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DIY Acoustic Panels

 
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AFryia



Joined: 09 Mar 2006
Posts: 921
Location: S.E. Michigan VPH-G70Q


PostLink    Posted: Thu Jul 31, 2008 8:49 pm    Post subject: DIY Acoustic Panels Reply with quote


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I'm looking for some design ideas for 4'x2' acoustic panels. Specifically how to build a frame with minimal cost.

I have 2 cartons of OC 703 1" I was planning on building 2" and possibly 4" panels and maybe some corner traps.
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garyfritz



Joined: 08 Apr 2006
Posts: 10624
Location: Fort Collins, CO


PostLink    Posted: Thu Jul 31, 2008 9:36 pm    Post subject: Reply with quote

If you want a basic rectangular panel, just get some 1x4's, build a box with a crossbrace, and put 2" of OC703 in it. I put some 1x1's behind the fiberglass to hold it in place. 2" OC703 doesn't need them, if it's cut to fit snugly, but 1" isn't as rigid and will probably need some help.

By spacing the OC703 2" off the wall, you'll get much better lower-end absorption than you get with 2" on the wall. 2" of air and 2" of OC703 is better than 3" of OC703, and almost as good as 4", if I remember right. http://www.bobgolds.com/AbsorptionCoefficients.htm has lots of absorption coefficients, though it doesn't have numbers for 2" off the wall.

I decided a rectangle would be boring so I made mine diamond-shaped. Smile Then wrap it in black grille cloth (Parts Express is cheap) or other cloth, and you're good to go.

See also Kal's panels.



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AnalogRocks
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Joined: 08 Mar 2006
Posts: 25456
Location: Toronto, Ontario, Canada

TV/Projector: Sony 1252Q, AMPRO 4000G


PostLink    Posted: Thu Jul 31, 2008 10:33 pm    Post subject: Reply with quote

garyfritz wrote:
If you want a basic rectangular panel, just get some 1x4's, build a box with a crossbrace, and put 2" of OC703 in it. I put some 1x1's behind the fiberglass to hold it in place. 2" OC703 doesn't need them, if it's cut to fit snugly, but 1" isn't as rigid and will probably need some help.

By spacing the OC703 2" off the wall, you'll get much better lower-end absorption than you get with 2" on the wall. 2" of air and 2" of OC703 is better than 3" of OC703, and almost as good as 4", if I remember right. http://www.bobgolds.com/AbsorptionCoefficients.htm has lots of absorption coefficients, though it doesn't have numbers for 2" off the wall.

I decided a rectangle would be boring so I made mine diamond-shaped. Smile Then wrap it in black grille cloth (Parts Express is cheap) or other cloth, and you're good to go.

See also Kal's panels.


Hey, trapaziod traps Thumbs Up

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garyfritz



Joined: 08 Apr 2006
Posts: 10624
Location: Fort Collins, CO


PostLink    Posted: Thu Jul 31, 2008 11:41 pm    Post subject: Reply with quote

Um, no -- rhombuses. Rhombi. 2 * rhombus. Smile
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AFryia



Joined: 09 Mar 2006
Posts: 921
Location: S.E. Michigan VPH-G70Q


PostLink    Posted: Fri Aug 01, 2008 1:51 am    Post subject: Reply with quote

So is it better to not have a back board?

I've read about the 2" panel stand off so the panel can absorb in two directions, as the sound wave comes then reflects.

I'm focusing on the frame because I have it in my mind that I need a frame so I can pull the speaker cloth tight.

Kal Idea looks very easy. Are there pro's and con's to having a back board vs. an open back?

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zGman



Joined: 22 May 2006
Posts: 599



PostLink    Posted: Fri Aug 01, 2008 3:58 am    Post subject: Reply with quote

Open back is best by far - you are are not wanting to create another reflecting surface. With
an open back you are getting higher efficiency (double pass) and spacing out from the wall
improves lower frequency attenuation - you will lose both advantages with a closed back.
Look into getting GoM fabric, it is acoustically transparent and has an architectural fire rating.
Also, some tightly woven "grill cloth" can actually tend to reflect the higher frequency energy.
G

Here is a link to some another type of panel construction - just scroll down the pic column

http://www.theaterdimensions.com/galleries/td_home_theater/index.html
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AnalogRocks
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Joined: 08 Mar 2006
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Location: Toronto, Ontario, Canada

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PostLink    Posted: Fri Aug 01, 2008 10:42 am    Post subject: Reply with quote

garyfritz wrote:
Um, no -- rhombuses. Rhombi. 2 * rhombus. Smile


Woulnd't that be a backwards trapazoid? OR am I forgetting my geometry ( 5 minutes after walking out of the classroom LOL )

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garyfritz



Joined: 08 Apr 2006
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Location: Fort Collins, CO


PostLink    Posted: Fri Aug 01, 2008 1:50 pm    Post subject: Reply with quote

Nope AR, see http://en.wikipedia.org/wiki/Rhombus and http://en.wikipedia.org/wiki/Trapezoid. Totally different shapes.

AFryia, my "open back" panels still have a back surface -- the wall. I don't think there's any difference between having an "open back" panel on the wall, vs. a "closed back" panel on the wall, assuming the material & spacing in FRONT of the back is the same. E.g. if you built a panel with 2" of OC703, 2" of air, and a hardboard back surface, I believe it would work 99% the same as my panel with 2" OC703, 2" air, and wall. The higher freqs will bounce off the backboard or wall about the same, and the low freqs will pass through them both about the same.

Kal's panels will have different performance not because of the back board, but because of the different material -- thickness and absorption properties -- and because of the lack of an air gap. Kal's panels should work fine for high freqs but will have much less absorption at lower freqs than mine.

I am no acoustic engineer, but: As I understand it, you have to contain a significant part of a cycle of a sound waveform within your absorber to significantly absorb it. It doesn't take much to absorb high freqs, which have very short wavelengths. Just hang a blanket on the wall and you'll absorb a significant amount of really high freqs. 10kHz has a wavelength of 34mm so thin panels like Kal's can soak it up pretty well. Absorbing lower freqs, which have longer wavelengths, requires thicker panels. The air gap behind my panels effectively makes a thicker absorber when the low tones bounce off the wall and pass through it twice. Deep bass has such a long wavelength (e.g. 50Hz = 6.7m or 22.5') that it's almost impossible to absorb. You can only contain deep bass with a massive barrier: concrete walls, things like that. I *think* stuff like Green Glue works by allowing the massive sheetrock to vibrate with low freqs. That absorbs the sound energy instead of letting it pass through. (Think of hitting a sheet of plywood with a hammer. If the plywood is hanging in midair it will absorb a lot of the energy. If it's rigidly connected to a concrete wall it will transfer almost all of the energy through to the wall.)

The tables at http://www.bobgolds.com/AbsorptionCoefficients.htm give you a good idea what these materials can do. 2" of OC703, on the wall, absorbs everything above about 300-400Hz. Space it off the wall and it lowers the freqs it absorbs.
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AFryia



Joined: 09 Mar 2006
Posts: 921
Location: S.E. Michigan VPH-G70Q


PostLink    Posted: Fri Aug 01, 2008 4:18 pm    Post subject: Reply with quote

So I think I'm going to pull the fabric all the way around to the back of the panel.

So I could build the frame out of any material as it will not show.

What material would be the most cost effective? OSD, MDF pine, other? I'm thinking a picture frame style open back.

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garyfritz



Joined: 08 Apr 2006
Posts: 10624
Location: Fort Collins, CO


PostLink    Posted: Fri Aug 01, 2008 4:24 pm    Post subject: Reply with quote

I just built a simple rectangle (or diamond) of pine 1x4's, then wrapped with fabric. The fabric also protects the wall in case I want to move them in the future.
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rabies_70



Joined: 15 Feb 2007
Posts: 1189
Location: Carlsbad, CA

TV/Projector: Sony G70Q


PostLink    Posted: Fri Aug 01, 2008 5:26 pm    Post subject: Reply with quote

These guys REALLY helped me when I was trying to treat my room. I have a pretty gnarly IB sub and with the panels and concepts I learned from the infinitely baffled crew my rooms acoustics are so much better. I built the units with the pegboard backs. They work really well. I'm no acoustic engineer by any stretch but a couple of my friends are professional musicians and they love the sound of the theater room even when there is no movie on. The room has a VERY different feel to it with the panels. I also use the tube style bass traps in the corners. I will eventually use the triangles in the corners too. Heres a link to the forum. You have to register but it's no big deal. Then there is a section on the forum about diy panels.

http://ibsubwoofers.proboards51.com/index.cgi

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