Additional transverse bracing in x bracing

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Tommaso Poggi
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Joined: Fri May 01, 2020 6:03 am

Additional transverse bracing in x bracing

Post by Tommaso Poggi »

Benedetto briefly talks about adding transverse bracing to an x bracing to reinforce the structure.
I don't get why they should be this much useful in reinforcing since x bracing is already stiffening in that direction but it's weaker along the grain compared to parallel.
What i do get is that they might help with coupling a bit more the movement of the x arms, so that when one goes down the other goes up and vice versa.
Also, wouldn't two more braces result in a heavier and too stiff of a top if you don't make the x thinner or lower? anybody experimented with it?

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Barry Daniels
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Location: The Woodlands, Texas

Re: Additional transverse bracing in x bracing

Post by Barry Daniels »

I thought that his suggestion for an upper transverse brace was to prevent the headblock from rotating due to string pressure. That part of the guitar adds little tonally so the effect should be minimal.
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Christ Kacoyannakis
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Re: Additional transverse bracing in x bracing

Post by Christ Kacoyannakis »

I recently saw a young maker's bracing pattern (I think it Tim Frick, who studied with Tom Ribbecke, but don't quote me on that) and there was a pretty substantial upper transverse brace there. I assumed that it was there, as Barry notes, to prevent the rotation of the neck due to the string pressure. However, in my mind, the archtop has a much different, and much more substantial neck block than a steel string acoustic. That, and the way the neck extension rest on top of the neck block for a good portion of it, should prevent any rotation.

The other maker who I am studying quite extensively is Ken Parker (is he polarizing in the archtop world? Because he seems to say things that fly in the face of what most are doing, but I find his reasoning very sound). Ken does not use "f" holes or a sound hole in front of the bridge, because he says "f" holes reduce the tonal size of the top, and the area where one would normally put a soundhole is an extremely active portion of the top. So, I am very seriously considering an offset sound hole on my next build.

Don't mean to derail this upper transverse discussion into a soundhole/Ken Parker discussion, so I apologize for that.

Alan Carruth
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Re: Additional transverse bracing in x bracing

Post by Alan Carruth »

Sound holes and bracing are interrelated, and really need to be discussed at the same time.

I got into arch tops via violin making, and as a student of Carleen Hutchins, I use her 'free plate' tuning methods (speaking of controversial...). After I get done carving the outer surfaces of the top and back I rough graduate them, and then check the resonant modes. The rest of the graduation process is guided by the mode shapes and pitches, with the objective of getting 'good' mode shapes at 'the right' pitches. On arch tops I try to get the pitches of the #5 'ring' modes to match in the top and back. It's hard to 'prove' that any of this really matters, but it does seem to work pretty well in practice.

I do an initial tuning of the plates before cutting the sound holes, trying to get proper mode shapes and pitches. Then I cut the holes, and some of the top mode shapes and pitches get totally messed up. I fit the top bracing, and then re-tune the top by shaving the braces. Generally speaking, you can get the mode shapes and pitches to come back to where they should be by getting the brace profiles right. I'll note that in round hole arch tops I have used a wide and shallow brace above the hole to compensate for the lost stiffness and strength there. F-hole tops don't need that.

This all makes sense. Arch tops get most of their stiffness from the arch shape and thickness graduation, and cutting holes any place, round ones or 'F' holes (or Tom Ribbecke's 'S' holes or 'A' holes...), messes up the stiffness. Proper bracing of the top gets it back to where it should be. This relationship seems to hold for all sorts of guitars in some respect. As Mark Blanchard says: "The sound is in the top". We use bracing on flat tops, and careful arching and graduation on arch tops, to add stiffness without adding too much mass, but there can be a cost. Bracing that is too heavy or too light or just in the wrong place can interfere with the way the top vibrates. The best bracing, then, is that which is designed to 'get out of the way', and the 'free' plate modes are, at least, sensitive indicators for that.

Sound holes are, of course, primarily a way for the internal air resonant modes to 'talk' with the world. You need to have some sort of hole someplace to have a Helmholtz type 'main air' resonance, which is the lowest pitched resonance that can effectively radiate sound. That's mostly a matter of the effective sound hole size, although the location can be a factor as well: William Allen's article on air resonances in American Lutherie #1 should be required reading. Aside from that the exact location and design of the holes determines which internal resonances they 'hear', and has an important effect on the overall timbre, as I found when exploring the use of 'ports'.

Me arguing arch top design and construction with Ken Parker invokes what's been called the 'American Question':"If you're so damn smart why aren't you rich?" Suffice to say that I don't always agree with him, but then, I'm not sure I always agree with everything I've ever said either. I do think I'm on solid ground when I say that the traditional designs have been optimized for certain uses over a long period of development, so it's hard to make changes that are universally accepted as 'improvements'. Also, of course, you run into the fact that any substantial change alters the timbre, and may well do so in a way that makes the instrument less useful for the standard repertoire. The violin world has pretty much backed itself into a corner in that regard: almost any change you make in the standard designs is 'wrong' in some way, so they're more or less stuck with making copies; the more exact , the better. We have more freedom, and time and the market will decide whether any particular change is a good one or not, assuming, of course, that people are still playing acoustic guitars in a hundred years....

Christ Kacoyannakis
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Re: Additional transverse bracing in x bracing

Post by Christ Kacoyannakis »

Alan, thanks for your reply. You always have good, well thought out responses. One of the reasons that I never got into violin making (although I did do quite a bit of research) is that you have to make copies. Sure, if you are making instruments for non-concert violinists, or just for fun, you can make whatever you want, but the concert violinist want copies of something 300 or more years old. I just didn't want to do that. I like guitarmaking, because I can push the envelope or be very conservative with designs, and nobody knocks your design for not being a copy of X, Y, or Z. Also, not really trying to make a living doing this, allows me to give something a try, just so see how it turns out, without worrying about missing a car payment if it doesn't sell.

Alan Carruth
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Re: Additional transverse bracing in x bracing

Post by Alan Carruth »

I've been having a little fun with fiddles of late; look in on my 'Harlequin' fiddle on the bowed string page.

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