Determining the proper scallop design and brace stiffness is a VERY subjective subject. So many methods and theories are used that a complete discussion on this page is not possible (and probably not worthwhile). Woody, and all luthiers, must start with a rational plan and then work from experience, intuition and feel. There is one piece of advice, however, that all instrument builders should follow... "Work scant". Proceed with carving braces only after drawing the intended shape of the scallop on the brace... stop...think about what you're doing... proceed again and stop again... Before you think that you've taken out enough... stop.
I've taken a couple of photos where the shadows of the braces show the length and depth of the scallops. Many, many guitars have been made without scalloped braces and they sound great! Some luthiers believe that scalloping braces is incorrect and weakens the brace unnecessarily. Martin used to scallop braces quite a while back (in the 1930's and 40's). Then they quit. I've restored several Martins from this vintage in which everything but the scalloped bracing needed repair. Martin now produces very good sounding instruments without scalloped braces, but their HD models feature scalloped bracing (something for everyone).
Scallops (briefly):
The idea of scalloped braces is to reduce the mass and stiffness of the brace thereby changing the resonant frequency of the top. This sounds logical. As a professional engineer, I am intrigued by the measurement and modification of resonance to influence tone. The resonance of the entirely assembled and finished box of the guitar, however, would seem to me to be the critical resonance impacting tone. The resonant frequencies of the free-standing component parts of the guitar (top, back, sides, neck, etc.) are significant, but how these components interface with each other and the volume of air within the box is the final test. The physics of the many variables impacting sound and vibration in guitar is beyond our scope here.
Back to the Basics:
Notice, in the upper bouts, the cross braces are not scalloped. This is the area of critical structural resistance to the torque generated by the pull of the strings on the top. Keep the upper portions of the cross braces at full height and strength.
October, 2015 Back and Top Braces
Top and Back Joining
Bracing Explained
The month of October flew past just as months do when one is busy. Guitar building is on-your-feet work and if you followed me around all day you would be tired. You know, a good tired, a sleep-well tired.
September ended with the completion of the initial joining and rough dimensioning of the top, back, sides and neck of the several guitars that Woody intends to complete in the next few months (before Easter). Please don't be confused, Woody and I are the same person. I just get tired of someone talking about my-self all the time.
The guitars under construction for this journal are all dread-naughts, not like the arch top Gibson shown above. I mentioned the Folk Festival and working on the Gibson as excuses for not getting as much done on the D's as planned.
Back Braces:
The shape and carving of flat-top guitar braces is unique to every guitar maker (as is almost everything). The common property of back braces is that they are all arched to produce and maintain the arch of the back.
Many guitar makers these days use a manufactured, bowl-shaped jig, lined with sandpaper to produce the arch in the back braces. This is a personal choice of the maker. When one thinks about the geometry of a concentric bowl, however, the arc north and south is the same as the arc from east to west. The arc from the heel to the butt of the guitar, however, is not the same as the arc from side to side. When produced in a sanding bowl, the brace cross-section will have the same arc across the bottom as it does down its length. This would seem to introduce a number of hard-to-control variables... I must not know everything that there is to know about sanding bowls.
When the bottom of the brace is perpendicular to the vertical centerline of the section, it is relatively easy to shape on a 6 x 48 stationary sanding machine, and then glue in place tightly. That is the method used here.
Woody prefers using six shaped braces to achieve the curve of the back. To do this, six brace molds (made from ordinary 1 x 2 pine) serve as templates for the braces. These molds then become cauls when fixed to a work board, as shown below. The final shaping of the braces can be done before or after the braces are glued to the back. I prefer shaping them before gluing. Here I glued them, then carved them to final shape. These back braces are 15 mm tall all the way across.