Guitar bracing performs two wildly different functions: strengthen the top of the guitar while allowing it to sufficiently vibrate to produce a warm and resonant tone. In a standard scale guitar with medium guage strings, the guitar’s top withstands approximately 185 lbs of constant tension. A thin top without bracing would buckle or warp in very little time. A top thick enough to withstand the pressure could not sufficiently vibrate and would result in a thin tone with little volume. Bracing a thin top then finds the best of both worlds.
Bracing plays a major role in determining the tone of a guitar as well. Although there are many theories about tone production as related to bracing, there is little disagreement on its importance. A luthier makes purposeful decisions about the placement, pattern, and physical shape of bracing because, in many ways, these decisions affect the final tone of the instrument as much as the actual tone wood. For instance, Taylor Guitars’ distinctive voice is heavily influenced by their bracing patterns. Likewise, vintage Martin guitars are prized for the shape of the braces and how they affect tone.
A final word on bracing in general:
Too much bracing = a dead, muffled tone
Too little bracing = poor structural integrity as well as an airy tone with no definition
X bracing, forward shifted, scalloped… all on a Colling Guitar in production in 2003.
The bracing pattern found in most steel-string dreadnoughts is the “X” pattern. Originally developed by C.F. Martin in the 1850’s, this pattern features the two main braces running in an “X” from the upper bouts to the lower bouts. The “X” crosses somewhere between the sound hole and the bridge. There are several auxiliary braces other than the main X-braces. This pattern provides the strength and well-balanced tonal palette that most builders find attractive.
Pre-war era Martins have a bracing pattern that many enthusiasts believe to be the best. In truth, Martin did two things differently. First, braces were scalloped in that era. That is, wood was selectively removed from certain areas of the braces to weaken the top enough to allow it to vibrate freely without weakening it so much as to make it structurally unsound. Scalloping opens up the mids and increases volume. Second, Martin used a forward shift of their X bracing. On most X-braced steel string guitars, the “X” crosses approximately 2″ below the soundhole. On pre-war Martins, the “X” crosses about 1″ below the soundhole. The result is that the bridge rests less directly atop the main X-braces and transfers more of its vibration to the top.
In the 1940s, Martin moved the X bracing away from the soundhole to its current position and quit scalloping braces. According to Martin, they did this due to the preponderance of players using heavy -gauge strings to boost their volume. The light bracing pattern coupled with heavy strings resulted in a high damage rate. Because pre-war Martins are becoming so rare and expensive, Martins along with many other builders are now producing guitars with scalloped and forward-shifted bracing. All builders recommend using light-guage strings on these instruments though.
Being closely related to the lute, construction of the modern classical guitar grew out of that tradition. Classical guitars tend to have thinner tops and much lighter bracing than steel-string guitars, since nylon strings generate less tension across the soundboard.
The most commonly used bracing pattern is fan bracing. In this pattern, ribs “fan” out from the soundhole toward the back of the instrument. Actual placement of these ribs is key to the tone of the instrument. Luthiers have experimented with variations on this pattern with varying degrees of success.
Braces in traditional build guitars are made of the same material used for the guitar’s top. That is, a spruce top guitar would use spruce braces. However, innovations in this area continue as well. For instance, Martin is now using a hybrid bracing system that combines an A-frame with an X-pattern for their 16 series guitars. They are also using wood composites for the X-bracing in the DX series. As a matter of fact, the DX series features only an X-shape as the bracing with no additional braces anywhere on the top. This is due the inordinate strength of the wood composite.
Other builders are going a step further. Garrison, Rainsong, and CA guitars all use a carbon composite for their bracing. The carbon is lighter than wood and tends to allow the top to vibrate more freely as a result. Garrison, by the way, builds a wood guitar using the carbon braces. Rainsong and CA build completely carbon composite guitars.