It's ironic that one of the most important elements of an acoustic guitar is a part that's rarely seen by the player -- the bracing. Here's an example:
In an earlier post, I wrote about the plates and how the material is carefully selected and prepared to be light, stiff and responsive. It seems a bit counterintuitive that we would then glue on other bits of wood to the inside surface.
Fact is, the plates, and particularly the top plate, are under enormous stress. The steel strings are under high tension and attached to the bridge, located in the middle of the top plate. Were it not for some carefully located structural elements, the top plate would fail. One of the primary goals of bracing is to supply structural strength to support the bridge area, but in such a way that the bridge can transmit the string vibrations into the plate. It's a balancing act.
There are a large number of bracing patterns used by guitar makers, but for steel string guitars many of them incormporate what's often called the "X brace". As you might expect, the brace elements are crossed in a lap joint and resemble an "X". The lower arms of the X cross under the bridge to help give structural support while retaining the ability for the bridge to rotate front to back, and thus "drive" the top. Also under the bridge, you'll find a patch of thin hardwood, which provides structural support to bridge area, while also providing a solid surface where the balls on the ends of the strings can have direct contact with the top, without causing damage.
Above the soundhole, one will usually find a beefy transverse brace which stiffens up the area where the finger board is usually attached, preventing distortion of the upper bout when under tension. Around the soundhole itself, you'll also find small braces or an extra layer of top material in the form of "donut" which provides to stiffness to the perimeter of the hole and some protection against cracking of the end-grain.
Other smaller, light braces are also usually located to provide additional stiffness to targeted areas of otherwise be unsupported. These provide some protection against cracking along the grain of the plate, and their stiffness and location can be adjusted to affect the tone and balance of the sound.
Generally speaking, we want the braces to be stiff, but without adding excessive mass to the vibrating top plate. When preparing the braces, the maker will usually orient the grain 90° to the surface of the plate, and will make the brace taller than it is wide, all to maximize stiffness of the brace, while minimizing mass. The braces can be further "tuned" by removing material from areas and decreasing the height of the brace where strength is less important. This is known as "scalloping".
All of the brace elements work together with the materials and design of guitar to give the instrument its individual sound. What might seem a random pattern of sticks glued to the underside of the top has an enormous impact on the sound. You can't see the guitar maker's skill in this department, but you sure can hear it in the finished product.
In the photo sequence below, I'll take you through the bracing of an OM-style instrument.
I've glued a circular patch over the area under the soundhole, just a hair larger than the rosette, and I'm thicknessing the patch with a drum sander.
Here the sound hole "donut" and the soundhole itself have been cut out. The other bracing elements are dressed to rough dimension, and then glued on to the underside of the top plate. Here you see a clever contraption known as a "go-bar" deck, where dowels are used to clamp the elements as they're glued.
Once all the elements are glued in place, comes one of my favourite steps in building a guitar -- shaping and scalloping the braces. My weapon of choice, a 1-inch chisel. By carefully shaping and whittling, while periodically tapping the top to monitor changes in the top's responsiveness, the braces take shape slowly.