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In music, an octave ((ラテン語:octavus): eighth) or perfect octave is the interval between one musical pitch and another with half or double its frequency. It is defined by ANSI〔ANSI/ASA S1.1-2013 Acoustical Terminology〕 as the unit of frequency level when the base of the logarithm is two. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems".〔Cooper, Paul (1973). ''Perspectives in Music Theory: An Historical-Analytical Approach'', p.16. ISBN 0-396-06752-2.〕 The most important musical scales are typically written using eight notes, and the interval between the first and last notes is an octave. For example, the C Major scale is typically written C D E F G A B C, the initial and final Cs being an octave apart. Two notes separated by an octave have the same letter name and are of the same pitch class. Three commonly cited examples of melodies featuring the perfect octave as their opening interval are "Singin' in the Rain", "Somewhere Over the Rainbow", and "Stranger on the Shore". The interval between the first and second harmonics of the harmonic series is an octave. The octave has occasionally been referred to as a diapason. To emphasize that it is one of the perfect intervals (including unison, perfect fourth, and perfect fifth), the octave is designated P8. The octave above or below an indicated note is sometimes abbreviated 8a or 8va (= Italian ''all'ottava''), 8va bassa (= Italian ''all'ottava bassa'', sometimes also 8vb), or simply 8 for the octave in the direction indicated by placing this mark above or below the staff. ==Theory== For example, if one note has a frequency of 440 Hz, the note an octave above it is at 880 Hz, and the note an octave below is at 220 Hz. The ratio of frequencies of two notes an octave apart is therefore 2:1. Further octaves of a note occur at 2''n'' times the frequency of that note (where ''n'' is an integer), such as 2, 4, 8, 16, etc. and the reciprocal of that series. For example, 55 Hz and 440 Hz are one and two octaves away from 110 Hz because they are 0.5 (or 2 −1) and 4 (or 22) times the frequency, respectively. After the unison, the octave is the simplest interval in music. The human ear tends to hear both notes as being essentially "the same", due to closely related harmonics. Notes separated by an octave "ring" together, adding a pleasing sound to music. For this reason, notes an octave apart are given the same note name in the Western system of music notation—the name of a note an octave above A is also A. This is called octave equivalency, the assumption that pitches one or more octaves apart are musically equivalent in many ways, leading to the convention "that scales are uniquely defined by specifying the intervals within an octave".〔Burns, Edward M. (1999). "Intervals, Scales, and Tuning", ''The Psychology of Music'' second edition, , p.252. Deutsch, Diana, ed. San Diego: Academic Press. ISBN 0-12-213564-4.〕 The conceptualization of pitch as having two dimensions, pitch height (absolute frequency) and pitch class (relative position within the octave), inherently include octave circularity.〔 Thus all Cs, or all 1s (if C = 0), in any octave are part of the same pitch class. Octave equivalency is a part of most "advanced musical cultures", but is far from universal in "primitive" and early music.〔e.g., Nettl, 1956; Sachs, C. and Kunst, J. (1962). In ''The wellsprings of music'', ed. Kunst, J. The Hague: Marinus Nijhoff.〕〔e.g., Nettl, 1956; Sachs, C. and Kunst, J. (1962). Cited in Burns, Edward M. (1999), p.217.〕 The languages in which the oldest extant written documents on tuning are written, Sumerian and Akkadian, have no known word for "octave". However, it is believed that a set of cuneiform tablets that collectively describe the tuning of a nine-stringed instrument, believed to be a Babylonian lyre, describe tunings for seven of the strings, with indications to tune the remaining two strings an octave from two of the seven tuned strings. Leon Crickmore recently proposed that "The octave may not have been thought of as a unit in its own right, but rather by analogy like the first day of a new seven-day week". Monkeys experience octave equivalency, and its biological basis apparently is an octave mapping of neurons in the auditory thalamus of the mammalian brain.〔"(The mechanism of octave circularity in the auditory brain )", ''Neuroscience of Music''.〕 Studies have also shown the perception of octave equivalence in rats (Blackwell & Schlosberg, 1943), human infants (Demany & Armand, 1984),〔Demany L, Armand F. The perceptual reality of tone chroma in early infancy. J Acoust Soc Am 1984;76:57–66.〕 and musicians (Allen, 1967) but not starlings (Cynx, 1993), 4-9 year old children (Sergeant, 1983), or nonmusicians (Allen, 1967).〔 While octaves commonly refer to the perfect octave (P8), the interval of an octave in music theory encompasses chromatic alterations within the pitch class, meaning that (unicode:G♮) to G♯ (13 semitones higher) is an Augmented octave (A8), and (unicode:G♮) to G♭ (11 semitones higher) is a diminished octave (d8). The use of such intervals is rare, as there is frequently a preferable enharmonic notation available, but these categories of octaves must be acknowledged in any full understanding of the role and meaning of octaves more generally in music. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「octave」の詳細全文を読む スポンサード リンク
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