May 5, 2007
In more ancient times it was sometimes said that the number of notes in a scale was dependent on the number of planets in the sky (All part of astrology and 'harmony of the spheres').
In the West, the visible planets (Wanderers) were Mercury, Venus, Mars, Jupiter, Saturn, the Moon and the Sun; therefore 7 notes. In China, the Sun and Moon were not considered planets and the principle scale had only 5 notes. There are exceptions in both cases.
Alternatively it might be argued that since there were only 5 or 7 notes in a scale, that was why there were only 5 or 7 planets in the sky.
I'm not sure why the Heavens should bear the blame for the decisions we make but it remains a fairly common theology today.
June 28, 2005
Musemath was cited in a discussion group on the question of "Why use only 7 notes in the scale?" One of the answers given involved a psychological principle "seven plus or minus two" that human beings readily distinguish only about 7 items in a continuum. The discussion was fairly old or I might have joined in with Wood's historical- mythical explanation of two Greek Tetrachords combined.- see below. It's a good question. Here are the results of some recent searches - they are not exclusive, I have seen other explanations in print media some time ago.
http://www.bikexprt.com/tunings/fibonaci.htm
The rule of seven plus or minus two
The seven-tone scales in
the twelve-tone system approach the limit of what the human mind can assimilate.
The rule of "seven plus or minus two" in sensory psychology states that for any
sensory continuum, humans describe between five and nine different categories:
to give an example, we describe the gray scale using the categories white,
off-white, light gray, medium gray, dark gray, near black and black. Though we
can discriminate more shades of gray when they are placed side by side for
comparison, we do not give names to them, or use the discrimination between them
as part of a conceptual structure based on unaided observation and memory.
The musical octave is regarded by sensory psychologists as a continuum like the gray scale, and it follows that 12 distinct pitch classes may be more than we can easily assimilate as elements of a musical structure.
"Between seven and nine is generally the maximum number of things that a person can comfortably hold in their short-term memory. If you flood people with more choices than that, they begin to lose track of the first choices."
http://www.andymilne.dial.pipex.com/Scales.shtml
The Tonal
Centre
Andrew Milne
Subdivision of the octave .
By choosing a limited number of notes the ear is given a structure that is simple enough to be understandable and whose notes are spaced apart enough to be easily heard as different. Ideally within any octave, each note is perceived to be fundamentally different from every other note - each note has a unique identity. When that identity is unique enough it allows for each note's pitch to be varied with vibrato and other decorative techniques without losing its identity and becoming confused with other notes.
The pitches used in purely melodic musics - such as classical Arabic and Indian are generally more flexible and complex than those used in the tonal harmonic music...
Alexander Wood’s (1879-1950)
The Physics of Music
Revised
by J.M. Bowsher
Seventh Edition, 1944,1975
Chapman and Hall,
Ltd.
The origin of our own major and minor scales can be traced with fair certainty to the music of the Greeks…The records of musical culture show that there was considerable development as early as 1200 B.C. The instrument most closely associated with the development of the scale was the tetrachord, an instrument of four strings as the name suggests. P173
(The basic historical idea here is that (in theory) two tetrachords together would amount to eight strings with eight different notes and seven intervals (ratio of one frequency to another) between them, the highest note would be an octave (double the frequency, a ratio of 2:1) above the lowest note. All the intervals between would total that one octave. By changing the starting note from one string to the next a different order of intervals could be obtained that would help keep the music interesting by providing some variety. The order of intervals is called a mode. .P174)
In the following discussion, the "8th note" is the octave.
http://www.foundationwebsite.org/Guitar.htm
How to Play the Guitar by Ear
(for mathematicians and physicists)
Joseph George Caldwell
The fact that most (Western) musical pieces involve only eight tones over an octave may trace its origin to the simple (traditional) flute. Since the human hand has eight fingers opposed to the two thumbs, it is possible to cover up to eight tone-holes on a flute...For players of harps, it would be convenient to cover a full octave (from one frequency to double that) with eight fingers. Whatever the reason, the melodies of Western music evolved with most songs based on eight tones to the octave.