The notion of microtonal music represents music that uses different tone ratios than the ones tradionally used in Western music.
It's a culture-historical notion and represents a striking collection of historic and contemporary musical movements, which have used different ways of ridding themselves from the limitations of twelve tone equal temperament, the tuning which has onesidedly dominated Western music since the eighteenth century.
Microtonal music lets the Western composer, musician and listener hear the possibilities and impossibilities of the musical tone-system and changes the outlook on the twelve-tone system.
Microtonal music is a collective name for various kinds of music that use tone systems different from what is customary in Western music. The normal Western scale one can imagine the best using a piano keyboard: from one octave to the other there are twelve keys present, which produce the same amount of tones. The distance from one key/tone to the next is called a semitone; twelve of these semitones make full an octave.
In microtonal tone systems this is different. To go from one particular tone to the next which is one octave higher, more, and often many more steps must be gone through. This means, evidently, that the distance from one tone to the next is smaller than in the usual twelve tone system. Such a smaller distance is called a microtone, or - which would be a slightly better word - microinterval.
There are numerous methods for building musical tone systems with microtones. More or less familiar systems are the quartertone system (where the octave is divided in 24 quarter tones) and the 31-tone system (where the octave is divided in 31 small steps which are called dieses). But there are also plenty of other systems, for example with 19, 43 and 53 tones per octave. For these said systems applies that they still have a relation in some way with the normal 12-tone system making them sound somewhat familiar. One can also go about in the opposite way by choosing an arbitrary microtonal system, examine its possibilities concerning scales, intervals, chords, etc. and using them in a microtonal composition.
Why would one choose a (complicated) microtonal tone system, while Western music has flourished for centuries with the relatively simple twelve-tone system? The answer to this question is twofold. In the first place microtonal music creates much more refined nuances with respect to pitch, intervals and chords, and it can be refreshing employing these new sounds and harmonies in musical compositions. In the second place, this aspect hasn't been mentioned here yet, in some microtonal systems certain traditional intervals can be realised much purer than in the twelve-tone system, which because of its restrictions had to make a compromise with respect to purity. Microtonal systems based on the latter point of departure, are called just intonations. These just intonations are often very complicated systems, which can also lead to the development of new instruments.
What sounds out of tune and what not is merely a matter of habituation. Something sounds out of tune if it's impure, but what's pure is a matter of agreement, of a certain norm. It is an agreement of a certain group of people in a certain cultural domain. The longer this norm serves as a value, the more one begins to believe it is the only true norm, like a physical law. For example in the 16th century people were used to meantone tuning, and found equal temperament to sound rather out of tune. Now that we are used to it, we sometimes have to get used to the special character of meantone tuning when we hear it.
Microtonality is not a certain style in music. It concerns only the material with which composers work. But by the use of microtonal tone systems usually music originates that has a wholly new sound, which cannot be brought under the usual denominators of the music of the twentieth century. Though, even if most microtonal works have been written in this century, in particular after around 1920, a small number is already of much older date, going back to the sixteenth century. In that century it were the theoretically interested composers, who in their attempts to revive the Greek enharmonic tetrachord, wrote the first microtonal music, if we leave the Greeks themselves out of consideration; of their music almost nothing has survived.
On a normal piano it is not possible to realise microtonality (unless the instrument is retuned drastically). On most wind instruments microtonality is applicable with the help of special blowing techniques and fingerings. For the voice, string instruments and the trombone microtones are mainly a matter of practice; these instruments can after all produce any desired pitch. But where microtonal music feels at his best are of course the microtonal musical instruments, instruments with a large number of keys, strings, pipes, frets, et cetera, each tuned to a certain pitch. And obviously in the world of synthesizers and computer music everything is possible for microtones.
In classical music microtones occur more often than one would probably think initially. Think for example about vibrato, glissando, small intonation adaptations by string players and microtonal ornamentations by singers. Still this doesn't make this music microtonal. This is the case if it is based on a microtonal tone system. We call a tone system microtonal if it contains intervals which are smaller than a minor second (semitone), or are not a multiple of it, in other words: "fall between the piano keys". So the tone system doesn't necessarily need to have more than 12 tones per octave.
Much non-Western music is microtonal: classical music from India, Turkey, Arabia and Persia, gamelan music from Indonesia, xylophone music from Africa, Byzantyne liturgical music, folk music from Middle- and Eastern Europe, the Caucasus, etc.
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