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Top and Back Radiius


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I'm not sure how many of you here build acoustics on a regular basis. I am not quite at the building acoustics continuosly stage yet(I do only electrics currently, although I've built one acoustic that I'm very pleased with) but I was wondering what radiius you all put in your tops and backs? Also do you do a different radiius in your braces to pre-load the top? When I start I'm planning on using 15ft for the back and 30 for the front with the X- bracing at 50ft.

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Some say that putting a flatter radius in the x-braces than in the top puts spring in the top so it has potential energy stored allowing it to snap into action quickly(the radius actually gets sanded into guitar sides/kerfing and the bottom of the braces). I'm not sure if that is entirely true but it makes a lot of sense to me. It's seems that it would make a guitar have more attack and less sustain. If that theory is true than the top would get going without much initial energy which would provide a quick, punchy attack, and slow down quicker due to the tension that's already there. I'm not a physics major by any means, I just know that the one I built sounds outstanding and that's how I built it. I was trying to see if anyone else does it differently or uses different radius measurements.

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I use 25' for the soundboard and 12' for the back. I sand the braces to the same radius as the top or back I am gluing them to.

The idea od stressing the braces and fitting them to the top by having to force them is a violin family technique (violins, cellos, double basses, etc.). These instruments are designed to be taken apart to have the braces refitted every 5 years or so because the braces will relax over time. A guitar is not made to be taken apart like that so it is not a good idea to try this idea unless you are prepared for a weakening of the instruments voice. This would not be good if you plan to sell them.

The idea of using a dome or arch is to provide the structure for a stronger soundboard that is less prone to warping. It has nothing to do with increasing response (over the life of the instrument anyway). As soon as you add the string tension you will achieve the stresses necessary to drive the top and make the guitar responsive. Why not focus on tuning the sounboard through brace placement and carving so that your guitar will start out responsive and stay that way?

Just my $0.02.

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  • 3 weeks later...

The arched surface will resist the string tension without warping better than a flat surface. The arched surface with the arched brace imposes a situation that forces the brace to stretch in length if the arch is falttened. Since this does not readily happen without the splitting of wood the arch is maintained.

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  • 2 weeks later...

An arched surface is also stiffer geometrically, so the resonant frequency (tap tone) will be higher, thus giving you more posibilities for tuning.

Follow the advice of Myka:

Why not focus on tuning the sounboard through brace placement and carving so that your guitar will start out responsive and stay that way?

Build the the soundboard to a relaxed state and tune it - else the tuning will change while the initial tension disappears.

(Wood is actually as close as you can get to polymers in the nature, the main difference to wood and polymers is the grain and thus directional character of woods. Mechanically they behave very similarly, and the tendency to relaxation is well known from polymers. Normal non-reinforced polymers have lost all initial tension in about a year or two in room temperature)

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Thanks guys. That's interesting. I'll definatly take what you said into consideration as I'm developing my basic design.

I'm just making sure I get what your saying. If I build a guitar with the braces at a flatter radius than the top, in a few years the guitar will actually be less responsive than if I would have just used the same radius on the braces as the radius on the top? If that is what your saying than I'd like to sit down with the people who taught me how to build and find out where they're coming from. Thanks again.

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