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Functionality of the Guitar

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A fundamental problem that occurs in guitar making is the sound design of the instrument. It takes years of experience for the guitarmaker to develop a feeling for planning, designing and constructing so that the finished instrument will show those qualities. Will the guitar have enough powerful basses? will the trebles produce the desired sweetness? will the timbre of my imagination go along with the taste of the musician?

The guitarmaker should be aware of these points before he starts his project. Without experience that becomes an insurmountable obstacle at the beginning as they keep closer to established concepts, approved in the past, which promise success. The basis of this concept was founded by Antonio de Torres in the mid-19th century. He is seen as the father of the classical guitar and almost all of today's guitar constructions are based on his work.

To meet the requirements of the 'modern' guitarists the known system developed by Torres has now become insufficient. Nowadays more volume and power is required. Criteria which can not be implemented by the traditional system. If you want to make such specific changes with the instrument or if you want to integrate certain intentions and imagination you suddenly see a countless number of regulation screws appearing in the whole instrument. Trial and error was the procedure of the old masters. However, this need not to be the only way you can take to gain a major awareness and a better understanding about the functionality of the guitar. Science, in recent years has become more and more integrated in guitar making, delivering new conceptions which follow a logical and physical pattern, guiding the guitar maker to a better understanding about the complexity of the guitar.

As an introduction for a better understanding about the functionality of the guitar I would like to start with an example of the vibrating string. A single guitar tone is far more complex than you can imagine because it is not only existing on one frequency but from fundamental and numerous, harmonical overtones that ultimately shape the timbre of the tone by their difference in sound level. For example, by plucking the string close to the bridge, the peaks of the high harmonics are much more pronounced than the lower ones, forming the timbre of this tone bright and sharp. However, if you pluck the string at the sound hole the peaks of the fundamental and the subsequent harmonics are more pronounced and the colour is changes into a dark timbre. The harmonics of the vibrating string could be also declared as so-called 'oscillation modes' which is of the utmost importance for the further understanding of the functionality of the body.

The keystone is found in the aspect of the entire construction which works in essence only as an amplifier for the vibrating string. Without this body construction the string would not be able to set enough air in motion to produce a sufficiently loud sound. Therefore, the string is coupled to the guitar body. The vibrating string transfers its energy over the bridge to the top, thus the top also starts to vibrate whereby this vibration is again setting the surrounding air in motion which is actually generating a note.

These vibrations, mainly generated by the top, are not visible to the naked eye but physically measurable and are can illustrated below.

 

FdG-Deckenschwingungen

 As you can see all top vibrations are mainly generated in the lower bout below the sound hole and in various ways. These vibrations or so-called 'modes' are divided into fields of vibrations and nodal lines. With increasing frequency, the areas of vibration are changing into much smaller parts and can be considered like the overtones of a vibrating string with the difference that the modes of the top do not appear in a harmonical relationship. As already mentioned, the vibrating string transfers its energy to the top, then it starts to oscillate and sets the surrounding air into motion by changing sound pressure. This air movement is essentially the mechanical amplification of the body which finally can be heard as a guitar note.

From this point of view it is absolutely necessary to pay primary attention in building a good 'air pump' that can provide sufficient sound pressure. The intensity of the sound pressure can be determined by special measurement methods (FFT-Analysis) for each mode, delivering in this way information about the tonal qualities of the guitar, such as a fingerprint. With specific manipulations of intensity and frequency position of the modes tonal differences occur, allowing one to point out certain characteristics of the guitar or to cut them off. For example, if the intensity of the high frequented modes are increased it would mean that the vibrating behaviour is enhanced in ways of a larger amplitude of the antinodes, the result will be a guitar with a bright and clear sound character. By strengthening the vibrational behaviour of the low modes you will achieve a dark timbre and voluminous sound.

FdG - Kurve

In the picture above you can see the entire frequency spectrum of a guitar with its respective modes represented as resonance peaks. Each peak typifies a mode, a body vibration, pronounced differently in their intensities that forms the timbre and character of the guitar. These peaks can be seen basically like the controller of a conventional equalizer which can control the timbre of the guitar by changing the intensities in certain frequency ranges.

A special feature that occurs in relation with the resonances are the so-called 'Wolfnotes' of which every musician is probably aware of. A wolf can usually be clearly distinguished from its neighbouring notes because its decay time is much shorter than the adjacent notes and in its sound character is rather disturbing. These wolves can be found all over the entire fingerboard. The two strongest are located on the low E string in the range of F sharp and on the third string on g. This also can be varying by one or two semitones from guitar to guitar. This phenomenon is easy to understand by regarding the frequency curve of the graphic above. When the frequency of a played note is hitting a resonance peak of a top vibration with a similar frequency a faster transfer of energy between strings and body is taking place and the note is dying more quickly. For example, in our diagram we can see a resonance peak detected at 192 Hz, the third string (g) has a frequency of 196 Hz, thus both frequencies are very close together. The vibration of this top resonance is extracting almost the whole energy of the plucked g-string in one fell swoop which can be recognized as the characteristics of a wolf note by a very fast attack and decay time. Accordingly, each resonance peak, visible in the diagram are potential wolf notes, which can more or less be noticed all over the entire range of the guitar. This peculiarity is inescapable, because the functionality of the guitar is based on these resonances and body vibrations, without the guitar couldn't sound. We have to accept the wolves which can be found in every instrument.

By using this physical knowledge you are then able to get a better understanding about the expertise existing in traditionally constructed guitars and thus receive a strong companion in addition to results achieved by trial and error. The luthier therefore has the ability to show and offer different tonal directions but ultimately it will be the player who decides in which direction the development will go.

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© 2013 by Dominik Wurth

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