November 3, 2011
Q: Assuming you’re looking for a back to work in tandem with the top, as opposed to a reflective back, should the back also be thinned till it “relaxes”, as you do on your guitars?
A: Ummmmm… this is a really interesting topic that very few people have done any thinking about — and most of the ones that have are classic guitar makers, not steel string guitar makers.
The matter is too complicated for me to write fully about in this format, especially as I have written about exactly this kind of thing in my book. Have you read my book’s chapter on the functions of the guitar back? If you haven’t, it’ll be useful for you to do so. Mainly, my answer is based in the proposition that the job of the guitar top is to generate an optimal mix of monopole, cross dipole, and long dipole signal… which gets converted into sound a bit further on down the line. The back has a different function — although, frankly, almost no one that I know of has ever considered making a back that might have a purposely dominant monopole, cross dipole, long dipole, or whatever.\
The back has not been studied like that. And one indicator of this circumstance is that while guitar tops have been made with all kinds of variants of “X” bracing, double-X bracing, fan bracing, lattice bracing, ladder bracing, Kasha bracing, radial bracing, and even the most oddball experimental bracing, over the years… 99.99% of all guitar backs have been made with three of four parallel braces since the back was invented. Period. So our information about the possibilities of the back is limited to one model of bracing that has been done over and over and over and over again. I show some experimental back-bracing ideas on page 91 of my book The Responsive Guitar; take a look at them.
Also, consider that it doesn’t matter how the back is constructed if it is not allowed to be active. For instance, Bluegrass guitars are played with the guitar’s back resting against the player’s body. These backs are significantly damped out. That is, they are prevented from participating in the dances of the frequencies. Would it matter to that kind of guitar that the back has been thinned to the relaxation point? Not at all. That back isn’t expected to do anything. The technique of playing the typical bluegrass guitar (standing up, strap around shoulder, guitar resting against player’s body) does not concern itself with the back’s doing anything in particular except maybe acting as a reflecting surface and otherwise keeping the dust out. And, as I say in my book, (at the risk of becoming unpopular): the use of a highly resonant and expensive wood on the back of a guitar that has no use for a functioning back is to waste the wood.
But aside from all this, to get back to your question, the short answer is “yes”. My prejudice is to make the back more flexible than other makers typically do. The reason for making both the top and the back flexible to begin with is that everything else you do to them does nothing but stiffen them up. You brace them, dome and stress them, and attach the perimeters to the guitar rims. Pretty soon, you’ve got something that you’ve (perhaps inadvertently) made really too stiff.
But too stiff for whom? For you? Maybe; or maybe not. For me? No, I don’t really care. For the strings and their work? Yes: they care.
I first got onto this idea, years ago, from an interview with David Rubio in [long-since disappeared] Guitar And Lute Magazine. Rubio recommended thinning the free (unclamped and unbraced) top until it had no tap tone of its own. If it still had an identifiable tap tone, it would be introduced into the guitar’s structure and responsiveness. But if one introduces a “tone-neutral” top (or back) into the system one could then build an appropriate tap tone back into it by bracing it, attaching it to the guitar, and bridging and stringing it. The basic equation is: if you start out with this, and then add that and something else, you wind up with this + that + something else = something greater than what you might think you have..