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Round hole archtop bridge bracing?

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Round hole archtop bridge bracing?

Postby Chris Richards » Fri Jul 07, 2017 6:25 am

Hi

Just wanted some thoughts regarding bracing for a round hole archtop. In the Benedetto book it shows that parallel bracing should go under the feet of the bridge, this becomes a bit of a problem with a round/oval sound hole since the braces would have to be too far apart, in which case would it be prudent to put some extra bracing in under the bridge position between the two parallel braces?

Thank you...Chris
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Re: Round hole archtop bridge bracing?

Postby Brian Evans » Fri Jul 07, 2017 7:13 am

I would probably do an X brace if I were doing a center round hole, or just splay the longitudinal braces (which aren't actually parallel in most applications) out around the center hole. The point of braces in an archtop is three-fold, as I think of it. One is strength - to help carry and distribute the vertical string tension load on the bridge. The second is also strength - to carry the tension of the longitudinal string load between neck and tail block. Finally tone - to help distribute the string vibrations around the top as directly as possible. When you put a traditional round hole below the neck in an arched top, you compromise two things. One is you take away from the natural strength of the arch to resist both longitudinal and vertical forces on the top by putting a big hole directly in the normal load paths. You might think about a mid transverse brace below the sound hole, an upper transverse brace above the sound hole and A braces to pick up the load from the neck block and distribute it to those two braces. The second compromise is you reduce the ability of the whole upper third of the body from vibrating and producing sound, very similar to how with a flat top guitar the sound plate becomes somewhat restricted to the lower bout only. That would change how I would carve, tune and brace the lower bout, to maximize it's potential. I might even do fan bracing with four or five very light braces around the lower bout. Finally; http://theartoflutherie.com/acoustic-ar ... benedetto/
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Re: Round hole archtop bridge bracing?

Postby Chris Richards » Sat Jul 08, 2017 2:47 am

Hi Brian and thank you for the information it is always interesting to read this sort of stuff. this wil be only the second archtop I've built and as a result I haven't got a wealth of experience to add, even reading Benedetto's book and other experienced builders accounts it's certainly far from being an exact science, I still don't see what you can gain by tap tuning a plank of wood but it does become interesting listening to the sound the plane makes when you're getting down to the final thickness of the top and back plate.

It's also contradicting when some of these experienced builders talk about what contributes to a good sound and then add on lots of embelishment which very much goes against the "grain" when it comes to sound quality. There's also an amount of documentation saying how the "F" holes improved the sound of the archtop guitar, then someone goes and builds a top quality guitar with the "F" holes replaced with a flower pattern somewhere else on the guitar...I guess it just goes to show that there's nothing wrong with a bit of diversity.

Regarding the top I've often thought, keeping in mind that the recurve area/nodal point is pretty important to getting the top to vibrate, how about cutting slot sound holes around the perimeter of the guitar in the trough of the recurve, obviously leaving wood between the slots, that way the top would be free to vibrate more and coupled with that have the back concave such that the sound is reflected towards the outside edge of the guitar (sound hole slots)......Maybe that's my next project but it will be a lot of work for a guitar that will probably sound awful!!!

Thanks again, Chris
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Re: Round hole archtop bridge bracing?

Postby Beate Ritzert » Sat Jul 08, 2017 7:17 am

If You give the guitar a centered hole You introduce a weakness that is hard to come by. The braces will have to take over the whole load. And there are only two of them. As an example of the kind of trouble one might expect You might remember this thread: viewtopic.php?f=8&t=1652&start=20#p33131

I would give it it an elongated narrow hole between two strong braces or place the hole in the upper left part of the body.
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Re: Round hole archtop bridge bracing?

Postby Alan Carruth » Sat Jul 08, 2017 5:57 pm

When I've made round hole arch tops I've used a relatively thin cross grain patch above the sound hole to help spread the stress. Although it has been a while, I think it was a fitted patch that ended up about 3-4mm thick and was 40-50mm wide. I've only made them with X-bracing so far. I picked that one up from a round hole D'Angelico mando, which, I think, was 'parallel' braced.
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