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From: Ross
Remote Name: 76.101.10.109
Date: 13 Jul 2007
Time: 08:11:55 -0400
Luthier’s Weblog 7/13/07 The question was: what does a brace do? The robot luthier’s answer is: manage load. An unbraced top is surely feasible from a structural standpoint, but the mass required to achieve the necessary stiffness to hold shape would present too much inertia for the propagation of sound. Besides, the top doesn’t need to be stiff all over, but only at strategic points. Couldn’t one leave the top thicker in areas that require stiffness, thinning the areas that don’t as needed? Alas, the spruce that the builder favors is straight grained, with the top plate being far stronger along the guitar’s longitudinal plane than along the lateral plane. Carbon fiber or some other composite may prove amenable to this approach, but the builder is too constrained by convention and training to pursue this avenue. Better, he realizes, to use a series of braces , orienting their grain direction to achieve and distribute the strength required to hold shape. There is, after all, a hundred and seventy-five pounds of force pulling against the bridge at all times. Having laid out his brace pattern, shaping of the braces begins. Through experience, the builder knows roughly what dimensions each brace requires, and has bandsawed the braces to an oversized profile, with a slight convexity to the bottom of each brace to achieve doming of the top. The doming will allow for a stronger structure, allowing less mass for a given strength. Further, it will allow for some climatic change, the dome flattening or rising without inducing top cracking. Gluing the braces in place, the builder then begins to sense what the overall top structure wants to be. Having previously thinned the top to a greater degree around the perimeter than in the center, the builder now seeks to continue this concept of stiff center graduating to compliant perimeter. At the center of the top is the bridge plate and X-brace junction. This relatively rigid area is the center of load stress. The bridge, sandwiching the top wood on the outside of the guitar, adds to the stiffness and strength of this area. If we cut out a pie plate sized piece centering on this area, we see that we would have a very stiff structure indeed. Continuing this stiffness outward, however, would result in a trebly sounding instrument, lacking in lower frequencies. The plan, then, is to shave the braces in such a way as to gradually achieve maximum top compliance. And maximum compliance means that the builder unloads ALL the string load carried by the braces. Noting that it is common to tuck the brace ends into receiving notches cut into the ribs of the guitar, the builder eschews this practice. From a point of maximum brace height, he tapers his braces to nothingness just at the point where the top joins the ribs. No energy will be unloaded into the ribs via the top braces in this way, and all of it into the top. The top will still maintain maximum compliance at it’s perimeter. It is interesting to note at this point that some manufacturers and builders will build in additional points of rigidity on their braces in the form of rising peaks, known as the technique of scalloping. The builder scratches his head and feels foolish as the virtues of this technique are explained by it‘s proponents. He cannot understand why a given area of the top would be made stiffer than an area of the top closer to the load center. Since lower frequencies generate physically larger sound waves, it seems to the builder that load needs to be dispersed as evenly as possible, reaching maximum top compliance short of the perimeter. Ross Teigen 9:43 a.m.