40edo
40edo, or 40 equal divisions of the octave (sometimes called 40-TET or 40-tone equal temperament), is the equal tuning featuring steps of (1200/40) ~= 30 cents, 40 of which stack to the perfect octave 2/1.
40edo can be considered a straddle-3, or dual-3, system, as it has both the 5edo fifth of 720¢, and a very flat diatonic fifth at 690¢, being the smallest 5n EDO to have a diatonic perfect fifth.
General theory
JI approximation
While 40edo has two intervals that can be considered a perfect fifth, its patent 3/2 is the flat, diatonic one. The 7th harmonic is similar, with the 7/4 inherited from 5edo (960¢) being a closer approximation compared to a very sharp mapping at 990¢; as is the 11th. However, 40edo approximates 5/4 rather well, with its 390¢ interval, and due to being a multiple of 10edo and 4edo, it represents the 13th and 19th harmonics through those EDOs' respective approximations.
Therefore, the case is not dissimilar to 29edo's treatment of harmonics 5, 7, 11, and 13, as 40edo's patent mappings of 3, 7, and 11 are relatively unambiguous, though damaged, and approximately equally flat. Combining this with primes 5, 13, 19, and 23, we find that 40edo approximates a rather broad subgroup of 2.5.7/3.11/3.13.19.23, and has a consistent slight sharp tendency for most of the basis elements in this group.
As 40edo approximates 9 better than it does 3, a slight extension of this group would be to treat 40edo as a dual-{3 7 11 17}, implying 9, 21, 33, and 51 as basis elements; this is the interpretation as a subset of 80edo. Of course, the patent approximations can still be used, an interesting consequence of which is that 6/5 is mapped to the quarter-octave (300¢), like it is in 12edo (though note that this is not the best 6/5, the 330¢ interval being slightly closer).
| Harmonic | 2 | 3 | 5 | 7 | 11 | 13 | 17 | 19 | 23 | 29 | 31 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Error | Absolute (¢) | 0.0 | -12.0 | +3.7 | -8.8 | -11.3 | -0.5 | -15.0 | +2.5 | +1.7 | -9.6 | -5.0 |
| Relative (%) | 0.0 | -39.9 | +12.3 | -29.4 | -37.7 | -1.8 | -49.9 | +8.3 | +5.8 | -31.9 | -16.8 | |
| Steps
(reduced) |
40
(0) |
63
(23) |
93
(13) |
112
(32) |
138
(18) |
148
(28) |
163
(3) |
170
(10) |
181
(21) |
194
(34) |
198
(38) | |
Edostep interpretations
In the 2.3.5.7.13 subgroup, 40edo's step has the following interpretations:
- 80/81, the negative syntonic comma (between 10/9 and 9/8)
- 512/507, the intertridecimal comma (between 16/13 and 39/32)
- 2187/2048, the chromatic semitone (between 256/243 and 9/8)
- 65/64, the wilsorma (between 5/4 and 16/13)
- 36/35, the mint comma (between 5/4 and 9/7)
- 49/48, the interseptimal comma (between 8/7 and 7/6)
Intervals and notation
In 40edo, a diatonic major third is submajor (about 16/13), and a diatonic minor third is supraminor (consistently 39/32). The 5-limit thirds are found by inflecting outwards from these, at the augmented third for 5/4 and the diminished third for 6/5 (which is shared with 12edo). Therefore, the # and b accidentals inflect by a single edostep.
Compositional theory
Chords
Scales
| View • Talk • EditEqual temperaments | |
|---|---|
| EDOs | |
| Macrotonal | 5 • 7 • 8 • 9 • 10 • 11 |
| 12-23 | 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 |
| 24-35 | 24 • 25 • 26 • 27 • 29 • 31 • 32 • 34 • 35 |
| 36-47 | 36 • 37 • 39 • 40 • 41 • 43 • 44 • 45 • 46 • 47 |
| 48-59 | 48 • 50 • 51 • 53 • 54 • 56 • 57 • 58 |
| 60-71 | 60 • 63 • 64 • 65 • 67 • 68 • 70 |
| 72-83 | 72 • 77 • 80 • 81 |
| 84-95 | 84 • 87 • 89 • 90 • 93 • 94 |
| Large EDOs | 99 • 104 • 111 • 118 • 130 • 140 • 152 • 159 • 171 • 217 • 224 • 239 • 270 • 306 • 311 • 612 • 665 |
| Nonoctave equal temperaments | |
| Tritave | 4 • 9 • 13 • 17 • 26 • 39 |
| Fifth | 8 • 9 • 11 • 20 |
| Other | |
