A DIY "Poly" Diffuser Array

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Postby Terry Montlick » Sun May 18, 2008 12:21 pm

bert stoltenborg wrote:what's that funny thing in the left bottom corner of the last pic?
:mrgreen:

I think it was called a "turn table," Bert. This was a device used in the previous century to reproduce audio from rotating vinyl disks. The audio was encoded along spiral grooves, in a crude analog fashion. But I think it was Charlton Heston who worked to overthrow and free us from this primitive ape technology.  8O

:mrgreen: :mrgreen:  :mrgreen:
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Postby MTB Vince » Sun May 18, 2008 4:01 pm

bert stoltenborg wrote:what's that funny thing in the left bottom corner of the last pic?
:mrgreen:


Terry Montlick wrote:I think it was called a "turn table," Bert. This was a device used in the previous century to reproduce audio from rotating vinyl disks. The audio was encoded along spiral grooves, in a crude analog fashion. But I think it was Charlton Heston who worked to overthrow and free us from this primitive ape technology.  8O

:mrgreen: :mrgreen:  :mrgreen:


Ooooh, everyone's a comedian 'round here!

Well perhaps the apes were onto something...   :) The most popular current-day music archival forms, compressed-to-death CDs and 128k MP3s, leave something to be desired.  :?

Happy Trails!
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Postby Zaphod » Sun May 18, 2008 6:37 pm

I was thinking of doing a poly array not unlike this one and
positioned about the same height.

Is that something you comedians would approve?  :mrgreen:

Vince,

Your theater is about the same height of mine, are you considering
an acoustic drop ceiling or you think the room is ok as is?
I try never to get involved in my own life. Too much trouble. :mrgreen:
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Postby Terry Montlick » Sun May 18, 2008 7:48 pm

Zaphod wrote:I was thinking of doing a poly array not unlike this one and
positioned about the same height.

Is that something you comedians would approve?  :mrgreen:

Yes!
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Postby Scott R. Foster » Sun May 18, 2008 8:45 pm

Is that something you comedians would approve?  


Get your hands off me you damned dirty ape!
SRF
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Postby Zaphod » Sun May 18, 2008 9:12 pm

We have a deal then   :mrgreen:
I try never to get involved in my own life. Too much trouble. :mrgreen:
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Postby J.F.Oros » Mon May 19, 2008 4:59 pm

I thought that is a food replicator  :mrgreen:
... studiOTipper ...
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Postby MTB Vince » Tue May 20, 2008 10:03 pm

Well I've found my damping material for the backs of the thin plywood Polys. As mentioned elsewhere in this thread, the required curve radii, necessitated the use of very thin 1/8" ply. The result when test-fit in place without adhesive was that the Poly's were pretty drum-like, not a good thing. So half the solution was to stiffen them by running a spine down the centerline of each forcing the resonance frequency higher and reducing its amplitude. The other half of the solution is damping the remaining vibration out of existance by bonding an acoustically "dead" material to the backside of each.

Pictured below is the subject of my experiments:

Image

When I couldn't find a source of thick (1cm+) felt, I tracked down the thickest  foam carpet underpad I could find. While wandering the aisles of my local Home Depot looking for other damping candidates, I came across some asphalt roofing shingles that appeared rather promising too.

Using some identical 15"x18" pieces of left-over ply I performed a highly scientific experiment. :D I glued a piece of each damping material to one side of the ply covering most of its surface area with a double application of 3M "Super-Tac" spray adhesive over the entire mating surface. The damping material was pressed firmly onto the ply and left alone for the glue to dry. In addition I had one untreated piece for comparison. Then each test piece was bent into a 4" radius curve and I knocked firmly on the apex with my knuckles while listening to the resulting sound.

And the winner is... The asphalt shingles, and by a large margin. The carpet pad damped piece of ply sang at a different pitch then the untreated  ply but the amplitude was almost the same and the tendency to ring was only slightly reduced. The test piece with a section of shingle affixed to it exhibited a higher frequency, much quieter, deadened tone in response to the knock with next to no ringing. In addition the lamination of the two materials was substantially stiffer than the ply alone or the underpad/ply combo. Freakin' awesome! Well its back off to the lumberyard for a bunch more shingles...

Happy Trails!
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Postby Scott R. Foster » Tue May 20, 2008 11:48 pm

Mass is our friend!

I used asphaltic sheets to dampen the metal panel resonances in my Pinzgauer per Eric's advice.   used a heat gun to form them to the crenulations of the panels and then glued em on... worked a treat.
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Postby jcgriggs23 » Wed May 21, 2008 1:28 pm

Scott wrote:

"Quote: Is that something you comedians would approve?    

Get your hands off me you damned dirty ape!"

It's a mad house - A MAD HOUSE!!!

BTW - Home Despot also sells rolls of a bitimous material (for patching flat roofs) that is self adhesive and might work as well as (or even better than) the shingles for damping - I believe the brand is "Black Knight" and you'll find it in 3 ft wide rolls in the roofing department.  I haven't used it for acoustical purposes but it patches roofs up a treat (I have the same background with shingles...)  It also has the advantage of not having grit embedded in the surface.

Regards,
   John
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Postby Scott R. Foster » Wed May 21, 2008 4:03 pm

Home Despot also sells rolls of a bitimous material (for patching flat roofs) that is self adhesive


The stuff i used fit this description.... no grit
SRF
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Postby MTB Vince » Wed May 21, 2008 5:56 pm

jcgriggs23 wrote:BTW - Home Despot also sells rolls of a bitimous material (for patching flat roofs) that is self adhesive and might work as well as (or even better than) the shingles for damping - I believe the brand is "Black Knight" and you'll find it in 3 ft wide rolls in the roofing department.  I haven't used it for acoustical purposes but it patches roofs up a treat (I have the same background with shingles...)  It also has the advantage of not having grit embedded in the surface.

Regards,
   John


Scott R. Foster wrote:
Home Despot also sells rolls of a bitimous material (for patching flat roofs) that is self adhesive


The stuff i used fit this description.... no grit


I think I came across the stuff you describe when I was looking. It was much thinner than the roofing shingles, and thus wouldn't provide nearly the damping effect or the rather fortuitous increase in laminated panel stiffness. The roofing shingles really worked great.

Happy Trails!
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Postby jcgriggs23 » Thu May 22, 2008 2:37 pm

MTB Vince wrote:

"I think I came across the stuff you describe when I was looking. It was much thinner than the roofing shingles, and thus wouldn't provide nearly the damping effect or the rather fortuitous increase in laminated panel stiffness."

The stuff I'm thinking of is definitely thicker than a standard asphalt roofing shingle (I'd guess a bit less than twice as thick) and heavier but not quite as stiff (although not exactly floppy either) - it's not underlay (which what I believe you saw), but patching material for flat roofs (torch down plastic, not mopped on bitumous).  It's shinier than underlay and is self-adhesive.  When I've seen it at Home Despot, it's been in 3ft wide rolls in a box and I am 99.9% sure it's made by Black Knight.

I just thought this might be easier to work with since it is self-adhesive and designed to be sliced and diced for patches.  But if shingles are doing the job, I doubt it's worth wasting any time or money on this stuff.

BTW, very nice lookng work!

Cheerz,
  John
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Postby MTB Vince » Thu May 22, 2008 2:49 pm

jcgriggs23 wrote:The stuff I'm thinking of is definitely thicker than a standard asphalt roofing shingle (I'd guess a bit less than twice as thick) and heavier but not quite as stiff (although not exactly floppy either) - it's not underlay (which what I believe you saw), but patching material for flat roofs (torch down plastic, not mopped on bitumous).  It's shinier than underlay and is self-adhesive.  When I've seen it at Home Despot, it's been in 3ft wide rolls in a box and I am 99.9% sure it's made by Black Knight.

I just thought this might be easier to work with since it is self-adhesive and designed to be sliced and diced for patches.  But if shingles are doing the job, I doubt it's worth wasting any time or money on this stuff.

BTW, very nice lookng work!

Cheerz,
  John


Aaah,

Well you're quite right, I didn't come across anything quite like you describe. Had I done so I would have experimented with it too. However, at almost 1/8" thick, the premium (25 year guarantee! :) ) shingles I did purchase do the job rather nicely- And the two sided application of 3M's strongest spray adhesive makes me confident that the damping material will stay where it belongs. I also suspect that their aggregate surface construction likely improves the shingles' damping characteristics.

Happy Trails!
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Postby Scott R. Foster » Thu May 22, 2008 3:28 pm

aggregate surface construction likely improves the shingles' damping characteristics


Well.. it has mass anyway... and its viso-elastic.. so it oughta work [as you have emperically proven].
SRF
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Postby MTB Vince » Fri May 23, 2008 3:47 am

Well today's job, bonding the asphalt shingle damping material to the backs of all the Polys was pretty damn time consuming. It took most of the day. Tools used were a drywall square, an Olfa knife, and a 4'x4' sheet of 1/4" hardboard as a "cutting board". In addition I used pretty much an entire package of shingles, five cans(!) of 3M Hi-Tac 76 spray adhesive, and a roll of painter's masking tape to mask off the portions of the Poly back in order to avoid glue over-spray. To ensure the shingles stayed put, I sprayed both surfaces, waited for them to tack up and then pressed them firmly together methodically using the heel of my hand, rather like a contact cement bond. Spraying both surfaces with 3M's strongest spray adhesive results in a crazy-strong lamination!

Image

Image

Image

Image

Tomorrow I've got everything prepped and planned to permanently mount the Polys, completing this build!

Happy Trails!
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Postby MTB Vince » Fri May 23, 2008 9:20 pm

Well the final installation of the Polys went smoothly and quickly. Each Poly was secured in place with a generous bead of No More Nails construction adhesive down the center stiffening "spine" and likewise generous beads of Weldbond Universal Adhesive in the frame slots where the Poly edges engage the frame. The latter glue dries crystal clear, the reason I didn't use No More Nails throughout. Below is a picture of the glue application just prior to the Poly being installed:

Image

Once each Poly was up in place and aligned perfectly, I used my pneumatic nailer/stapler to drive a couple of 1" brad nails into the spine, securing the Polys in position while the glue set. Careful work with finishing nails, a hammer, and a nail setting tool could substitute for the nail gun of course. The air tool just makes the job easier. After an hour, once the glue had set up nicely, the finished Polys are very stiff and quite dead, even the large 3'wide x 4'high x 8" deep Poly on the rear wall. No matter how heroic  the output of my sub's, my Polys won't be singing along. The final picture is a shot from beneath a completed Poly showing the laminated damping material:

Image

In retrospect, the only thing I might have done differently would have been to either stain or paint the reinforcing spines dark grey or black. Despite cutting them so as to minimize their view, from certain viewpoints  the spines in the horizontally oriented Polys are just barely visible. I may still mask the nearby wall surface and stain the visible parts with a rag. Depends how much it bugs me. Well, that about wraps up this build. In the immortal words of my toddler's favorite TV show:

Mission Completion!

Happy Trails!
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Postby Bob » Fri May 23, 2008 10:48 pm

Did you polyurethane the shingles?

a) they're an outdoor product -- might off gas
b) when you turn the volume up, the poly is going to vibrate, and the stones might fall out.

Yes, I'm wondering if you poly'd your poly's.
Regards
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Postby MTB Vince » Fri May 23, 2008 11:19 pm

Bob wrote:Did you polyurethane the shingles?

a) they're an outdoor product -- might off gas


I did a little research in thar particular regard. The shingles themselves don't though the traditional adhesive/sealers used when installing them on roofs do. Not a problem as I used spray adhesive.

Bob wrote:b) when you turn the volume up, the poly is going to vibrate, and the stones might fall out.

Yes, I'm wondering if you poly'd your poly's.


Before gluing 'em in place I did a very thorough job of brushing any loose aggregate free. I've managed to achieve a remarkable degree of stiffness with the reinforcing "spines" and laminated shingles now that the Polys are glued in place. I've left the ends wide open rather than sealing them and creating resonant cavities.  As such I expect very little independent vibration if any at all even at room shaking volumes. Hell, nothin' fell off 'em when I was over-bending them to pop the panels back into the frame slots so I'm not worried at all.

Happy Trails!
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Postby Baldin » Sun Jun 01, 2008 3:50 pm

Looks realy good.
Congratulations.

Can you hear any difference with the polys installed?

I'm working on some larger polys myself, which will be closed and half filled with Rockwool for them to act as a panel absorber as well.

Kind regards Baldin
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