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Old 03-23-2013, 10:38 PM
gitnoob gitnoob is offline
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Location: Emerald City
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I'm looking forward to the responses, but allow me to make a noob attempt at it.

Let's start from the top and work our way down:



The brace labeled "top plate" is also known as the "popsicle brace." It's literally a band-aid. I think Martin added it after they noticed cracks forming in that area under the fingerboard extension. The fingerboard wood expands/contracts at a different rate than the spruce, so that band-aid tries to hold the spruce together cross-grain-wise.

The brace labeled "number 1" is also known as the upper transverse brace, and it's probably the most important structural brace. It takes the load from the neck as it tries to rotate into the sound hole from string tension.

The "soundhole strips" are poorly designed IMO. Martin probably added them to support the exposed end-grain from the sound hole, but that hole in the top also makes it very weak in that area. Taller braces would be much better structurally.

The X does provide structural support, but it also enables several vibrational modes of the top and gives us the tone we all love. You'll notice that the legs of the X cross the wings of the bridge -- the strings set the bridge in motion, and the bridge sets the X in motion.

The side tone bars are sort of optional IMO. They don't provide much structural support, but they add stiffness to an otherwise large open area. Some designs only use a single tone bar on each side, but basically they help with higher-frequency response by adding cross-grain stiffness.

The bridge plate primarily keeps the spruce from getting chewed up by string ball-ends, but it can also help resist bridge rotation.

And finally, the mysterious bass bars or lower tone bars or lower face braces....

You'll notice that the slant gives you a more open area on the bass side. Somebody thought that this would enhance the bass response from the bass strings, but that's not how it really works. So really the slant is a historical curiosity, and plenty of builders do something different.

They also add some resistance to bridge rotation in the lower bout as well as cross-grain stiffness (which actually reduces bass response).

Computer models have shown that the mass effect of those bass bars is pretty significant, so they probably reduce responsiveness more than anything else.
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