4 Main dimensions

Playability

Do you want to have a big guitar or a small one? A big guitar gives you possibilities to end up with a guitar with a large sound volume and strong, durable, sustaining basses. A small one may lose power, compared to the big one, but still have sparkling trebles and be handy to transport. A small one may also be preferred if you have small hands, and find it hard to play specific accords on a big guitar.

Let's assume that you want to design a guitar, to construct and play it yourself. The first thing you should do is to have a look at the guitar(s) you have already played. Is playing easy? I mean do you have problems with accords, which ask for a large reach of your fingers, especially at the higher positions? No problem? Or do you like to have the frets closer to each other? May be just a little bit. For some people, due to the size or the restricted movability of their fingers a shorter distance between the frets may be desirable. If the distances between the frets are just fine you should keep it that way, so measure the distance between the nut (or 0-fret) and the 12th fret. The scale length of that specific guitar will be twice as long as that length and also the maximum for the guitar you want to construct. Of course if you have no trouble at all playing challenging accords look around for another guitar to try out and to find your own maximum or optimum scale length.

Now for the next step, let's assume that a large (sound) volume and rich basses are the characteristics you are most interested in. Then you fix the scale length on the maximum you just decided on or just below that. "Small" changes, e.g. between 63 and 65 cm give small changes in volume, but may change a lot in terms of easy playing.

Because it is easy to talk about a specific length and other dimension let's for the rest of the design process choose for a scale length of 648 mm. But keep in mind that at this starting point of the design you are completely free to make your own choice.

Because the 12th fret is just in the middle between the nut and the saddle the distance between the 12th fret (exact the edge of the guitar body) and the saddle is also fixed. In our example 324 mm.

Other design characteristics that are solely based on playability are the width of the neck and the dome of the fret board (curvature perpendicular and parallel to the neck). Comparative thick fingers are a good reason to choose for a comparative large width for the neck and a stiff, bowed forefinger on your left hand asks for a curvature of the fret board. For these choices please see chapter The neck and head.

Acoustical considerations

The main dimensions of the guitar body however are more based on acoustical considerations. Let's think about the function of the guitar body as a (Helmholtz) resonator. The dimensions of the guitar body or box determine which frequencies are favored for sound waves inside that box. Some people argue that the dimensions should be chosen such that the important tones (to you or to me?) are amplified. Or, in other words, the resonance frequencies of the guitar box should harmonize with the frequencies you like to be sustained at the most. On the other side you can imagine that favoring specific tones will bring the balance of the sound spectrum we strive for, in unbalance. However the practical classical guitar box with its feminine fluent profile has a whole spectrum of inside dimensions and every tone within that spectrum may, so to speak, find its place to resonance for a while. That may be the case, but it is still wise to give the basses we are looking for a preferred status because they are mainly influenced by the main dimensions.

Many designers choose the dimensions of the guitar body like the width of upper and lower bout, waist, height etc. as a fraction of the overall length of the guitar body. Just to realize that the "partials" also easy find a place to resonance for a while.
For that reason we take the distance between the saddle and the bottom as 1/3 of the total length of the top (L_top).

So: L_top = 324 + 1/3 * L_top, resulting in a total length of 486 mm. Taking into account small corrections in view of edges and linings we end up with a total outside length of 489 mm.

Now starting from an inside length of the guitar box, say 486 mm, you may raise a lot of partial lengths, like 1/2, 1/4, 1/3, 2/3, 3/4 x 486 etc. But the choice is not free! The guitar should look like a guitar and for constructability reasons the sides have to follow a smooth curve. Therefore there are only a few possibilities. For the lower bout we choose a fraction of 3/4, for the width of the upper bout a fraction of 5/9 and for the waist a fraction of 1/2.

Again after corrections for linings we end up with the following dimensions (for the EB-guitars):

Length	489 mm
Width lower bout	368 mm
Width upper bout	273 mm
Width waist	245 mm

Final decisions on main dimensions

If you wish to have 18 full length frets, as is usual for a classical guitar, and you position the upper transverse bar and the waist bar 5 mm separated from the edge of the sound hole, the other main dimensions can be derived easily. The image below indicates these dimensions as applicable for the EB-guitars.

Dimensions of "Torres-made" guitars

Now having fixed the main dimensions of your guitar you may wonder to what extent these dimensions differ from or are equal to the dimensions of other guitars, preferably those constructed by famous luthiers. So let's have a detailed look at the main dimensions of guitars built by Antonio de Torres. Of the ~ 300 guitars constructed by or under direct guidance of Torres the construction and dimensions of about 80 guitars are reasonably well-known. Derived from ref. 11 I put all main dimensions of Torres-build guitars in spreadsheets. A discussion on these spreadsheets, findings and conclusions are published in De Bouwbrief. Because it is obvious that a real small guitar (in length) will also have small widths and heights all these dimensions are normalized on a total body length of 500 mm. The results are shown in the image below. The dimensions of the "EB-guitars" are given as dotted lines.

Of a limited number of these guitars (~17) it is known that they were of exceptional class. They are indicated by ♦. It seems that there is no strong relationship between this high quality and the dimensions. However the dimensions of the group of "exceptional class" fall in general in the upper range. The length of the bodies did not really change during the time (1850-1892) Torres built these guitars and the variations of the ratios between the different width and the lengths were minor.

It is thanks to the curved shape of the guitar body, that allows many different frequencies to feel happy inside, that you can build with large and with smaller bodies.

The height of the body for these guitars always shows the effect that the height at the top is smaller than at the bottom, varying between 5 and 10%. Also taking into account convenience of holding the guitar and the wish to give preference to strong basses the height of the EB-guitars is taken as 90 mm at the top up to 100 mm at the tail block.

The diameter of the sound hole for the Torres-made guitars varied between 84 and 90 mm with a few exceptions up to 95 mm. When you decrease the diameter of the sound hole of your guitar by partially blocking the opening you will hardly be able to hear the difference as long the variation is not too big. That would be a reason to decrease the sound hole diameter to e.g. 60 mm. This would contribute to the strength of the upper bout and a little bit to the acoustical performance of that part of the soundboard. However, by doing so you will not be able to perform final adjustments or repair activities inside the box. This last point is the main reason that for the EB-guitars a diameter of 88 mm is chosen.

In chapter The soundboard I will further discuss the acoustic characteristics of the top and the positioning of the other construction and sound bars.