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Saw this Diffusor a while back - looks good, but will it work?
http://home-and-garden.webshots.com/photo/1180936902058980058cbMuYT
I guess the root of my question is will having fiberglass behind the wooden front panel do anything for absorption? I would think it wouldn't absorb much bc the sound would reflect off the front. And if that is not the case, is it really working as a diffusor in the first place?
Thanks for the help!!
Brent
Follow Ups:
Those look great. I think I would do something like that as well in my new listening room. It kind of reminds me of the M. Lavigne room design but in a smaller scale. If they used the diffusor in the Lavigne room the same way as you plan on as well, then I think you will have some sort of the same properties that you wish for, both bass damping and diffusion at a way lower cost.
Ciao,
Audioquest4life
> Saw this Diffusor a while back - looks good, but will it work?
That's not really a diffusor, but it's better than a bare wall.
> I guess the root of my question is will having fiberglass behind the wooden front panel do anything for absorption? I would think it wouldn't absorb much bc the sound would reflect off the front. <
Exactly. A much better way to make a diffusor is to use thick shiny card stock, or really thin balsa wood etc, to reflect at frequencies the diffusor is effective and to absorb at frequencies below.
--Ethan
"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
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
Check out Everest's "Master Handbook of Acoustics". The device in your link is what he describes as a polycylindrical diffusor and there are full instructions in the book though they happen to be hidden in the chapter on absorption rather than diffusion and they are called "polycylindrical absorbers" and "polycylindrical diffusors" at different points of the discussion.
bogatg suggests that the wood will be reflective and that you need uncovered insulation material for absorption. That's the stock 'gut reaction' but it isn't the whole of the story.
You can use a panel of plywood as an absorber, a 'panel absorber' to use Everest's term, and you 'tune' their effective bandwidth with the thickness of the panel and the depth of the enclosed air space behind it. Placing some insulation material in the space behind the panel improves the effectiveness of the device as an absorber.
The curved ply surface of the polycylindrical device in your link is stiffer and less effective as a panel absorber but it still provides some absorption at lower frequencies and, as with the panel absorbers, placing insulation material in the enclosed space behind the plywood surface improves effectiveness. The absorption curves shown in Everest for panel absorbers and polycylindrical absorbers/diffusors show similar curves for empty devices and higher absorption coefficients coupled with narrower bandwidths for the panel absorbers compared to the polycylindrical devices. They probably won't be as effective at bass absorption as a proper bass trap but they will certainly assist in bass absorption. From what Everest says, they're most effective at absorption if the cavity is actually filled with absorption material so putting more fibreglass in the cavity than displayed in the photos in your link would be beneficial.
At higher frequencies they are reflective and the curved surface reflects sound over a wider angle of dispersion than you obtain with a flat surface. That will provide useful diffusion at mid to high frequencies.
So the devices really act provide a combination of absorption at frequencies below, say, 500Hz and diffusion at mid to high frequencies. They're not the most effective devices at either function but they are certainly effective at both and it's a useful combination if you can use them in your room.
David Aiken
Fiberglass alone provides absorbtion and would likely only work with acoustic fabric.
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