Guitar bracing refers to the system of wooden struts which internally support and reinforce the soundboard and back of acoustic guitars.
Soundboard or top bracing transmits the forces exerted by the strings from the bridge to the rim. The luthier faces the challenge of bracing the instrument to withstand the stress applied by the strings with minimal distortion, while permitting the top to respond as fully as possible to the tones generated by the strings. Brace design contributes significantly to the type of sound a guitar will produce. According to luthiers W. Cumpiano and J. Natelson, "By varying brace design, each builder has sought to produce a sound that conformed to his concept of the ideal."
The back of the instrument is braced to help distribute the force exerted by the neck on the body, and to maintain the tonal responsiveness and structural integrity of the sound box.
Materials
Braces may be made from top woods (spruce or cedar), balsa wood or, in high-end instruments, carbon fiber composites.
Nylon string guitar bracing
Fan bracing
This is the standard bracing pattern on the classical guitar, dating to the work of Antonio Torres Jurado in the 19th century. Although the originator of this bracing style has not been reliably established, the earliest known use is by Spanish luthier Francisco Sanguino in the mid to late 18th century.
Kasha Bracing
In the 1970s, scientist Michael Kasha radically overhauled every aspect of guitar design to incorporate principles such as mechanical impedance matching.
Lattice bracing
The Australian guitarmaker Greg Smallman introduced guitars with an extremely thin soundboard, which is supported by bracing in the shape of a lattice. Smallman combines this with heavier, laminated back and sides with a frame. Smallman's guitars are used by John Williams.
Smallman's design was inspired by research by Torres who made a guitar with a papier mâché back and sides to show that the soundboard was the most important factor in guitar sound projection. Smallman also uses two 45 degree pole supports in the frame running from the bottom of the guitar to the waist to prevent the string tension from distorting the body and sound board.
Steel string flat-top guitar bracing
In all steel-string instruments, the ends of the top braces taper at the edge of the soundboard. In most factory built guitars the brace tops are given a round profile, but are otherwise left unshaped. This produces a stronger top and may reduce the number of warranty claims arising from damage, however, over-built tops are less responsive. Braces are usually made from Sitka Spruce (Picea sitchensis). Some luthiers use Adirondack Spruce, also known as "Red Spruce" (Picea rubens), in high end instruments.
X-Bracing
The tops of most steel string acoustic guitars are braced using the X-brace system, or a variation of the X-brace system, generally attributed to Christian Frederick Martin between 1840 and 1845 for use in gut string guitars. The system consists of two braces forming an "X" shape across the soundboard below the top of the sound hole. The lower arms of the "X" straddle and support the ends of the bridge. Under the bridge is a hardwood bridge plate which prevents the ball end of the strings from damaging the underside of the soundboard. Below the bridge patch are one or more tone bars which support the bottom of the soundboard. These abut one of the X braces and usually slant down towards the bottom edge of the soundboard. The top tone bar butts against a portion of the bridge patch in most instruments. Above the sound hole a large transverse brace spans the width of the upper bout of the soundboard. Around the lower bout, small finger braces support the area between the X-braces and the edge of the soundboard.
Double X-bracing
In this system, two overlapping X shapes form a diamond surrounding the underside of the bridge plate. Some luthiers prefer it where additional strength is required, for instance for twelve string guitars. This bracing does not allow the top to move or vibrate as much as it normally would but offers more strength and prevent bellying around the bridge area.