(NOTE: this article is still in progress)

© 2013 Ervin Somogyi

Top thickness is, along with bracing, the most debated and tinkered-with area of guitar making. It is so for two absolutely important reasons. The first is that the physical characteristics of the top set the stage for tone — along with the corollary that the lighter the construction of the top is, the better the sound. The second is that there’s a minimal top thickness/stiffness that must be respected if the plate is not to cave in under string load. If sound is one’s objective, then the luthier’s balancing act is in finding the correct balance point between the imperatives of ‘light construction’ and ‘not too light’.

In my work, I take my tops to a target deflection under a standard weight rather than to a predetermined, formulaic thickness. I’ve worked like this for a long time now and have written about my thinking and techniques at length. Still, my method may not work for everyone. There are a lot of guitar makers out there who swear by specific target measurements, and I’m not sure I have the right to say they’re wrong to do so; my own preferred method is simply different. The question comes up, then, of what is the proper justification for focusing on one or another specific number for top thickness? And, what would that number be? Well, it seems to me that a good place to begin would be to have some idea of where the measurements that we do know about, read about, have heard about, and use come from.

PAST GUIDANCE AND WISDOM

Many of my generation of American luthiers got our start by reading Irving Sloane’s seminal book Classic Guitar Construction, which appeared in 1966. This was, after A.P. Sharpe’s 32-modest-pages long Making the Spanish Guitar (published in 1957) the first available ‘real’ book on guitar making. Sloane advised the reader to make his tops 3/32” thick — which measurement is equivalent to.094”, or 2.34 mm. Mind you, this instruction appeared before any of the two-dozen-plus books on lutherie that are now available, and before the plenitude of secondary sources of information that now exist. How did Mr. Sloane — who was not only writing very early in the game but had, as far as I can ascertain, only built a few guitars on his own then — come up with this number? Well, perhaps by reading Sharpe’s book (he recommended the same measurements) and very likely by measuring some guitar tops and by talking with some makers.

He probably didn’t speak with Vicente Tatay, one of the early Spanish luthier-transplants to the U.S., though. Tatay came from a prominent Valencian family of guitar makers and presumably knew what he was doing, guitar-making-wise, even before he took his plunge into the New World¹ . Once here, he wound up working out of a store in Greenwich Village and became, by so doing, one of Mr. Sloane’s fellow New Yorkers. There’s a wonderful article by Steve Newberry, published in American Lutherie (“Vicente Tatay and His Guitars”, issue #66, Summer 2001, pp. 47-49) about the state of lutherie and its lore in the U.S. many years ago. It is told from the point of view of the author who, as a teenager, became fascinated by Mr. Tatay’s work and talked him into being allowed to hang out in Tatay’s shop after school hours and be of some help by sweeping, cleaning, etc. In exchange he got to observe Mr. Tatay at work, of course. This turned out to be a mixed pleasure: Mr. Tatay is described as having been a gruff, cantankerous, cranky and closed-mouthed chain smoker who had an explosive temper and spoke only Spanish. Still, one afternoon toward the end of the Summer, in an uncharacteristic moment of expansiveness and letting down his guard, Mr. Tatay motioned the young Newberry over to his workbench and, using hand gestures and some coins, indicated to him that the secret to his lutherie was to make the guitar top about the thickness of a nickel in the middle, and the thickness of a dime at the edges. (I should add that a lot of Spanish guitar making in those days was done just like that: by skilled feel and eye, and with amazing accuracy.) Tatay might or might not have known the numerical values of his thicknesses but he certainly knew how to work to such tolerances at the workbench. Incidentally, the breadth of a nickel and a dime are .075” and .050” (i.e., 1.9 mm and 1.34 mm), respectively. Give yourself a treat and look that article up; it’s as well written as anything Mark Twain ever wrote.

Four other books on guitar making followed Irving Sloane’s pioneering work on guitar building. Classic Guitar Making by Arthur Overholtzer, published in 1974, immediately doubled the available information on this subject² . The other three were Donald Brosnac’s The Steel String Guitar; Its Construction, Origin, and Design (1973), David Russell Young’s The Steel String Guitar; Construction and Repair (1975), and Irving Sloane’s follow-up book Steel String Guitar Construction (1975). These were the first sources of published information on the steel string guitar and their recommended guitar top measurements were 3/32” (.094”), 3/32” (.094”), and 7/64” (.109”), respectively. Overholtzer’s top measurements took into account wood density: for classic guitars his recommendations are 3/32” (0.094”) for soft spruce and 1/16” (.062”) for hard, dense spruce. For steel string guitar tops he recommends 3/32’ to 1/8” (.094” to .125”).

With the exception of Mr. Overholtzer, who had been a violin maker for some years previously, the others were pretty much acting as novice discoverers, craftsmen, and pioneers — as I myself was, except that I hadn’t written a book yet. I think it’s safe to assume that these young makers/authors were following each others’ and the Martin Company’s leads; and I was certainly following theirs. The Martin Guitar Company comes into this discussion because it was the premier steel string guitar producer of that time and would have been everyone’s main point of reference for making that kind of guitar. Mr. Sloane, whose second book Guitar Repair (1973) focused on steel string guitar repair procedures, was surely on this track: the book was photographed on the Martin Guitar factory premises, and the repair procedures that are described were carried out on the Martin company’s workbenches. Ditto Mr. Brosnac; I asked him, in a recent conversation, where he got his book’s recommended measurements from; he told me that he got them from Jon Lundberg, the legendary Berkeley-based Martin guitar retro-voicing pioneer, who was in those days possibly the world’s leading expert in that guitar³ . Both Overholtzer and Sloane seemed to take a lot of cues toward their classic guitar making from the work of Robert Bouchet (1898-1986), a noted and innovative French builder.

In 1987, twelve years after the last of the above books was published, the bibliography of guitar making took a major step forward when William Cumpiano and Jon Natelson published Guitarmaking: Tradition and Technology. This was the first book to address making both classic and steel string guitars and its recommended top thicknesses were the most comprehensive yet in recognition that not only does size of guitar and species of wood used make a difference, but that different makers have significantly different building designs and ways of using their materials. Accordingly, top thicknesses are suggested rather than instructed. Top thickness targets for classic guitars are given as around .100” (2.5 mm) for spruce and .110” (2.8 mm) for softer wood such as cedar. For steel string guitar the recommendation is 1/8” (.125”, or 3.17 mm) for a first-time project, but otherwise ranging from .095” up to .130” (2.4 mm to 3.30 mm) depending on size and shape of instrument as well as species of wood used. One can see that thinking about top thickness was getting more sophisticated.

CURRENT RULES-OF-THUMB FOR TOP THICKNESSES

So, according to published instructions to those dates, top-measurement for classic guitars are⁴ : 1/10” (.100”) to 7/64” (.110”), or 2.5 mm to 2.8 mm; 3/32” (.094”), or 2.34 mm; And for steel string guitars, they are: 3/32” (0.094”/.095”) to 7/64” (0.109”), or 2.38 mm to 2.77 mm; and from 1/8” (.125”) to a fat 1/8” (.130”), or 3.17 mm to 3.30 mm

Does this get us anywhere? Well, sort of. It tells us that, at least in the classic guitar, one can go as thin as 1/16” (about 1-1/2 mm) and still have the instrument hold together. That’s useful to know — as is the fact that Overholtzer is in a minority in promoting such thinness; he and contemporary luthier Greg Smallman go remarkably thin, but very few others follow suit. As for steel string guitars, we have no published accounts of whether there is a top-thickiness limit that’s below 3/32”; if anyone one has tried to push that envelope they haven’t written about it.

I’ll address some additional specifics further below, but for starters you should know that Tatay’s top-shaping approach is the traditional one used by Spanish classic and flamenco guitar makers: the top is made to its target dimension in the middle but it is thinned in the outermost inch and a half or two of the lower bout, from the waist down. We know this because work of this type is found in the instruments of established classical guitar makers whose work has been carefully measured and studied. Experts can even date certain classic guitars through specific variations in their measurements, which will have been documented from the various periods of their makers’ careers⁵ .

Flamenco guitars, unfortunately, lack the social and academic respectability of their rosewood-built brothers and have not received such formal attention; they get played a lot but not studied. Ditto steel string guitars. And speaking of these, Sloane’s and Overholtzer’s recommendations of uniformly thick classic-guitar-top measurements, previously cited, actually come out of the steel string guitar making tradition in which the top is the same thickness throughout, without any selective tapering or thinning.

VARIATION AND INCONSISTENCY

While both steel string and nylon string guitar makers tend to follow their own top thickness recipes, the former work to top measurements that are far less agreed upon or consistent than are the target measurements for the latter. Therefore those measurements — ranging, as we’ve seen, from .094” to as much as .130” — are not so useful to rely on as guides. This great variation is attributable to six main influences, the most important two of which I consider to be the following:

First, there needs to be a lack of dimensional consistency from maker to maker in steel string guitars because steel string guitars come in so many shapes and sizes. This itself is a function of industrial priorities of (1) needing to make one brand of guitar distinguished from another in the marketplace (hence different physical parameters), (2) different orchestral uses for the guitar as the provider of mass musical entertainments, and (3) the need to make mass-market products durable. There is a legitimate logic for producing workhorse/beater guitars in a mass-entertainment culture: consider the fact that there is nothing like Willy Nelson’s guitar among classic musicians.

And second, in the absence of a craft tradition in which independent luthiers ongoingly seek ways of refining their work, the newer generations of steel string guitar makers have — knowingly or not — been copying copies of copies of copies of copies of copies of copies . . . of mostly Martin guitars, but also Gibsons, Guilds, Harmonys, Epiphones, etc. ⁶

While copying — or imitation, as Oscar Wilde put it — is the sincerest form of flattery, it does cut down on investigation, discovery, originality, increased understanding, and improvement. Nonetheless, copying copies of copies of copies of copies of copies has worked well enough for a long time, and the top thickness measurements put forward in various books and plans are generally taken more or less as givens without being questioned. For that matter, how could it be any different unless one has had any other experience to compare against?

More important than this blind acceptance, though, is that, more or less by default, these guitars’ sound is attributed to this or that variation of “X” bracing (or fan bracing) rather than to any more reasoned and optimal thicknesses of soundboards. As far as steel string guitar making goes, ways of refining and fiddling with “X” bracing and its offshoots have consequently received lots of attention. Look in any modern guitar magazine for pictorial examples of this: every brand has its own version of the “X” with different angles, different scalloping and profiling of the main legs of the “X”, different height of their intersection, variously profiled finger braces, differently spread tone bars, etc. No one ever mentions differential top thickness, basic plate tapering, etc.⁷

MY OWN EXPERIENCE

I’ve made my steel string guitar tops thinner and lighter over time; I’ve found others’ typical construction to be too heavy. I’ve used many variations of “X” bracing in them, and even tried fan-bracing on a few. I’ve also made my Spanish guitars with thinner and thinner tops; I’ve mostly used traditional fan bracing on them but have done a little lattice bracing also, and even some “X” bracing.

The upshot of this trajectory is that I like the sound of my steel string guitars with comparatively thin tops and coupled “X” bracing, more than I like the sound of my (and others’) classic guitars that have thin tops plus either fan bracing or lattice bracing. I find that I can get a rich, deep, and pleasing sound from my thinnish-topped steel string guitars. But classic guitars with thin tops — both my own and others’ — have a quality of sounding a bit sharp, or harsh, or spare, and in general musically uninteresting to my ear, even though they may be loud. I like a richer, mellower, more complex sound. If any of you have heard the sound of a Friederich [also spelled Friedrich much of the time] guitar you’ll know what I’m talking about.

These are, admittedly, my personal preferences. But they are also shared by many others. Matters of tone have both subjective and objective components, of course. The objective part has to do with the things that tonewoods are known to realistically do when worked to this or that thickness. The subjective part has to do with musical tastes and with whether or not these woods produce sound and tone coloration that give pleasure.

In this regard, in the matter of nylon string guitars, we can return our attention to the matter of the differences between Sloane’s and Tatay’s recommended Spanish guitar top thickness. You’d expect that guitars with tops .094” thick would produce sounds different from those produced by guitars with tops tapered from .075” to .050”, wouldn’t you? But, oddly, along with the various instruction to “do it like this” or “do it like that” that appear in various books there’s no accompanying explanation of just exactly what it is that you get if you follow those recommendations. Sometimes, in the more scientific presentations, there are graphs or photographs of testing for monopole, dipole, and tripole Chladni patterns; these show that these guitars do have clear monopoles and dipoles, etc. [See Chladni photos from p. 121 of Engineering The Guitar] It is useful to know where various vibrational areas are most dominant, and at approximately what frequencies these tend to be most active. Most readers will not be sufficiently sophisticated to get more than this fundamental sense of how the average guitar works, though; I’m certainly not. And one is still left to infer many things from the sizes and shapes of the various blotches and wiggle-patterns in the photos. I have found them to not be of as much use as I would have liked in trying to understand some of the more specific aspects of frequency response.

An important clue is contained in Steve Newberry’s article, previously cited, when he states that Tatay’s guitars were loud (emphasis his; he really wanted to make a point). Interestingly, other words that are used to describe an impressive sound are: ‘powerful’, ‘brilliant’, ‘projective’, ‘full’, ‘rich’, ‘resonant’, ‘piano-like’, and so on. “Loud” merely suggests volume — a quality that is basic and not likely to imply character or complexity. I mean, when is the last time you heard any kind of explosion or crash described as being, say, rich or resonant? Also, the sound of an exploding volcano or an avalanche would probably be described as a roar instead of merely loud, which suggests the preponderance of a certain segment of the frequency spectrum, so I’m of the opinion that colloquial speech carries more information that one might at first think. In fact, many “sound” words such as bang, roar, thunk, and crash are onomatopoeic; that is, the word captures something of the actual sound it’s identifying. But before exploring this further — which we will do further below in the section titled ‘Correct Top Thickness’ — let us take a brief look at how woods do their tonal work.

WOODS’ AND GUITARS’ VARIOUS ACOUSTICAL TASKS

Tonewoods, by definition, make a sound — all by themselves. You have only to tap the good ones to get a surprisingly bell-like ring, when they are suspended in the air while held from just the right nodal spot. Compared with ordinary woods that merely go thud, thunk, or boink regardless of how or where they are held, such a response indicates a liveness and, especially, a high-frequency capacity. Indeed, tonewoods are sometimes described as being vitreous, which means glass-like — and of course having the ringing and sustaining vibrational quality associated with that material. If you tap these same woods while holding them at different nodal points they will also give you a lively and sustaining low-pitched hum. Such woods can do it all. Many rosewoods, spruces, cedars, redwood, cocobolo, wenge, padauk, etc. are bona fide tonewoods⁸ . Bubinga, teak, maple, cherry, oak, ash, African blackwood, zebrawood, Goncalo Alvez, ebony, olive, myrtle, koa, walnut, bocote, ziricote, and mahogany are generally not — or very little, at best.

There are significant differences between steel and nylon string guitars. The woods might all be the same; but the stringing, structure, and mechanical tensions these guitars operate under are hugely different. Steel string guitars want to produce a bright sound, not a bassey one, as a function of their basic construction and stringing. The natural voice of the fan-fretted nylon strung classic guitar, on the other hand, is the opposite: the bass is normally stronger than the treble. This is likewise a function of its basic design, construction and stringing.

Finally, I admit that I’m giving voice to my prejudices with a bit of factual information to justify them. The fact is that all kinds of really successful guitars have been made with exactly such “unsuitable” woods. I’m merely describing gradations of qualities, not absolutes. The real key is not what selection of wood you may have made, but what you’ll do with it. Keep in mind that beauty contests of all kinds, in which there’s a “best” followed by a bunch of “runners up” is one of the great artificialities of human culture. If this weren’t so, then only the lucky man married to the one single “best” woman would ever be happy and all the rest of us would get assorted runner-ups and rejects. Along those lines, I believe that Donald Trump believes that he has the best of the best in everything.

Yet, these are not at all the desired target sounds for these instruments. In any discussion about classic guitars it is essential to recognize that the ‘best’ instruments have treble notes that sound brilliant. They not only stand up to the bass notes, but they have their own very clear identity: that’s the standard by which these guitars are judged. ‘Best’ is here defined by the ‘romantic’ standard that Andres Segovia created, and which standard is still applied even to the newer classic guitars with thinner tops (about which I’ll say more further below). When an experienced classic guitar player puts his hands on any guitar that he’s never played before, his left hand immediately goes to the twelfth fret position and the first notes he plays will be the high ones; it’s the acid test, pretty much the first thing one does. It’s sort of like stepping into a new racing car and immediately revving the engine to get a sense of its power.

And what is this brilliance? Well, listen to some of Segovia’s early recordings in which he plays expressively and romantically. He emphasizes some of the high notes in such a way that their smoothly accented ping becomes part of the romantic sensibility of the song. Those notes are rich, very musical, and they sparkle.

On the other hand, in any discussion about the steel string guitar, the ‘best’ ones are those that have a full, good, solid, vigorous, punchy, present, and open low end response. Historically, the quest for a strong bass response has been the main factor behind the creation of the larger steel string guitar bodies such as the dreadnoughts and the jumbos. Low-end response is important in the steel string guitar; but smaller soundboxes can’t give it easily. (It is interesting to note that the Spanish guitar, in spite of having every opportunity to grow physically bigger along with its metal-strung cousin has — with only one technical exception — not done so. That exception is the Mexican mariachi bands’ bass guitar, the guitarron — which has a specific target sound and muscal use that is its own. The traditional classic guitar has long since found its optimal size.⁹ )

I think it is to the guitar’s credit that while its various standard designs and stringings produces sounds that are not, as I said above, the ideal target responses, the guitar’s design has sufficient internal and dynamic flexibility that any soundbox can be tweaked so as to bring out and emphasize the target frequencies. This is where the luthier’s skill comes in — and within the larger context of making Spanish and steel string guitars, the luthier’s challenges in making either one of these models of the guitar are directly opposite. I repeat: to achieve a good target sound in the steel string guitar the maker has to ‘build in’ a good bass response, which the instrument will normally lack. In the nylon string guitar — to achieve a good target sound — the maker has to ‘build in’ a good treble response, which the instrument will otherwise lack. (NOTE: these things are precisely the topic of chapter 32 of my book The Responsive Guitar.)

TONE PRODUCTION AND THE LOGIC OF MATERIALS USE

Bass response is associated with a top membrane that is loose enough, while also sufficiently ‘held together’ with bracing, to move as a single unit. This can be visualized as a sail that is billowing in and out under the wind. A thin, relatively flimsy top that is held together by any interconnected latticework of bracing will be able to billow back and forth, in unison with itself, and at relatively low frequency. In the guitar, this is called monopole movement¹⁰. Furthermore, any specific high-frequency potential or behaviors of the topwood — i.e., of the material itself, independent of the interconnected bracing lattice — are not so relevant to this mode. This is because metal strings themselves, by virtue of their own mass and stiffness, will bring plenty of high frequency signal into the system. One doesn’t need the wood to bring its own additional high-frequency contribution into the soundbox.

On the other hand, treble response is associated with a top membrane that is stiff enough to allow high-frequency/low amplitude motion, and which is not simultaneously ‘drowned out’ or overshadowed by dominance of monopole movement. The more the monopole is suppressed, and the top is prevented from moving like a sail or undulating like gentle waves — and the more it is enabled to move in rippling fashion in small-to-tiny sections — the better the high end. This is usually identified in the literature as dipole and tripole movement. Put in different words, the more that the top discharges its energy by billowing in and out like a bellows or a sail (monopole), the less energy is left over for the high end (dipole and tripole). And vice-versa. As with electronic speakers, it takes much more energy to produce low-frequency sound than it does to produce high-frequency sound.

The trick, obviously, is to not make the plate so loose that you lose the high-frequency end, nor so tight that you lose the low-frequency end. You want both, and the luthier’s task essentially becomes one of management-of-energy-budget. And thus, at this point, the question of ‘correct stiffness’ can finally meet up with some numbers that are associated with ‘correct thickness’.

SOME PROBLEM-SOLVING TOOLS

I’ve discussed the Cube Rule enough that I don’t have to repeat it here, except to remind us that it applies to length as well as height or thickness. In the matter of length, however, the Rule is inverted. The longer something with a weight on it is, the more it sags ; the shorter something with the same weight on it is, the less it sags — all in accordance with the Cube Rule. And, we should be speaking of deflection instead of stiffness in these matters: stiffness is, strictly speaking, a quality that is independent of dimension. Furthermore, if we’re comparing stiffnesses and deflections, the fact is that you don’t get the same difference going “up” from smaller to bigger as you do when going “down” from the bigger to the smaller. Cubed quantities don ‘t yield multiplicative proportional differences like that. For instance, reducing 100 by 10% brings you to 90, but increasing 90 by the same 10% doesn’t get you back to 100; it only gets you up to 99. You can get different results with the math if you’re not careful¹¹.

Let’s assume that you’ve been making pretty successful dreadnought guitars with tops at .090”. Dreadnought soundboxes are 21” long and have scale lengths of 25.4”. Let’s also assume that you’ve been commissioned to make an 18” long parlor guitar with the same scale length, and it of course needs to also sound good. It would make sense to figure out the logical top thickness of that 18” length guitar, based in your current 21” guitar criteria; you’d want a number that represents equivalent deflection; these guitars would both, after all, be functioning under the same string load. Interestingly, there are two distinct methods whereby one could arrive at an answer: an intuitive one and a mathematical one. In the interest of comprehensibility, I’m going to describe only the former; the latter is full of complicated mathematical formulas.

THE INTUITIVE METHOD

Let’s start with the fact that Guitar A is 21” long and Guitar B is 18” long, and that the difference in lengths is 3”.

• 3/21 = proportional difference in length, from the point of view of Guitar A, is about 14%
• 3/18 = proportional difference in length, from the point of view of Guitar B, is about 16%

It won’t work to assume that differences of 14% and 16% can be considered to average out at 15% for the sake of convenience. Guitar B is 14% shorter than Guitar A, and guitar A is 16% longer than guitar B. We need to work with real numbers and we can’t get around this.

We could do some math around the above quantities, again keeping in mind that (1) deflection changes geometrically with thickness, and (2) geometrically as the inverse of length, and (3) that the math will give you different numbers depending on whether you’re going from smaller-to-larger or larger-to-smaller. A 14% loss or a 16% gain in length means that these guitars can be designated as having lengths of 1.00 and .86, or 1.00 and 1.16. (It would be a bad idea to label these guitar tops as 1.16 and .86 respectively; we’d be counting the difference twice.) On the other hand, the math for this involves both direct and inverse Cube relationships and it gets just a little a bit tedious.

So, instead, one could cut to the chase by recognizing that, precisely because we are dealing with Cube and Inverse Cube quantities, the change in measured deflection from a 14% decrease in length will be “cancelled out” by a 14% decrease in thickness. The Cubed loss/gain of one will match the inverse of the Cubed loss/gain of the other. As a basic example, if you make something twice as long you weaken it to 8 times the original deflection; if you make it twice as thick you increase the measured stiffness to 1/8 as much deflection (even though thinking of “increasing stiffness to less deflection“ sounds confusing). In any event, 1/8 x 8 = 1, and net gain or loss are cancelled out.

I repeat: the longer something is, the more it sags under a weight (larger deflection number); the shorter something is, the less it sags under the same weight (smaller deflection number). Now, remember that we’re at 14% and 16% levels of size difference, depending on which direction you’re looking at this from. If we’re making 21” guitars at .090”, then we’d make 14% shorter guitars 14% thinner for them to have equivalent measured deflection. The .090” top would become a .0774” top. That seems easy. But that’s not the whole story: the guitar top’s width also has some bearing on the top’s stiffness.

FACTORING IN THE PLATE WIDTH

An 18” long guitar is 86% the length of a 21” guitar; and it will probably also be narrower by some proportion. Let’s assume that the 21” guitar has a 16” lower bout and the 18” guitar has a 15” lower bout. I repeat yet again: thickness/height varies as the Cube; length varies as the inverse of the Cube; and width affects stiffness in a linear way.

The math for making these adjustments with respect to equalizing stiffness is interesting because translating width measurements into thickness measurements (as when a narrower guitar top needs to be thicker in order to maintain constant deflection, or when a wider top needs to be thinner in order to maintain constancy of deflection) involves translating a linear quantity into a cubed or cube-root one. Finally, the 16” to 15” shift is an approximation because these are not rectangular plates.

We can deal with these numbers as follows:

• Guitar A is 16/15 (106%) the width of Guitar B,
• and Guitar B is 15/16 (94%) the width of Guitar A;

Therefore, as far as plate width influencing plate thickness goes:

Guitar A, being wider than B, needs to be thinned by the cube root of that 106%. Reciprocally, guitar B, being narrower than Guitar A, needs to be left thicker by the cube root of the percent of difference. These calculations will yield small numbers — something on the order of .002” .

One can more easily affect these numbers by how one braces the top: it’s otherwise very difficult to remove exactly .002” of wood. Metal, yes: machine shops do that kind of work all the time; but wood shops, not so much. Otherwise one can get calculation-happy very quickly by trying to figure out these balancing acts mathematically. I can tell you, however, that after a while one simply develops a feel for what is right. And the math is still a useful, if cumbersome, guide for whenever one has a project that is way outside of one’s experience. If you really want to go ahead and remove small amounts of thickness forget about using sanders and learn to use a hand plane.

Finally, one would think that a smaller guitar will be more stressed per inch of top than a larger guitar — because the considerable pull of the strings is spread out into a smaller top plate; each inch of top has to hold up to more pull. That certainly sounds logical, yet it is incorrect — because of the inverted Cube-Rule relationship between area and resistance to deflection. As we’ve just been learning, a larger top plate is looser than a smaller one of the same thickness, in direct proportion to the Cube Rule. Therefore one can legitimately say that a larger guitar top will be more stressed per inch than the one on a smaller instrument, because its top will be more yielding to the strings’ pull. Each square inch of a larger top has less ability to hold up to string pull than each sauare inch of a smaller top of the same thickness. What we’re seeing is that smaller surfaces have enormously more resistance to deflection in an inverted Cube-Rule way. It gets wonderfully complicated.

[AS I WROTE ABOVE, THIS ARTICLE IS STILL UNDER CONSTRUCTION]

FOOTNOTE 1) Officially, Tatay wasn’t making ‘classic’ guitars. According to authority Richard Brune, this was in the days before Spanish guitar makers recognized any difference between ‘classic’ and ‘flamenco’ guitars; that distinction didn’t take hold until as late as the mid 1950s. Until then, the guitar makers simply made ‘guitars’ to order — either with cheap domestic cypress or expensive imported rosewood, depending on the client’s budget. But while Tatay, who lived from 1889 to 1942, would not ‘officially’ have been making ‘classic’ guitars he was certainly doing so technically: he was following the techniques that informed the creation of soundboxes that merely weren’t yet being called that.

FOOTNOTE 2) The earliest photograph of me as someone involved in woodworking appears in this book, on page 21. It was taken in 1972 in back of MacBeath Hardwoods, in Berkeley. The store had received a shipment of Brazilian rosewood and had given me a call to drive over and pick out some. About a hour after I arrived a van full of the Overholtzer contingent pulled up, disgorged itself like the proverbial Thousand Clowns, and they started going through the same pile. The man standing with Mr. Overholtzer is Mr. MacBeath senior, the owner. I’m the bearded guy in the background. The planks of wood on the scale (it was being sold at a princely $1.25 per pound!) next to these gentlemen were the ones I’d already picked out.

FOOTNOTE 3) I was living in Berkeley myself in those days and knew Jon Lundberg; he owned and was running Lundberg’s Music store, a great magnet for friends of the steel string guitar. Richard Johnston, who has recently written two comprehensive books on the Martin guitar’s history, was working for Jon Lundberg at the time; I remember exditedly walking into that store and showing Richard the very first guitar I ever made. Don Brosnac was living in San Francisco then. All of us in this small community knew one another.

FOOTNOTE 4) For the record, and chronologically, here’s what later authors have recommend for top thickness:

• 1971, The Classical Guitar, by Donald Mcleod and Robert Welford: “between 2 and 3 mm” (This is a British book, unknown to Americans until much later than its date of first publication)
• 1981, Make Your Own Classical Guitar, by Stanley Doubtfire: “2 mm at the minimum”.
• 1986, A Guitar Maker’s Manual, by Jim Williams: .125” (3.2 mm) for steel string guitars, and .100” (2.5 mm) for nylon string guitars
• 1993, Making Master Guitars, by Roy Courtnall (classic guitars), including:
  • (1) Daniel Friedrich: 2.1 mm in the middle; 2.2 mm at the periphery; 2.5 mm in upper bout (note that this is the only maker on this list who makes his tops thinner in the middle! I know that luthier Dake Traphagen has worked the same way, but this technique has to be the subject of a separate article)
  • (2) Jose Romanillos: appx. 2.75 mm in the middle to 2.0 or 1.9 mm at the edge
  • (3) Robert Bouchet: 2.0 to 2.1 mm thickness overall
  • (4) Roy Courtnall: no less than 2.5 mm in the middle, or 2.0 to 2.3 mm at the edge
• 1996, The Guitar Maker’s Workshop, by Rik Middleton (classic guitars): 2.5 mm at the center to slightly less than 2.2 mm at the edge, but not less than 1.5 mm
• 2004, Build Your Own Acoustic Guitar, by Jonathan Kinkead* (steel string guitar): “1/8” (3 mm)” * also spelled Kinkade throughout the book
• 2006, Step-By-Step-Guitar Making, by Alex Willis (steel string): 3/32” (2.5 mm) at center, and 5/64” (2 mm) at perimeter
• 2007, Classical Guitar Making, by John Bogdanovich (classic) .100” under the bridge, and .090” to .095” otherwise 1/16” (.0625”), or 1.59 mm;
• Mr. Tatai: 0.075” to 0.050”, or 1.9 mm to 1.34 mm (the thicknesses of a nickel and a dime)

FOOTNOTE 5) I should add, as a caveat, that the canonic ‘Spanish lutherie tradition’ is Andalusian and Madridean — where the most famous Spanish luthiers worked — but not Valencian. Valencia, the home of the Tatay family, is on Spain’s East coast; and it seems to have been more a center of production-oriented lutherie. According to Google, the Tatay Company has grown into a concern that currently produces 40,000 instruments annually. As far as I know, no Andalusian or Madridean maker operates at that level. Also, in illustration of the importance of the Valencian school in Spanish guitar making, the Casa Zavaleta’s (Zavaleta-guitarras.com) inclusion on Google cites more than two dozen historical guitar makers of that school and region.

FOOTNOTE 6) A strong third reason — besides the absence of a strong crafts tradition, but closely associated with it — is the absence of a strong teaching tradition. I won’t beat the existing guitar schools up for doing the best they can: they are all, after all, fairly new arrivals on the scene. But their efforts don’t (and cannot) extend past a beginner’s level education in making-and-assembling-guitar-parts. This education lasts as little as ten days to as much as several months. It’s a great starter kit but, necessarily, cannot be more than that.

In comparison, there are respected schools of furniture making that turn out competent journeymen craftsmen and which put their students through several years of training — which includes design, proportion, a variety of woodworking techniques, history, joinery, and finishing. The better violin-making schools have a four-year curriculum! A large part of the problem is that many people simply don’t know that there’s any more to making a guitar than its just being a more complicated woodworking project than, say, making movie sets. You know: looking good but nothing substantial behind the façade. I think you can appreciate that just learning to put a guitar together — with very little actual joinery (sand-flat-apply-glue-line-it-up-and-then-clamp-it is not a difficult skill to master) or tone-making savvy going on — is not going to provide a realistic foundation for any kind of success. A hobby, maybe; but not an income.

The other important influences are:

• Even if we were to consider that a viable craft tradition in this area has by now been established, there has been an absence of individual makers whose work is important enough to have set a standard worth studying. Lutherie by skilled individuals is too new. The importance of a viable craft tradition is that craftsmen — if they are paying mindful attention to the work and their materials and not simply working to recipe formulas — are in effect continually seeking and prototyping new designs.
• Any interest in the qualities of steel string guitar construction and its relation to sound has been a scientific backwater. These instruments have been mostly considered folk instruments, designed to be bought and played, period, and uninteresting enough to be seriously looked at. They have lacked the cachet of having been subject any systematic, serious examination by scientists. Almost all of the studies that have been published are about classic guitars.
• The whole kit-and-caboodle-issue of the relationship between structure and sound has been bypassed by a focus, among manufacturers as well as players, on the use of amplification. Who needs to worry about the fine points of dimensional optimizing when one knows that consumers will expect to get their sound by plugging their guitars in and setting the dials of their amps and effects modules?
• Finally, there’s the bedrock influence of the Industrial Revolution. Three paragraphs before this footnote citation I’d mentioned that Sloane and Overholtzer’s recommendations of uniformly thick classic-guitar-top measurements actually come out of the steel string guitar making tradition in which the top is the same thickness throughout, without any selective tapering or thinning. This itself is rooted in the Modern Tradition of Industrial Production in which the wood is put through a sander, followed by the braces being glued onto the thinned-to-a-target measurement plate that the machine spits out at the other end. There’s much less craftsmanship, hand-work, or time-consuming concern with the fine points and subtleties expended in what are, basically, mass-produced products for a mass-market. The academic and intellectual implication of this is that if and when such instruments are formally studied, the results are based in the study of instruments that have all been made under these conditions, with no control group of a different architecture to compare against.

FOOTNOTE 7) This is a generalization, of course: there have been Spanish guitar factories cranking out guitars just as efficiently and formulaically as anything that these steel string guitar factories have done — with similar results as far as tone goes. But I believe this generalization holds up as containing some useful truth.

Also, the phenomenon of no one ever mentioning top thickness is merely a public one and, in private, it is not true that these dimensions go unquestioned. Many luthiers are fascinated by the idea of “correct” top thickness and live with a nagging suspicion that there may be ‘better’ top thicknesses out there than the ones they’re using. When luthiers get together the question ’how thick do you make your tops?’ is frequently asked. And makers often feel protective of that specific piece of information, if theirs differs from the norm.

And other makers don’t. An example of this comes from a conversation that I once had with flatpicker extraordinaire Dan Crary. He told me that when Bob Taylor — whose guitars Crary has long played and endorsed — took him on a tour of his production facilities, Taylor explained to him the tonal reasons for his guitar tops’ being made to exactly .109” thickness.

Incidentally, none of the methods, techniques, procedures, or measurements so far mentioned are “wrong”. Far from it. All of them are merely an account of How Things Have Been Done At This Or That Time. And all of them offer a peek at Truth. One can appreciate this by noticing, for example, that none of them urge that guitar tops be made 1/4” thick.

FOOTNOTE 8) Brazilian rosewood (dalbergia nigra) was originally used to make marimbas: the sections were simply cut to size and length that would produce a specific musical note!

There is, in addition, a separate category of guitar making woods that are also called ‘tonewoods’ but really aren’t so in this sense of the word. That is, they are used for making guitars and will of course therefore make sounds, but they don’t have anything like the vitreousness of true tonewoods — or perhaps only a little bit. There are also selections of normally ‘live’ woods such as rosewood, spruce, cedar, etc. that don’t give you much sound: that’s where proper wood selection comes in. A large separate category of notvery-live woods, furthermore, is made up of the visually spectacular species such as the figured/ornamental maples, walnuts, and mahoganies. Figuring is a direct function of plentiful movement and irregularity in the grain; the greater the figure the crazier the grain. This feature always makes such woods less stiff than a straightgrained sample of the same material is, and therefore less able to vibrate in a vitreous, sustained manner; they’re ropey and floppy rather than brittle. The sheer beauty of such woods sometimes makes up for their less-than-full sound, but the fact is that such materials serve, mechanically, to absorb string energies rather than to move with them. The sound will consequently be shorter in duration (less sustain), and will be mellower, less rich, and with less bite and sparkle. Nonetheless, under string load, all of these will make sound.

FOOTNOTE 9) The physics of sound-producing energies dictates this. It takes much more energy to generate bass response than it takes to generate high-frequency signal, and the nylon string guitar has a much smaller energy budget than the steel string guitar does. If you designed a nylon string guitar to use that limited energy for bass response (as in the guitarron), you wouldn’t have much treble response at all.

FOOTNOTE 10) To further underline the importance of looseness to the billowing action, imagine a ship with sails made of plywood; the billowing action will pretty much cease. While loss of monopole is not disastrous in a sailboat (it merely needs adequate surface area of sail, without it being so heavy that the boat will capsize), a guitar needs topwood that will move.

FOOTNOTE 11) You can get a fuller sense of this by looking at a table of cubed quantities. You can immediately see that the Cubed intervals are bigger in one direction and smaller in the other.

by Ervin Somogyi

I’ve been building guitars for 22 years. I started out building flamenco guitars, which are my first real love. I still play them. Flamenco sank its harpoon into me early on and hasn’t let go; it’s a wonderful music.

The flamenco crowd, I soon learned, is not able to support a luthier. They have enough money to buy themselves Gaulois cigarettes, but that’s about it. The next logical step was to make classic guitars. I quickly found that the classical players are, to the luthier, not a very user-friendly group. They are picky and critical, and since I basically didn’t know what I was doing I found it impossible to please them. It was not a happy experience.

Steel string guitar people have a very different mind set. They are by and large prone to being uncritically friendly, accepting, and encouraging. Their comments are liberally peppered with statements like “Wow!” and “That’s great!” I found their company very appealing. I was able to thus delude myself that I was doing something worthwhile — until 1977, when I was invited to participate as one of seven exhibiting luthiers at the important Carmel Classic Guitar Festival. It was a turning point for me. Seeing my instruments next to those of serious and competent luthiers forced me to reevaluate the quality of the work I had been doing. Up to that point I had managed to live a fantasy and make a very meager living at it. I’ve solved one of those problems since, but I’m still working on the second one.

In spite of their critical stance, the classical crowd has a very useful tool: a disciplined approach which is in large measure missing among steel string luthiers. The classical guitar people that I meet really seem to pay attention to what they’re doing in an organized way. They measure, they listen, they interact much more freely and much more sophisticatedly with guitar players. They have a greater vocabulary in common about tone color, what the guitars do, what they don’t do. Steel string guitar people do not yet have the tradition of this kind of discipline, but I think that will change when the more freeflowing character of the steel string guitar world recognizes the benefit and advantage of it.

The guitar is a relative newcomer to the stringed instrument scene. Before the guitar, the lute was absolutely the most popular plucked string instrument in the Western world. The lute served a very specific musical purpose, which it was no longer able to fulfill as the nature of musical tastes and entertainment changed with the rise of the European middle class. The lute became less and less a courtly chamber instrument and was more often expected to perform for the new bourgeoisie in larger halls and rooms. That was when the problems began. Lutes were really not loud enough. Instrument makers kept making lutes bigger and adding strings, but there were many problems with these approaches. The piano came into being in this period for the same reason: its sound could carry.

As far as I’m concerned, the guitar replaced the lute as the most popular plucked string instrument because it was able to solve the problem that the lute could not: being heard in a large room. It did so through the development of three specific design features: the bridge with a saddle positioned in the middle of the soundboard; the vibrating back; and longitudinal bracing.

Let’s talk about the bridge first. The lute’s bridge is simply a tie block. The forces acting on the bridge of the lute are almost exclusively in line with the pull of the strings. This drives the face in a specific and limited way, tending toward high-frequency, low-amplitude motions. (See Figure 1) The significant thing about the design of the contemporary guitar bridge is that not only is there a longitudinal force acting, as in the lute, but by virtue of the break angle of the strings as they pass over the saddle a vector force is created which actually pushes downward on the face. This drives the face in a different way, creating different tonal possibilities. (See Figure 2)

The fact that the guitar bridge is in the middle of the lower bout creates yet more tonal possibilities, but I’ll get back to this point later.

The saddle, in addition to helping to create the strings’ downward push, is an important coupling mechanism between the strings and the guitar face. If you do nothing to the guitar except change the saddle from a tight fit to a loose fit, you will absolutely lose volume. The fit is critical. Physicists have found that the guitar is a very inefficient sound-producing system. On the average, for each stroke of the strings, about 95% of the kinetic energy thus generated goes to mechanical vibration and is also dissipated as heat and friction. Only about 5% of the moving energy of the strings becomes sound — and if you lose some of this energy at the saddle you will get a disproportionate loss to the instrument. I am concerned that the material the saddle is made of be noncompressible, hard and stiff. I don’t use plastic because I feel it will absorb and damp some of the string vibrational energy. I’ve almost always used bone, except at one point where I was using melamine from cut-up dinner plates.

The average saddle is 3/32″ to 1/8″ wide. I use saddles that are 3/16″ or more wide because it makes a saddle wide enough to offer adequate intonation compensation on a steel string guitar. The rationale for intonation compensation is the subject of another talk, but I want to say one thing about the contact characteristics between the strings and the saddle. If you have two virtually identical guitars one of which has the strings resting on a single high point of the saddle, and the other with the strings resting on a significant portion of the saddle’s top, I think you’ll find this second guitar works better. There is a better and more efficient coupling of kinetic string energy to the saddle by virtue of that extended contact. I can’t prove this, but I suspect if you pay attention to it you’ll get better results. (See Figure 3)

The second feature of the guitar that made it more successful than the lute was the vibrating back. The lute has a sound chamber with its own natural resonant air frequency, as does the guitar. But by virtue of its construction the lute’s sides and back are one piece and very rigid. If you have a second vibrating diaphragm, which the guitar back is, more possibilities come into play because the guitar’s back is active — it actually does something. Try this: put your guitar in its case and gently tap on or near the bridge and listen to the sound you get. This is the sound of the top only, with the back damped. Then lift the guitar partially out of the case so it’s not lying on its back and tap it the same way again. You’ll hear a very different sound. This is the sound of the top interacting actively with the back. In a similar way, when you are playing a guitar the strings excite the bridge and the face and then the air mass, which in turn excites the back, and the back starts to vibrate in some frequency relationship to the movement of the top. The most successful guitars have the back tuned in relationship to the top so that they act together and make a guitar that really projects. In physics this phenomenon is called constructive interference. When the top and the back are mismatched in vibrational activity they are in effect fighting each other, and this is called destructive interference. It is the same phenomenon you observed in high school physics class when you hung a weight from a rubber band and tried to move the weight by tugging on the rubber band: you had to do it at a certain frequency, and when you found that frequency the weight moved with little effort on your part. Otherwise the mass of the weight created resistance to its movement. In this example you are like the guitar top — the driver — and the weight is the guitar back. In the guitar the back contributes to projection and sustain — or relative lack of them, if the top and back plates are working against each other.

The third factor in the success of the guitar was the pioneering use of longitudinal bracing, in tandem with a more centrally located bridge. Lutes had only ladder bracing, with the bridge at 1/6 the length of the face from its bottom. Longitudinal bracing allows the bridge to couple to a larger portion of the soundboard than otherwise and impel it to movement. Positioning the bridge nearer the middle of the soundboard helps in this, because a central point of initial impetus usually means you can drive a larger plate and larger air mass. This simply translates to greater volume. Tap at the edge of any guitar soundboard, and then tap in the middle and listen to only the loudness of response, regardless of tone: you’ll immediately hear what I mean.

The importance of longitudinal bracing is central to the success of the guitar because the guitar is basically an air pump, and in lutherie we need to concern ourselves with how efficiently the guitar can pump air. All other considerations, such as choice of woods and how pretty they are, must be subordinated to this if you want to make a successful guitar.

One way you can get a reading of this air-pumping function is to gently tap a guitar top at the bridge while holding your other hand in front of the soundhole. You will feel a displacement of air from within the sound cavity. You can feel the guitar breathe on you. On better guitars you can feel more air coming out because the top is more responsive; it responds more to the energy of your finger. Dead-sounding guitars won’t be found to breathe on you as much.

A second way of getting a reading as to how freely the guitar is able to pump air is in the sound of the tap tone you just delivered, if you did this exercise. Again, this is very subjective, but still a very useful comparative way to get some sense, some keying in as to what is going on. If the guitar top is tight, stiff, overbuilt, heavy past certain limits (which most commercial guitars are), it’ll sound somewhat like a table top — high, tight, and solid. If the guitar top is free to move, the pitch of the tap tone will go down and the sound emitted will be markedly more open. Obviously there are limits in making soundboards light past which you shouldn’t go because you’ll destroy the integrity of the guitar, but most guitars are so overbuilt that I don’t think you have to be afraid of going in this direction. If you will go around the guitars in the exhibition hall, I’ll be very surprised if you don’t find that the better ones are just more responsive when you do something as simple as tapping.

There are yet other ways of keying in as to how sensitive the guitar is, how sensitive to the vibration and energy of the strings the guitar top and back can be. One fun way is to use a so-called super ball. If something has high internal damping then energy put into the system — in this case the kinetic energy generated via gravity by dropping this ball to the ground — is dissipated by mechanical distortion into heat and internal friction, and this ball won’t bounce back up very high. This particular ball bounces up off the floor almost to its starting point, so it does not have high internal damping. You can get one of these balls and go around the exhibition hall… (laughter) …and bounce this off the tops of some guitars. Some guitars will produce a higher bounce than others. This is kind of a childishly dumb thing to do but it’s a lot of fun (laughter). Some guitars are instructive and you can really learn something from this: the higher the bounce, the more solidly the top is constructed; the less the bounce, the more yielding and responsive the top is. This is really not much different from what the strings do, except that strings are expected to make music when they move the soundboard.

While I believe the degree to which a guitar is successful is in direct relation to the extent that you can free the soundboard up to pump air, this is only part of the story. There are many specific ways in which the guitar top moves, vibrates and flexes in its use of string energy so as to generate a wide spectrum of tones. The lower bout, the area surrounding the bridge, is the main arena for this activity. Let’s examine some of what goes on here when the bridge moves.

How does the bridge work? In what way will the bridge couple with the face? In what way does the bridge transmit the kinetic energy of the strings to the rest of the system so we can ultimately hear music? The guitar bridge moves in three modes: (1) it pumps up and down as a unit; (2) it rocks backward and forward in line with the pull of the strings; and (3) it seesaws sideways around the center line, at right angles to the strings. (See Figure 4) You get a hands-on sense of how much the bridge moves laterally in the following way. Put your fingertips very lightly on one end of the bridge, then tap on the other end. Unless the guitar is heavily overbraced you’ll feel lateral displacement as the bridge rocks from side to side. You’ll feel the motion on one end of the bridge as you tap the opposite end. You may feel a lot or you may feel only a little, but you will feel something. Classic guitar bridges move laterally a lot more than steel string guitar bridges do, by virtue of both construction and bracing.

I’m going to bypass entirely the subject of top selection so let’s assume for the time being that we’ve picked a good piece of wood for the top. Then we have to figure out how to work and shape it. I’m going to try to simplify for you my image of how the top vibrates. I have the innards of a little music box here. This is dime-store stuff but it’s a useful indicator of a fundamental principle of guitar dynamics. (Winds up music box and holds up to the audience). Now this thing here, nobody can hear it. You’re not supposed to hear it. There’s no resonator on it. It’s not exciting enough air for anyone to hear it. The instant you put this on a surface that can take its vibrational energy and excite more air… well, let’s see what happens. (Places the music box on the surface of a table) Maybe you people in the front rows can hear this? Now this isn’t a guitar, it’s a piece of fine furniture. (Laughter). When I place the music box on the top of this guitar the sound can probably be heard a few more rows in the back. So, the guitar will take the energy and excite yet more air with it. And what of the specific sound quality? To make a long story short, what I hear when I put this gizmo on the perimeter of a guitar top is a bright and tinny sound: the high notes are really kind of shrill and piercing. The bass notes in the song are not very distinguishable in quality from the high notes. Now I put the music box, this sophisticated frequency generator, on this guitar near the bridge — a very important place. I notice that there is much more range. Placed over the bridge I can hear lows as well as the highs. This is not a $4000 signal generator, but it shows what a guitar will do when stimulated in different places. If we did this enough we would conclude that driving of the midportion of the guitar face results in a generally fundamental, bassy, full, and loud sound. Activation of the perimeter results in treble activity. This makes perfect sense, as the center of the soundboard is the most yielding part and is thus able to support high amplitude, low frequency activity. The edge, being more rigid, is the logical place for high frequency, low amplitude activity to occur. The top can move as in Figure 5, acting more or less as a unit. That’s bass. The top can also move as in Figure 6, shaking and quivering like a bowl of jello left on your washing machine during the spin cycle. That’s treble.

This is an important clue to brace design. In planning a bracing system, these are the kinds of top motion which you have to plan for, and plan for with respect to a point of primary excitation of the soundboard, in addition to considerations of selection of woods, thicknessing, and bracing. That is, whatever else you do, you have to plan it with regard to where the bridge will be.

Historically, the bridge was placed smack-dab in the middle of the lower bout of early guitars, with a twelve-fret neck sticking out of the body. (See Figure 7) In the 1930s, in response to musicians wanting a more fully accessible fingerboard, steel string guitar makers discovered that players could have an extra two frets worth of fingerboard by making a fourteen-fret-to-the-body neck. This involved shifting the point of bridge attachment that much nearer the soundhole, but bypassed the need to install a cutaway into the instrument. It was a quick fix, and a successful one. But it also required planning the bracing around the new point of bridge attachment. If you install a bridge at the fourteen-fret neck position on a guitar previously braced for a twelve-fret neck you will have acoustic interference from the bracing, and a less than successful guitar. Bracing work involves positioning, as well as shaping.

Bracing is a complicated and never-ending puzzle. I’m sure that throughout my lifetime, and maybe even my children and grandchildren’s lifetimes, controversy will rage about what is the best bracing system. The fact of the matter is that successful guitars have been built with just about every conceivable bracing system. Superb guitars are built with symmetrical bracing systems. Wonderful guitars are built with asymmetrical systems. I’m led to believe that a bracing system as a recipe approach has little virtue. It’s best when it’s part of a context, an thought-out process. To say only that I use X bracing, or Sitka spruce, is by itself relatively meaningless. I will touch on this again in a few minutes.

Traditionally, guitars were strung with gut and came out of a European tradition of individual people like ourselves working in small shops. They paid a lot of attention to what they were doing, the skills being passed on by the best makers into their families for generations. The tradition of making steel-string guitars, however, is American and has almost always been a factory one. Until recently there have been very few independent luthiers making steel string guitars.

You’ve all seen this kind of guitar top before: this is a Martin guitar top. This is the standard today. It is the pattern, the most common way to construct and brace a steel-string guitar in the world. Almost everyone copies it. This is the Somogyi version of the X braced guitar top. Let me talk to you about how these two differ. Before I get into this, though, I want to say that I’m not picking on the Martin guitar design; I have this Martin top because the Martin people were nice enough to give me one when I asked. Other companies I asked a top from, didn’t.

To understand my thinking about guitar soundboard design I want to talk to you about the strength-to-weight ratios of woods and about basic guitar dynamics. The strength-to-weight ratio is a number that expresses how strong or stiff something is, per unit of mass. It is my opinion that it is important to know this about the woods you use. I place weights on my woods when taken to certain standard sizes and measure the deflection; weigh in grams for a measure of mass. Guitar dynamics are the study of how a guitar top moves.

In his book Understanding Wood Bruce Hoadley says that the load bearing capacity of a piece of wood is reduced by 50% when you reduce its height by 20%. This is a pretty startling statistic. So if you take, as an example, a floor joist that is a 2 x 10, and then take another that is a 2 x 8, the latter has only half the load bearing capacity of the former. This strength-to-height relationship has some bearing on the design of guitar braces.

This is a cross section of your average guitar brace. (See Figure 8) Usually it’ll be quartersawn or close to quartersawn. The possibilities for designing braces are virtually endless, ranging along a continuum from low and flat to high and thin, and complicated by whatever contour of scalloping and high points exist along their lengths. What I want to achieve when I make a guitar is done in part by picking woods that have a favorable strength to weight ratio. I want woods that are strong and lightweight. Therefore one way in which my braces are different from standard lies in my choice of materials. Secondly, I and other luthiers like myself are concerned with maximizing the strength to weight ratios of our soundboards. We want to make the guitar as strong as we can while making it as lightly constructed as we can. We don’t want to reduce the height of braces significantly, because as Mr. Hoadley points out, the cost of this to brace strength is considerable. But we want to reduce the mass. I would like to invite you to later step up here and pick up both of these sample tops and compare just how much wood is in each one. You’ll notice that one feels noticeably heavier than the other. My way to achieve this difference is to leave the height on the brace but to lessen mass by making the brace slimmer. This probably does remove some strength, but I believe that it does so in a favorable relationship to the weight that is lost. My approach is to have thin, high braces throughout my guitars. (See Figure 8)

Fred Dickens: You seem to be using the terms stiffness and strength interchangeably. Is that your intention?

I take my woods down to a standard thickness which is greater than the final thickness, put a weight on it and measure how much deflection there is. I call this stiffness and do use this word interchangeably with strength.

About guitar dynamics: are some fundamental differences between steel string and classic guitars, which affect bracing. In part because the steel string guitar is driven by a much heavier metal string, the tendency for it is to be very bright and trebly. The nylon string guitar, on the other hand, wants to be bass-heavy (within the potentials of nylon string response) when left to its own devices. The job of the luthier is to work the wood so as to shift the response spectrum in the desired direction. It’s very hard to make a well balanced classic guitar that has a clear, ringing treble: that’s the whole trick. Likewise, it’s really difficult to make a steel string guitar that has a rich, deep, satisfying bass. You have to do specific things to the soundboards to achieve these things. Furthermore, a luthier has to figure out how to achieve a balanced sound — how to get energy input from the point of primary excitation of the face — the bridge — in both types of guitars to adequately effect both high frequency, low amplitude vibrations and low frequency, high amplitude movement. While there are endless things to be said about bracing systems, plans, and distribution, as well as the selection of bracing woods, I want to remind you of the indicator that we got earlier from that little music box where we heard bright, shrill notes from the edge of the soundboard and heard mellower notes from the soundboard’s center. It reveals that one should pay attention to the perimeter if one wishes to manipulate the high end frequency response of the guitar, and pay attention to the area near and around the bridge if one wishes to manipulate the low end. (See Figure 9) For these reasons you will notice that the profiles of the braces in my guitar top differ from the profiles of the braces in the Martin guitar top, even though their layout is about the same in both cases: my X is lower and my perimeter bracing is higher.

In the steel string guitar, an X brace system is almost always notched together at the intersection of the two main braces. This is an important load-bearing point, and it takes us back to our thinking about strength-to-weight ratios and Bruce Hoadlely’s formula. The main effect of notching the legs of the X brace so they can be fitted together is to turn two large, strong braces into the equivalent of two much smaller and weaker — but still massive — braces. (See Figure 10) This circumstance is bound to affect the response you get from a guitar. The typical factory way of capping the otherwise open X notch is to glue a little piece of muslin cloth on top. If you are concerned with strength to weight ratios and the load bearing capacity of your braces, I think it makes no sense to shoot yourself in the foot by cutting a huge notch in your braces, after all the work you’ve done, without trying to reestablish their original strength by bridging the notch cut. (See Figure 11) Bridging, or capping, the X notch will tie one part of the interrupted brace to the other part, and will prevent the opening and closing of the notch under soundboard movement.

While the X brace is ostensibly designed to have a certain strength, but the minute you cut a notch into it and leave the notch uncapped it’s much, much weaker than before.

One way that I know this is important is by testing it. I made a guitar some years ago which had a capped X brace, but its sound was not satisfying to me. I thought that the bracing was too stiff for the sound that I wanted and I concluded that I would get better results if I shaved the X brace down. I reached in through the soundhole with a palm plane and shaved about 3/16″ off, right through the cap. With the newly-lowered X braces and opened X joint, the character of that guitar’s bass response changed radically. I had changed the strength to weight ratio of the X too much. I recapped the X brace at its intersection and again the sound changed noticeably, for the better. What an astonishing difference removing and regluing a fraction of an ounce of wood made, so long as it was in the right spot. Cutting through that little cap turned out to be a useful mistake for me to have make.

There are a lot of people now capping their X brace joints, including the Japanese. They, by the way, seem to pay more attention to details in the construction of their guitars, even the factory ones, than any American manufacturer I’m aware of.

Audience: have you ever taken the cloth patch from the intersection of the X brace and replaced it with a wood bridge?

I have, but you have to cut back quite a bit to get a flat enough gluing surface to get a cap onto. It’s more of an operation than you might think. One thing you’ll notice on my guitar top is that even though my X braces are tapered in cross-section they are rectangular in cross-section at their intersection: this makes a really tight and very efficient joint. (See Figure 12) I don’t take any wood from inside the notch, as happens when braces are rounded before they are notched together.

Audience: you talked about the shape of the braces but could you talk a bit on the angle of the intersection of the X brace?

My way of thinking is to concentrate on the bridge activity in relation to brace position. These always work in relationship to one another. Let’s take the Martin X brace pattern for an example: the wings of the bridge are normally coupled onto the legs of the X brace as you can see here in Figure 13. Do you recall my example some minutes ago of tapping on one wing of a bridge and sensing the movement of the other wing? In all guitars this is an important bridge motion. In the steel string guitar this coupling is defined by the angle of the X and allows certain degree of bridge rocking. This motion will support the creation of a steel string guitar sound. This sound, then, is made as a function of the effectiveness of the bridge-brace coupling.

In the classic guitar the bridge has more freedom to move in this rocking mode, because the classical guitar usually has longitudinal braces which more or less allow this lateral movement. Those of you who were at Robert Ruck’s workshop might have heard him say that some of his models that had angled fan braces responded differently. It is my belief that these angled braces are dynamically and acoustically replicating the anchoring work of the X brace, which acts to inhibit bridge rocking compared with longitudinal bracing.

Therefore, if we change the angle of the X on a steel string guitar so that the bridge is not so fully held back in its rocking movement, if the bridge is freed or otherwise encouraged to move more actively in this way, then the response spectrum of the guitar will be shifted toward that of a classic guitar — you will get a higher, brighter sound. So that’s my way of answering that.

Colin Kaminski: could you comment on bridge patch size, selection and stiffness and what kind of tone you can expect from these variables.

I had a very instructive disaster some years ago. had built a guitar that had the most wonderful, heavenly bass of anything I had ever made. The treble was OK but the bass had a gorgeous, lovely tone like honey, full and rich, and unencumbered by harmonics that you could distinguish. I loaned it to a friend who mistreated it. He brought it back after a month, during which time he had had another luthier cut the bridge down. This was unforgivably stupid of him and I was very upset. But mainly, the guitar no longer had the tone. I replaced the bridge. I weigh everything: I think one of the most useful lutherie tools is a triple beam balance. Working with strength-to-weight ratios means needing to know how much things weigh, and I don’t want my bridges to be heavier or lighter than a certain optimal range of about 35 grams. Anyway, this other luthier had messed up my bridge so I put a new one on, reestablishing an original parameter of the instrument. But the tone didn’t improve. I didn’t know what else to do so I hung the guitar on the wall. One day I was walking by it and noticed that the light from the windows reflected in the lacquer of the face revealed a noticeable deformation. I became aware of it then because the light was focused on it. The face was markedly distorted and dimpled specifically at the bridge and all around it. What had started as a straight, flat piece of wood had become bent all around the bridge by the torquing action of the strings. And this had been allowed to happen because there was virtually nothing under the bridge to guard against this kind of deformation. At that point I started to install bigger bridge patches on my guitars. (See Figure 14)

One of the things I’ve learned to do in lutherie is to think long term. The guitar is not going to be the same down the road. In a year or so woods will have settled, strings will have pulled something into or out of alignment, and this is one of the forces which guitars are subject to. The distortion in this guitar was caused because I had put in a very small bridge patch, and I don’t do that any more. It cost me the most wonderful bass I’ve ever had, but I learned something from it.

Audience: Did you ever know what caused the loss of that great tone?

No, I never know anything for certain. Everything in lutherie is intelligent guesses. My guess was that because I had noticed a sudden, marked deformation of the face around the bridge, a physical change had occurred which affected the tensile strength of the face and affected its movement. My response t it was to replace the bridge because it had been cut down, and I chose not to put a new bridge patch under it later. I don’t believe it would have distorted as it did had it had a larger bridge patch to begin with — and perhaps it would not have had the same tone then, as the mass would have been different. But whatever it had, it would probably have retained its sound. I kept the guitar on the wall and it was sold to someone who was wonderfully happy with it.

This is another factor in lutherie. The instruments you’re going to like, other people might not; the instruments you might feel indifferent to, others may not be able to live without. So that story had a happy ending. Someone got a good guitar and I learned something.

Audience: you say anything about the domestic woods which are being used instead of the traditional imported woods?

Yes. Do you remember my comments on the nature of the constructive interaction between the guitar top and the guitar back, in a better guitar? Well, the shift from the traditional rosewoods into the “alternative” woods will have to come to terms with this. These are not going to be bad woods, but they will do different things and we need to come to grips with that.

Brazilian rosewood is like glass, and it goes “pinnnnnnngggg” when you tap it. It has very little internal damping, which means that when you put energy into it in the form of a tap (or playing on a string) it’s not dissipated away in heat and internal friction. It’s retained in the form of mechanical vibration, excitation of air, and hence sound. The maples and mahoganies and walnuts have much less of that. So as the back begins to move, if the back has more internal damping, you’ll get a different sound, one characterized by shorter duration of tone. To the degree that you can couple the action of the back with the action of the top you’ll have projection, but this relationship needs to be worked out for each wood individually. There’s no reason you can’t have a good non-rosewood guitar. But I find that talking categorically about woods is not very useful because there can be such great variation within a species. maples are wonderful; others I’m not interested in. The same with walnut. Some koas are as dense as rosewood and some are practically so light that if you sneeze the board blows over. Mahogany, likewise, is much more different from board to board than rosewood is. That’s the virtue of rosewood: it’s much more uniform in working properties. I can only say, good luck to you in your quest for the best sound you can make. Good luck to all of us.

(reprinted from American Lutherie, #36, Winter, 1993)