The guitar is a set of stretched strings, free to vibrate for most of their length, a body which amplifies the sound of the vibrating strings while having the strength to hold them in tension, and a neck which allows the player to vary the length – and hence the acoustic frequency – of the strings when vibrating.
The luthier faces several challenges –
- to produce a beautiful sound of sufficient volume
- to keep the overall structure as rigid as possible while allowing the amplification and projection of the sound
- to make the overall structure both stable and light
The body is a box with front sides and back and when looked at from the front it is figure of 8 in shape. The narrow middle is generally called the waist; the slightly wider upper part the “upper bout” and the widest lower part the “lower bout”.
The front of the box is variously called the “top” and “soundboard”. It is acoustically the most important part of the instrument
The soundboard and back are glued to the sides and some strength is added internally with blocks at the bottom and in many designs at the top.
The neck is attached to the body’s top block (or in some designs it actually protrudes into the inside of the body – “Spanish Heel”).
At the top of the neck is the head or “headstock” and this carries the “machine heads” or “tuners” which secure one end of the strings and have a mechanism to tighten them.
The strings pass from the head, through the “nut” with a slot cut in it for each string and then extend over the “frets” set into the “fingerboard” and on to the bridge. (Occasionally guitars may have a “zero fret” set very close to the nut.) On a classical guitar there are typically 19 or 20 frets. And also on a classical guitar the neck joins the body at the 12th fret (the 12th being located at half the length of the vibrating string).
The “bridge” is a piece of wood glued to the soundboard. In it is set the “saddle” over which the strings pass before they are secured through tie-holes drilled through the bridge.
The distance between the nut and the saddle is called the “scale length” and it is the length of the freely vibrating section of the strings.
There is a hole – almost always round – cut into the soundboard at the end of the fingerboard called the “soundhole”. This allows air to pass relatively freely in and out of the body cavity. If the box was sealed it would restrict the free vibration of the soundboard. Some guitars have a second hole in the body – a “soundport” – typically cut on the bass side of the upper bout.
The soundboard both takes the strain from the tension of the strings and also has to vibrate freely so that the instrument produces a sound. If it were thick it could more easily take the strain but would not vibrate that freely. If it is too thin, it might make a beautiful sound – but only for a short while as the string tension would cause it to break. To give it extra strength there is usually a system of bracing on the inside.