For percussion containing wood or metal bars with free ends, the pattern of harmonic partials is irregular. Sethares (2010) calculated the pattern of the first 6 partials as:
f, 2.76f, 5.41f, 8.94f, 13.35f, and 18.65f
P.1
Then, I calculated and draw the dissonant curve:
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The dissonant curve
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After that find the frequencies with consonant intervals, the green frequencies below come from the simple ratios from the harmonic pattern (P.1) and the blue one comes from the curve. The middle column is the ratios between each consonant step.
consonant freq. | ratio b/w each | log(octave)/ log(r between) |
220 | | |
259.2 | 1.178181818 | 4.126597564 |
302.5 | 1.167052469 | 4.380129661 |
329.6296296 | 1.089684726 | 7.878224355 |
362.3290203 | 1.099200399 | 7.154014395 |
432.8 | | |
| | |
avg | 1.133529853 | 5.884741494 |
The main concept of finding the best number of equal temperaments is to capture all of the consonant steps as close as possible with limited steps within a reasonable range. (For example, 120 steps with an octave can possibly capture all the consonant intervals, but not practical for actual performance.)
To look at individual consonant frequency ratio between each and come a good number.
If our destination is 6m away and each consonant step is 0.5 m in length, the best equal steps are 12. In this module, each step is equal-distance. However, in the interval above, as the milestones in the journey, ratios between each consonant interval are not equal. We have to find a middle ground. The log(octave)/log(ratio in between) converts the distance into how many steps. For example, the ratio of 1.178 takes 4.12 steps to the octave (1.178^4.12=1.967). The ratio between 432:362 is omitted since other intervals have been compared to 432 in order to prevent the data from duplicating.
The average number of steps is 5.88. The number has to be an integer or at least very close to it, we cannot take half step. By doubling it, splitting a step in half, 12-TET is a balance option.
12 tet | diff % |
220 | |
232.7616328 | |
246.2635351 | |
260.5486478 | 0.5203116609 |
275.6624032 | |
291.6528685 | |
308.5708995 | 2.006908916 |
326.4703018 | -0.9584477626 |
345.4080024 | |
365.4442301 | 0.8597737309 |
386.6427077 | |
409.0708544 | |
432.8 |
.By continuing splitting a step in half, 47-TET is very close to the integer steps.
| steps |
1 | 5.884741494 |
2 | 11.76948299 |
3 | 17.65422448 |
4 | 23.53896598 |
5 | 29.42370747 |
6 | 35.30844896 |
7 | 41.19319046 |
8 | 47.07793195 |
9 | 52.96267345 |
10 | 58.84741494 |
11 | 64.73215643 |
12 | 70.61689793 |
Calculation in this google
sheet. In the tab of PERCU.
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