2 Acoustic principles
There is a fundamental difference between a classical guitar and a violin in the way the sound is produced. In both instruments the energy of the vibrating strings has to be transported via the bridge to the soundboard. However finger picking the guitar strings leads to a short excitement of the strings, where in the case of a violin the energy is more or less continuously added by the fiddle stick. In the case of a violin the vibration of the strings brings the bridge into a movement, perpendicular to the top. Some people describe the same phenomenon for a guitar but in fact it is the wriggling movement of the bridge that gives rise to the waves in the soundboard of the guitar. In designing a guitar understanding this difference is of primary importance.
Because the wriggling movement of the bridge is the main way the soundboard is excited the dimensions and the other characteristics of the bridge of a guitar play a radical role in the design. Because of the limited energy of separate notes of the guitar string every bit of energy should be used to generate the sound we want to hear. But unfortunately the efficiency of that process is low: for a "standard" classical guitar only 5 to 7% of the energy in the string is converted into sound waves. It will be clear that one of the main challenges in the construction of a guitar is to look and fight for every possible improvement of that efficiency. Prevention of every useless loss is a must.
Image: Lay out of the classical guitar
Firstly, the connection points of the strings have to be fixed. The saddle has to be a hard fixed point related to the nut. That asks for a rigid neck, head and nut. In fact also the construction parts, connecting the neck with the bridge and directed parallel to the longitudinal axis have to be inflexible. Even the saddle and the nut have to be shaped in such a way that the string will not lose energy to the construction itself. The bridge itself is only a tool to transport the energy of the strings to the soundboard and it should not absorb any energy by itself. Not by a large momentum of its own and not by internal heat absorption by compressing or bowing motions. So the bridge and saddle have to be as light and inflexible as can be (see chapter The bridge).
The soundboard forms the vibrating membrane, which makes the music. The waves, introduced in the soundboard should on one side dispose their energy by introducing airwaves, but on the other side fade away slowly to show the desired sustain for the pitch involved. As for the bridge this asks for a piece of wood, that is very light, but with enough strength not to be deformed by external forces it is subjected to.
Here it is important to mention two different construction principles, sometimes described as contradictive. Some guitar builders highlight the essential role of the soundboard by following the frame principle. In that case the guitar body is seen (and constructed) as a solid, inflexible frame on top of which the soundboard is glued. That is in line with the idea that every "unnecessary" loss of energy should be prevented. So the only acoustical function of the guitar body in that case is to reflect the acoustic airwaves. This idea will lead to relatively thick back and sides and makes the construction of these parts relatively easy. However there are some "secondary" effects, which are in favor of a more flexible construction. It is well known, that some famous luthiers in the past, e.g. Torres, Ramirez and Hauser produced instruments with thin and flexible backs and sides. Such an approach has also advantages, of which the following are important. First a thin sidewall combined with a flexible lining allows the edge of the soundboard to turn over a little bit and thus enlarging the "effective" surface of the soundboard. This may give, let it be small, an increase of the sound volume. A second advantage of flexible back and sides is that some standing waves in the guitar body are supported. The end-effect of these different approaches can be characterized as "more volume with flexible linings and more sustain with fixed linings".
As in life it is good to go for the best in both approaches and instead of choosing one of the two we will look for a combination of rigidity and flexibility of back and sides (see chapter The soundboard, chapter The back and chapter The sides).
For a good understanding of the construction principles as described in this article some basic knowledge of possible vibration modes of the guitar top is needed. The traditional classical guitar top vibrates in many ways, but some fundamental frequencies below 500-600 Hz are of prime importance. These are the (0.0), (1.0), (0.1), (1.1) and (2.0) modes as shown in the image below.
Image: Primary modes of the guitar top
These images of basic characteristic oscillations hold for a symmetrical top without sound bars installed and will be used for some design steps in chapter Sound bars and chapter The back. Interesting and useful background information on these vibration modes, their relationship and effects on the guitar can be found in the references 1, 2, 3, 4, 5 and 23.