Acoustic Guitar Construction

I have 4 pieces here some tight grain, medium, and wide. Which pieces would be better for top and the back or does it matter. I will be fixing the angle of the grain on my band saw. Thanks guys for chiming in. The two on the right are really tight you can't even see it!

All good -- does not matter -- focus on the deflection and tap tone of the sound board assembly. You'll know when the sound board is responsive otherwise it sounds like a table top or piece of cardboard. As you build more you'll decide what "you" like to hear.

Some may think the following is overly scientific and not worth the trouble, and I won't argue with that since I've only built a few guitars so far and to each his/her own after all, so feel free to tune out if you don't like this kind of approach but here's what I do. In the Gore & Gillet book there is a simple procedure for measuring the Young's modulus of brace material. Basically, square up the piece and measure its dimensions, weigh it, and calculate the density (it's an Australian book so all metric units here). Then hold it about a third of the way down and tap in the center to excite the "marimba" vibratory mode. I record this into a PC, and measure the peak frequency with Audacity software (free). The Young's modulus can be calculated with a simple formula from the frequency, dimensions and density. You can also do it using deflection but the frequency approach is really fast once you have it set up plus it requires no fixture other than your fingers - it only takes me about a minute to measure a piece of wood.

A simple and reasonable measure of "goodness" for brace material is the ratio of Young's modulus divided by the density, which is loosely speaking its stiffness-to-weight ratio. To put it another way, that measure is inversely proportional to the weight of a brace if one reduces its width (not height) to get a given stiffness so if you want a given stiffness in a brace then E/d is inversely proportional to how heavy the brace will end up. For the most part heavy=bad, so with E/d bigger numbers are better and I've seen a large variation among different samples of sitka spruce I have measured. The best pieces I use for the main top braces, making them somewhat narrower according to E/d (as if I had it down to that much of a science which I don't!), middling ones for the smaller braces, and so-so ones for things like the upper popsicle brace that doesn't move much. It's not that one can really engineer a soundboard so precisely, and what Ken says about listening to the top still makes good sense, but I'm trying to make the best use of my materials by using the stiffer+lighter wood for the braces that matter the most and the lesser wood where it has less effect. Usually the denser-grained pieces have tested better but not always. I've gotten one or two pieces that were just off the scale bad, basically quite dense wood but also with very low stiffness, and those worked well as kindling. :-)

To understand my (Ervin Somogyi) 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)

I'm still a newbie so I mostly try to learn from the experience of others, and I've been making braces shaped like the one on the right - but does anyone really make them like the one on the left anymore? I just haven't come across examples of that so maybe enough people have read Somogyi's book and others like it that tall-and-narrow is the norm now. I dimly recall reading about one manufacturer claiming that the gently rounded tops of their braces produce a smoother sound than sharp-edged braces. Ahh, I sure love that mellow round-brace sound! You can't say they lack a sense of humor. I suppose a parabolic brace cross-section would help focus the sound waves for greater projection, provided that one also used acoustically transparent hide glue and sound-lubricating oil varnish. ;-)

but does anyone really make them like the one on the left anymore?

Of course there are --. I've said and wrote it many times "in the realm of guitar making its a good idea to refrain from and deciding or proclaiming what's best" -- guitar makers simply apply their preferences. Also note that even Somogyi gets questioned about some of his fuzzy not so conclusive scientific demos. And I believe you'd be hard pressed to find an established Luthier that changed a single aspect of their building procedure based on the info in the Gore books.

I too enjoy experimenting and doing research -- but quickly pass over any material that claims to have found the guitar making silver bullet or holy grail.

For me I've found -- keep it tight and light and it will be responsive. BTW I believe it was Steve Klein that once wrote an article that appeared in one of the guitar publication stating that in his opinion brace shape and pattern have little to do with tone other than keeping the guitar from falling apart. And too Roger Siminoff mentioned that scallop brace peaks are a waste of time.