There are two types of structural elements commonly used in arched guitar tops. In the 1920s Lloyd Loar, the designer of the Gibson L-5, the first arch top guitar with F holes, used two tone bars that are roughly parallel, running from near the neck block lengthwise along the top to near the tail block, with the spacing widening slightly towards the tail block. Then in 1934 with the first Super 400, Gibson used X braces like Martin had already been using on its dreadnaught guitars for about a decade; the arms and legs of the X run from the upper bout on one side to the lower bout on the opposite side, crossing just before (on the neck side of) the bridge. Both types of structural elements have advantages and disadvantages. X braces are inherently weak at the joint where one brace is notched over the other, but sound warmer and darker. They extend over a wider area. Parallel tone bars are strong and create a top that projects well, but can sound cold or overly bright. They are better at resisting string pressure. Nearly all arch top guitars have used either parallel tone bars or X braces. These structural elements have both been in use for the better part of a century now.
For my arch tops, I wanted to the advantages of both types of structural elements, but without the disadvantages of either. I wanted to create tone bars that would start near the upper and lower bouts, like X braces, but not cross; they would curve to be parallel in the critical area near the bridge, and then continue to curve back out to the other bout on the same side. I wanted to implement this idea without having to bend or laminate the tone bars, or get too extravagant in dedicating a big piece of spruce to carve each tone bar as a whole separate structure. I invented a solution that gives me the configuration I wanted, and I believe it has some other significant benefits. I call my design “integrally carved curvilinear tone bars”.
I recalled that some violin family instruments have carved-in bass bars, and I found that the typical height of the structural elements used on arch top guitars is within the margin of wood that I would be removing from the inside of the top. I intended to use a CNC router to do the rough carving of both inside and outside surfaces of my tops and backs, and I realized that I could add the shape of the tone bars that I wanted to the 3D model of the inside of the top, and let my CAM plugin (MadCAM) figure out how to carve them for me. Actually, that's a little more complicated than may sound, but the result is integrally carved tone bars that are strong like Lloyd Loar’s parallel tone bars near the bridge, increasingly flexible as they get closer to the periphery, and have a broader coverage area like X bracing.
One additional benefit of my design is that there are no glue joints or wood grain transitions involved in these structural elements. Aside from the center seam, the whole top structure is one integral piece of wood. To the extent that glue joints and wood grain transitions are barriers to string energy, my integrally carved design is that much better at string energy propagation. Another benefit is that the integral tone bars are not just as tall as their highest point above the inside surface of the top; the same vertical grain extends all the way to the outside surface of the top. If anything, I believe they are stiffer for their size than conventional tone bars, in the critical central area. They are also more flexible than conventional tone bars as they fan out to the periphery of the top. I believe this integrally carved curvilinear tone bar design is a big part of why my arch top guitars sound the way they do.
The top shown on this page is for an 18” Skylark in Alaskan Sitka Spruce.
Integrally carved curvilinear tone bars modified for a surface mounted pickup on the Pannonica model. The area around the pickup cavity is slightly thicker for strength along the edges. Small braces will be added across the grain alongside the pickup cavity.
The cross grain braces have been added. This top is shown at the rough carved stage; some hand finishing remains to be done.
A second generation Pannonica top ready to be glued on to its body.