I seem to have answered several times the question "why 12 tones per octave?"
Here are the posts, with some email and followups.
dave
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Date: Wed, 28 Jul 93 16:06:05 CDT
Wrom: UIVOTQNQEMSFDULHPQ
To:
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Subject: log3/log2
I use log3/log to explain the importance of the 12-note scale of Western music.
Here is my reasoning.
One note does not music make (the "One Note Samba"
notwithstanding). Now, which notes sound best with a note of a given
frequency? The ancient greeks more or less decided it was those whose
frequencies were integer multiples of the first (Indeed, those other
frequencies are present in practice because the Fourier expansion of a single
note on an instrument includes those other frequencies with small but not
tiny coefficients).
Now, unless you play nothing but octaves, say, you have two frequencies which
are multiples of the first but not of each other. The most audible will be
the lowest two, which are in a 3:2 ratio. This is the pure fifth, and still
makes a pleasing chord. Other ratios can be tried but as a rule the larger the
integers necessary to describe the ratio, the worse the sound.
If we build fifths above the fifths, we get more tones in the scale (typically
we reduce by an octave, i.e., a factor of 2, whenever producing a tone
of more than twice the original frequency). This of course is the construction
of the circle of fifths. I have gotten pretty good at using this to tune
pianos.
Unfortunately, the process never terminates: no power of (3/2) is ever a
whole number of octaves (or indeed any integer multiple of the first frequency).
I make this observation whenever teaching about Unique Factorization. Thus,
we introduce more and more tones describing more complex ratios which, as I
noted above, sound worse and worse.
So we fudge the fifth to make the equal-tempered scale: find a ratio r
roughly equal to 3/2 so that some small power of r is a power of 2. This
amounts to finding good integer approximations for the solutions of
(3/2)^x = 2^y, which we rewrite as 3^k=2^l, or l/k=log3/log2=1.584962501...
The theory of continued fractions tells us how to do this: Form the continued
fraction expansion of this real number, stop at certain points, and reevaluate
the fraction l/k which will approximate log3/log2. Lots is known about this
process, but two facts are useful here: The fractions so attained are better
approximants than any others with smaller denominators; and the approximations
are unreasonably good iff we stop just before a big term shows up in the cont.
frac. expansion.
So here it is: log3/log2= cont.frac[1,1,1,2,2,3,1,5,2,23,...], which gives the
following optimal approximations:
1/1, 2/1, 3/2, 8/5, 19/12, 65/41, 84/53. 485/306, 1054/667,...
(I stop here because the next term to use, 23, is really large, so that
1054/667 is a much better appproximation than the 667 leads you to expect;
the next best approximation has a huge denominator).
Musically, these numbers tell us that by building a pure fifth repeatedly
we get closer and closer to a real octave if we use 1, 2, 5, 12, 41, 53, ...
fifths; other numbers of fifths offer no particular advantage.
I find it a curious twist of nature that the highly-divisible number 12 shows
up here; had it been 11 or 13, music would have developed a lot differently
as you noted in your news post. Also I think it is interesting that the 12
shows up corresponding to the [1,1,1,2,2] part of the fraction; the next
number (3) is larger so 19/12 is a pretty good approximant to log3/log2,
considering the size of its terms; the 65/41, by contrast, is less impressively good. I guess nature just provided for us well.
I think this explains the prevalence of the 5-note and 12-note scales. I
have heard compositions written in n-note scales where n was in the range of 10
to 20; all were (necessarily?) Bach-like and kept suggesting 12-note music
which was "slipping" from time to time. From the mathematics of it I would
expect that no really good new music would result until we tried a 41-note
scale. Just offhand I would expect a keyboard with 3 or 4 levels of blackness
of keys instead of the current 2 (white and black), which would be used with
decreasing frequency (no pun intended) to correspond to the fact that they
corresponded to the higher powers of 2^(24/41) ("higher" meaning mod 41 I guess)
As my electronic music equipment is limited to the world-class IBM PC speaker,
I am not in a position to try this out, but I think it would be a real
kicker to turn a 5-row computer keyboard into this kind of musical instrument.
I am a better mathematician than musician, but I have a good time linkning the
two anyway. Ask me sometime about the shape of a harp.
dave
[email protected]