According to legend, Pythagoras discovered the foundations of musical tuning by listening to the sounds of four blacksmith's hammers, which produced consonance and dissonance when they were struck simultaneously. According to Nicomachus in his 2nd-century CE Enchiridion harmonices, Pythagoras noticed that hammer A produced consonance with hammer B when they were struck together, and hammer C produced consonance with hammer A, but hammers B and C produced dissonance with each other. Hammer D produced such perfect consonance with hammer A that they seemed to be "singing" the same note. Pythagoras rushed into the blacksmith shop to discover why, and found that the explanation was in the weight ratios. The hammers weighed 12, 9, 8, and 6 pounds respectively. Hammers A and D were in a ratio of 2:1, which is the ratio of the octave. Hammers B and C weighed 8 and 9 pounds. Their ratios with hammer D were (12:8 = 3:2 = perfect fifth) and (12:9 = 4:3 = perfect fourth). The space between B and C is a ratio of 9:8, which is equal to the musical whole tone, or whole step interval ().
The legend is, at least with respect to the hammers, demonstrably false. It is probably a Middle Eastern folk tale. These proportions are indeed relevant to string length (e.g. that of a monochord) — using these founding intervals, it is possible to construct the chromatic scale and the basic seven-tone diatonic scale used in modern music, and Pythagoras might well have been influential in the discovery of these proportions (hence, sometimes referred to as Pythagorean tuning) — but the proportions do not have the same relationship to hammer weight and the tones produced by them. However, hammer-driven chisels with equal cross-section, show an exact proportion between length or weight and Eigenfrequency. Earlier sources mention Pythagoras' interest in harmony and ratio. Xenocrates (4th century BCE), while not as far as we know mentioning the blacksmith story, described Pythagoras' interest in general terms: "Pythagoras discovered also that the intervals in music do not come into being apart from number; for they are an interrelation of quantity with quantity. So he set out to investigate under what conditions concordant intervals come about, and discordant ones, and everything well-attuned and ill-tuned." Whatever the details of the discovery of the relationship between music and ratio, it is regarded as historically the first empirically secure mathematical description of a physical fact. As such, it is symbolic of, and perhaps leads to, the Pythagorean conception of mathematics as nature's modus operandi. As Aristotle was later to write, "the Pythagoreans construct the whole universe out of numbers". The Micrologus of Guido of Arezzo repeats the legend in Chapter XX.
Contents of the legend According to the oldest recorded version of the legend, Pythagoras, who lived in the 6th century BC, sought a tool to measure acoustic perceptions, similar to how geometric quantities are measured with a compass or weights with a scale. As he passed by a forge where four (according to a later version, five) craftsmen were working with hammers, he noticed that each strike produced tones of different pitch, which resulted in harmonies when paired. He was able to distinguish Octave, fifth, and fourth. Only one pair, which formed the interval between fourth and fifth (a major second), he perceived as dissonant. Excitedly, he ran into the forge to conduct experiments. There, he discovered that the difference in pitch was not dependent on the shape of the hammer, the position of the struck iron, or the force of the blow. Rather, he could associate the pitches with the weights of the hammers, which he measured precisely. He then returned home to continue the experiments. He hung four equally long, equally strong, and equally twisted strings in succession on a peg attached diagonally to the corner of the walls, weighting them differently by attaching different weights at the bottom. Then he struck the strings in pairs, and the same harmonies resonated as in the forge. The string with the heaviest load of twelve units, when paired with the least burdened string carrying six units, produced an octave. Thus, it was evident that the octave was based on the ratio 12:6, or 2:1. The most tense string yielded a fifth with the second loosest string (eight units), and a fourth with the second tightest string (nine units). From this, it followed that the fifth was based on the ratio 12:8, or 3:2, and the fourth on the ratio 12:9, or 4:3. Again, the ratio of the second tightest string to the loosest, with 9:6, or 3:2, yielded a fifth, and the ratio of the second loosest to the loosest, with 8:6, or 4:3, yielded a fourth. For the dissonant interval between fifth and fourth, it was revealed that it was based on the ratio 9:8, which coincided with the weight measurements carried out in the forge. The octave proved to be the product of the fifth and fourth:
3 2 ⋅ 4 3 = 12 6 = 2 1 = 2 {\displaystyle {\frac {3}{2}}\cdot {\frac {4}{3}}={\frac {12}{6}}={\frac {2}{1}}=2}
Pythagoras then extended the experiment to various instruments, experimented with vessels, flutes, triangles, the Monochord, etc., always finding the same numerical ratios. Finally, he introduced the commonly used terminology for relative pitch.
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