Equal temperament
Equal temperament is a scheme of musical tuning in which the octave is divided into a series of equal steps (equal frequency ratios). The best known example of such a system is twelve-tone equal temperament (sometimes abbreviated to 12-TET), which is nowadays used in most western music. Other equal temperaments do exist (some music has been written in 19-TET for example), but they are so rare that when people use the term equal temperament it is usually understood that they are talking about the twelve tone variety. The rest of this article details 12-TET before presenting the more general case and then some examples of other equal tempered systems.The distance between each step and the next is aurally the same for any two adjacent steps, though, because steps form an arithmetic sequence the difference in frequency increases. A linear sequence of one frequency difference would create ever smaller intervals (ratios), such as the harmonic series.
In twelve tone equal-tempered tuning, a semitone interval is exactly half a tone interval. This causes every step s to be 21/12 times the frequency of the note a semitone lower, or a ratio of approximately 1.05946:1. Thus twelve equal-tempered semitones make exactly one octave. This can be seen from the fact that
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Twelve tone equal temperament was designed to permit the playing of music in all keys with an equal amount of mis-tuning in each, while still approximating just intonation. This allows much more complex harmonic motion, while losing some subtlety of intonation. True equal temperament was not available to musicians before about 1870 because scientific tuning and measurement was not available. Instead, they used approximations that emphasized the tuning of thirds or fifths in certain keys, such as Mean tone temperament. J. S. Bach wrote The Well-Tempered Clavier to demonstrate the musical possibilities of well temperament. There is some reason to believe that when composers and theoreticians of this era wrote of the "colors" of the keys, they described the subtly different dissonances of particular tuning methods, though it is difficult to determine with any exactness the actual tunings used in different places at different times by any composer.
More generally, every step in n tone equal temperament is 1200/n cents. However, if one wishes to create an equal tempered scale that does not repeat at the octave, a scale with n equal steps in a pseudo-octave p is based on the ratio r
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Wendy Carlos created two equal tempered scales for the title track of her album Beauty In The Beast, the Alpha and Beta scales. Beta splits a perfect fourth into two equal parts, which creates a scale where each step is almost 64 cents. Alpha does the same to a minor third to create a scale of 78 cent steps.
The equal tempered version of the Bohlen-Pierce scale consists of the ratio 3:1, 1902 cents, conventionally an octave and a just fifth, used as a tritave, and split into a thirteen tone equal temperament where each step is
