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glyph 7 4.png
JI glyph for 7/4


7/4
|-2 0 0 1>
968.82591 cents
sound sample

Frequency ratio 7:4, measuring approximately 968.8 cents, has been given the name "harmonic seventh." It represents the interval between the 4th and 7th harmonics in the overtone series. It is also called a "septimal subminor seventh" -- the word "septimal" referring to the presence of a 7 as the highest prime in the ratio, and the word "subminor" referring to the harmonic seventh's narrowness compared with a traditional minor seventh (such as 9:5 or 16:9, 12edo's 1000-cent interval, or a minor seventh found in a meantone system).

7:4 has seen use in blues music, barbershop quartet music, and some musical traditions of the world, but has mostly not been recognized as a "consonance" in Western music theory. In most Just Intonation systems, the harmonic seventh is treated as a fundamental consonance in its own right, with its own distinct quality.

Harmonic Seventh Chord

7:4 appears in an otonal tetrad that forms the basis of much JI music, commonly called a "harmonic seventh chord." It consists of a major triad (4:5:6) plus a harmonic seventh: 4:5:6:7(:8). This tetrad, a hallmark of blues and barbershop harmony, not to mention modern Just Intonation practice, represents a sequence of overtones from the fourth to the seventh. (8, being a doubling of 4, represents an octave above the root.) The intervals between adjacent members of the chord decrease in size:

5:4
approx. 386 cents
major third

6:5
approx. 316 cents
minor third

7:6
approx. 267 cents
septimal subminor third

8:7
approx. 231 cents
septimal supermajor second


This chord is similar to the "dominant seventh chord" in 12edo, the most significant difference being the mistuning of the harmonic seventh. In 12edo, the interval closest to the harmonic seventh is the minor seventh at 1000 cents. The difference of about 31 cents is striking, and especially noticable when the chords are presented next to one another. While the "dominant seventh chord" of 12edo is treated as a dissonance that needs to resolve (usually in the chord pattern V7 to I), the "harmonic seventh chord" has a much different flavor and is often treated by composers in Just Intonation as a consonance.

Another interval found in a harmonic seventh chord is the septimal tritone of 7:5, which represents the interval between the major third (5) and the harmonic seventh (7). This interval, at 583 cents, sounds distinct from 12edo's half-octave tritone of 600 cents. In just intonation, 7:5 is treated as a consonant tritone, and has a much mellower and sweeter sound than the 600-cent tritone we are used to hearing.

Since 12edo does not distinguish between a minor and subminor third or a major and supermajor second, the intervals between adjacent members of the chord do not have the pattern of decreasing step size which characterizes the harmonic seventh chord:

5:4 becomes 400 cents.
6:5 becomes 300 cents.
7:6 becomes 300 cents.
8:7 becomes 200 cents.

Meantone Augmented Sixth

In meantone systems -- which are generated by repeatedly stacking a slightly flatted (from just) perfect fifth such that four fifths gives a near-just major third -- there is sometimes a good approximation of the harmonic seventh in the form of an "augmented sixth". Quarter-comma meantone (aurally identical, for most intents and purposes, to 31edo) is one such system. In quarter-comma meantone, the interval of C to A# approximates a harmonic seventh, and is a distinct interval from C to Bb, a meantone minor seventh (falling somewhere between 16:9 and 9:5). The augmented sixth appears in tonal harmony in the "augmented sixth chord," and is treated as a rare and special dissonance. The so-called "German Sixth," in quarter-comma meantone, would approximate the harmonic seventh chord of 4:5:6:7(:8).

Note that a good approximation of the harmonic seventh is not available in every meantone system. In 19edo (aurally identical, more or less, to 1/3-comma meantone), the "augmented sixth" is an interval of 947 cents -- about 22 cents flat of 7:4, and so less effective as a consonance.

See: Septimal Meantone Temperament on Wikipedia.

Approximations

EDOs containing good approximations of the interval 7:4 are (ordered by relative delta):

EDO
Abs Delta
Rel Delta
Prominent Multiples
26edo
0.40486 ¢
0.87720
78edo
83edo
0.15121 ¢
1.0459 r¢
166edo
57edo
0.40485 ¢
1.9231 r¢

31edo
1.0839 ¢
2.8003 r¢

5edo
8.8259 ¢
3.6775 r¢
10edo, 15edo, 20edo, 25edo
21edo
2.6026 ¢
4.5547 r¢

88edo
0.6441 ¢
4.7233 r¢

47edo
1.3868 ¢
5.4319 r¢
94edo
73edo
1.0371 ¢
6.3091 r¢

36edo
2.1592 ¢
6.4777 r¢
72edo
16edo
6.1741 ¢
8.2321 r¢

68edo
0.14686 ¢
9.9865 r¢

11edo
12.992 ¢
11.910 r¢
22edo
89edo
1.9606 ¢
14.541 r¢


http://en.wikipedia.org/wiki/Harmonic_seventh