z8000783 wrote:Are you sure you're not just making this stuff up?
John
Nope. The twelve tone scale of western music can be built in a number of ways:
you can stack twelve fifths on top of each other, corresponding to (3/2)^12. This is fairly close to 2^7. 2 corresponds to an octave - the doubling of a tone. The difference is (3/2)^12/2^7 ≃ 1.01. In western music, we distribute this error equally over the twelve 3/2s, so instead of 3/2 being 3/2, it's represented by ((2^7)^1/12) = 2^(7/12).
Due to a kind of awkward mathematical fact known as the fundamental theorem of arithmetic, no two different primes to any non-zero power will ever be equal, so 3^n/2^n will never give an integer, as 3 will cycle through the final digit being 3, 9, 7, 1, 3, ... and 2^n will cycle through the last digit being 2, 4, 8, 6, 2, ...
We'll get near misses, though, and that's where we attack the problem.
However, this only really gives us good approximations of ratios with low powers of three (and any power of 2, as 2 is represented by a perfect 2) in them (and some weird random ratios of other primes - 19/16 happens to get a very good approximation.
We could just as well approach it by noticing that 2^(n/12), for different n gives good approximations of 3/2 and 4/3, less good approximations of 5/4 and 6/5 (and for an obvious reason 5/3 and 8/5), about as good approximations to 15/8, 16/15 and 9/5 and 10/9 as those of 5/4. 9/8 is very well approximated. Notice how the very good approximations all have factors no higher than three. 5 is badly approximated, 7 even worse- so seven-limit harmony isn't used in western, tempered music. We could use temperaments that approximate 7 and 5 better than currently - in fact, the most prevalent scale in the 16th and 17th centuries in the west, quarter-comma meantone, if remade so that all keys are playable, and it's somewhat tempered, comes very close to 31-tone equal temperament. A system where chords of the form 4:5:6:7, 5:6:7:8, (as well as the 7/7, 7/6, 7/5 inversion) all work. (When it was in use, alas, quarter-comma meantone only was used in a range of about 12 tones, so only a few keys worked in it, but in some of the further away keys, you got these alien yet overtone-like intervals.)
Anyways, uh, music theory is how I learned to calculate approximate logarithms in my head (to three decimals accuracy for very many numbers, numbers with higher prime factors might get a bit worse as far as accuracy goes), so ...trust me, I know this shit.