Official Luthiers Forum!

Hey guys. Im building a j-185 right now, but Im extending the scale length to 25.5 and Im making it a 12 fret join to bring the bridge farther into the lower bout. This means I need to change the brace locations/positions. In order to have the X-brace properly positioned under the bridge, I either need to shift the bracing backwards about an inch and a half, or I need to change the angle of the x-brace. What do you guys recommend? -Alex
?

Move the x's...

Possibly shift your sound hole as well...

What's the angle on the X? If it's more than 90°, I'd consider reducing it a little & not shifting as much.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

When I made a 12 fret J200, I added about 1/2" to the upper bout, and slid the X back the remainder of the distance. Worked for me, and I think the shape looks well balanced, like the earlier 12 fret version of that 'Jumbo' body shape.

I actually started drawing p some new plans with the x-brace slide back, but the modification left the soundhole too far away from the brace. Would I be safe by sliding the soundhole back with the x-brace? If that wont have any negative structural/tonal properties then I think im good to go!

If you move the sound hole too far back, the instrument begins to sound more like a trumpet than a guitar.

Just kidding. Shouldn't make a bit of difference.

_________________
http://www.CostaGuitars.com
PMoMC

I believe moving the soundhole back will affect the main air resonance, but I'll let someone that knows how it will affect it confirm that.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

Roger, why does moving the sound hole effect the main air? The box volume stays the same and the hole diameter is the same.

_________________
Jim Watts
http://jameswattsguitars.com

I don't know, but I found this post by someone that does:

It occurred to me that William Allen might have said something about this in his article on air resonances in American Lutherie #1, so I dug it out. Nope. He does have a lot of interesting stuff, though, such as a discussion of how the sound hole location changes the Helmholtz resonance. Nothing's ever simple.... Al Carruth

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

This is a gross simplification:

It is because an enclosed body of air will exhibit not only Helmholtz resonance, but also pipe resonances. It's a little counter intuitive because a guitar body is not shaped like a pipe, but still they are there. Imagine waves bouncing around in a small enclosed pool. Anyway, as the exit (soundhole) gets closer to the end of the 'pipe', the output tends to increase in magnitude slightly. Also, the phase characteristics change. Practically speaking, moving the soundhole an inch or two one way or the other is not likely to give you 'more bass', but it may change the frequency response slightly due to phase differences between the spectral response coming from the sound hole, and the output from the top resonances. This might be audible if you have good ears.