Front Wall Treatment for Bipolar/Dipolar Fronts - AVS Forum
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post #1 of 14 Old 04-04-2011, 12:18 AM - Thread Starter
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After some extensive reading on the Master acoustic's thread I'm still left with some lingering questions. I do realize this topic has been rehashed over and over, but I would like a specific answer in my situation, leaning toward music rather than HT which isn't quite as mentioned in the thread.

I'm using a pair of Kappa 9's with dipole midranges (midrange back shooting backward) and a bipolar tweeter shooting backward toward the wall. The room is about 14x18 with the speakers on the 14' wall about 9 feet apart and each speaker about 2-3 feet from either wall.

Formerly, I had been using monopolar speakers and per the advice of many experts here, I treated the entire front wall or the majority of it with 4 2'x4' OC703 1" panels. However after upgrading my speakers to bipole/dipole speakers, the ambience of the speaker and the room seem rather dead. I suspected as such I was absorbing too much of the backwave from the speaker (I know is desired in HT). So I took down the 2 panels directly behind the speakers (they are the two panels closest to either side wall), but I had left the panels on either side of the TV (the panels closest to the center). After listening to music and judging with my own ears, there was clearly more richness and ambience to music and while the center image seemed only a tad less focus, the soundstage grew by a bit, which I preferred for music (unpreferred for HT).

So down to my question, should I play with diffusion directly behind the speaker rather than absorption. Seeing that my room is relatively small, I'm not sure leaving it bare is really an option acoustically. Unfortunately I don't have any ficus plants to play around with to position behind my speaker. Or should I just use diffusion on the entire wall for music? The occasional movie will be played but it is not of primary concern. I also have 2x4 oc703 panels at the first reflection point and will definitely use diffusion behind the main LP when I get the chance.
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post #2 of 14 Old 04-04-2011, 12:12 PM
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^^^ Yes, diffusion there would be much better than a bare reflecting wall.

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post #3 of 14 Old 04-04-2011, 08:56 PM - Thread Starter
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Quote:
Originally Posted by Ethan Winer View Post

^^^ Yes, diffusion there would be much better than a bare reflecting wall.

--Ethan

Thanks for the reply Ethan. After doing some more research on the matter, perusing 1D QRDs and 2D diffusors, I've stumbled upon a most novel solution a polycylindrical diffusor. From my understanding you of all people would probably recommend a 1D QRD to diffuse horizontally rather than dissipate energy into the ceiling or the floor. So ideally a 1D QRD is ideal, but I have read that a polycylindrical diffusor would be ideal in the sense that it can diffuse just as well horizontally if there was enough distance from the listener to the diffusor which is not a problem if the diffusor were to be placed on the front wall. However the proximity to the speaker is of concern about 2 feet. I'm not interested in using multiple cylinders just one shaped sine wave shape about 2ft wide and 4ft high.

However, behind the LP, I will use a traditional 1D or 2D diffusor. Has anyone accomplished a polycylindrical diffusor? I'm reading on the subject and it's rather confusing to extrapolate measurements for one to DIY. I'm getting things such as a "half sine wave," angle of incidence of 30 degrees. If anyone can clarify that'd be great.

Onto more research into the matter!
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post #4 of 14 Old 04-04-2011, 09:31 PM - Thread Starter
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I've stumbled on quite an informative thread, http://forum.studiotips.com/viewtopic.php?f=1&t=3533. But I'm having difficulty extrapolating numbers for actual construction.

Terry Montlick is trying to explain the theory or construction of a polycylindrical diffusor. However would this diffusor be ideal for placement directly behind a bipolar/dipolar speaker for diffusion. I understand it will not diffuse to the lowest of frequencies but the only rear firing elements i'm interesting in diffusion are the tweeter and midrange. Is there any difference in the diffusion between a 1D diffusor of let's say 6" in depth to a polycylindrical diffusor.
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post #5 of 14 Old 04-05-2011, 05:26 AM
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Here is a reasonable approach ...

Make your diffusers from 1/4" peg board. Take a 4x8 sheet, cut it in half.

Starting on the room center line, from the ceiling down, orient your first diffuser vertically. Directly below that, install one oriented horizontally ... and so on down to the floor (you don't need to go all the way to the floor .... whatever works with the spacing.

Next, you do the same to the two rows to each side of the center; but, your first cylinder would be horizontal, not vertical. Carry on following that pattern and ideally your last row to each end would be a single, floor to ceiling cylinder straddling the corners.

You might want to cover the pegboard with fabric for appearance sake. Behind each cylinder install 3.5" of non-faced fiberglass. (Let the math wizards take a crack at that).

That is the simple way. The more complex way is to vary the radii (meaning for spacing you'd have to calculate the chords so they match), horizontally, you'd have three rows of cylinders and vertically, you'd have five or seven columns.

(Much more than a 2nd order differential equation.)

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post #6 of 14 Old 04-05-2011, 11:36 AM
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Quote:
Originally Posted by aboroth00 View Post

I've stumbled upon a most novel solution a polycylindrical diffusor.

That's more of a deflector than a diffusor. The problem with polys, at least in small rooms, is they don't reduce comb filtering the way real QRD diffusors do. This video lets you hear several types of acoustic surfaces close up:

All About Diffusion

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post #7 of 14 Old 04-05-2011, 03:41 PM - Thread Starter
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Quote:
Originally Posted by Ethan Winer View Post

That's more of a deflector than a diffusor. The problem with polys, at least in small rooms, is they don't reduce comb filtering the way real QRD diffusors do. This video lets you hear several types of acoustic surfaces close up:

All About Diffusion

--Ethan

Ethan,

I actually watched the video prior and read the debate between you and Terry Montlick on the topic of polycylindrical diffusors. I do understand the problems of polycylindrical cylinders such as the ones you constructed. However, to avoid the problems of focusing, usually I see diffusors not placed adjacent to each other in the same orientation (vertical next to vertical) and the radii and arc are varied between. As for comb filtering, my understanding is a little poor, but by having a curved surface does it not reduce comb filtering? because it is reducing the amount of parallel surface?

However I'm not sure I understand why this is not a diffusor and just a deflecter. Would the curve not scatters sound just as would a QRD in the higher frequencies while not absorbing sound as would a 1D QRD. Here's the post referenced: http://www.avsforum.com/avs-vb/showt...l#post10280423.
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post #8 of 14 Old 04-05-2011, 03:50 PM - Thread Starter
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Quote:
Originally Posted by Dennis Erskine View Post

Here is a reasonable approach ...

Make your diffusers from 1/4" peg board. Take a 4x8 sheet, cut it in half.

Starting on the room center line, from the ceiling down, orient your first diffuser vertically. Directly below that, install one oriented horizontally ... and so on down to the floor (you don't need to go all the way to the floor .... whatever works with the spacing.

Next, you do the same to the two rows to each side of the center; but, your first cylinder would be horizontal, not vertical. Carry on following that pattern and ideally your last row to each end would be a single, floor to ceiling cylinder straddling the corners.

You might want to cover the pegboard with fabric for appearance sake. Behind each cylinder install 3.5" of non-faced fiberglass. (Let the math wizards take a crack at that).

That is the simple way. The more complex way is to vary the radii (meaning for spacing you'd have to calculate the chords so they match), horizontally, you'd have three rows of cylinders and vertically, you'd have five or seven columns.

(Much more than a 2nd order differential equation.)

Thanks

Thanks for the reply Dennis, that's a very novel solution. However it might be beyond my reach for the time being. I was looking for a more simple solution, as time during the year isn't readily available. I do understand that the solution proposed would probably be best.

However what would you think of placing perhaps 2 diffusors each behind the speaker while still using absorption on the wall? Are you recommending that the diffusors be cylindrical, as in a semicircle? or more like an arc? What dimensions do you propose for the diffusor, I'm not quite clear on that.
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post #9 of 14 Old 04-05-2011, 08:29 PM
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You could use a couple of inches of Quest PerfSorber.

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post #10 of 14 Old 04-06-2011, 12:28 AM - Thread Starter
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Quote:
Originally Posted by Dennis Erskine View Post

You could use a couple of inches of Quest PerfSorber.

Huh... I sort of feel like a noob, but after googling it and searching Quest's site, I don't see the product or details on the product. I was hoping to DIY whatever I need to save some bucks, but I don't think I can DIY a product that doesn't exist!!

But on a less kidding note, I believe Quest doesn't offer the product or at least doesn't list it as one of the products they sell.

Would the Perfsorber functionally work the same way polycylindrical diffusor using pegboard and fiberglass behind it? I can't find too much on it. The pegboard idea does sound compelling and I've actually read some of your earlier posts on the matter such as plugging some of the holes on the pegboard in a systematic manner to help diffusion. However some dimensions on the arc would be very helpful.
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post #11 of 14 Old 04-06-2011, 05:12 AM
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I can understand the issues ... as well, Quest is a tad behind getting their new products up on their website. The PerfSorber is a 2" product (available as ") which provides both absorption and diffusion. You can get the raw material and cover it yourself or buy it already fabric covered. A bit more expensive that a figure it out yourself project; but, in the end, it does work.

If you'd PM me with your email address, I'll send you the technical data and product sheet.

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post #12 of 14 Old 04-06-2011, 12:05 PM
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Quote:
Originally Posted by aboroth00 View Post

I see diffusors not placed adjacent to each other in the same orientation (vertical next to vertical) and the radii and arc are varied between.

Yes, all of that helps.

Quote:


by having a curved surface does it not reduce comb filtering? because it is reducing the amount of parallel surface?

Comb filtering is caused by reflections, which is different from flutter echo caused by opposing surfaces. So comb filtering is reduced by a poly only if the curve is very severe. These posts explain well why polys are not real diffusors:

http://www.gearslutz.com/board/bass-...ml#post1721345
http://www.gearslutz.com/board/bass-...ml#post4342028

More on comb filtering if you care:

Comb Filtering Article
Comb Filtering Video

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post #13 of 14 Old 04-06-2011, 02:54 PM - Thread Starter
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Quote:
Originally Posted by Ethan Winer View Post

Yes, all of that helps.



Comb filtering is caused by reflections, which is different from flutter echo caused by opposing surfaces. So comb filtering is reduced by a poly only if the curve is very severe. These posts explain well why polys are not real diffusors:

http://www.gearslutz.com/board/bass-...ml#post1721345
http://www.gearslutz.com/board/bass-...ml#post4342028

More on comb filtering if you care:

Comb Filtering Article
Comb Filtering Video

--Ethan

Hmmm... I was under the impression that poly's are diffusors which diffuse sound by deflecting it in various directions. However, I also realize that what the poly fails at is: temporal diffusion, however what it is successful at is: not absorbing. However, there is a rather large discussion whether polys can be successful in a smaller room.
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post #14 of 14 Old 04-06-2011, 04:27 PM
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I am very happy with a pair of naked QRD diffusors I purchased from Gik Accoustics for $178 pair. They are very light weight and were easy to spray paint and mount to the wall behind my magnepan speakers.
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