Of top arches and domes and their relative merits

I'd like to briefly discuss the relative merits of two different top construction techniques. Antonio Torres (1817-1892), a Spanish builder known as the father of the modern guitar, introduced the technique of arching the lower bout of the guitar top in order to allow him to build a light top that remained strong enough to resist the pull of the strings. A light top allows a finer and more nuanced response from the instrument, and Torres quickly became famous for the wonderful musical qualities of his guitars. Torres life and work is documented in an excellent book by the classical builder Jose Romanillos called Antonio de Torres Guitar Maker - His Life and Work (ISBN 0-933224-93-1). This practice of giving the top strength with an arch has continued to this day, and a couple of different construction methods have evolved to facilitate it. This note briefly discusses two of these techniques, and looks at their respective merits.

The first technique is the one used by Torres and virtually all classical guitar builders from his time onwards. It is the one i use in my building practice. Torres built his instruments on a solera, or workboard, face down. Torres imparted an arch to his top in the area between the bottom harmonic bar below the sound hole to the tailblock thru the expedient of a solera which was concave in this area. The top would be placed face down on the solera, and the fan braces would be glued to the top and, when dry, the fan braces hold the top in the shape of the concave solera. Torres employed what is now known as the Spanish method of construction and his neck would be attached to the sides and top through the mechanism of a 'Spanish foot'.

Here's a close-up photo of the Spanish foot of a classical guitar i built showing the top, sides and neck in place on the solera. In the Spanish method the Spanish foot receives the sides, top and back of the guitar. Builders who adopt this method typically do not use a neck set. The top and neck are in a single plane, and a flat fingerboard remains flat when glued to the guitar. When they do adopt a set, the neck often tilts forwards a degree or so to obtain a lower bridge height and mass, and when this is done the solera is often progressively thinned or tapered from the top of the soundhole to the top of the body so that the neck and top half of the sound board remain in one plane.

The second technique is one that is popular amongst steel string acoustic guitar builders and some other small stringed instrument builders. This technique domes the entire top, from the top to the bottom. The top is placed face down on a concave form and the fan braces and harmonic bars are glued to the top, similiarly causing it to retain the shape of the concave form. Unlike the first method, the entire top is domed, not just the area from the lower harmonic bar to the tail block. In this method, the bottom of the harmonic bars are arched to match the dome of the top. Builders who have adopted this method typically use a neck block instead of a Spanish foot, and the neck is attached after the body is finished using some form of dovetail joint or mechanical fastening method. Builders using this method typically give the neck a set backwards by a degree or two to align the plane of the neck with the rising dome of the top half of the sound board, so that they remain a plane surface to accept the bottom of a flat fingerboard.

Here is a picture of a typical fan braced classical guitar top, showing the harmonic bars and fan braces. Many small stringed musical instrument builders have also adopted this style, using 2 or 3 fan braces instead of 5 or more. In this picture the two harmonic bars and 5 fan braces are clearly visible. The 2 harmonic bars (running vertically in this photo) cross the top, one above and one below the sound hole, providing strength in this area. The combination of 2 harmonic bars and the fingerboard keeps the top rigid from the area of the bottom of the sound hole to the top of the guitar body. The small areas to the left and right of the fingerboard provide small high frequency resonators, but by and large the upper part of the sound board can be considered rigid and not free to vibrate. Every builder has developed his own top bracing method and there are many variations, but the use of fan bracing and harmonic bars is common, though there are exceptions. Steel string acoustic builders generally adopt what is called an X-bracing system instead of fan braces, but in this bracing style variation the top half of the soundboard is also held rigid by the fingerboard.

In practice, the energy from a vibrating string is transfered by the bridge to the top, causing it to vibrate and produce sound. But to be responsive it must be free to vibrate. Because the top half of the sound board is restrained by the harmonic bars and soundboard, it is the bottom half of the sound board that does most of the work. And work it must, because it must be free to vibrate while resisting the pull of the strings, which would cause it to collapse if they could.

Some builders view the top conceptually as a rigid plate suspended from the sides by a compliant area (the recurve, or thinner parts of the top near the edges). But if you look at Romanillos's analysis of Torres graduation methods, or think a little about the problem, you'll see that strength is needed in the top mostly in the area between the center of the bridge and the bottom of the sound hole, because this is the direction is which the static tension of the strings pull. So in this conceptual model, we don't just have a simple rigid top suspended by a compliant recurve, we have something that looks more like the reed of a stringed instrument starting at the lower harmonic bar and extending down the center of the top to the bridge area and slightly further, with a compliant recurve continuing around the lower bout to the lower harmonic bar, thus forming a diaphram centered on the bridge and occupying the lower bout of the top.

Lets look now at these two construction methods and their consequences. In the Spanish method, its very easy to establish a plane surface for the fingerboard. In fact, you must work hard to avoid it. In the second method, you must correctly set the neck angle or/and flatten the fingerboard after it is glued on. The domed method is very frequently found in a factory production setting where bodies and necks are being manufactured separately and assembled, though individual builders have adopted it. The simple concave forms used to shape the top can be purchased from suppliers in quantity, ready made. The Spanish method is found more commonly amongst individual builder who are responsible for construction of an instrument from beginning to end and who are interested in understanding the ways and methods of those who came before them and are willing to build a solera and experiment with its rich potential.

The Spanish method differs in an important way from the doming method in another regard, its effect on the sound of the instrument. If you imagine a cross section of a domed top just below the lower harmonic bar, you'll see that it is most arched at this point. If you look at a top built using the Spanish method, you'll see that it is most arched near the bridge and the area just below the lower harmonic bar is flat. This difference has very important consequences to the way the top behaves and consequently to the tone and sustain of an instrument. There is an optimum degree of compliance in this area necessary for the top to remain responsive and still retain the strength to resist the static pull of the strings. For any given compliance, a domed top must be thinner in this area than a top whose arch is highest near the bridge. And the smaller the doming radius used, the thinner this area must be to achieve this optimum degree of compliance. In general, doming of the soundboard requires thinner soundboards for optimum compliance than the traditional method of construction. I hope to find time in a future note to cover the effects on tone and sustain of these two different methods.

This is a topic about which there is much controversy. I hope that builders who use the domed method of top construction will take the time to think these remarks through and experiment with the traditional method of construction. There are fashions in all things, and doming is a contemporary fashion in some circles of lutherie. I'd advise anyone beginning upon this path of adventure and discovery called lutherie to explore the methods of those great builders of the last 150 years, learn to understand and appreciate their collective wisdom and the potential of their methods before chasing fashion or attempting innovation to something as highly evolved by many people over many many decades as the classical guitar and its smaller relatives.