Simulating stiffer back, closed box

[Craig Bumgarner]

Any thoughts on simulating a stiffer back on a closed box? Adding mass simulates less stiffness, but what about stiffer? My case is complicated by a soundhole which is too small to get a hand through, so it is unlikely that I can make a permanent change to the guitar but I wonder if there are non-destructive ways of simulating a stiffer back to see what changes would sound like, and look like in FRC.

[Trevor Gore]

It depends where you are in the building process.

For simulation purposes, with an unfinished guitar you can glue braces on the outside and shave and trim at will, then remove all traces and finish the guitar. If you want to make a permanent change it gets difficult, as you say! On a finished guitar, it may be possible to glue a brace on the outside with a water based glue and soak it off afterwards, but I've never tried that.

If you just want to look at the low order resonances, then a reasonably calibrated 4DOF model of the guitar type you're looking at can yield some useful insights, example here, but you don't get to hear it. However, if you've tapped enough of your guitars you form an impression of what a particular FRC (from one of your guitars) will sound like.

[Craig Bumgarner]

For simulation purposes, with an unfinished guitar you can glue braces on the outside and shave and trim at will, then remove all traces and finish the guitar.

That what I've been thinking too. My backs are laminated with 0.5mm veneers so I have to be a little careful, no room for glue staining, but I usually epoxy grain fill, so I'm thinking I'll do that first and then epoxy glue the temporary braces to the outside with the same glue and when done testing, shave & scrape down to the epoxy layer again.

If stiffening the back makes a significant improvement, I can open the box up to do the work necessary, this is a spec guitar, so I don't have anyone breathing down my neck.

If you just want to look at the low order resonances, then a reasonably calibrated 4DOF model of the guitar type you're looking at can yield some useful insights

I'm beginning to see a 4DOF model is in my future. good winter project.

Thanks

[printer2]

Would holding the box down on a solid surface with some padding take the back out of the picture? I know not ideal but not hard to do.

[Craig Bumgarner]

Would holding the box down on a solid surface with some padding take the back out of the picture? I know not ideal but not hard to do.

It might, holding the guitar tight to the body does deaden the back some, but I don't want to take the back out of the picture, I want it in the picture, only stiffer. Right now, the back resonance is too close to the top and a stiffer back is what I think I need, I just need to prove it. Adding temporary mass to the back can simulate less stiff, but as far as I know there is no way to simulate stiffer with something simple like added mass. There are probably some steps I could take if I could get my hand inside, but that is not possible on this guitar.

To be more clear, what I want to do is raise the back resonance, B(1,1), so the corresponding T(1,1)3 resonance will be closer to the recommended 4 semi-tones of separation between T(1,1)2 and T(1,1)3. The only way I know to raise B(1,1) is be making it lighter or stiffer. I don't believe lighter is practical in this case.

[Craig Bumgarner]

Here's what I ended up doing. Epoxy filled the back until the grain was fully filled and smooth. Then glued a couple pieces of scrape 4mm cedar top material to the back with five minute epoxy. The laminated back is the first I've made with an violin back shape. I thought with the shape and one brace, it would be stiff enough, but not so.

I started with pieces about 30mm tall and shaved them down until I got the peak where I wanted it.
The test braces should be able to be carved off and the residual epoxy feathered out eventually.

The guitar is definitely improved with close to four semitones of separation between the back and top. So, now all I have to do is figure out how to get the braces inside a 54 x 70mm sound hole. I'll probably have to take the top or the back off, but if anyone has a better idea, I'd love to hear it.

Could exterior braces be the next Big Thing? "Keeps the guitar is the proper position off the player's abdomen." "You can tune the back at home, with nothing more than a block plane" I've thought about some uni-directional carbon fiber rovings laid in epoxy on the back like braces. Wouldn't look too good, but at least they would be unobtrusive.

BTW, found this handy little Hz to Semitone converter on line. Works with any two fields filled and the third blank. http://users.utu.fi/jyrtuoma/speech/semitone.html

[Trevor Gore]

Nice work, Craig. I can't help with how to get the braces inside though! There's some nice binding work that will have to be undone, it seems.

I'm interested in the positions you chose for the braces. Do you already have a lower bout brace inside?

[Craig Bumgarner]

Right, I had a 7x18mm ladder brace at the max width of the lower bout already. Molded archtop like backs with a single brace were used on some vintage French gypsy guitars. This was my first go at it and I misjudged the stiffness. I thought the shape would lend more stiffness, but it was less than I guessed. The inside brace plus two outside braces makes three in the conventional locations for a Selmac.

[kiwigeo]

Always handy to get in a bit of practise at back surgery 101...

[Craig Bumgarner]

A number of the French vintage gypsy guitars had soft backs, molded (laminated) to a dish or violin back shape, some with only one or no braces at all. Soft being a back with B(1,1) lower than T(1,1)2. Deflection under a 1kg load can be over 0.30mm! The result is usually light weight, high volume, quick attack and good sustain. On the negative side, there can be an excess of resonance, ringing and lack of clarity and nuance.

I've been paying more attention to the backs on gypsy guitars and find some I like best have backs that are 2-3 semitones higher. In my test guitar above, the back was at 200hz and the top at 235, fairly common for a soft back gypsy guitar. The added exterior braces took the back to 265. I thinned the top edges a little and added 8gr to the bridge and got the top to 212. Pretty drastic change, eh? From 2.7 semitones below to 3.8 above.

The result is equally dramatic. Louder, more clarity, less resonant, better balance and more nuance. This guitar is a little too brassy and this did not change. Darker strings will fix that I think.

Ha, the doctor will see you now! Taking the back off is probably easiest way to go, the only (interior) brace is not socketed, so with bindings off I can probably zip the back off with a hot knife without too much trouble. The linings are fairly stout which should help as well.

[Trevor Gore]

I'm interested in the positions you chose for the braces. Do you already have a lower bout brace inside?

Right, I had a 7x18mm ladder brace at the max width of the lower bout already. Molded archtop like backs with a single brace were used on some vintage French gypsy guitars.

OK. Thanks Craig.