In music, 31 equal temperament, which can be abbreviated 31-tET, 31-EDO, 31-ET, is the tempered scale derived by dividing the octave into 31 equal-sized steps. Each step represents a frequency ratio of 21/31, or 38.71 cents.
Division of the octave into 31 steps arose naturally out of Renaissance music theory; the lesser diesis — the ratio of an octave to three major thirds, 128/125 or 41.1 cents — was approximately a fifth of a tone and a third of a semitone. On this basis, Nicola Vicentino produced a 31-step keyboard instrument, the Archicembalo, in 1555, but it was not until 1666 that Lemme Rossi first proposed an equal temperament of this order. Shortly thereafter, having discovered it independently, famed scientist Christiaan Huygens wrote about it also. Since the standard system of tuning at that time was quarter-comma meantone, in which the fifth is tuned to 51/4, the appeal of this method is immediate, as the fifth of 31-et, at 696.77 cents, is only a fifth of a cent sharper than the fifth of quarter-comma meantone. Huygens not only realized this, he went farther and noted that 31-ET provides an excellent approximation of septimal, or 7-limit harmony, which was an advanced insight for its time. In the twentieth century, physicist, music theorist and composer Adriaan Fokker, after reading Huygens's work, led a revival of interest in this system of tuning which led to a number of compositions, particularly by Dutch composers.
The following are 21 of the 31 notes in the scale:
The remaining 10 notes can be added with, for example, five "double flat" notes and five "double sharp" notes.
Here are the sizes of some common intervals:
|interval name||size (steps)||size (cents)||just ratio||just (cents)||difference|
|undecimal tritone, 11th harmonic||14||541.94||11:8||551.32||9.38|
|tridecimal major third||12||464.52||13:10||454.21||-10.30|
|undecimal major third||11||425.81||14:11||417.51||-8.30|
|septimal major third||11||425.81||9:7||435.08||9.27|
|undecimal neutral third||9||348.39||11:9||347.41||-0.98|
|septimal minor third||7||270.97||7:6||266.87||-4.1|
|septimal whole tone||6||232.26||8:7||231.17||-1.09|
|whole tone, major tone||5||193.55||9:8||203.91||10.36|
|whole tone, minor tone||5||193.55||10:9||182.40||-11.15|
|lesser undecimal neutral second||4||154.84||12:11||150.64||-4.20|
|septimal diatonic semitone||3||116.13||15:14||119.44||3.31|
|diatonic semitone, just||3||116.13||16:15||112||-4|
|chromatic semitone, just||2||77.42||25:24||70.67||-6.75|
This tuning can be considered a meantone temperament. It has the necessary property that a chain of its four fifths are equivalent to its major third (the comma 81/80 is tempered out), which also means that it contains a "meantone" that falls between the sizes of 10/9 and 9/8 as the combination of one of each of its chromatic and diatonic semitones.
One property of 31-et is that it equates to the unison, or tempers out, the syntonic comma of 81/80. It can therefore be considered a meantone temperament. It also tempers the 5-limit intervals 393216/390625, known as the Würschmidt comma after music theorist José Würschmidt, and 2109375/2097152, known as the semicomma.
In addition, it also tempers out 126/125, the septimal semicomma or starling comma .. Because it tempers out both 81/80 and 126/125, it supports septimal meantone temperament. It also tempers out 1029/1024, the gamelan residue ., and 1728/1715, the Orwell comma .. Consequently it supports a wide variety of linear temperaments ..
31-et also tempers out 99/98.
Many interesting chords of 31-et are discussed in the article on septimal meantone temperament. Chords not discussed there include the neutral thirds triad, which might be written either C-D-G or C-F-G, and the Orwell tetrad, which is C-E-F-B.