Acoustic Guitar Bracing

[Portland Guitar]

Hi There,

I wrote this for my blog and thought people here might be interested. There's too much to post here so you can check out more here: guitar bracing

There have been many bracing designs used over the course of Jay’s building career. When he started out with a kit, the bracing used was an x-bracing pattern A good learning experience. There were difficulties present. Shaping each brace to the curvature of the body was a tedious and repetitive task. Cutting them out to the right shape and height for strength and voicing was an imprecise process. A mechanical engineer, Jay searched for a better way. As more guitars were built there were improvements in design.
The current guitar bracing patterns that we use 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 inspired by two Australian luthiers. The key is to use a curved bracing system instead of straight pieces. The curved pieces are bent into shape then placed onto top or back. There are six braces in the guitar 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 read to the bridge and up the sound hole. This provides the maximum amount of support over the axis that the tension is applied while keeping the amount of tension in the minor axis minimal. It is very flexible along that axis this makes it 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 in making the braces. The braces are made from three pieces. Two pieces of mahogany and a strip of carbon fiber. They 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 an acoustic guitar bracing dimension of 0.3” that’s making contact. Now, we have a strong brace that is providing the most amount of support under the bridge and sound hole.
The chances of this guitar undergoing bellying, where the area beneath the bridge swells. This effect occurs due to the tension over long times pulling it up. With the falcate bracing system there’s enough support to minimize this effect. This is one of the advantages of a Portland Guitar
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 circular piece. Since the top and back want to be bowled surfaces at the end, 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.
The last innovation with the braces is the process of tuning. 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 shaping of the braces tapered down to a point is a good practice. Our process is to simulate the tension from the strings and then sand the braces. 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. We know that at one meter or so and at 2 inches of deflection with a 10-pound weight we can simulate the 200lbs of tension on the top. If the deflection is less than two inches than we know there is too much strength in the braces. The braces are sanded down at their modal points 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 vibrational response. This improves the tone and responsiveness of the guitar. We built this with a jumbo body and the feeling on the body was incredible.
Here is the top in our contraption:

This is the lever arm attached to the top which simulates the tension. When the weight drops two inches, 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

In a comparison between our guitar and a Collings guitar we can see differences due to the acoustic guitar bracing. We can see a quantitative difference in these two-frequency response graph the Collings guitar does respond on some notes, but our guitar is responsive consistently on every note to a great amount. The dip that is seen is how much the body resonance interferes with the note that we hear. This is where the guitar becomes special in how it sounds. The amount of variance indicates that the guitar is moving quite a bit and that the air cavity is coupled very well to the top and thus responsiveness.

Collings Guitar

Portland Guitar

There are many different variables in this experiment, bracing being one of them. First the woods were different, which could have an effect. The Collings had rosewood with a spruce top, while the Portland Guitar had ebony with a redwood top Second, the Collings was a dreadnought while the Portland Guitar was an OM. Despite these differences there must be an explanation for the spike that we see across all these strings. Some of the responsiveness is inherent in the top that is used. The rest I’m willing to contribute to the bracing pattern. This phenomenon contributes much to how we interact and feel the guitar.

[Ten]

Very interesting, nice guitars on your website. Any demos of these vs. comparable x-brace guitar?

[mc1]

We can see a quantitative difference in these two-frequency response graph the Collings guitar does respond on some notes, but our guitar is responsive consistently on every note to a great amount.

...the Collings guitar does respond on some notes...

I'm not following, can you elaborate?

[varve]

Glad to see The Gore/Gilet Falcate bracing used - kudos to Portland Guitars! I believe that the only other North American luthier using this bracing system is GR Bear of Vancouver, BC, who makes superbly responsive instruments.

Trevor Gore holds lutherie workshops in this hemisphere every year, regrettably not this summer due to our current situation. Once the world begins to turn on its axis once more, tho, I can heartily recommend these workshops, held at Robbie O’Brien’s shop in Parker, Colorado, and I believe in Toronto as well. Hopefully next summer these can resume.

Cheers, Dave Olson

[Scolaguitar]

We're in the same neighborhood! You came up in a search I was looking for set up work but you obviously are way beyond that. If it weren't for this pandemic, I'd be interested in checking out your guitars. Thanks for sharing

[Beakybird]

Very interesting blog. Thanks!