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And now, time for a completely different view. I have read many accounts of tuning tops, backs and braces to specific notes, and have not found these notions useful in my work building and modifying guitars. It is my view that guitar balance can be achieved when the top and back are allowed to make ALL of the frequencies a guitar can produce, and at this point, specific body resonances fade into the overall sound. Of course, balance can also occur when there just enough dead spots to evenly dampen all of the sounds.
The brace shapes Martin used during its golden era have all been compromised by approximations and compromises needed to have a machine do the shaping. And some, completely forgotten. My background evolved out of studying guitars and their braces reshaped by Berkeley's Jon Lundberg, who "shaved" braces to bring out the sound of instruments played by the Dead and many other Bay Area musicians, in the 1960's through '80's. Lundberg duplicated Martin's earlier hand shaped curves, both on top and back braces. There were also problems, too much scalloping or dead strings. I bought a '37 Gibson Advanaced Jumbo Jon had worked on in 1977, and soon noticed a very quiet D string. Later, Jon said they were all that way, and nothing could be done about it.
When I evolved my theory of how and where each string makes its sound in a guitar in 1989, I was able to further modify top and back braces to bring the D up to full volume, enhance the treble, as well as overall volume and sustain.
Integral to my theory is the loudspeaker analogy mentioned above. My work reshaping braces and bridges on guitars by all of the major manufacturers and over a dozen individual builders has consistently confirmed the utility of analyzing why any given string or range is quieter than others using the loudspeaker analogy.
But first, consider how a loudspeaker works. When any specific frequency excites the coil in the cone center, the whole cone doesn't vibrate, a specific ring vibrates, one that resonates with the frequency. The higher the frequency, the smaller the diameter, and vice versa. It's why bottom end speakers have large diameters. I demonstrated this to myself in 1989, plucking different strings, and feeling the metal 10" cone of a Dobro with the cover plate removed.
In a lightbulb moment, it occurred to me that a guitar top works in the same way. Basically, if a brace has more mass than needed to support the string tension, it dampens the top where it is glued. Most top bracing patterns can be determined simply by gently percussing the top with the fleshy part of a finger. The braces sound like studs in a wall, and do not allow the top to vibrate to its full potential. The interaction of this pattern with the various rings generated by any and all notes on the guitar determine which notes are more, or less, impacted by dampening, affecting overall balance.
On a steel string guitar, the center of these rings is the crossing of the X brace. On a classical, the transverse brace below the soundhole forces the lower bout to operate independently of the upper, and the rings' center is in about the center of the bridge.
Braces permitting, the back can act to reinforce the top's vibrations, with the lower bout adding to the lows and mids, the upper, the mids. I have thinned and streamlined the lower two braces, greatly enhanced the bass end, and noticed that the Helmholtz frequency raised from an F# to a G#.
A more complete discussion can be found on my web site, vanlingeguitars.com., in the pages on "parabolic braceworks" and "bio and brace talk". I will be giving a workshop at the GAL convention this June in Tacoma.
I have seen many mathematical discussions, graphs and diagrams of how a guitar works. But none, for example, could tell why the F# (high E, second fret) was quieter than its neighbors on a Larrivee I worked on last year (that's, was).
Happy tapping...
Scott
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