Alpha scale

The α (alpha) scale is a non-octave-repeating musical scale invented by Wendy Carlos and first used on her album Beauty in the Beast (1986). It is derived from approximating just intervals using multiples of a single interval without, as is standard in equal temperaments, requiring an octave (2:1). It may be approximated by dividing the perfect fifth (3:2) into nine equal steps (3:2)^​1⁄₉,[1] or by dividing the minor third (6:5) into four steps (6:5)^​1⁄₄.[1][2][3]

Minor third (just: 315.64 cents Play ,
12-tet: 300 cents Play ,
Alpha scale: 312 cents Play 

Chromatic circle

Comparing the alpha scale's approximations with the just values

Twelve-tone equal temperament vs. just

The scale step may also be precisely derived from using 9:5 (B♭, 1017.60 cents, Play ) to approximate the interval ​^3:2⁄_5:4 (=6:5, E♭, 315.64 cents, Play ).[4]

Carlos' α (alpha) scale arises from...taking a value for the scale degree so that nine of them approximate a 3:2 perfect fifth, five of them approximate a 5:4 major third, and four of them approximate a 6:5 minor third. In order to make the approximation as good as possible we minimize the mean square deviation.[4]

${\displaystyle {\frac {9\log _{2}(3/2)+5\log _{2}(5/4)+4\log _{2}(6/5)}{9^{2}+5^{2}+4^{2}}}\approx 0.06497082462}$ and ${\displaystyle 0.06497082462\times 1200=77.965}$ (Play )

At 78 cents per step, this totals approximately 15.385 steps per octave, however, more accurately, the alpha scale step is 77.965 cents and there are 15.3915 per octave.[4][5]

Though it does not have an octave, the alpha scale produces, "wonderful triads," (Play major  and minor triad ) and the beta scale has similar properties but the sevenths are more in tune.[2] However, the alpha scale has, "excellent harmonic seventh chords...using the [octave] inversion of ​⁷⁄₄, i.e., ​⁸⁄₇ [Play ]."[1]

interval name	size (steps)	size (cents)	just ratio	just (cents)	error
septimal major second	3	233.90	8:7	231.17	+2.72
major third	5	389.83	5:4	386.31	+3.51
perfect fifth	9	701.69	3:2	701.96	−0.27
harmonic seventh	12	935.58	7:4	968.83	−33.25
octave	15	1169.48	2:1	1200.00	−30.52
octave	16	1247.44	2:1	1200.00	+47.44

See also

• Bohlen–Pierce scale
• Beta scale
• Gamma scale
• Delta scale

Sources

1. Carlos, Wendy (1989–96). "Three Asymmetric Divisions of the Octave", WendyCarlos.com. "9 steps to the perfect (no kidding) fifth." The alpha scale, "splits the minor third exactly in half (also into quarters)."
2. Milano, Dominic (November 1986). "A Many-Colored Jungle of Exotic Tunings", Keyboard. "The idea was to split a minor third into two equal parts. Then that was divided again."
3. Carlos, Wendy (2000/1986). "Liner notes", Beauty in the Beast. ESD 81552.
4. Benson, Dave (2006). Music: A Mathematical Offering, p.232-233. ISBN 0-521-85387-7. "This actually differs very slightly from Carlos' figure of 15.385 α-scale degrees to the octave. This is obtained by approximating the scale degree to 78.0 cents."
5. Sethares, William (2004). Tuning, Timbre, Spectrum, Scale, p.60. ISBN 1-85233-797-4. Scale step of 78 cents.