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Xenon · 28th September 2009

Thanks Collo,

Couldn't have done it without your input. Your advice raised my game enormously. And QRDUDE is an awesome tool. Yes N23 is very pretty

I'm not sure about innovations here, but can say that 1) using XPS (EPS) blueboard foam seems an excellent solution: cheap, light, easy enough to cut, and measure. And it's difficult and very time-consuming to 'block up' heights with wood spacers (we tried--lots of extra cutting/measuring). Wells are perfectly sized and spaced and the well volume 100% filled with acoustically absorbant material! Also with XPS some LF absorption is gained below the effective diffusor LF. Another key point is that attaching 4cm x 91.5cm faces of 2.5mm becomes a simple affair, and the result is a perfectly flat face along its length. We used white glue for everything. This is not the best for the faces as the ends can curl up. We had to use 'spacers' and occasionally a little weight, while curing. Rubber cement reacts with XPS, and we don't have ready access to other cost-effective glue options. In North America, there's construction adhesive or liquid nails, which might be better face glues, if they don't react with XPS foam.

Using a well (vertical) length = to the foam panel width makes for a nice looking and well fitting unit. The key is cutting the frame and fins pretty exactly. We cut our fins 1mm less high than the frame (frame inner width = 91.5cm, foam width = 91.5cm, fin height = 91.4cm).

If I were doing this again, I would redesign my dimensions with the face wood-depth in mind. We must minus -2.5mm for every face (wood-depth), and had to work with -0.5mm in measurements all the time. Better to make, for example, the zero-well depth 13.75cm (frame depth = 14.cm). This means foam depth (a cut) for a zero well = 135mm. (not 132.5mm as in our case, since we must subtract for the face depth). We designed with nice round numbers for well depths in QRDUDE, but this wasn't the best approach! For those with more money to invest or time, nicer frame wood and 45deg. corners would look very sharp.

2) Using the frame member as part of the first/last wells: With successive periods, this way, there is no 'frame gap' between them. This seems a good thing. Looking at various N periods, most can be arranged so that a zero-well and near-zero well are at the ends. For our N23 we have a zero-well, and a -7mm well at the ends. So we rip the left-right frame sides (and use 30mm foam instead of 40mm foam for the wells, as the frame takes up 12mm). To do this nicely, you want the top/bottom frame members sized for the entire length, while left/right frame members fit inside these top/bottoms (this would be very difficult to do with 45deg. corners).

Another thing to mention -- we are using a 2.5mm backing (advantages are best-max.-well-depth, and lightness). We had problems with it flexing during assembly. 3) So we tacked three frame boards vertically behind (one left/right, one middle). This kept the backing even. Also it's important to work on a flat surface. After assembly we take them off, place a flat ply panel behind the unit and bungee cord everything together, and place over bricks, for at least 18 hours after gluing. (Take a look here.) One concern is that with a 13.75cm deep frame, there is some bowing of the long frame members. This will disappear when mounted vertically, but would be a concern if the frame was longer/deeper (requiring a top-to-bottom wood strip cross-member in a well of near-zero depth, near the middle of the frame, lengthwise). Using plenty of glue and the overnight bungee method resolved this issue for us.

And we 4) had to add 0.6mm length for each well-segment over the theoretical, according to actual fit.

Collo -- come to think of it, The ability to input an 'actual fit' well-depth constant would be a handy advanced feature in the QRDUDE Calulator.

I think overall, 12mm plywood is a great choice for a frame if the depth isn't too deep or long, but for larger/deeper units, I will go with something thicker or try finished lumber. I'm also thinking that 5) it would be easy to build a double-length frame if you have (near) a zero-well near the middle. Use your frame cross members for the (near) zero well in that case -- and bowing problems would be solved. I'll add that we got all the wood but the fins cut at the home center, which saved us time -- however, some trimming here and there may be involved. We cut our own fins and foam -- which wouldn't be so easy for one person, at least with our table saw.

If I could, I'd start over, with a somewhat better (easier and faster-to-build) design. Maybe this information will help someone avoid the errors we made. I think for the DIYer in ya, 6) our final-result N23 unit is 99cm x 94cm (@ 3'3" x 3'), weighs 11.7kg (25.7lbs.) and cost @$49.USD equivalent, in materials, in Japan.

28th September 2009	#60
Xenon Gear nut Joined: Jul 2008 Location: Japan Posts: 77 Thread Starter	Thanks Collo, Couldn't have done it without your input. Your advice raised my game enormously. And QRDUDE is an awesome tool. Yes N23 is very pretty I'm not sure about innovations here, but can say that 1) using XPS (EPS) blueboard foam seems an excellent solution: cheap, light, easy enough to cut, and measure. And it's difficult and very time-consuming to 'block up' heights with wood spacers (we tried--lots of extra cutting/measuring). Wells are perfectly sized and spaced and the well volume 100% filled with acoustically absorbant material! Also with XPS some LF absorption is gained below the effective diffusor LF. Another key point is that attaching 4cm x 91.5cm faces of 2.5mm becomes a simple affair, and the result is a perfectly flat face along its length. We used white glue for everything. This is not the best for the faces as the ends can curl up. We had to use 'spacers' and occasionally a little weight, while curing. Rubber cement reacts with XPS, and we don't have ready access to other cost-effective glue options. In North America, there's construction adhesive or liquid nails, which might be better face glues, if they don't react with XPS foam. Using a well (vertical) length = to the foam panel width makes for a nice looking and well fitting unit. The key is cutting the frame and fins pretty exactly. We cut our fins 1mm less high than the frame (frame inner width = 91.5cm, foam width = 91.5cm, fin height = 91.4cm). If I were doing this again, I would redesign my dimensions with the face wood-depth in mind. We must minus -2.5mm for every face (wood-depth), and had to work with -0.5mm in measurements all the time. Better to make, for example, the zero-well depth 13.75cm (frame depth = 14.cm). This means foam depth (a cut) for a zero well = 135mm. (not 132.5mm as in our case, since we must subtract for the face depth). We designed with nice round numbers for well depths in QRDUDE, but this wasn't the best approach! For those with more money to invest or time, nicer frame wood and 45deg. corners would look very sharp. 2) Using the frame member as part of the first/last wells: With successive periods, this way, there is no 'frame gap' between them. This seems a good thing. Looking at various N periods, most can be arranged so that a zero-well and near-zero well are at the ends. For our N23 we have a zero-well, and a -7mm well at the ends. So we rip the left-right frame sides (and use 30mm foam instead of 40mm foam for the wells, as the frame takes up 12mm). To do this nicely, you want the top/bottom frame members sized for the entire length, while left/right frame members fit inside these top/bottoms (this would be very difficult to do with 45deg. corners). Another thing to mention -- we are using a 2.5mm backing (advantages are best-max.-well-depth, and lightness). We had problems with it flexing during assembly. 3) So we tacked three frame boards vertically behind (one left/right, one middle). This kept the backing even. Also it's important to work on a flat surface. After assembly we take them off, place a flat ply panel behind the unit and bungee cord everything together, and place over bricks, for at least 18 hours after gluing. (Take a look here.) One concern is that with a 13.75cm deep frame, there is some bowing of the long frame members. This will disappear when mounted vertically, but would be a concern if the frame was longer/deeper (requiring a top-to-bottom wood strip cross-member in a well of near-zero depth, near the middle of the frame, lengthwise). Using plenty of glue and the overnight bungee method resolved this issue for us. And we 4) had to add 0.6mm length for each well-segment over the theoretical, according to actual fit. Collo -- come to think of it, The ability to input an 'actual fit' well-depth constant would be a handy advanced feature in the QRDUDE Calulator. I think overall, 12mm plywood is a great choice for a frame if the depth isn't too deep or long, but for larger/deeper units, I will go with something thicker or try finished lumber. I'm also thinking that 5) it would be easy to build a double-length frame if you have (near) a zero-well near the middle. Use your frame cross members for the (near) zero well in that case -- and bowing problems would be solved. I'll add that we got all the wood but the fins cut at the home center, which saved us time -- however, some trimming here and there may be involved. We cut our own fins and foam -- which wouldn't be so easy for one person, at least with our table saw. If I could, I'd start over, with a somewhat better (easier and faster-to-build) design. Maybe this information will help someone avoid the errors we made. I think for the DIYer in ya, 6) our final-result N23 unit is 99cm x 94cm (@ 3'3" x 3'), weighs 11.7kg (25.7lbs.) and cost @$49.USD equivalent, in materials, in Japan.