Brace yourself, for a little scallopini
Acoustic guitars, the kind that most people know, are commonly called “flattops”, to distinguish them from the archtops found in the jazz world. They are not really flat; well, not completely. In order to appear flat, the tops (and bottoms) are actually arced slightly. This helps strengthen the box construction; and, if they were built completely flat, they would look slightly concave. So, all of the bracing has a slight arc built in to them; no more than 1/8″ deviation from flat.
With all of the braces prepared, arced, and glued, its time for the next fun step: carving them. The bracing adds a lot of stiffness to the top, giving it a bright, clear ringing sound when tapped. But, it makes it too stiff, there would not be enough resonance; the sounds of the strings would not sustain well. So, after adding all that stiffness, we need to carve away some of it, to achieve more vibration. Its a big balancing act. You have to carve enough away for a good sound, but leave enough structure for strength…and a good sound. Here are the braces before (mostly) carving.
The transverse brace at the top get the first shaping, thin at the ends to fit under the side kerfing, but thick in the middle to support the neck. Here’s where a good paring chisel gets a workout. That’s part of the fun of guitarmaking; you get a lot of hand tool practice. A delicate touch is critical here.
A little sanding, and ready for the next one. The hole, by the way, is for the allen wrench used to adjust the truss rod.
The lower brace, and the two small side braces, are all about the sound. They don’t really do anything for the structure, they are there to help transmit the string’s vibrations throughout the top. As such, they don’t need to be too massive, and receive a more interesting scalloped treatment. Thinned at the ends, they come to a peak – thinned horizontally – with another, central scalloping for the longer, lower brace, similar to a suspension bridge.
The final braces are the most important. These are the X-braces that Martin developed, that really enabled the development of the steel string acoustic guitar. Previous bracing layouts were meant for gut strings, and would be too weak too counteract the tensions that steel strings put on the guitar. With the X-bracing, the force is spread out to the entire top. But, again, we don’t want to make it so stiff, that the sound is deadened. So we need to scallop the ends, while keeping the central X, near the bridge plate, solid.
The final touch to the bracing will be a linen patch glued over the lapped X joint. It doesn’t add a lot of strength, but it is very traditional. I may fiddle a bit more with a brace or two, tapping all the time, trying to find that elusive “perfect” balance.
Next up, will be the bending of the sides. Strike up the barbie!
Acornhouseworkshop 
August 6, 2009 at 1:27 am
I’ve always been curious about what the trusses inside an acoustic looked like.
One question – why is there a solitary support at the bottom that is not symmetrical on the long axis of the guitar? Are you going to add electronics and need it to support weight?
August 6, 2009 at 2:14 am
Hi Steve,
That asymmetric lower brace supports the soundboard between the bridge and the bottom. It is a normal part of the X-bracing system, in fact, on full size guitars, there would be two, running in parallel, but still asymmetric. I believe it goes back to lute construction, where it is know as the bass bar; so I suppose it provides additional acoustic support for the low register. (I do see a lute in my future!) I am thinking about adding a pickup, but it doesn’t need any additional bracing.