FAQ #6: Bracing, Thickness, or Both
December 18, 2011
Q: In my limited experience with classical guitars there seems to be a need to have a more flexible top on the bass side and a more stiff top on the treble side, giving the warm and low sound on the bass and more sustain on the treble, as well as preventing the percussive anvil sort of trebles . The flamencos seem to have a somewhat less flexible bass and a flexible treble side, which gives them a somewhat percussive sound with rapid decay in sound. This is at least consistent in the examples I have and have had the opportunity to play.
If one were to attempt to make a classical guitar as stated above would a change in top thickness to accommodate the additional stiffness required on the treble side be in order, or would changing or stiffening the bracing on that side be a better option than making a guitar with a top that is not of uniform thickness?
So…essentially, bracing or thickness or both?
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A: Your question addresses the primary issue of whether it is desirable to design a plate that relies on symmetrical design, or an asymmetrical one, for optimal functioning. The matter is clouded by the fact that some remarkably good guitars of both types have been made. Also, some pretty unimpressive guitars of both designs have been made. So there’s no clear winner, and I’m not convinced that the mere fact of symmetrical or asymmetrical construction is the most important consideration. I mean, if it were, one of these designs would produce consistently better results than the other.
As I read your question, I am translating it (to myself) into language that I am most comfortable with. So my answer might put a different spin on things than you’re used to. Let’s see if this makes sense to you.
To my knowledge, flamenco guitars achieve their characteristic sound by having looser, more flexible tops in general than classic guitars. Specifically, they are so loose that the monopole (the base) is discharged quickly — thus giving those instruments the characteristic traditional “dry” sound without a lot of sustained presence; their sound is more percussive. Usually, if one believes in asymmetrical construction, the treble side is made a bit stiffer than the bass side. In any event, compared to classic guitars, flamenco guitars have a more prominent cross-dipole. They are “looser” in that mode, in which the top moves side-to-side across the centerline and the bridge teeter-totters with one wing going up as the other goes down, and back. You can get a sense of this looseness by simply pressing down on one of those tops with your thumb: you will probably feel them “give’ fairly easily. You can also test for cross-dipole compliance by lightly putting one or two fingers of one hand on one wing of the bridge and lightly tap on the other wing with your other hand. You should feel a definite and instantaneous displacement as the bridge see-saws relatively freely. Classic guitars will be built less loosely, and will accordingly have less mechanical “give”. Neither one of these designs is “good” or “bad”, by the way; they are simply different — and different for a reason.
To my knowledge, these characteristics of flexibility and movement in a guitar top are best achieved by careful BUT SYMMETRICAL calibration of the plate — and I do not try to build any mechanical or dimensional asymmetry into my own guitar tops. But I acknowledge that other makers of successful guitars use dimensionally asymmetrical faces, so I’m inclined to believe that the motions of the cross-dipole (or any other mode) apply to a variety of architectures. At that point, the issue becomes that of basic calibrating so that one is “in the ballpark” and doesn’t overbuild or underbuild. I go into several detailed discussions of this concern in various chapters of my book The Responsive Guitar. You’ll undoubtedly find some of my ideas from there worth at least thinking about.
Your question addresses the question of whether it’s appropriate to add or subtract bracing in order to accommodate to changees in top thickness — as a matter of making material stiffnesses — and hence vibrational action — be consistent. The matter of achieving a balance between stiffness and/or looseness through top thickness vs. bracing mass is central to lutherie. And in this balancing act, there are two factors that inform my thinking.
First, in the traditional approach, if one were to make part of the top thinner, one would indeed want to “compensate” for it by making the bracing a little stiffer. This is assuming that the maker’s goal is to have AN EVEN GRADIENT OF MECHANICAL AND VIBRATIONAL STIFFNESS FROM THE BRIDGE TO THE PERIMETER, IN ALL DIRECTIONS. This is certainly my goal. In purely practical terms, this is surprisingly tricky to do until you begin to understand what you’re doing and have some practice at it; after that, it’s surprisingly easy. So an experienced hand and eye are really useful to have. I should add that, incidentally and technically, the gradient that I visualize in my work is only even in the sense that there are no lumps or irregularities of localized stiffness between the center and the perimeter; but it is not the same slope on all axes, in the sense of being identical. The longitudinal gradient is stiffer than the lateral gradient.
Be all that as it may, the second factor is, I think, just as important. It’s also interesting, subtle, and elegant — and obvious. So much so that it took me years to see it. It’s the “water running downhill” principle; you know: that water will find a way to run downhill regardless of trees, rocks, or irregularities of slope or terrain — because that’s the nature of water. In fact, such downhill movement of water cannot be prevented short of putting up a barrier or obstacle that exceeds the power of gravity over water.
Interestingly, sound energy is the same, except that instead of running downhill it wants to radiate off a guitar top — with all the freedom of water running downhill. It’s the nature of sound energy to dissipate into its surrounding medium, be it air or water. If we think of sound energy as seeking its easiest path “out”, as water wants to find the easiest path “down”, then it’s a short step to seeing physical unevenness in a guitar top as being analogous to unevenness in downhill terrain. And unless any of this unevenness is significantly huge, both water and sound will continue to flow and radiate. Tweaking any of the minor irregularities of slope, terrain, or structure will by no means stop any of the flow or radiation; they’ll find a way to get from here to there.
Let’s take a look at how this might work in a guitar. Let’s assume that you have a dimensionally asymmetrical plate, as you described above. And let’s further assume that the structure — irregularities and all — is “in the ballpark” as far as not undermining the monopole, cross-dipole, and long dipole. Or, saying the same thing with different words, that the irregularities are such that they allow the capacity of the plate to engage in these modes without messing any of them up). The top will flex and bend and seesaw and vibrate just as all the theories, diagrams and Chladni patterns suggest. The question then becomes: what makes you assume that such a top and its vibrational modes have to function symmetrically around the guitar’s centerline? Or that the various vibrating quadrants and subsection of the face will map out as being active with elegant evenness, symmetry, and consistency? I mean, no one expects water to flow downhill over a natural terrain in a straight, even, consistent, and predictably regular line, do they?
Your question cites differential side-to-side construction. This is the axis of the cross-dipole, which is (in theory) a see-sawing action around the centerline. If we were to imagine two kids on a playground see-sawing up and down on a teeter-totter, that device will be pivoting on its center point as a matter of the manufacturer’s design. But, suppose one of the kids is heavier than the other one? That would introduce an irregularity into the flow of their play. The manufacturer of the teeter-totter wouldn’t care about that, of ocurse; only the kids would. And to anyone to whom the kids’ fun was important, they could compensate for this disparity in mass (in the “playing field” or “gradient”) of that teeter-totter by simply adjusting the fulcrum point to a somewhat off-center position. Then, being better balanced, the kids could see-saw happily and without strain: same mass, same energy, same frequency, easier and more harmonious oscill.ation.
This is close to my sense of how the guitar works. To repeat, using other words: if there is sufficient unevenness in the top because of any design idea of the maker, and the design variable isn’t so huge as to throw a monkey wrench into the natural functioning of the guitar, then that “uneven” top will accommodate to the needs of the energy flow of that irregular structure all by itself. It’ll adjust, within some limits (of the natural capacity for flexibility of its woods), and perhaps wind up fulcruming, say, 1/16″ off the centerline. It can do that because the guitar lacks a fixed fulcrum in the way that a teeter-totter has one. Therefore, the vibrating quadrants of the top may be a little bit lopsided or asymmetrical in actual movement, etc.. But, as far as dissipation of strings’ energy is concerned, nothing has been prevented; it’s simply found its way out via an alternate path from what “the manufacturer’s blueprint” might have suggested.
This doesn’t fully answer your question yet, but my answer required me to have sketched in this background before addressing it specifically. This background, to repeat once more, is that tonewood that has been worked to more or less optimal dimensions has a certain innate flexibility of vibratory potential. There are no fixed fulcrums or vibrational nodes. And it may not matter that you’ve made an irregular plate — as long as you have not made it so uneven that you’ve pushed the plate past some limit of being able to perform its principal vibrational tasks.
So, to sum up: my answer to you is in four parts:
First, that yes, if you make a top thinner on one wing, which necessarily weakens it, then there’s a logic to adding bracing stiffness to it to make up for that weakening. I believe that one should aim toward at least some standard of evenness of physical/mechanical/tonal gradient if one’s goal is to make better and more reliable guitars.
Second: I believe that these maneuvers work most effectively if the top and braces are “in the ballpark” as far as optimal mass and stiffness are concerned — rather than the system being overbuilt as is often the case. Or underbuilt, if you’ve gone too far in thinning. If you’re overbuilding, then the thinning and bracing work that you are considering might be nullified or overshadowed by the fact that the structure is still too stiff. Or maybe the part that you’ve thinned will work fine, but the part that y ou haven’t thinned will be inhibited. But you won’t get 100% cooperation from such a top.
Third, you will probably produce minimally uneven tops no matter what you do; guitars in the real world always have something or other that’s not optimal.
Fourth, as with the example of water cited above, and if your gradient is not too unevenly made to begin with, then what you’ve constructed or misconstructed probably won’t matter. Or at least it won’t matter very much within the context of the flexibility of vibrational potential that the top has. The top will bend itself (sorry about the pun) to work in any way it can, to release its load of sound energy. It’ll modulate itself physically and vibrationally. As I said above, the vibrationally active areas of the top may functionally be a little bit asymmetrical, things may be a bit off-center, vibrational patterns might not be quite mirror-image, etc. But this is no big deal: the top plate has the capacity to function at least a little bit like that in order for the system — as it is physically constructed, with its unevenness and irregularities — to engage in an adequate monopole, cross-dipole, and long dipole. Finally, the sound will get out, sometimes because of, and sometimes in spite of, and sometimes without being much bothered one way or the other by, the work you’ve done. And I think this is where a bit of the magic comes in.
I hope this makes some sense.