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Music theorists sometimes use mathematics to understand music, and although music has no axiomatic foundation in modern mathematics, mathematics is "the basis of sound" and sound itself "in its musical aspects... exhibits a remarkable array of number properties", simply because nature itself "is amazingly mathematical".^{[1]} Though ancient Chinese, Egyptians and Mesopotamians are known to have studied the mathematical principles of sound,^{[2]} the Pythagoreans of ancient Greece are the first researchers known to have investigated the expression of musical scales in terms of numerical ratios,^{[3]} particularly the ratios of small integers. Their central doctrine was that "all nature consists of harmony arising out of numbers".^{[4]}
From the time of Plato, harmony was considered a fundamental branch of physics, now known as musical acoustics. Early Indian and Chinese theorists show similar approaches: all sought to show that the mathematical laws of harmonics and rhythms were fundamental not only to our understanding of the world but to human wellbeing.^{[5]} Confucius, like Pythagoras, regarded the small numbers 1,2,3,4 as the source of all perfection.^{[6]}
To this day mathematics has more to do with acoustics than with composition, and the use of mathematics in composition is historically limited to the simplest operations of counting and measuring.^{[citation needed]} The attempt to structure and communicate new ways of composing and hearing music has led to musical applications of set theory, abstract algebra and number theory. Some composers have incorporated the golden ratio and Fibonacci numbers into their work.^{[7]}^{[8]}
Without the boundaries of rhythmic structure – a fundamental equal and regular arrangement of pulse repetition, accent, phrase and duration – music would be impossible.^{[9]} In Old English the word "rhyme", derived to "rhythm", became associated and confused with rim – "number"^{[10]} – and modern musical use of terms like meter and measure also reflects the historical importance of music, along with astronomy, in the development of counting, arithmetic and the exact measurement of time and periodicity that is fundamental to physics.
Musical form is the plan by which a short piece of music is extended. The term "plan" is also used in architecture, to which musical form is often compared. Like the architect, the composer must take into account the function for which the work is intended and the means available, practicing economy and making use of repetition and order.^{[11]} The common types of form known as binary and ternary ("twofold" and "threefold") once again demonstrate the importance of small integral values to the intelligibility and appeal of music.
The word "rhyme" was not derived from "rhythm"^{[contradiction]} (see Oxford and Collins dictionaries) but from old English "rime". The spelling of "rime" was later affected by the spelling of "rhythm", although the two are totally different.
A musical scale is a discrete set of pitches used in making or describing music. The most important scale in the Western tradition is the diatonic scale but many others have been used and proposed in various historical eras and parts of the world. Each pitch corresponds to a particular frequency, expressed in hertz (Hz), sometimes referred to as cycles per second (c.p.s.). A scale has an interval of repetition, normally the octave. The octave of any pitch refers to a frequency exactly twice that of the given pitch. Succeeding superoctaves are pitches found at frequencies four, eight, sixteen times, and so on, of the fundamental frequency. Pitches at frequencies of half, a quarter, an eighth and so on of the fundamental are called suboctaves. There is no case in musical harmony where, if a given pitch be considered accordant, that its octaves are considered otherwise. Therefore any note and its octaves will generally be found similarly named in musical systems (e.g. all will be called doh or A or Sa, as the case may be). When expressed as a frequency bandwidth an octave A_{2}–A_{3} spans from 110 Hz to 220 Hz (span=110 Hz). The next octave will span from 220 Hz to 440 Hz (span=220 Hz). The third octave spans from 440 Hz to 880 Hz (span=440 Hz) and so on. Each successive octave spans twice the frequency range of the previous octave.
Because we are often interested in the relations or ratios between the pitches (known as intervals) rather than the precise pitches themselves in describing a scale, it is usual to refer to all the scale pitches in terms of their ratio from a particular pitch, which is given the value of one (often written 1/1), generally a note which functions as the tonic of the scale. For interval size comparison cents are often used.
Common name  Example name Hz  Multiple of fundamental  Ratio within octave  Cents within octave 

Fundamental  A_{2}, 110  1x  1/1 = 1x  0 
Octave  A_{3} 220  2x  2/1 = 2x  1200 
2/2 = 1x  0  
Perfect Fifth  E_{4} 330  3x  3/2 = 1.5x  702 
Octave  A_{4} 440  4x  4/2 = 2x  1200 
4/4 = 1x  0  
Major Third  C♯_{5} 550  5x  5/4 = 1.25x  386 
Perfect Fifth  E_{5} 660  6x  6/4 = 1.5x  702 
Harmonic seventh  G_{5} 770  7x  7/4 = 1.75x  969 
Octave  A_{5} 880  8x  8/4 = 2x  1200 
8/8 = 1x  0 
5limit tuning, the most common form of just intonation, is a system of tuning using tones that are regular number harmonics of a single fundamental frequency. This was one of the scales Johannes Kepler presented in his Harmonice Mundi (1619) in connection with planetary motion. The same scale was given in transposed form by Alexander Malcolm in 1721 and by theorist Jose Wuerschmidt in the 20th century. A form of it is used in the music of northern India. American composer Terry Riley also made use of the inverted form of it in his "Harp of New Albion". Just intonation gives superior results when there is little or no chord progression: voices and other instruments gravitate to just intonation whenever possible. However, as it gives two different whole tone intervals (9:8 and 10:9) a keyboard instrument so tuned cannot change key.^{[12]} To calculate the frequency of a note in a scale given in terms of ratios, the frequency ratio is multiplied by the tonic frequency. For instance, with a tonic of A4 (A natural above middle C), the frequency is 440 Hz, and a justly tuned fifth above it (E5) is simply 440×(3:2) = 660 Hz.
Semitone  Ratio  Interval  Natural  Half Step 

0  1:1  unison  480  0 
1  16:15  minor semitone  512  16:15 
2  9:8  major second  540  135:128 
3  6:5  minor third  576  16:15 
4  5:4  major third  600  25:24 
5  4:3  perfect fourth  640  16:15 
6  45:32  diatonic tritone  675  135:128 
7  3:2  perfect fifth  720  16:15 
8  8:5  minor sixth  768  16:15 
9  5:3  major sixth  800  25:24 
10  9:5  minor seventh  864  27:25 
11  15:8  major seventh  900  25:24 
12  2:1  octave  960  16:15 
Pythagorean tuning is tuning based only on the perfect consonances, the (perfect) octave, perfect fifth, and perfect fourth. Thus the major third is considered not a third but a ditone, literally "two tones", and is (9:8)^{2} = 81:64, rather than the independent and harmonic just 5:4 = 80:64 directly below. A whole tone is a secondary interval, being derived from two perfect fifths, (3:2)^{2} = 9:8.
The just major third, 5:4 and minor third, 6:5, are a syntonic comma, 81:80, apart from their Pythagorean equivalents 81:64 and 32:27 respectively. According to Carl Dahlhaus (1990, p. 187), "the dependent third conforms to the Pythagorean, the independent third to the harmonic tuning of intervals."
Western common practice music usually cannot be played in just intonation but requires a systematically tempered scale. The tempering can involve either the irregularities of well temperament or be constructed as a regular temperament, either some form of equal temperament or some other regular meantone, but in all cases will involve the fundamental features of meantone temperament. For example, the root of chord ii, if tuned to a fifth above the dominant, would be a major whole tone (9:8) above the tonic. If tuned a just minor third (6:5) below a just subdominant degree of 4:3, however, the interval from the tonic would equal a minor whole tone (10:9). Meantone temperament reduces the difference between 9:8 and 10:9. Their ratio, (9:8)/(10:9) = 81:80, is treated as a unison. The interval 81:80, called the syntonic comma or comma of Didymus, is the key comma of meantone temperament.
In equal temperament, the octave is divided into twelve equal parts, each semitone (halfstep) is an interval of the twelfth root of two so that twelve of these equal half steps add up to exactly an octave. With fretted instruments it is very useful to use equal temperament so that the frets align evenly across the strings. In the European music tradition, equal temperament was used for lute and guitar music far earlier than for other instruments, such as musical keyboards. Because of this historical force, twelvetone equal temperament is now the dominant intonation system in the Western, and much of the nonWestern, world.
Equallytempered scales have been used and instruments built using various other numbers of equal intervals. The 19 equal temperament, first proposed and used by Guillaume Costeley in the 16th century, uses 19 equally spaced tones, offering better major thirds and far better minor thirds than normal 12semitone equal temperament at the cost of a flatter fifth. The overall effect is one of greater consonance. 24 equal temperament, with 24 equally spaced tones, is widespread in the pedagogy and notation of Arabic music. However, in theory and practice, the intonation of Arabic music conforms to rational ratios, as opposed to the irrational ratios of equallytempered systems. While any analog to the equallytempered quarter tone is entirely absent from Arabic intonation systems, analogs to a threequarter tone, or neutral second, frequently occur. These neutral seconds, however, vary slightly in their ratios dependent on maqam, as well as geography. Indeed, Arabic music historian Habib Hassan Touma has written that "the breadth of deviation of this musical step is a crucial ingredient in the peculiar flavor of Arabian music. To temper the scale by dividing the octave into twentyfour quartertones of equal size would be to surrender one of the most characteristic elements of this musical culture."^{[13]}
The following graph reveals how accurately various equaltempered scales approximate three important harmonic identities: the major third (5th harmonic), the perfect fifth (3rd harmonic), and the "harmonic seventh" (7th harmonic). [Note: the numbers above the bars designate the equaltempered scale (i.e., "12" designates the 12tone equaltempered scale, etc.)]
Note  Frequency (Hz)  Frequency Distance from previous note  Log frequency log_{2} f  Log frequency Distance from previous note 

A_{2}  110.00  N/A  6.781  N/A 
A♯_{2}  116.54  6.54  6.864  0.0833 (or 1/12) 
B_{2}  123.47  6.93  6.948  0.0833 
C_{3}  130.81  7.34  7.031  0.0833 
C♯_{3}  138.59  7.78  7.115  0.0833 
D_{3}  146.83  8.24  7.198  0.0833 
D♯_{3}  155.56  8.73  7.281  0.0833 
E_{3}  164.81  9.25  7.365  0.0833 
F_{3}  174.61  9.80  7.448  0.0833 
F♯_{3}  185.00  10.39  7.531  0.0833 
G_{3}  196.00  11.00  7.615  0.0833 
G♯_{3}  207.65  11.65  7.698  0.0833 
A_{3}  220.00  12.35  7.781  0.0833 
Below are Ogg Vorbis files demonstrating the difference between just intonation and equal temperament. You may need to play the samples several times before you can pick the difference.
Musical set theory uses some of the concepts from mathematical set theory to organize musical objects and describe their relationships. To analyze the structure of a piece of (typically atonal) music using musical set theory, one usually starts with a set of tones, which could form motives or chords. By applying simple operations such as transposition and inversion, one can discover deep structures in the music. Operations such as transposition and inversion are called isometries because they preserve the intervals between tones in a set.
Expanding on the methods of musical set theory, some theorists have used abstract algebra to analyze music. For example, the notes in an equal temperament octave form an abelian group with 12 elements. It is possible to describe just intonation in terms of a free abelian group.^{[14]}
Transformational theory is a branch of music theory developed by David Lewin. The theory allows for great generality because it emphasizes transformations between musical objects, rather than the musical objects themselves.
Theorists have also proposed musical applications of more sophisticated algebraic concepts. Mathematician Guerino Mazzola has applied topos theory to music,^{[citation needed]} though the result has been controversial.^{[citation needed]}
The chromatic scale has a free and transitive action of the cyclic group , with the action being defined via transposition of notes. So the chromatic scale can be thought of as a torsor for the group .
This section does not cite any references or sources. (December 2011) 
James Tenney reconceived his piece "For Ann (Rising)", which consists of up to twelve computergenerated tones that glissando upwards (see Shepard tone), as having each tone start so each is the golden ratio (in between an equaltempered minor and major sixth) below the previous tone, so that the combination tones produced by all consecutive tones are a lower or higher pitch already, or soon to be, produced.^{[citation needed]}
Ernő Lendvaï analyzes Béla Bartók's works as being based on two opposing systems: those of the golden ratio and the acoustic scale. In Bartók's Music for Strings, Percussion, and Celesta, the xylophone progression at the beginning of the 3rd movement occurs at the intervals 1:2:3:5:8:5:3:2:1. French composer Erik Satie used the golden ratio in several of his pieces, including Sonneries de la Rose Croix.^{[citation needed]}
The golden ratio is also apparent in the organization of the sections in the music of Debussy's Image, "Reflections in Water", in which the sequence of keys is marked out by the intervals 34, 21, 13, and 8 (a descending Fibonacci sequence), and the main climax sits at the φ position. "Prelude to the Afternoon of a Faun" also reaches a climax point, marked by the entrance of the antique cymbal, at the φ position.^{[citation needed]}
Many of the important musical events in Krzysztof Penderecki's "Threnody for the Victims of Hiroshima" occur at φ positions.^{[citation needed]}
Boards of Canada have discussed using the golden ratio and Fibonacci numbers in their work.^{[15]} Song titles such as "Music is Math", and numerous songs featuring samples of people counting or discussing numbers, also illustrate the influence of mathematics on their music.^{[citation needed]}
Australian composer Scott Sanders, in his piece "Sweets from Dr Phil" uses modular arithmetic to limit the Fibonacci series to a set of digits modulo 2 to modulo 16, performed by 15 instances of the same timbre to reveal a pseudomelody apparent in the upper notes of the combination of these instruments. A system of directly mapping digits to pitch classes and durations determines the notes each instrument plays.^{[16]}^{[17]}^{[18]}
The song, "Lateralus", by Tool incorporates the Fibonacci sequence.^{[19]} The theme of the song describes the desire of humans to explore and to expand for more knowledge and a deeper understanding of everything. The lyrics "spiral out", refers to this desire and also to the Fibonacci spiral, which is formed by creating and arranging squares for each number in the sequence's 1,1,2,3,5,8,... pattern, and drawing a curve that connects to two corners of each square. This would, allowed to continue onwards, theoretically create a neverending and infinitelyexpanding spiral. Related to this, the song's main theme features successive time signatures 9/8, 8/8, and 7/8.^{[20]} The number 987 is the sixteenth integer of the Fibonacci sequence.^{[21]}
