with their use has been a scarcity of published absorption coefficients but then states that a Russian acoustician, V.S. Mankovsky, has addressed that shortage with his published work.
It would seem that some professional acousticians regard them as as quite a deal better than a bare wall.
The data shown in Everest's book, mentioned in my post below, shows that filling the cavity with absorbent material can actually increase the absorption coefficient from around 0.25 to 0.4 at 200 Hz for a 20" wide device and from around 0.4 to an estimated 0.6 for a 45" wide device. Depending on the width of the device, performance below 200 Hz or so starts to roll off somewhat. As I said in my post, they're not as effective as a device designed specifically for bass trapping but they also aren't bad by any means. A bare 1/2" thick gypsum board wall has an absorption coefficient of 0.1 at 250 Hz so the absorption coefficients I've quoted for these devices range from roughly 2 to 5 or 6 times as efficient as a bare wall at that frequency. Absorption has dropped considerably by 500 Hz and size of the curve seems to have little effect on results above that.
The curved surface will certainly promote the creation of a diffuse sound field, but not as quickly or effectively as a lot of other diffusors, but those more effective diffusors tend to offer little in the way of absorption.
I think the big problem with them as far as home listening rooms go is simply size. You aren't going to want to use only 1 of them per wall, you're going to want to use several, and at sizes ranging from 20" wide on up to 45" wide for the ones data is given for in Everest, that means you're going to take up a lot of wall space so I suspect they work better in bigger rooms. The curved surface will certainly promote the creation of a diffuse sound field, but not as quickly or effectively as a lot of other diffusors, but many of those more effective diffusors tend to offer little in the way of absorption.
Yes, they do appear to be a compromise solution in some ways but the published data indicates better performance than you give them credit for. I tend to think they're a more viable option for larger rooms than for the average listening room and in the average listening room I'd want broader band absorption than they offer which is another argument for using them in larger rooms.
David Aiken
Follow Ups:
David,
Always nice to read your thoughts. :-> )
> Everest observes that they are in use in some top-flight recording studios <
Yes, and this brings up a good point. The type of studios that benefit most from poly reflectors are much larger than most home listening rooms. The issue isn't so much that a poly doesn't scatter sound, because clearly it does. And by extension a poly can reduce or eliminate flutter echo without requiring absorption.
The problem with polys in small rooms is they don't reduce comb filtering as much as QRD type diffusors. Comb filtering is a big problem in smaller rooms because in smaller rooms the reflecting surfaces are near to the listener. It's common to see a couch right in front of a reflecting sheet rock wall, so the listener's ears are literally only a foot or two away from the wall. In this situation a QRD is much better than a poly because it scatters sound and also reduces comb filtering.
> I think the big problem with them as far as home listening rooms go is simply size. You aren't going to want to use only 1 of them per wall, you're going to want to use several <
Yes, exactly, and this brings up another problem with polys. You could use one very large poly on a wall, but it would have to extend into the room many feet to be effective. So instead you'll see people put a bunch of them adjacent on the wall. And this causes a new problem. Imagine this crude ASCII drawing is a top view of a wall with adjacent polys mounted on it:
_ _ _ _ _ _ _ _
UUUUUUUU
Between each convex portion that scatters the sound (good), there's another concave section that focuses the sound (bad). This also gives a boxy, hollow sound due to the concave cavities resonating. Try "talking into" some side-by-side stacks of filled soda cans, and I'm sure you'll immediately hear how affected the reflections sound. A QRD well diffusor also has chamber resonances, but all the chambers are different so the boxy sound is reduced and all the resonances tend to average out.
> those more effective diffusors tend to offer little in the way of absorption. <
Not to turn this into an ad, but this is a big feature of the QRD diffusors my company sells. For DIY'ers, These are built more or less as I described above, using material that reflects at mid and high frequencies but passes bass frequencies. Behind the reflecting material is rigid fiberglass so the net result is substantial bass trapping.
--Ethan
i'm not trying to muddy the waters or be oppositional but you raised an interesting point about 'comb filtering' effects for qrds.
i seem to remember coming across an obscure reference in some of rpgs tech literature a long time back. i can't remember the details excactly but it was something about needing 3 'path lenghts' of the frequency to be diffused to avoid peceiving comb filtering effects. if i've remembered wrongly my apologies. so if you wanted to diffuse a frequency such as 1000hz, IF the reference is correct you would need 3x1000hz length between you and the diffuser.
independently of the reference (and its correctness or not) could you nevertheless comment on the issue of how much space you need behind qrds for them to 'work' and if tests have been done on how listeners perceive varying the amount of space.
I believe the "three-wavelengths" rule is more about scattering than avoiding comb filtering, though I suppose one could consider them to be the same. I've done a lot of listening tests at various distances, and a good diffusor (not simple curved wood!) is much better than a bare reflecting wall even at a few inches away. I made a video a few weeks ago to demonstrate the difference between three surface types at very close distances. It's near the bottom of the list on my company's Videos page.
--Ethan
David Aiken
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