Ingenious design 
Apart from it being a bit of a PITA to construct it, i can see a few advantages. Low weight (which is your design goal) & possibly a decent reduction in radiation from the enclosure of midrange from the back of the 12" drivers.
The foam filling should damp this to a reasonable extent & the thin walls will also tend to pass it less easily than a single sheet of thicker wood.
Keep up the good work I popped in yesterday for a look, but couldn't resist finally posting today!
Nice job.
Mark
Apart from it being a bit of a PITA to construct it, i can see a few advantages. Low weight (which is your design goal) & possibly a decent reduction in radiation from the enclosure of midrange from the back of the 12" drivers.The foam filling should damp this to a reasonable extent & the thin walls will also tend to pass it less easily than a single sheet of thicker wood.
Keep up the good work
I popped in yesterday for a look, but couldn't resist finally posting today!Nice job.
Mark
pourable foam
Try this link for foam, since no one else has spoken up, very good people to deal with.
Raka, Inc. 772-489-4070
Look center at the bottom of the page. Under the accessories tab.
Neat concept on the construction, your test panel did not look to have the same arc on both sides, is this accidental, or purposeful, or optical illusion? If accidental, I'd suggest you add a 1/2 width filler to the bottom of your clamping jig to even out the final bends. Don't rely on gravity, the bending modulus of that material can vary considerably. You stated this was to be "Hardboard" have you considered using " Tempered Hardboard" Aka HDF ? That would alleviate some of your concerns about moisture.
I'll just watch now.....
John
Try this link for foam, since no one else has spoken up, very good people to deal with.
Raka, Inc. 772-489-4070
Look center at the bottom of the page. Under the accessories tab.
Neat concept on the construction, your test panel did not look to have the same arc on both sides, is this accidental, or purposeful, or optical illusion? If accidental, I'd suggest you add a 1/2 width filler to the bottom of your clamping jig to even out the final bends. Don't rely on gravity, the bending modulus of that material can vary considerably. You stated this was to be "Hardboard" have you considered using " Tempered Hardboard" Aka HDF ? That would alleviate some of your concerns about moisture.
I'll just watch now.....
John
Ingenious design
Keep up the good work
Nice job.
Mark
Thanks Mark,
Never hesitate to leave a comment, it saves me from thinking I'm talking to myself (again).
I have high hopes but I'm a long way from the finish line. If I have a VERY productive weekend I might have one nearly assembled. The big obstacle right now is the foam. The one place I went to today only had it in quart cans and it cost WAY too much. I'll try another place tomorrow.
Hi John,
Thanks for the link.
The arc is exactly the same on the inside as outside so it must be an optical illusion.
I can't find tempered hardboard in 1/4" thickness. This material is still HDF, just not tempered (tempering is coating the sheet in linseed oil then baking it).
TBH I'm not overly concerned about moisture. With these panels filled with foam they will be hard pressed to lose their shape. I'm not worried about loss of tension either as it is the shape of the panel that is important.
So, I have 2 sides assembled and one for the top. The parts are cut for the bottom panel and I'll get that put together tomorrow.
I'll need to re-design the back panel to be flat on the outside in order for everything to line up. I will probably make it solid and brace it through to the front baffle. This makes sense and better distributes the weight front to back.
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To be expected I guess.
I did an experiment - I tried sprayfoam in a can:
This worked very well on this shorter section but it would be a bit trickier on the long one - I'd have to drill holes to inject the foam at the middle.
Each void of the long section has a volume of ~20 litres and it would be very expensive to fill these this way. Since I've had zero luck finding a reasonably priced local source for this foam, I've gone ahead and ordered it from US Composites. Their price is the best I've found so far - nearly 4 times less than the equal amount at a local supplier.
There's not much I can do until I get that foam, just finish assembling the panels and getting them ready to fill.
If I learned one thing from the sprayfoam filling, it's that this WILL work - my Gods how rigid is that section now!
John Gods,
small wonder, foamcore sandwich is one, the double-convex geometry added leaves just the compressibility of polyurethane foam as main squeeze.
Very smart to come up with this, curious what you've got in store to connect the edges in the corners.
(scuse the vote, poll options like that should be prohibited, i decline responsibility for the four follow-ups)
Thanks Jacco,
Joining the corners will be a delicate procedure. I originally thought of using a piece of solid wood shaped with the double curve to act as blocking - miter the corner, scoop out the foam and glue in the wood.
I have come up with a different plan and the best way to illustrate it is with actual pictures when I'm doing it. Basically, it has the top coped into the side and the backbones at this 90 degree joint doweled and epoxied together. I will scoop out some of the foam at the ends of the top and bottom sections and reinforce these with high density foam. I have ordered some of the 16lb foam along with the 2lb for this (and other) purpose.
Should be bucket loads of fun.
Rather than a tap test...
I'd like to see the panels anchored and directly driven from the center by an exciter. Sweep and measure different panels as John did with boxes in the building materials thread.
It will indeed be interesting to see what solution you have for the joints, surely that was many readers' thought when they first saw the proposal. Given previous project pictures, I have confidence it will be done well.
As Jacco mentioned, a convex foam sandwich is stiff and strong indeed. It's a technique sometimes used on rocket fins. Passing through mach subjects them to large stresses due to pressure, where failure mode is vibration followed by shredding.
I'd like to see the panels anchored and directly driven from the center by an exciter. Sweep and measure different panels as John did with boxes in the building materials thread.
It will indeed be interesting to see what solution you have for the joints, surely that was many readers' thought when they first saw the proposal. Given previous project pictures, I have confidence it will be done well.
As Jacco mentioned, a convex foam sandwich is stiff and strong indeed. It's a technique sometimes used on rocket fins. Passing through mach subjects them to large stresses due to pressure, where failure mode is vibration followed by shredding.
I just hope it's not the point where he's hitting his head on it saying WHY...WHY...WHY...
That is actually one of the top or bottom panels, not a test piece. I didn't want to waste all that sprayfoam.
The edges are indeed solidly glued.
I will be doing some tests once the first box is fully constructed. I figure if this method does not work as predicted, I can still add bracing to the inside through the woofer holes. The outer shape is how I want it to be regardless.
Unlike the materials tests, this time I'll use my homemade accelerometer:
It is very sensitive, has very wide response and gives good clean readings.
Apparently there is still a misunderstanding as to the objective here. I'm not trying to damp this panel, so "deader" is not an issue. I'm trying to create an incredibly stiff panel, one that resists flexing. If it can't flex, it can't resonate.
Totally eliminating movement is nearly impossible but if the resonant frequency is driven up high enough, there will not be enough energy in the box to excite it (thanks Dave
). I didn't tap on it, I swear!
I changed the construction slightly - the back panel is now flat like the front. The side, top and bottom (the broadest panels) are the curved ones and the curve goes from front to back.
I was trying to draw this up in Sketchup the other night but didn't finish. Windows 7 did the most annoying thing ever by downloading an update, installing it and restarting the fracking computer while I was asleep.
No, I didn't save my work.
If I get bored later I'll try again.
No misunderstanding. Driving up resonant frequency is only a good plan if there is more loss at higher frequencies. I have abstained from the poll, for what it's worth.
Driving up the resonant frequency is always a good plan - it is the reason why a speaker should be extensively braced.
A higher frequency resonance is easier to damp, wouldn't you agree? The typical music signal has much less energy at the higher frequencies, therefore less to excite resonance.
I'll disagree with you there. The higher the frequency, the less energy there is to exite it. If it is not excited it is as if it does not exist.
In a multi-way if you can push the panel resonances above the XO point it is MUCH less likely to get excited.
Further, the higher Q you can make the resonance the less audible it will be.
dave
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