A DIY "Poly" Diffuser Array

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A DIY "Poly" Diffuser Array

Postby MTB Vince » Sat May 17, 2008 4:38 am

Hey Lads,

I'm cutting and pasting the majority of this from where I've got it posted in my local AV Forum. So I'll post it as a series of post in the order things were accomplished...

I have a wicked slap echo in the upper reaches of my 20'3" long x 14' wide x 11' tall room. To hear it one just has to reach above your head while standing and clap your hands together. My current arrangement of absorptive panels centered on a seated listener's ear height eliminates the problem down lower. In the pic of my room below, the Poly array will be centered between the tops of my  fabric covered absorption panels and the lighting/HVAC soffit above it. Each of the two Poly arrays will consist of three vertically oriented and two horizontally oriented elements.

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The frame in which the curved plywood Poly elements are mounted in is made from doubled up strips of 1/2" MDF. I had two 4'x8' sheets of MDF ripped into 8'x3" strips on the lumber store's panel saw as my small table saw won't handle that large of material. Then using my chop saw with an 80T carbide finishing blade, I cut all the individual pieces that made up my frame design. Here I am making lots of sawdust. Speaking of which when machining MDF wear a mask. There is stuff in the MDF binding agents which aren't good for you!

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The two layers of MDF strips which make up the frame, overlap completely and interlock so that the parts of the frame which can't be screwed to an underlaying stud, will be screwed to a frame section which is. Next a photo of the two frames laid out as a mirror image pair along their long (horizontal) axis.

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Once the interlocking frame was completed and the parts laid out to verify they all fit together precisely and as intended, it was back to the table saw to bevel some of the edges. The outside-most top and bottom horizontal strips are beveled at a 45 degree angle in the interest of aesthetics. In addition, the outer layer of MDF strips were bevelled at either a 30 or 45 degree angle wherever they engaged an edge of the the curved plywood polys. Finally all the pieces were sanded with 220g using a combination of my orbital palm sander and a hand block. Below is a pic of all the cut and stacked pieces.

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The next part wasn't so much fun. Over the next three days I PMAO (Painted My Ass Off)! -Two coats of high quality tinted latex primer and two coats of  premium Benjamin Moore Acrylic Latex in the same dark Grey as my base boards and room trim with a light hand sanding with 220 grit between each coat. Fortunately the weather gods have been co-operating and I've been able to perform the finishing work outside. In the shot below I'm about half way having completed priming and just begun the first coat of the finishing colour.

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Friday I made a trip to Exotic Woods in Burlington and picked out some appropriate material for the curved Poly surfaces. I'll be using a special 1/8" clear birch unidirectional plywood. This ply has all the grain of the plies oriented in the same direction making it much more bendable. I intend to stain the light blond birch veneer with a transparent grey stain and then spray it with multiple coats of a semi-gloss poly-eurathane. I'm after a finish reminiscent of the stained maple "graphite" finish offered on some Sonus Faber loudspeakers. If I can pull it off, my Poly array should look quite architecturally stunning. However, the plywood waits until I can actually get the frames on their respective side walls. Then I'll test fit slabs of corrugated cardboard bent into a curve to mimic the ply. This'll permit me to accurately dimension the plywood to achieve the curve I'm after. Well, this has been my progress to-date. Updates to follow as the project evolves.
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Postby MTB Vince » Sat May 17, 2008 4:40 am

Saturday morning I completed the final light sanding and second finishing coat of dark gray paint. It took a few hours to tear down and clean up my work area on the deck. The weather was gorgeous and my jet lagged spouse was very pleased to get our deck back to just kick back and chill. :D All the painted framing parts took up residence in neat rows on our guest room floor for a few days for the paint to cure somewhat before the install. I'm told by my professional painter friend that I should wait a month until they are fully cured but I'm an impatient man!

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For Wednesday I've lined up the rental of this crazy spinning laser level on an adjustable tripod. It projects a continuous level line on all four walls. This should tremendously simplify the installation of the 16' long Poly arrays. A buddy with strong arms and back will be assisting and we'll be installing my just completed Skyline diffusor at the same time. More to follow...

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:41 am

Well today was a long day! Its after 1am and I've been working on the DIY Poly build since about 11am when I picked up the super cool gyro stabilized laser level pictured below at my favorite tool rental house, Windmill in Dundas. The level allowed me to generate an around-the-room line at the correct height for the bottoms of the Poly arrays.

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Things started out a little sketchy when I realized that my super-duper stud sensor wasn't reading a single thing(!) but upon further reflection, it didn't matter. When I put up my room, the studs were skinned with 5/8" ply prior to the drywall to stiffen the walls up. The underlaying ply would provide plenty of purchase for my screws and I'm using a bead of "No More Nails" construction adhesive between the wall and the two layers of the frame as well. When I install my "Mother-of-All-Subwoofers" IB subs later this year, I don't want anything moving besides cones and air! After much piloting and counter-sinking (and the assistance of my bike shop's co-op student  :D ) I had the first layer of frame up on the left side wall by about 4:40pm.

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After a quick run to return the laser level, I completed about half the first layer on the right hand on my own in only an hour. Practice makes perfect and all that. I had to take a short break to teach a Spinning Class in my shop and a bite to eat. By 8:30pm I was back at it with my mountain-biking buddy and close pal Mark. In short order we had the first layer of framing complete on the right hand wall despite this install being complicated by the equipment laden gear racks along this wall. It was back to the left hand wall to put up the second outer layer of framing. This took a little longer than I'd anticipated 'cuz you had to precisely measure and evenly space apart your holes to avoid those in the first layer. A few pieces of this layer also ended up being slightly long and required trimming on the chop saw. Not a big deal- an 1/8" here and a 1/16" there but I'm pretty anal about such things. In any event by 1:00am the left hand wall was complete.

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It looks really good in person! Tomorrow I hope to enlist the assistance of my co-op guy again and complete the right hand wall. Then I'll cut up a bunch of big sheets of cardboard as planned in a prior post to template the pieces of curved plywood which will form the Polys themselves. I plan on filling all the countersunk screw holes with Bondo along with one or two other small gaps before sanding and hitting the outermost layer again with a coat or two of paint. I'm pretty psyched about getting the frame up as it was a pretty big step. Stay tuned...

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:42 am

This morning sandwiched between a physio appointment and grocery shopping, I got out and hit the last local paint shop in my search for transparent grey interior wood stain. I struck out at Benjamin Moore too. The salesperson suggested the same Toronto wood finishing specialist that the guys from Burlington Exotic Woods had- Goudey Manufacturing in TO. A Google search followed by a quick call solved the problem. The required grey stain would be shipped immediately and arrive in a day or two. Woo-hoo!

I got to spend the afternoon and part of the evening working on the Poly arrays. The remaining outside layer of the right hand wall was completed quickly just after lunch with a little help from our shop co-op kid. This finishes the major construction phase of the Polys. The remainder should go pretty easily and not require any additional helpers. After a brief trip to the doctor's office for a round of immunizations for the little guy :cry: , I Bondo'd all the countersunk screw holes. That was actually rather time consuming as you have to mix the stuff up in small batches and use it fast! Otherwise, like epoxy, it hardens before you've got the job done. Bondo is much sturdier than wood filler though, and I've used it on the advice of my favorite contractor.

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Finally, I made up corrugated cardboard templates for the ply Polys to figure out the final dimensions of the ply when its curved and fitted to the frame. Now I can order my unidirectional Birch ply from Exotic Woods. I decided it would be easier to have them cut the pieces to size for me. Getting there...

Image

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:43 am

MTB Vince wrote:...This finishes the major construction phase of the Polys.


Perhaps I spoke to soon. Craaaap! Sanding the Bondo smooth on 100+ screw holes IS NO FUN AT ALL! :P I've got this handy little Bosch Corner and Detail Sander for the heavy work but the Bondo plugs up the pads real quick and if you're not payin' real close attention, its easy to sand right through the finish back to bare wood. I follow up with 220 grit wet/dry by hand. I've got freakin' blisters on my fingertips. :x Ouch.

I got through about 75% of the sanding tonight (3+ hours so far) but I've run out of sanding pads for the Bosch. I guess I'm rippin' up to the 'Depot in the morning. I double checked all my cardboard template measurements for the "bend-y" ply and then called Exotic Woods to order 'em. Rick is gonna hand pick the most highly figured (grainy) sheets he can. The pieces will be cut to size by Exotic Woods as my table saw is too small to easily, accurately get the job done. The ply should be here by the middle of next week.

The overall size of the arrays was dictated to some degree by doorways along one wall and the need for left-to-right symmetry. I did have some leeway however in the individual Polys making up the array. I didn't do a cut plan beforehand 'cuz I couldn't calculate how much wider the piece of ply needed to be than the frame to achieve the desired arc. The result is that there will be a lot of waste from the 5'x5' sheets. Oh well, I guess I'll have lots spare to practice getting the wood stain applied properly with. If any reader goes ahead with a similar project, you should try and plan out the individual "cells" of the array to making more efficient use of the material. I stuck with a consistent finished depth of the bent ply arcs of about 4-1/2" inches off the wall at the outermost point in the bend.

FWIW for future DIY Poly constructors-  With my frame design featuring a  3/8" deep "V" groove notched into the surrounding frame to engage the straight edges of each Poly:

-A 24" width of frame opening and a 4.5" arc required a 27-1/8" wide piece of ply.
-A 34-3/4" width of frame opening and a 4.5" arc required a 37" wide piece of ply.
-A 31-1/2" width of frame opening and a 7" arc required a 36" wide piece of ply.

For other approaches to securing the bent ply or different widths of frame, you're on your own!

I guess that about wraps it up for tonight.

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:47 am

Postscript to the "Adventures in Bondo" described above:

My favorite go-to guy for contractor-type advice was in the shop today gettin'  his son's bike tuned up. In passing I mention just how much fun using Bondo to fill the screw holes turned out to be... NOT! He sez "I didn't tell ya to fill the holes with Bondo! I told ya to skim-coat any exposed mdf end grain with Bondo prior to painting it. Hole filling with Bondo is way too hard. Use spackle for that." :oops: Somewhat mortified... There is a day of my life I'll never get back.

I did finish sanding the Bondo smooth again and am in the process of priming and finish coating the patch spots. All the painting will likely be complete tonight so I'm just waiting on the arrival of the pre-cut unidirectional birch ply from Exotic Woods and the grey wood stain from Goudey. Almost done.

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:47 am

Well the pre-cut unidirectional ply arrived from Exotic Woods in Burlington today. Woo-hoo! As Rick had promised, the hand selected sheets of material exhibit much more pronounced wood grain in the birch veneer than usual- perfect. At the last minute the decision was made to go with 1/8" ply rather than 1/4" as originally planned. The 1/4" material was simply too difficult to bend and would have likely required steaming the ply to achieve the required arcs.

After a couple little buzzes with my handy Porter Cable mini belt sander, I was able to friction-fit all the Polys into their V-notched recesses in the frames. Below are photos of the left side wall array and the single large 3'x4' Poly mounted immediately above my recently completed 2D QRD "Skyline" Diffusor on the back wall. The Polys look quite architecturally striking in place.

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A quick bout of hand clapping with hands held overhead confirmed that the nasty slap echo which once occupied the upper reaches of my room has been completely banished! I love it when a theory proves out in practice. :D My next order of business today will be to run some high level low frequency test tone sweeps through the subs to ensure the Polys don't vibrate along with my music and movies. If they do, depending on the severity I'll either template some curved 1/2" ply stiffening spines to glue behind them or glue some dense closed cell foam to the back sides. I'll keep ya posted on that one.

The next step will of course be practicing and perfecting my wood staining technique on some of the scrap material, then finishing the veneer properly. Not far left to go...

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:49 am

Well after a short break (caught up on some movie viewing and a couple of 360 games :D ), Its time to get these finished up. The panels do exhibit some "drum head" resonances as I'd suspected they might and on the larger panel widths the resonance is pretty low in frequency. Simply gluing damping material to the backs ain't gonna cut it either. I need to stiffen the curved panels up some too.

My original plan was to make one or two evenly spaced "bulkheads" for each Poly by tracing each cross-sectional curve onto a cardboard template, then using the templates and a jigsaw to cut the bulkheads out of 3/4" ply. After spending 3 + hours with pencil, scissors, a box knife and beaucoup cardboard, I only had two templates completed to my satisfaction. I realized that this was way too much work! Instead I decided to make a longitudinal spine for each of the Polys as This required a much simpler measurement and no templates. I figure dividing the unsupported span in half for each poly out to make 'em much stiffer. This will both reduce the amplitude of airborne induced vibration while increasing the resonant frequency. The latter will make it easy to damp any residual vibration out completely by spray gluing carpet backing felt to the backs of the Polys. Below is pic of all the just completed spines ready for installation.

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Last week I experimented some with the transparent grey wood stain I intend to finish the Polys with on some scrap pieces of birch ply. After much experimentation I've accepted the fact that Sonus Faber's "Graphite" grey stained maple perfection is beyond my talents. :lol: To get a splotch free finish with the Goudey stains I have, with or without conditioner or a pre-coat of transparent stain base, I've gotta stick with a really light application. This leaves the blond wood color "coming through" the grey stain. Oh well, it'll still look cool, just not stunning. I suspect Sonus Faber is actually using an Aniline dye on the "Graphite" finished Cremona series rather than a wood stain.

To be continued...

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:52 am

Well here is the first Poly "spine" glued and screwed into place, a rather easy job.  Note the cut away shape of the bottom of the spine. This ensures that the spine is not visible from any reasonable viewing angle.

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And here is the first of the eleven Polys stained and test fit for appearance, prior to application of the felt damping material to its backside and being glued in place.

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And the remaining Poly stiffener "spines" glued and screwed in place along the left side wall-

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And right side wall-

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The grey stained Poly looks really cool and fits in much better with the room's  neutral grey colour scheme. Holy crap though, staining 'em is a lot of work. The Goudey Mfg wood stain I'm using goes tacky really fast, leaving very little time to wipe the excess back off. Of the first 4 Polys I sanded then stained, two had to be sanded back down and stain re-applied 'cuz I wasn't fast enough! :o

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 4:54 am

Well today I got a bunch more done. A trip down into the basement warehouse of Alexanian's Flooring produced some big scraps of the heaviest carpet underlay the have (Thanks Alene Alexanian!  :D ). Then off to Home Depot to purchase spray bombs of Minwax clear satin Polyurethane and a super cool accessory handle that attaches to spray bombs providing far more control during application. I decided to use a low sheen finish on the Polys. They grey stained panels would probably look better with a glossy finish but I've been very carefull to avoid and reflective finishes to maximize picture quality. Thanks to Andrew, my favorite contractor, for the spray varnish and handle recommendation. While at the Depot I picked up a couple of asphalt roofing shingles too to experiment with as a potential bonded damping material.

When I got back home it was back to the grind literally, while I took the palm sander through 120 grit, then 180, and finally 220  on all the larger panels that hadn't been prepped yet. I guess practice must make perfect but because staining 'em all went far more smoothly this time round. After waiting for the stain to dry, out came the spray bombs of varnish and the brilliant accessory handle. I'll never varnish with a brush again! It took longer to shake the cans in preparation for use then it did to actually apply the finish. This stuff is the s**t!!! :D A couple hours later in the evening, after a light scuff sand by hand with 220 grit, the second coat went on. I think thats all they'll need and I'm done for tonight.

Image

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sat May 17, 2008 5:04 am

So with the previous post, that brings my progress up-to-date. Not much left to do. Once I figure out the most effective damping material to bond to the backs of the curved Polys, I'll glue 'em in place and "thats all she wrote". I've got 4 identical scrap 18" squares of my 1/8" birch ply to experiment with insofar as damping materials go. I'll use 3M Super Tack 76 spray adhesive to bond the damping material to the un supported expanses of the Polys' backsides. So far I've just got the 3/8" thick "crumbly" sponge carpet underlay and the not-quite 1/8" thick roofing shingles to try. Any other suggestions?

Happy Trails!
Vince@FReewheelcycle.com
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Postby Bob » Sat May 17, 2008 5:11 am

I used stain applicating cloth from home depot for staining. Wipe a lot on, wipe some off. Worked fine.
See pictures about 40% of the way down http://www.bobgolds.com/LivingRoomPlasm ... n/home.htm

Are you going to fill the space behind the poly's with fiberglass? (And perhaps 'side' pieces.)

Have you considered laminating another layer of wood over the poly's. I think stiffer is usually better than damped in a poly, right up to the point where the weight makes em fall of the walls.
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Postby MTB Vince » Sat May 17, 2008 5:43 am

Bob wrote:Are you going to fill the space behind the poly's with fiberglass? (And perhaps 'side' pieces.)


Nope, I hadn't intended to do either. My test fitting of the unfinished Polys pictured above, completely eradicated the slap echo  previously present in the upper third of my room. I really get the sense that I've got enough absorption in the room already from the mid-bass up. The Meridian pre-pro's MRC waterfall curve plots suggest this and I've got a pretty good ear. After an aborted attempt to build curved "bulkhead" stiffeners for the Polys, I've accepted that  unlike my imagination, there are limits to my carpentry skills. :D  Thus going with a spine splitting each curve in half at the mid point rather the bulk heads. Not quite as ideal but once each Poly is glued into the corresponding grooves cut into the frame along the opposing edges and then glued and nailed (with brads) to the spines on the center axis, The panel resonant frequency ought to be pretty high and thus more easily damped. Ne pas?  

Bob wrote:Have you considered laminating another layer of wood over the poly's. I think stiffer is usually better than damped in a poly, right up to the point where the weight makes em fall of the walls.


I'd actually originally planned to use 1/4" 5 ply unidirectional birch for the Polys but experiments with bending the stuff suggested that it was simply too stiff for the 12" "radius" of most of my Polys. It would likely have been a better choice for the 32" and 36" wide versions however. As the Polys' outer finishing is complete, there is no way I'd consider a solution involving cutting slightly larger pieces to glue and fit over top of the work I've just completed. I'd toyed with the idea of gluing "ribs" of additional 1/8" ply at intervals to the backs of the Polys but I couldn't figure out a practical way to accomplish it. I figured if I glue 'em on prior to bending the ply and fitting it in place, the tighter radius the inner rib will be forced into will likely shear the glue bond off! Fitting them into place after the fact would only be possible on the single largest Poly at the rear of the room as all the others are recessed into the frame on all sides. Only the 3'x4' Poly at the rear of the room is open at top and bottom. For this one I may try cutting ribs to follow the inner curve and force fit them between the perimeter frame and the spine on the big one with some white glue that doesn't tack up as fast as the construction adhesive I've used throughout. I'm sure the big Poly would most certainly benefit from further stiffening in light of its larger size. Your thoughts/suggestions on the specifics of how to accomplish?

Happy Trails!
Vince@Freewheelcycle.com
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Postby Scott R. Foster » Sat May 17, 2008 9:12 am

Another nice set of polys... with pics added to the FAQ.

Woot!
SRF
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Postby avare » Sat May 17, 2008 10:51 am

Great build diary!

Regarding filling the diffusors with fiberglass, this will reduce the peak value of absorption that they have and broaden the range of absorption.  In other words it will probably improve the room sound.  I write probably because I haven not read any mention of calculations for the room acoustics.

Andre
Last edited by avare on Sat May 17, 2008 10:55 pm, edited 1 time in total.
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Postby Zaphod » Sat May 17, 2008 10:23 pm

Vince,
The ceiling, how high is it?

Neat work, i'd say  :D
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Postby MTB Vince » Sun May 18, 2008 12:57 am

Zaphod wrote:Vince,
The ceiling, how high is it?

Neat work, i'd say  :D


As per the opening lines of the opening post to this thread :wink: :

MTB Vince wrote:I have a wicked slap echo in the upper reaches of my 20'3" long x 14' wide x 11' tall room.


Its also just under 10' to the bottom of the lighting and HVAC soffits which run the length of the sidewalls which limited by bass trap stacks to about 9.5' tall. Thanks for the kind words, I'm rather pleased with how my diffuser projects have turned out.

Happy Trails!
Vince@Freewheelcycle.com
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Postby MTB Vince » Sun May 18, 2008 1:03 am

So close...

Polys temporarily up in place while the poly-urethane finish cures for a day or two. All thats left to do is bonding the damping material (?) to the back surfaces and gluing 'em permanently in place. I'll be experimenting with damping materials this weekend.

Leftt Side-Wall:
Image

Right Side-Wall:
Image

Rear Wall:
Image

Happy Trails!
Vince@Freewheelcycle.com
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Postby J.F.Oros » Sun May 18, 2008 9:10 am

Impressive work and good looking too ! Thanks for sharing this here with us, Vince !  :D
... studiOTipper ...
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Postby bert stoltenborg » Sun May 18, 2008 10:18 am

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