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The following articles cover some of the basics of classical guitar construction and set-up.
Soundboard strutting systems
What are struts (braces)?
The struts are lengths of wood glued to the inside of the top (soundboard), which act to support the top against the tension of the strings, and to control the overall sound that the guitar produces. They are usually made of the same material as the top, although often cedar top guitars will have spruce struts.
By far the most common pattern used for the strutting is the fan brace system, so called because the struts are arranged somewhat like the ribs of a fan. The struts all radiate from a single point, usually somewhere around the 12th fret, although this point can vary significantly, making the struts more (or less) parallel to each other.
Before Torres popularised the fan bracing system (contrary to popular myth, he did not invent it) most guitars used a simple "ladder" bracing system - with just a few struts running across the soundboard, i.e. perpendicular to the grain direction.
Although the majority of guitars still use a variation of the fan bracing system, there have been a number of alternatives tried over recent years. Perhaps one of the more radical looking systems was that developed by Michael Kasha and Richard Schneider (see lower image). Dr. Kasha used his scientific training to develop an entirely new system of strutting, and Kasha and Schneider collaborated for many years on this design.
More recently, lattice bracing systems have become more popular, largely as a result of John Williams' patronage of Greg Smallman guitars. The lattice system employs a number of crossed struts, usually joined at the intersections, to form a "lattice". This form of strutting is very stiff, and can be made very light, and allows the top be made significantly thinner than on traditionally braced guitars. This can help to improve the efficiency of the instrument in generating sound, which can give the guitar more volume. Lattice-braced guitars also tend to have a distinctive tone (which is not to everyone's taste), but this is more a result of the lightness and stiffness of the top than the system of strutting itself.
Radial bracing systems have also been used, where each brace radiates out from the centre of the lower bout. Makers have experimented with other bracing systems, but at the moment variations on the fan bracing or lattice bracing systems are by far the most common.
Some stuff about tonewoods...
What is a tonewood?
A tonewood is any timber that has appropriate acoustic properties for musical instrument making. In terms of tonewoods, the most critical parts of a guitar are first the top (soundboard), and second the back and sides.
Most (but not all) tops are made from softwoods, and by far the most common are spruce and cedar. Cedar tends to produce a guitar with a warmer, fuller sound, whereas spruce has the advantages of clarity, good separation, and a bell-like tone.
Occasionally you see a classical guitars with unusual tonewoods used for the top. I recently played a guitar with a cypress top, and was sufficiently impressed that I'm thinking of building one myself soon.
The back and sides of the guitar have less effect on the guitars tone than the top, but they are still important. By far the most common tonewood used is rosewood. Traditionally, Brazilian rosewood was believed to be the best, but many luthiers now prefer Indian rosewood - and not only because of it's much lower cost.
Flamenco guitars traditionally have back and sides of cypress (flamenco blancas), although rosewood (flamenco negra) seems to be increasing in popularity.
A great many other tonewoods have been used for the back and sides of classical guitars, including walnut, cherry, cocobolo, Macassar ebony, mahogany, padauk. My personal favourite, however, is maple. Bird's-eye maple is an incredibly beautiful wood (see photo), and gives the guitar what I describe as a 'creamy' tone - if that means anything!
Action and set-up
One of the most frequently asked questions on guitar forums (and eslewhere) is what is the correct action for a classical guitar.
First we need to understand what is meant by action. The action of a guitar is a measure of the height of the strings above the fingerboard. The normal way to measure the action is at the 12th fret, measuring from the top of the fret to the underside of the string.
A lower action tends to make the guitar easier to play, but there is an increased risk of the strings "buzzing" against the frets further up the fretboard. A higher action reduces this risk, but makes the strings harder to fret, particularly in the higher positions.
The perfect string height will depend to some extent on the player. A player who strikes the strings hard will need a higher action than one who plays relatively gently. Typical values for action on a standard classical guitar are 4.0mm for the bass E, and 3.0mm for the treble E. I would say that the normal range of acceptable action is between 3.5 and 4.5mm for the bass, and between 2.5 and 3.25mm for the treble. Anything outside this range requires some attention.
If a guitar is buzzing at relatively low volumes, then raising the action may help, but it is important that the levelling of the frets is checked first. If buzzes occur at only a few places on the fretboard (and particularly if they occur at the same fret on more than one string), then it is likely that the frets will need levelling before the action is adjusted. This is a job that should only really be undertaken by a qualified luthier.
If the action at the 12th fret seems OK, but you're still finding the action difficult (particularly at the first few frets), then there may be a problem with the action at the nut. Check this by pressing down each string between the 2nd and 3rd frets, and looking at the gap between the bottom of the string and the first fret. For the 1st, 2nd and 4th strings (top E, B, and D) there should be almost no gap at all (less than the thickness of a piece of paper). For the other 3 strings (G, A and bottom E) the gap should be about 1/4mm (slightly more for the bottom E). If the gaps are much greater than this, then the guitar would benefit from adjusting the action at the nut - usually a job for a qualified luthier again.
12-Hole Tie Blocks
Why are twelve holes better than six?
More and more luthiers are using 12-hole tie blocks as standard on their classical guitars. The 12-hole tie block gives a more consistent, and greater break angle for the strings over the saddle. This can allow the action to be lowered further before the break angle becomes too low. It also looks very neat!
It is important to know how to tie the strings correctly (especially the trebles), to avoid any painful (for the guitar) accidents. If treble strings are not tied securely, they can slip, and the high tension means that the end of the string whips round the tie block, and can make quite a deep dent in the top of your guitar. The same is true of 6-hole tie blocks, but most players are familiar with how to tie these securely (if not, there are some good instructions here:). Instructions for tying 12-hole tie blocks correctly can be found here.