The inner portion of the acoustic guitar's top and back is reinforced to protect them against the strings' pressure. The reinforcement is done through an arranged system of wooden struts called the "braces." It is common to use Spruce (Sitka Spruce or Adirondack Spruce) for top bracings and Mahogany for the braces at the back. But, Other woods may also be used.
A bracing strut is normally built as a long and thin rectangle that may be beveled at the top to form a crest and tapered at the ends.
Top Bracing or Soundboard Bracing
Top bracing provides stiffness to the top and prevents it from damage against the string tension. These braces transmit the string forces from the bridge to the rim of the instrument. The bracings are designed by the luthiers to allow the top to provide a full response to the tones generated by the strings while allowing minimal distortion to it.
Bracing design and patterns significantly impact the type of sound an acoustic guitar produces. However, the relationship is complex and acoustic engineers don't fully understand the acoustic function of the braces, as yet.
There is a clear disagreement among the luthiers on whether the braces lead the sound from the bridge to the soundboard or not, with opinions divided almost equally. Studies using holography, oscilloscopes, and graphite dust scattering have shown the vibration of the guitar top to be very complex. Some of these non-understood facts about the vibration pattern of the top of the guitar include:
Types Of Traditional Bracings
Three major types of traditional bracings for acoustic guitars are Transverse bracing, Fan bracing and the X bracing. In addition, there have been other experimental, transitional, and proprietary bracing patterns from many manufacturers. Let us discuss them in some level of detail below.
Transverse Or Ladder Bracing
Transverse bracing, also known as the "Straight Bracing" or the "Ladder Bracing" was used in the earlier years of acoustic guitars. The braces were placed straight across or at a slight offset and were perpendicular to the grain of the soundboard. Guitars by the popular French luthier Rene Lacote carried ladder bracing.
The contribution of ladder bracing to the strength or acoustic performance of the instrument was minimal. Transverse bracing was in use till the 1960's when Japanese X braced instruments wiped them out.
Fan bracings are suitable for the 70-pound tension of nylon strings in classical guitars. But, they can't sustain the 150-pound string tension of the steel string guitars.
In the early 19th century, manufacturers developed patterns similar to the modern fan bracing, but with three or five struts. Antonio Torres in the middle of the 19th century started the system of seven main fans supported by the secondary braces. This system is in use even today and forms the basis of nylon string guitar bracing design.
The brace orientation is in the direction of the grains and not perpendicular. A larger body, thinner soundboard, and fan bracings contribute significantly to the warm, earthier sound of the classical guitars with a very strong bass response.
X Bracing or its variations forms the traditional bracing pattern for steel string instruments and has two braces placed crossing each other and form an "X" shape just below the sound hole. The arrangements may have several secondary, transverse, and flat struts in addition.
The two lower arms of the X brace formation support the two ends of the bridge. The hardwood bridge plate, placed under the bridge, prevents the damage from the ball end of the steel strings to the underside of the guitar's soundboard.
The tone bars are placed below the bridge plate and support the bottom of the soundboard. These tone bars butt against one of the X braces and slant down towards the edge of the lower edge of the top. The top bar abuts the bridge plate in most instruments. A large transverse bar runs along the entire width of the upper bout of the acoustic guitar above the sound hole. Small finger braces known as fans support the area between the X brace and the edge of the top of the guitar on either side.
X bracing system was developed by C. F Martin in the middle of the 19th century. By the end of the century, all the Martin acoustic guitars exclusively used X brace in their guitar design, even before the advent of steel strings.
Scalloped X Bracing
Early Martin guitars had their braces chiseled at the center into parabolic curves, known as scalloped bracings. The reduced weight of the brace made them more flexible, allowing the top to vibrate more freely. This type of bracing with a higher crossing point location of the X bracing gave early Martin instruments a distinctive sound.
However, the strength of the bracing reduced, leading to arching or bellying of the tops at the bridge upwards. The effect became more pronounced with the advent of the steel string guitars. Martin discontinued shaving the braces and moved the crossing point of the two braces away from the soundhole and nearer to the bridge.
In the 1970s, many players began to custom shave the braces for that distinctive sound. Martin soon re-introduced scalloped X bracing on some of their select models in the mid-1970s. Original high brace position also returned on some select acoustic guitar models.
There are three different placements of the X-bracing on the Martin Guitars - The Standard placement, forward-shifted and rear-shifted. In the forward-shifted arrangement, the crossing point in the X is shifted about half an inch towards the soundhole to improve the bass response. In the rear-shifted case, the crossing is moved by one-third ich away from the soundhole to have improved projection.
In present times, many manufacturers offer these features. However, these enhancements make a real difference only on a finely made instrument. On a mass-produced or pre-shaved instrument, it is not very useful and may only be a marketing gimmick. Also, if the top is not good, focussing on the bracing style is meaningless.
It has been established from various studies that long, thin braced patterns impart the greatest structural strength while allowing the top of the guitar to vibrate more freely.
Voicing is a sensitive hand-shaving process to perfectly tune the braces to the soundboard. The process involves listening to the sound of the wood on tapping and then gently shaving the brace with a sharp chisel to improve its tuning. This is done to maximize the responsiveness of the top while maintaining its structural integrity.
Other Bracing Patterns
Apart from the above traditional patterns, many other bracing patterns are in use. Let us look at some of them below.
Double X Bracing
Two X shapes are placed in an overlapping arrangement to form a diamond surrounding the bridge plate. This bracing pattern provides additional strength like in twelve-string guitars but restricts vibration of the top.
There is more than one design of guitar bracing pattern designated as the A bracing.
V Class Bracing
V class bracing style was developed by Andy Powers from Taylor guitars. It results in louder and longer sustaining notes in the acoustic guitar. Andy Powers had earlier successfully developed the Advanced Performance Bracing used in the 800 Series of Taylors and wanted to overcome the limitations of the traditional guitar bracing systems with a completely new bracing design.
X Brace system resulted in a trade-off between volume and sustain as these elements depended on two counteracting properties, the flexibility, and stiffness of the top of the acoustic guitar, respectively.
V class bracing allows the guitar tops to be stiff in the direction of the strings to produce notes that usually last longer. Additionally, guitars with V class braces are more flexible on both sides of the top to produce an orderly motion and much higher volume. As per Taylor guitars, the design results in better harmonic intonation between the top and the vibrating strings.
Two long braces diverge from the tail block on the guitar to either side of the sound hole towards the neck side. These main braces take the shape of a "V".
A lateral brace spanning the width of the guitar top is placed between the bridge plate and the soundhole. A set of fan/tone braces are positioned roughly perpendicular to each of the longitudinal V brace symmetrically. The arrangement provides the required strength and support to the center of the guitar and its bridge area while allowing the sides to vibrate freely.
Guitars with lattice braces in their top first appeared in the late 1970s and are still popular among many players. The rigidity of the pattern allows the use of much thinner wood in the soundboard and still withstand the tension of the strings, allowing the luthiers to reduce the total weight of the guitar.
Proprietary C Class Bracing was developed by Taylor for their smaller GT body shapes. It uses some of the concepts of V Class braces to enhance the sustain and volume at the same time. These have an asymmetric cantilevered arrangement that emphasizes the lower frequencies to produce a powerful bass response even from a much smaller shape guitar.
Back Bracing And Sides Reinforcement
Guitar backs are braced to provide structural strength and to keep the body stiff to make it act as a resonating chamber. The main decision while designing them is whether the back will take an active role in the frequency response characteristics of the guitar.
Normally, these braces include a combination of high & thin studs and low & flat ones. Other designs including X Bracing have been tried in an effort to turn the backs into a second soundboard.
Sides are made from bent woods and can be subjected to knocks, making them very vulnerable to lateral cracks. Hence, they are given additional strength by either reinforcing them with a cloth soaked in glue or by pieces of wood like mahogany, cedar, or spruce. These reinforcements are placed at the strategic locations on the guitar sides where the stresses are expected to be maximum to localize grain cracks and prevent the sides from splitting along the grain.