Acoustic Guitar Construction

From Cumpiano's Blog:

There are almost as many schools of thought regarding bracing systems and patterns as there are luthiers. My approach is different from the great majority of builders. The more common view is that besides limiting the distortion of the tension-loaded soundboard, the particular pattern or scheme molds or “guides” the sound to some undefined ideal, and insist that the traditional “Martin” x-brace arrangement produces the iconic sound on steel string guitars and the “Torres” fan bracing system produces the iconic sound on nylon string Spanish or Classical guitars. Beyond that, most luthiers will add or subtract braces, or create braces that arch over other braces or play with the angles between them or their size and numbers in an effort to reach some sort of elusive or ill-defined sonic goal. The vast majority of builders are however, unschooled in even the most fundamental tenets of wave physics or acoustics, so they are flying blind or expecting to be winners in some sort of sonic lottery. Others fall into the trap of chasing traditional myths, such as the belief that the soundboard produces all the sound of the instrument [it produces only a limited part] and that it can be divided into bass and treble halves, i.e. that the bass response can be manipulated by altering the braces under the bass strings and the treble response the braces under the treble strings. So the field of guitar making is largely populated by finish carpenters who think they can handily control and mold the resonances of an impossibly complex vibrating system.

My having been mentored by an acoustics scientist has freed me from much of that baggage. Accordingly, my approach has been a stark departure from the norm. My approach is to work within the traditional forms while striving to achieve what I call minimum adequate structure. What that implies is that the guitar is not the creator of the sound, that the true source of all of its musicality is the string's harmonic series. Thus Mario Macaferri's dictum, the string is the heart of the guitar. Indeed, the guitar actually gets in the way, clouding and diminishing the string’s perfect signal—because it is essentially a cultural artifact, not an optimized energy transformer. The result of this fact is that it is a crude and inefficient energy transformer, succeeding in transforming only a small fraction of the string’s kinetic energy into acoustic energy. The major portion of the string’s kinetic energy is frittered away by the guitar as heat. So the luthier’s dilemma is how to take this fetishized cultural artifact—which due to its traditional form and structure, is inherently a poor acoustic machine—and create out of it a usable, hopefully desirable, tool for musical expression.

The way I reconcile this dilemma is to try to get as much of the guitar out and away from the strings, that is, to make its impediment to the strings' signal as minimal as possible. I’ve reduced as much mass and structure from the traditional model as is possible, while focusing primarily on the architectural requirements imposed on the structure by string tension. This is what I call minimum adequate structure. If the structure is excessive, the beauty is muffled, limited, impaired. If the structure is inadequate, the guitar will slowly fold under tension and become unplayable.
So my guitars, then, emerge as the inevitable product of my unique approach. You’d have to hear and play them to know what that means exactly. This is admittedly a long answer to your request for bracing information. If you’re using carbon fiber instead of spruce you have to be aware of 1) the different stiffnesses/mass ratios of the two materials 2) the amount of tension stress which is being dumped onto the soundboard and 3) that little detail that luthiers usually ignore, that the distortion of a plate under load increases as a cube of its span. Obviously, the width of the soundboard plate and the actual load at the bridge are crucial factors that must be taken into account if you’re optimizing the system.
Posted by William. R. Cumpiano No comments: Links to this post

The article is a bit condecending, but I agree with his message.
Don't be a unique builder, make good guitars. If it is about the guitar.
Herman

"fetishized cultural artifact", brilliant!

I agree with condescending, I am not sure I buy most of what he says especially ...

the string is the heart of the guitar. Indeed, the guitar actually gets in the way, clouding and diminishing the string’s perfect signal—because it is essentially a cultural artifact, not an optimized energy transformer.

If that were the case he should make electric guitars or very very stiff acoustic guitars like ovations with amplified pickups then the strings with their great harmonics and perfect signal can ring cleanly without the cultural artifact of the guitar body.

I view it the opposite of what he lays out so I guess I am one of the cabinet makers, to me the guitar with its inefficiencies and complex resonances creates the beautiful voices we love in a guitar which the string can excite. The voice is not the string but the guitar body and its interactions with its parts including the strings.

I do get building lightly and efficiently as the strings especially nylon are only able to carry a limited amount of energy from a pluck.

John, you are not a "cabinet maker". Indeed, you are on target to call out against Cumpiano's assertion it is all about strings, in which case electrics would be the alpha and omega of guitar design, as you say. Instead, it is all about what wood does to modify as well as transmit the sound of the raw strings. I agree Cumpiano and others here that the least amount of wood necessary to meet the various requirements is a generally sound approach to design and construction. However, Brian Burns has discovered that his heavier instruments, made heavy by laminating the sides to make them less able to vibrate, results in a better concert instrument, capable of filling large rooms, due to the greater movement of the top necessitated by the resistance of the sides. For small rooms, where most people play, his lighter sides work well enough to fill the room, and transmit some of their vibrations to the body of the player, as well feel pleasantly lighter to hold, and so are often preferred.

And as far as bracing goes, the reason we have traditions is traditions are proven to work. That does not snuff out innovation, especially innovation that augments the good qualities of tradition. Somogyi's tall, thin X-bracing coupled with very low mass tops (.050 inches in some cases) is a good example.

made heavy by laminating the sides to make them less able to vibrate, results in a better concert instrument, capable of filling large rooms, due to the greater movement of the top necessitated by the resistance of the sides. For small rooms, where most people play, his lighter sides work well enough to fill the room, and transmit some of their vibrations to the body of the player, as well feel pleasantly lighter to hold, and so are often preferred.

John, I love this point. I do not often see it pointed out that concert instruments and intimate setting instruments can and should make different trade offs. I see this especially with classical guitars like a smallman guitar basically a very light top mounted on a massive plywood framed body. Very loud and efficient. In your living room or playing for your own enjoyment an instrument that has an active back and lighter overall can make a wonderful sounding and complex voice. Sure not quite as loud but as loud as you would want in an intimate environment.

In steel string world stage hands and performance artists really like ovation acoustic guitars as they are built so heavy they have a clean voice, the sound guys can use it as input and do whatever they want without having to suppress the instruments voice and feedback. They are very boring played in a more intimate setting.