Acoustic Guitar Bracing Science
Over the course of the years different bracing designs and modifications have been used. Braces add most of the stiffness to the top. This is necessary to prevent the top from destroying itself from the tension of the strings. A traditional bracing pattern is the x-brace. Found in many kits, it proves a good learning experience. There are difficulties present, such as shaping each brace to the curvature of the body. It is a tedious and repetitive task. Cutting them out to the right shape and height for strength and voicing is an imprecise process. Following the principles of mechanical engineering a better way was sought to make braces. As more guitars are built more improvements are made.
The current guitar bracing pattern used is called a falcate bracing pattern. This is our acoustic guitar soundboard bracing
This is vastly different than a traditional x-bracing
There are myriad reasons to use this bracing system. It is a design by two Australian luthiers Trevor Gore and Gerard Gilet described in their book Contemporary Acoustic Guitar Design and Build. The key is to use a curved bracing system instead of straight pieces. The curved pieces are bent into shape then placed onto the top and back. There are six braces use for the back or top, three sets of two. They are placed symmetrically on the back in pairs. The direction they are lying is such that the longest part of them is in line with the tension the strings are providing. Running from the end to the bridge up the sound hole. This provides the maximum amount of support over the axis that the tension is applied while keeping the amount of strength in the minor axis minimal. It is very flexible along that axis, this makes a more responsive top as it can vibrate more freely. There is less impedance to some of the modal resonances and provides a more clear and articulate tone.
Another innovation is the use of graphite to make a composite brace. The braces are made from three pieces. Two pieces of mahogany and a strip of carbon fiber. Sitka Spruce or pine can also be used, any material that will undergo bending for the curves and is light but stiff and quarter-sawn will work. There is a lot of variability from wood to wood and piece to piece.
The pieces are epoxied together and then sliced laterally. The grain of the wood is facing straight up, and the carbon fiber is along the same axis, this provides a significant amount of strength. These are hard to break. Even flexing them, not along the strong plane, is hard to do. They are significantly stronger now the pieces are alone. In addition, the surface area of the brace is minimal, width of 0.3” or less. Now, there is a strong brace that is providing the most amount of support under the bridge and sound hole.
A major problem is bellying or deformation of the top where the area under the bridge swells and the area in front collapses. This effect occurs due to the tension over long times and the humidity changes from aging. The falcate bracing system provides enough support to minimize this effect. The carbon fiber does not change with the humidity. It is rigid this forces the top to stay in place as it goes through the seasons. This is one advantage of the falcate bracing style.
Here are the pieces that are used to create a brace
The braces as they are placed onto the back or top, smaller in the final form
The next benefit of the acoustic guitar bracing patterns is a building advantage. Each of the curved braces is already part of a curved shape. Since the top and back are bowled surfaces, the braces can be placed directly onto top or back and glued into place. There is no sanding of the bottom of the braces to conform to the curvature of the top. The braces are already conic sections that lie within the bowl. Once they are being glued on in the go-box the top or back is forced to take the round shape of the mold and hold it in shape. This reduces the amount of time spent on repeating sanding by a lot and frees up time to spend on detailing.
Despite the bracing style used the goal of a top is to provide two degrees of deflection. The deflection is the amount that the strings bend the bridge and top of the guitar. If there is more than the top will be unstable and tear itself apart over time. Less deflection will provide less amplitude and response from the top.
There is a misconception in the luthier world that tap-tuning the top will account in better voicing and sound. Sanding away parts of the braces in special spots doesn’t have a large effect on the tone. The thickness of the top has more of an effect then small places here and there in the braces. The top holds most of the mass compared to the braces while the braces provide more stiffness.
Our process is to simulate the tension from the strings and then sand the braces to optimize the deflection. The top is put in a jig with the sides supported to simulate a guitar body and top. The top has a lever arm with a weight hanging off it. This is how the torque is applied. By using what is known about the mass, length of the lever arm and the angle of deflection wanted, the amount of bend to the lever can be calculated. If the stiffness of the braces is overdone, causing a less than 2-degree bend. These would need sanding. the We know that a lever arm of around 28” undergoing 1” inches of bend with a 5-pound weight is 2 degrees of deflection. This equation is shown in the diagram.
The precise measurements that we use get us in the right ballpark but are not exact. If it’s slightly overbuilt the top will be sound of structure.
In this way we can simulate the 160-200lbs of tension on the top. If the deflection is less than one inch than we know there is too much strength in the braces. The braces are sanded to bring the tension down. This ensures a minimally braced top and thus one that has the best tone. The act of sanding down the strength of the braces allows for greater flexibility and more vibration response. This improves the tone and responsiveness of the guitar. The torsion arm technique to measure deflection is described in Contemporary Acoustic Guitar Design and Build Volume 1: Design on page 4-33 by Trevor Gore and Gerard Gilet. As well they state that 10Nm is the torque applied from the strings.
This is the lever arm attached to the top which simulates the tension. When the weight drops one inch, we know that the right amount of force is being applied to counteract the tension.
Here’s what it looks like to sand the braces
This build was for an acoustic bass guitar, so the braces are rather thick
Here is the guitar back bracing. The back is not load bearing. It does not have the string tension and requires much less support to function. A stiff back will change the frequency response of its resonant point. The rule of thumb is that a lighter back will produce a more “live” tone.
An X-bracing Guitar and a journey through past methods
One of the main trade-offs in making a bracing system are strength and flexibility. The more of one that is put in the less of the other is left. A thick wide stick in the middle of the guitar makes it more stiff in that direction. It cuts out the flexibility of the wood top to move along the major and minor axis of the top resonances. Those being down the center and across the center. A great sounding, responsive guitar is built with the minimal amount of bracing as necessary, prioritizing flexibility.
This is some guitar bracing theory. To gain greater flexibility Jay changed the design of his braces. A tall thin piece of wood was used for the brace. Its surface area was minimized, and strength was increased. More room for the top to vibrate. As well, holes were drilled through the side of the braces. This decreased the weight of the braces while keeping their internal strength. The braces were shaped near the ends into a pleasing taper as the middle section is the most important to brace.
X-bracing has many problems that make it less than an optimal bracing pattern. The bars crossover the top from side to side. The tone bar is restrictive
Let’s think about the forces on the guitar. We have the strings at 160-200lbs of tension All of this tension comes from the top and the tuners down the fretboard and into the saddle and bridge. Now all that tension is placed squarely, or curvedly, on the bridge. This is what causing belling in older guitars. The braces didn’t hold the bridge down and it raised up. This is a common sight to see on guitars that are factory made, they don’t hold up over time. That’s the consequence of using a poor and insufficient bracing system. The guitar destroys itself bit by bit the longer and harder it’s played.
The X-bracing pattern does almost nothing to prevent this problem. The main bars over the center of the top do provide structural support, but it’s not optimized. It’s like supporting a coin over a cup of water with two toothpicks. If they are crossed, they do support the coin and it will rest undisturbed. If we start to press down on the coin one of the edges. It’ll be strong where those beam bars are and much weaker in the spots between. The coin will tip over when it’s pushed in on the edge. The guitar works in much the same way the pushing down is the tension over the bridge and the toothpicks are the braces supporting the coin. The bridge is pulled off by the tension of the strings. The top isn’t supported underneath the bridge directly so it’s susceptible to being pulled up by the string tension. The x does support the top but it’s a cross support over that straight force.
One part that standard x bracing does well is the sound hole braces and in the guitar top bracing. Sound hole braces are a good element and do provide optimal support in the correct direction. They are placed in the direction the strings are going reaching out from the main braces to the front of the top. Since they are in the same direction as the strings, they are set up to vibrate on the same axis and support that force. An interesting note here is that if the force were applied perpendicular to the strings, these braces would not provide almost any support. In the X-bracing system when the force is applied on the minor axis these braces still provide support! Part of those braces are going the same direction, so they still function as braces. This is not a good feature. All the bending and vibration that the top does along that axis in its resonance mode is denied. This impedes the sound and makes a less responsive top. The tone bars as well serve to dampen the top as they go across the body.
Sanding the x-braces into shape is inconvenient. They must be run back and forth over the bowl mold. The shape is never exact but does get close. It is tedious to do this with all the braces. That limitation and extra time taken is time that could be spent detailing and working on other aspects of the guitar.
These are the reasons that Portland Guitar uses a falcate bracing system as our acoustic guitar bracing. Thank you for reading. Please reach out with any comments or questions.
Here is a video of the guitar in use:
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