If bracing is one of my favourite elements of building acoustic guitars, then shaping and profiling the braces is one of my favourite elements of the bracing work. This is the moment when you really start to craft the tone of the instrument by removing material from specific points of the braces, and shaping them to a final profile that is pleasing to both the eye and the ears. As I’ve mentioned before, shaping the braces is an act of juggling structural integrity and acoustic resonance, and ultimately it comes down to feel, using your ears, and understanding how string tension is applied to the soundboard. Numbers alone are of very little use here!
With the braces all now glued to the soundboard, I planed the transverse and X braces down to their nominal height (13mm and 14mm respectively). This gave me the first opportunity to assess the effect of the bracing on the tap tone of the soundboard, and as expected the bracing at this point was too heavy, meaning that the soundboard did not resonate as much as I would like. But no bother, as there was plenty of material still to be removed.
Broadly speaking, the area of the soundboard from the soundhole up contributes less to the sound of the guitar, but is critical for the structural integrity of the guitar. The braces in this area are therefore left heavier than the area from the bridge and below, as this lower area is the part of the soundboard that makes the largest contribution to the sound of the instrument.
My first step when tuning the soundboard braces is to shape the X braces. There are a number of options available here, including scalloping the lower legs of the X, or planing a gentle curve across the length of each X brace. I prefer to curve the X braces rather than scallop them, and my tool of choice for this is my trusty low angle block plane. The lower legs of the X brace had the most material removed, with the very end of each brace curving down to 3mm in height, while only a very gentle curve was planed into the top legs of each brace so as not to reduce the structural strength of the soundboard round the soundhole and upper bout. All the time while planning the curve into the X braces (and in fact removing any material from any brace) I was checking my progress first by gently flexing the soundboard across its length, and also holding it between forefinger and thumb and tapping the lower bout to assess how responsive the soundboard was. This is something that comes with experience, but essentially I listen for the point when the tap tone starts to open up and resonate. As you get closer to this point it is important to progress very cautiously, as removing too much material can loosen the soundboard up too much, which can pose the risks of both the instrument folding under string tension, or developing a flabby bass heavy sound with little definition between notes.
Next I tapered the ends of all of the braces, as I had previously done for the ends of the finger, tone, and soundhole reinforcement braces where these connected with other braces. I carve a taper of just less than 1/3 of the length of each brace , save for the X braces where the taper is roughly 1/6 of the length of the brace, and the finger braces where only 1/4 of the length is tapered. The taper results in the tip of each braces brace being reduced to 1.5mm in height. These tapered ends fit into the kerfed linings when the guitar is assembled.
Having removed further material by carving the tapers at the end of each brace, I was getting closer to a point where the soundboard had the desired responsiveness. Taping and flexing the soundboard I felt that the transverse braces only needed minor tweaking, and shaving a further 0.5mm off their nominal height brought them down to a good final height. The height of the tone brace was also lowered a little from the nominal height of 13mm
The curve on the X braces and tweaking the height of the transverse and tone braces had opened the tap tone up, but I felt there was a little more to come off. Rather than increase the curve of the X braces any further, my next step was to shape the profile, or cross section, of the braces. Some builders profile the braces after they have tuned the bracing for the optimum balance of strength and resonance, but I prefer to use the profiling as a means to tune the braces. Because after all, profiling involves removing yet more material from the bracing. The profile is a matter of personal preference, and some builders use a gentle rounded profile while others aim for a more pointed, almost gothic, shape. There really is no right (or wrong) answer, providing that the optimum amount of material is removed from the braces, so I profile the braces until I am happy with the appearance and the tap tone indicates that no further material should be removed.
I profile the braces using a 25mm long thumb plane, slowly knocking off the corners of the braces and gently bringing them to the desired shape. A sharp ½” chisel is also useful for working into tight spaces between braces, both used bevel down, and bevel up with the chisel back pressed against the line of the brace. I prefer to leave my braces shaped from the plane and chisel blade, rather than sanding out any tool marks. Again this is personal preference, and while I like to finish the exterior of the guitar to as high standard as possible, it pleases me to think that internally there are some signs that my instruments have been made by hand rather than machine. The braces for this soundboard ended up with a gothic arched profile, which looked sharp and also lightened the bracing a little further.
Having profiled the braces, the resonance of the soundboard was very close to where I wanted it, but not quite 100%. The final touch was to gently scallop the middle of the tone bar by 3mm, and this resulted in a lively and responsive soundboard which still had plenty of stiffness. It just goes to show how a little spruce can go a long way, when you consider that the scalloping did not remove much material from the tone bar but definitely had a significant impact on the sound. Scalloping after the profiling leaves a nice crisp transition between the different cross sections of the tone bar, although again there are no hard and fast rules about how to do this and some makers will scallop before they profile the braces.
The only remaining task for the soundboard is fitting the bridge (which is in effect the final brace) and I will be writing about this soon.