# Curved Sided Cabinets



## fredk (May 14, 2008)

I'm doing a small set of bookshelves, .5 cu.ft. and wanted to do a curved sided cabinet.

I've done some basic woodworking and casework, but have never attempted anything like this.

From the reading I have done, I can use thin mdf, or bendy ply. I had though of using thin (say 1/8 or 1/4) BB, but nobody seems to do this.

I was figuring on 3 or 4 vertical frame pieces out of ply to define the shape and a couple of horizontals to hold everything in shape. I need to work out the size and work up a template.

I've searched forums and found a few threads, but if any of you have done curved sided cabs I would appreciate your input.


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## 1Michael (Nov 2, 2006)

Some people have used 3/4" BB and cut slots 1/2" deep every inch or so apart. This makes it somewhat bendable.


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## fredk (May 14, 2008)

I've seen that, but I wonder how this affects cabinet resonance.

I'm going to talk to a local specialty wood products company about bending thin BB ply. I'm surprised this is not done, but there may be good reasons.


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## fredk (May 14, 2008)

I'm also looking for a reality check on costs. My alternative is using a pre-built cabinet at about $250 by the time you add taxes and shipping.

I figured I could do this for aroung $100 in materials including a nice veneer of my choosing. Does that seem reasonable?


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## dyohn (Apr 17, 2008)

Kerfing 3/4" MDF or ply is better than using thin bendable plywood for resonance issues IMO. In either case, to deaden the curved sides after bending try using fiberglass resin inside the enclosure.


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## fredk (May 14, 2008)

OK, found the thread I was looking for on using 1/8" bb ply. Wohoooo!!!

Dead Sexy!!










The original thread is here. Now I need to figure out rough dimensions and do some costing.


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## 1Michael (Nov 2, 2006)

I would not use that thin of wood...ever. You can also do layers of 1/4 inch, one on top of each other to get the correct thickness.
Yep I forgot about doing the same thing with MDF, that works also:T


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## BuddahX (Jan 6, 2010)

3/4" MDF and kerf it will be the best. the walls will flex if too thin. Kerfing Isn't hard to do.


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## fredk (May 14, 2008)

buggers said:


> I would not use that thin of wood...ever.


If you build up layers to the appropriate thickness, why not?? Thats all ply is, thin layers of wood glued together.

1/4 would be a more desirable option because it involves less work. I don't know how thick I can go with this method. The big question I have is of band clamps will provide enough pressure over the entire sheet to properly laminate sheets.

I know I could use MDF, but ply seems to be the preferred choice when price is not an issue.

I could go real fancy and use both mdf and ply and do a constrained layer cabinet, but I don't know that I really need to go that far. I may just explore that option for fun though.


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## fredk (May 14, 2008)

BuddahX said:


> 3/4" MDF and kerf it will be the best. the walls will flex if too thin. Kerfing Isn't hard to do.


Maybe I didn't explain myself very well. I'm looking to build up to a 1/2" to 3/4" thickness here.

It seems to me laminated 3/4" anything is going to be better than the same material kerfed unless there is a structurally sound way to re-bond the kerfed material. Yes? No?

There are probably many ways to skin this cat since labour costs are irrelevant.


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## BuddahX (Jan 6, 2010)

LOL. I think doing layer's would work out well. I just looked at how he did it and I think That would work out pretty well. Cool looking design. I say go for it and let us know how it come's out. building it yourself and staining the wood your own color will be rewarding. Just make sure and do internal bracing and you'll be good to go.


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## fredk (May 14, 2008)

OK, I didn't think I was 'way out there' on this one, but you never know. :T


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## fredk (May 14, 2008)

Played with Sketchup for a bit tonight and came up with this:










Add the front baffle and the cabinet back and I should have a good frame to wrap the side walls onto. I figure I can set the two curved frame pieces into dados in the front and back pieces to help square things up and make assembly a little easier.


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## mwmkravchenko (Jul 11, 2009)

Hi Fred

I can see that there are quite a few opinions being posted. So I'll add mine to the list. Go with your idea. Use the 1/8th" ply in what ever build up thickness you care to do. IT will become as stiff as all get up when it is cured. It will work like a charm. The clamping idea from the thread you sited is also quite good. You will need more clamps by the way. On a box your size you can either make cauls and use regular clamps or clamp it with band clamps. In a pinch I have used the strapping that holds large bundles of lumber together. But that requires a bit of skill. 

And by the way most of my career as a cabinet maker has been in doing the weird and wacky world of curved work. Been at it for 23 years. Done some real neat stuff. I have quite a few sheets of 1/8th" baltic birch plywood in my storage shed behind my shop for just such a project. But the bends will be a smidge more interesting. :bigsmile:

The kerf cut mdf will work as well but it will not be as stiff. So will the kerf cut plywood. But depending on the plywood you will cut most of the way through before you can get it to bend.

Mark


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## fbov (Aug 28, 2008)

fredk said:


> ...
> It seems to me laminated 3/4" anything is going to be better than the same material kerfed unless there is a structurally sound way to re-bond the kerfed material.
> ...


You missed the post telling you to coat the kerfed area with fiberglass resin after the bend is set, to fill in all the kerfs and eliminate any potential for structural weakness or resonance. Kerfed MDF with a nice veneer is the cheap way to go (depending on the veneer) but layered BB has a certain visceral appeal. 

Have fun,
Frank


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## 1Michael (Nov 2, 2006)

fredk said:


> If you build up layers to the appropriate thickness, why not??


Building up is fine. 3/4" being minimum. My point is not to have a 1/8 thickness for the walls...
For the baffle use at least 1.5" thick and using inner layer of MDF and outer layer BB works quite well:T


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## fredk (May 14, 2008)

Frank. I did see that. Having worked with fiberglass a little I'm not a big fan of using it unless you have a well vented place to work. I would also be concerned about flowing the resin into each curf properly and voids.

It may end up not being an issue, but it seems to me that laminating will be easier to manage properly.

Hi Mark. I appreciate your comments as I know you do this stuff for a living. I'll probably have more questions as I get closer to the build


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## mwmkravchenko (Jul 11, 2009)

> For the baffle use at least 1.5" thick and using inner layer of MDF and outer layer BB works quite well.


Hi Fred

Michael's idea is good. One of the things that a custom box can do much better than a bought box is not make any sound of it's own if it is properly done. Your constrained layer sides will still resonate but it will be quite high up in frequency. A layer of ice and water sheild will dampen that out. But the front baffle is where all the goings on takes place. If you provide a firm foundation for the launch of your music it will indeed sound cleaner. In the midrange for sure and even in the upper bass. The best cabinets I do have a 2" thick front baffle. For the size your considering 1 1/2" is plenty. Personally I would make a sandwich of Baltic birch and MDF or whatever you have on hand. It will distribute resonances better if you use different materials. I basically mean that your box will vibrate less if you have a heavy front baffle.

Mark 

Mark


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## fredk (May 14, 2008)

> One of the things that a custom box can do much better than a bought box is not make any sound of it's own if it is properly done.


That was one of several reasons for going with a custom cabinet. Unless I use an appropriate glue, I don't think what I am doing qualifies for constrained layer. I need to figure this out as it might just be worth while trying to do a constrained layer design since I'm doing the curved sides already.



> Your constrained layer sides will still resonate but it will be quite high up in frequency.


Hmm, what must one do to stop the cabinet from resonating?

Actually, I wonder how much the tweeter will put out into the cabinet in resonance since it is essentially sealed driver. If that is true, I would want to push the cabinet's resonant frequency up above the crossover point. I think that will be in the 2-3 Khz area for this woofer.


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## 1Michael (Nov 2, 2006)

fredk said:


> Hmm, what must one do to stop the cabinet from resonating?


At least a double baffle and lots of internal bracing...


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## mwmkravchenko (Jul 11, 2009)

> I don't think what I am doing qualifies for constrained layer. I need to figure this out as it might just be worth while trying to do a constrained layer design since I'm doing the curved sides already.


Hi Fred

A constrained layer structure can also mean that the entire built up thickness is under constraint or pressure. Your layers of plywood left to themselves will want to flatten out. But you keep them in a curved position by means of a glue layer. You will indeed have that accomplished. It produces a stiffer structure which will always raise the frequency content of the structure born resonances. You can increase the stiffness somewhat by the choice of glue you use. A white type wood glue has quite a bit of elasticity. Yellow PVA wood glue is a bit more stiff. Polyurethane is very stiff. My own experience will point you to polyurethane. It is slow drying but makes a very good bond. It has to be worked with gloves on or you will be wearing it for two weeks!



> Hmm, what must one do to stop the cabinet from resonating?


You really can't. You can dictate how much or how high in amplitude ( how loud ) and how high in frequency. Unless you built a battleship design with sand filled panels which I have done and they do sound marvelous but they weigh a ton. A more finessed design uses the approach your taking with some evil little tricks. Resonances are another term for vibrations. 

A vastly simplified explanation of structural born resonances deals with a single frequency that you don't want your cabinet to resonate at. That never really happens. There are always a host of resonances that are present. The good news is they are usually present in a fairly narrow band that is determined by the materials and the size of unsupported panels you have in the enclosure. In your case the size of unsupported panels is tiny. The signals large enough to cause you problems come from the woofer primarily. What you want to do is beat them down with small sticks and little bits of mushy materials. When all else fails you go to the pillow fight method. So go with a thick baffle. Brace the interior. Each brace will cut a larger panel into two pieces as far as a structurally carried resonance is concerned. It will therefore raise the resonant frequency. Half the size will obviously double the frequency where the panel will resonate and dampen the panel for resonances. The higher the unwanted frequency that you have to deal with the easier it is to quiet it down. A cracked bell never rings right? So what you have to do it introduce either a crack or a large bit of anti ringing material. Look in a car or pay attention to car audio and you will see numerous examples of structural dampening in the form of soft sticky panels bonded to the sides of the car. This stuff is not cheap. If you know someone in construction they use in roofing a wonderful thing called ice and water shield which has all the qualities of the car stuff with a lot smaller price. It is super sticky on one side and has roofing granular material in the other side. Here a 60 foot by 3 foot roll costs $85. A layer on the inside will work wonders. Pillow fight? Yep fiber fill will absorb sound frequencies up to four times it's thickness when it is setup as a loose fibrous tangle within a box. So how low will the effective absorption go? If you box is 1 foot deep and full of loosely packed fiber fill it will absorb down to around 128 hz. If it is 6 inches deep it will work down to about 256 hz. Look up a length to frequency calculator on the net and you will get some ideas as to the wave length of sound.

All you can clean up is the midrange to high bass. But that is where most of the music is in the first place. Your structural design will take care of the low end. As for your tweeter causing problems. A structural born resonance needs to looked at in a number of ways. The primary two are frequency and amplitude or how loud it is. When we look at how loud a tweeter gets there is not to many things that can be done in the enclosure to dampen tweeter resonances. Pick a good tweeter and the work will be done for you. Or pick one you can tailor like the Neo 3 from BG. It comes as a flat panel that you can attach to a plate. Behind the plate you can put a container like a area and cover it with a couple layers of dampening material and stuff it with fiber fill. The amounts of fill and the volume behind the tweeter will tailor the sound quite a bit. Time for some Experiments with stuff!

Mark


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## fredk (May 14, 2008)

Hi Mark. I'm somewhat familiar with resonance and how its applied to audio.

I think that you and I are using the term 'constrained layer damping' a little differently.

What I was thinking of is where you put a thin elastic layer between two stiffer layers. The elastic layer creates shear force between the two stiff layers with the result being resonant energy converting to heat.

Bending wood works a little differently. You put the inner and outer layers of the wood under tension (stretching and compression) this stiffening the wood and driving up the resonant frequency.

Thanks for the tip on glue stiffness. I didn't even know there were polyurathane based glues

So, to apply the three things above together here is what I would do:

Three layers of 1/8" ply bonded by poly glue followed by a layer bonded by something elastic like green glue and then two more layers bonded by poly. That _should_ give me a sidewall stiffer than strait 1/4" thick wood with a constrained damping layer in the middle.

Its the practical application I really struggle with. What glue/material should the damping layer be hand how thick?? Supposedly both too thick and too thin reduce the effectiveness of the damping.


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## mwmkravchenko (Jul 11, 2009)

> Three layers of 1/8" ply bonded by poly glue followed by a layer bonded by something elastic like green glue and then two more layers bonded by poly. That _should_ give me a sidewall stiffer than strait 1/4" thick wood with a constrained damping layer in the middle.
> ​


Just consider how thin a good glue line is and tell me how much of a layer of more flexibility you will end up with. A decent glue line is around 0.010" that's about the thickness of a hair. It won't help much. And from what I have read up on actual constrained layer dampened panels for military ( submarines ) use it is not that effective for the size of panel that you are building.

My advice is go for the stiff and dampen it on the inside of the panel. If you are really determined you may find a product called tremco tape that is used to bond glass thermal panels into wood frames. It is very soft and sticky as it gets. It comes as a tape and could be used as a true viscous layer. But it does creep. So you would have to glue up two layers of two laminations without glue in between layers 2 and 3 then bond layers two and three with the tremco tape.

My opinion on that is ist's not really worth all the effort in a panel the size you are doing. 

Mark


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## mwmkravchenko (Jul 11, 2009)

> Three layers of 1/8" ply bonded by poly glue followed by a layer bonded by something elastic like green glue and then two more layers bonded by poly. That _should_ give me a sidewall stiffer than strait 1/4" thick wood with a constrained damping layer in the middle.
> ​


Just consider how thin a good glue line is and tell me how much of a layer of more flexibility you will end up with. A decent glue line is around 0.010" that's about the thickness of a hair. It won't help much. And from what I have read up on actual constrained layer dampened panels for military ( submarines ) use it is not that effective for the size of panel that you are building.

My advice is go for the stiff and dampen it on the inside of the panel. If you are really determined you may find a product called tremco tape that is used to bond glass thermal panels into wood frames. It is very soft and sticky as it gets. It comes as a tape and could be used as a true viscous layer. But it does creep. So you would have to glue up two layers of two laminations without glue in between layers 2 and 3 then bond layers two and three with the tremco tape.

My opinion on that is it's not really worth all the effort in a panel the size you are doing. 

Mark


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## lsiberian (Mar 24, 2009)

If you want to use contraint layer dampening your best bet is 1/4" thick peel-n-seal applied with a heat gun. This is used to kill the energy transfered to the box from the driver. Rockwool is use to kill the energy coming from the back of the driver. Curved plywood can be purchased from tapease in varying lengths.

For baffle step I suggest 4" radius be used. Then you can use concrete, oak, or ply(if going cheap) to make the inner layer. don't worry about the rounded edges. Just apply the layer onto the front and sides. Bracing at every 3 to 4" is suggested along every axis. If you want less that's fine. If you want easy then just use some mitered bracing on every corner every 3 feet to brace. Then line the walls with R11. 

Doubling the baffle is best to allow for flush mounting. I always use birch ply for my project because it's light easy to work with and comes already venered.


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## robbo266317 (Sep 22, 2008)

I have been prototyping a curved speaker as well using ⅛ ply glued with polyurethane.
The first layer I also screwed into the bracing.

Here is the framework:
 

I kerfed a piece of mdf to spread the pressure evenly:
 

Completed unit:


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## lsiberian (Mar 24, 2009)

This thread has given me some ideas. 

Crazy ideas. I spy some curved speakers in my future.


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## fredk (May 14, 2008)

That looks great Robbo. It looks like you cut the panels slightly over sized and then trimmed them down after lamination?


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## robbo266317 (Sep 22, 2008)

fredk said:


> That looks great Robbo. It looks like you cut the panels slightly over sized and then trimmed them down after lamination?


Yep, I used an edge trim router bit to do the sides. When I trimmed the front and back baffle I created an mdf baseplate for the router shaped like a C so I could use a trimmer blade and slowly wind it down till it just took off the material without cutting the baffle. I also made sure the blade was cutting from the outside of the cabinet towards the inside - otherwise it tears pieces of laminate off. :rolleyesno:

The prototype is only ½" thick (4 ⅛" laminations) and because it is stressed by being bent I don't think you need to go much thicker than that for a main speaker. 
If you rap it with your knuckles it is very "dead"


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## mwmkravchenko (Jul 11, 2009)

Hi Bill 

Nice job and a good example of how to make and use a clamping caul!

Mark


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