Modern Music

Contrary to what one might think, the strings of a classical guitar set do not follow an equal tension profile.

Tension evaluation method:

The most common method used to evaluate the degree of tension of a string on a guitar, is to apply a pression, using the right hand fingers, right near to the bridge: a set is considered well balanced if all the strings oppose the same force to the pressing finger. One might come to the conclusion that all strings have been calculated with the same working tension.

But this kind of evaluation is actually a ‘tactile sensation’ of the tension, and not the real tension expressed in kilograms or pounds, as one could measure using appropriate instruments. Following a law of Physics, two strings that have the same lateral movement by means of the same pressure applied on the same point (as could be the pressing finger), will also have the same tension, expressed in Kg (or lb). But this same tension does not correspond with the one used in the necessary calculation to determine the gauges of the string.

The physical nature of the strings:

When they are exposed to traction, the strings stretch (this is particularly more evident when turning the peg during the tuning); this implies, as a consequence, a progressive thinning of the diameter. But together with the reduction of the diameter, there will also be a reduction of the working tension, proportional to the originally calculated tension. The stretch amount is not the same for all strings; its maximum is observed on the chanterelles, it will be less on the second, and even less on the third: it is well known that the number of turns on the peg of the chantarelle is way higher than the ones needed for the third string. As a consequence, a set that has been calculated using an equal tension, once tuned correctly would be completely unbalanced.
This fact is inevitable: a guitar not only has strings of heterogeneous physical nature (the first three are synthetic monofilaments, while the basses, instead, are composed of two paired materials, like a synthetic core and a metallic wounding), but its diameters are different and each string is used at a different “Working Index” (that is expressed as the product of the frequency and the vibating length of the string). As a result of all this, the achievement of a homogeneous tactile feel of the tension among the strings is actually a more complex thing as compared to the simple theoretical calculation, where the parameter for the tension is assumed as a constant.

Conclusions:

The scaling of the tension is therefore a compensation process that is carefully studied and applied by the string-maker in order to neutralize, string by string, the thinning of the diameter caused by the stretching of the string during traction: once stabilized in their tuning, each string will decrease its value in percentage until they reach a working tension similar to the other strings. Thus all the strings will show the same bending, when pressed with a finger. From a practical point of view, if the chanterelle will experimentally decrease of a 2%, the diameter in the starting calculations needs to be increased by a 2%, and so on with all the remaining strings.

But, in practice, the gradient of the tension profile needs to be even more emphasised: the aim of this exaggeration of the scaling of the tension is to more efficiently oppose the increase of frequency of the thicker strings on the higher frets (on the second string and even more on the third). In the past, when gut strings were used, this exaggeration was not needed because thicker strings were automatically produced using a higher torsion (making them more elastic), while with modern synthetic monofilaments each string has the same stretching coeffficient.
This measure for synthetic strings, however, has been proven not to be completely enough; a series of compensatory measures on the bone of the bridge is therefore often needed, such as changing its inclination.