Just Hammond

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The article features just intervals created by the mechanical tonegenerator of the classical Hammond B-3 Organ model.

Design of the Hammond B-3’s Tonegenerator

Since 1935 the Hammond Organ Company’s goal was to market electric organs with 12-tone equally tempered (12edo) tuning. The mechanical tonegenerator of the Hammond B-3 Organ is based on a set of 12 pairs of gearwheels that make twelve driven shafts turn. The corresponding twelve driving gearwheels are mounted on a common shaft and turn all at the same rotational speed n₁. Certain gears reduce, others increase rotational speed.

Every chromatic pitch class has a separate driven shaft. Pure octaves are generated by dedicated tonewheels (with 2, 4, 8, 16, 32, 64 or 128 high and low points on their edges) that rotate with the driven shafts. Each high point on a tone wheel is called a tooth. When the gears are in motion, magnetic pickups react to the tonewheels’ passing teeth and generate an electric signal that can be amplified and transmitted to a loudspeaker.

For each pair of gearwheels the ratio of rotational speed n₂/n₁ is determined by the inverse ratio of the gearwheels’ integer teeth numbers Z₁ and Z₂:

[math]\displaystyle{ \frac{Z_1}{Z_2}=\frac{n_2}{n_1} }[/math]

To calculate the rotational speed n₂ of the driven shaft we write

[math]\displaystyle{ n_2=\frac{Z_1}{Z_2}\cdot n_1 }[/math]

Table 1: Pairings of Gearwheels / Ratios and Intervals

Pitch Class	HAMMOND Organ Mechanical Gearing		Gear Ratio (canceled)			Conversion of raw ratio to interval	Deviation from 12tone equal temperament (12edo)	Intonation (Note A renders standard pitch, if shaft (A) is rotating @20 rev./sec)
	(A)	(B)	(C)	(D)
	driving Z1[teeth]	driven Z2[teeth]	Fraction		Ratio (C)/(D)	[cents]	[cents]	[cents]
C	85	104	85	104	0.8173077	-349.26	-49.26	-0.576
C#	71	82	71	82	0.8658537	-249.37	-49.37	-0.684
D	67	73	67	73	0.9178082	-148.48	-48.48	0.200
D#	105	108	35	36	0.9722222	-48.77	-48.77	-0.088
E	103	100	103	100	1.0300000	51.17	-48.83	-0.145
F	84	77	12	11	1.0909091	150.64	-49.36	-0.681
F#	74	64	37	32	1.1562500	251.34	-48.66	0.026
G	98	80	49	40	1.2250000	351.34	-48.66	0.020
G#	96	74	48	37	1.2972973	450.61	-49.39	-0.707
A	88	64	11	8	1.3750000	551.32	-48.68	0.000
A#	67	46	67	46	1.4565217	651.03	-48.97	-0.285
B	108	70	54	35	1.5428571	750.73	-49.27	-0.593

(Purple colored cells contain prime numbers)

Just Intervals

When we associate “ratios of the gearwheels’ integer teeth numbers” with “frequency ratios between partials” we realize an intrinsic just interval determined by integer teeth numbers within such mechanical gear - even without turning the shafts! Although the Hammond Organ pretends to generate a 12edo scale, the instrument in fact creates a high prime limit just scale.

Tuning

The whole set of frequency ratios is fixed by the design of the gear mechanism. The driving shaft’s (A) rotational speed n₁ determines the instrument’s (master-)tuning. Rotating at exactly 1200 rpm (which equals 20 rev./sec), the pitch of note A equals precisely 27.500 Hz or one of its doublings. Therefore the instrument aligns note A with a concert pitch of 440.0 Hz.

<math>f_A=20.0/\textup{sec}\cdot\frac{88}{64}\cdot(2^4)=440.0/\textup{sec} = 440.0\textup{ Hz}<\math>

[...WILL BE CONTINUED]