Dome height

[Tommaso Poggi]

Hi, i'm currently working on a benedetto 17" and my next project will be a 16", i bought a plan of an old L5 and i'm adding a cutaway and some frets, i will use x bracing and i'm also probably going to use different fholes. I noticed that on this plan the back dome is less tall than the top, while on the benedetto is the opposite. What is the "industry standard"? Also how do you compare the height between a 17 and a 16"? Do you do a proportion like h1:h2=16:17 or is it better to compare absolute values?

For what i got it seems to me that a flatter dome is a looser contruction, and i imagine you'd get a more jazzy sound instead of a percussive one from a lower top, but i might be wrong and i don't have the slightest idea of what it means for the back, also i wasn't able to find any article on this matter online

[Alan Carruth]

It turns out that arch height scales best with the thickness of the plate, rather than the length or width. It's a balancing act, of course. You use both arch height and thickness to get the stiffness you want, so a small increase in arch height along with a little more thickness gives a much stiffer plate.

Back when I made my first arch top classical guitar I figured I could use a high arch to get the needed stiffness, and make the top thin to save weight. Structurally it worked fine, but the sound left a lot to be desired. I made another, nearly as bad, and then read Schelling's article on 'The Violin as a Circuit' in the IEEE Journal. He included the bit about scaling the arch to the thickness as a footnote. I tried it, and it worked.

I use 'Chladni patern' tuning, a tech version of 'tap tones', and had trouble getting the plates to work right. The high arches cause them to 'buckle' around the edges, in a sense, when they'e too thin: there has to be a certain amount of stiffness to keep things under control. The thin, high arched plates had an 'edgy' and unpleasant sound.

On the subsequent guitars I used the lowest density top wood I could as a way to keep the mass down. I figured I could get away with a 3 mm (1/8") thickness and still carry the down load of the bridge OK. Violins use an arch height of 15 mm (measured from the bottom of the plate to the top of the arch at the bridge) for that thickness, so that's what I went with. They tuned up easily, and the sound was much better.

As usual, there are probably about a jillion variables in all of this. We haven't worked them all out, and you have to start someplace, so...

[Tommaso Poggi]

Hi Alan, thank you for the reply, i knew some of your work before posting and i really apreciate your research and willingness to share.
The plan i have calls for 14,5 mm for the top and 12 for the back, measured from the upper edge of the plates (the 0 of the recurve). Benedetto instead uses 17,5 and 20,6 for the top and back. To me it seems that the difference in height between the two soundboards is still in a reasonable range, 3mm is relevant but not drastic, while the 8,6mm difference on the back looks like two totally different philosophies of building, so i was trying to understand what kind of choice was behind that. I find it very interesting that the arch is so closely related to thickness, i'll surely read into that, i might compare the ratios used in violins with the dimensions of benedetto and also get an estimate of the thickness i should keep on the l5 if i don't change the arch, sadly my set of plans came without the graduations shown

[Marshall Dixon]

Hello Tommaso,

Though it doesn't address your question directly you may find this conversation of interest.

viewtopic.php?f=5&t=5922

[Tommaso Poggi]

Thanks, i found that to be really interesting, there is a lot of good information on this forum!

[Barry Daniels]

Welcome to the MIMF Tommaso!

[Alan Carruth]

AS for the difference in arch height from top to back, which I failed to address, your guess is as good as mine, I think. It seems to differ from one maker to another. Violins tend to start out with the top arch about 1 mm higher than the back, but that may be related to the use of a sound post. The post pushes the back out, and helps keep the top from collapsing, so maybe thongs even out a bit over time? We don't have measurements of a 'new' Strad or Guarnari to compare, so it's hard to say.

There is a sense in which arching acts as an 'instant upgrade' on the wood, particularly the back. An arched maple back acts more like a flat rosewood one, with higher stiffness and relatively lower damping than a flat maple back of the same thickness. The higher the arch the more it pushes things in that direction. This has advantages and drawbacks, and you can have too much of a good thing, of course. I suspect different makers have diifferent ideas about what they like to hear, and that probably has some bearing on their choice of arch height

[Marshall Dixon]

... and you can have too much of a good thing, of course. I suspect different makers have diifferent ideas about what they like to hear, and that probably has some bearing on their choice of arch height...

I previously built steel stringers with 12' radius back and have been using 25' for classical. For no particular reason. I brace them lightly because I think the arch adds a great deal of stiffness somehow related to beam height in a brace.

Now I'm going to see what a 25' radius will do. I'm getting good results with western broadleaf maple. I love the stuff for steel string and classical both. But I'm now wondering if the greater arch will give it some ethereal attribute. One of my last two classicals had Broadleaf and the other European maple. I like the European maple better, but the Broadleaf has a 10% deeper box, plus other major differences.

Anyway I hope that much change in radius is going to do something.

[Alan Carruth]

This thread has been about carved arches; I'm not sure how that relates to a bent-with-bracing dome.

[Marshall Dixon]

This thread has been about carved arches; I'm not sure how that relates to a bent-with-bracing dome.

OK, sorry. Just gives me more ideas.

[Alan Carruth]

Marshall:
I'm sure that the dome height on a braced 'flat' back has some effect on the sound; it's just hard to say what's doing what. The braces add a lot of cross grain stiffness, probably more than the lengthwise arch, and some mass of course. The shape of the arch matters as well. On most guitars these days the back arch is more or less spherical, with no recurve around the edges as a carved arch would have. How you do that recurve makes a difference. I've seldom seen a braced 'flat' back with as much dome as even a low carved arch, and then only on 'historical' instruments. In some ways, then, the carved arch is a bit easier to understand, and that makes it hard to evaluate a braced 'flat' back by comparison.

[Clay Schaeffer]

Has anyone tried doing an arched bent stave top on a guitar like some of the old viols had?

[Christ Kacoyannakis]

There was a guy in Los Angeles CA, USA, who was doing bent 2 piece tops for archtops. I'll have to look around for his site. Really nice guy. Basically, he was using a flat top, and bending it to the arch shape, and then jointing the two halves, and gluing them together. It saved a ton of wood. I went to visit him, and he was very open and sharing, and really nice person. Great guitars. There are a ton of ways to accomplish anything. There is no right answer.

[Alan Carruth]

That technique goes 'way back; they used it on viols da gamba. Often the tops were of five peices; a central bent strip, two more outside of that, and split wedges toward the edges. The bent strips would staet out at about 5 mm thick, a scant 1/4", which allowed for some carving.

Smallman uses a somewhat different method to make the arched backs for his guitars. Again, the wood, (in this case, BRW) is cut thick and over sized, but roughly to shape, for the back. The pieces are steamed, and clamped together into plywood cauls that are half-outlines of the guitar. Wedges are put in between the pieces to bend them outward into the rough arch shape, and the wood is allowed to cool and dry. The two halves are then removed and joined along he center line before the final carving. He ends up with backs around 5 mm thick, or so I'm told; the one I got to handle certainly felt heavy enough, so he must be starting with fairly thick stock. That would hold the heat long enough, and BRW does bend well.

There are lots of ways to make a guitar.