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Chapter 26 EVB3 481
Additive Synthesis With Drawbars
The Hammond B3 is the classic drawbar organ. As with an acoustic pipe organ, the
registers (drawbars, or “stops” on a pipe organ) can be pulled out, in order to engage
them. But in contrast to a pipe organ, the B3 allows seamless mixing of any drawbar
registers. The more you drag the drawbars down, the louder they will become.
Despite characteristics such as key clicks, intonation undulations, distortions, and
crosstalk (which are emulated by the EVB3), playing a single note, with a single register,
results in a pure sine tone. Mixing harmonic sine tones results in more complex spectra,
and is known as “additive synthesis”. Organs—even acoustic pipe organs—can be
regarded as additive synthesizers. There are, however, several limitations that need to
be considered before viewing the instrument in this way. These limitations, on the
other hand, constitute the character of any real musical instrument, loaded with charm.
The naming of the drawbars is derived from the length of organ pipes, measured in
feet ('). This naming convention is still used with electronic musical instruments.
Halving the length of a pipe doubles its frequency. Doubling the frequency means
nothing other than: one octave up.
The lowest register, 16' (far left, brown drawbar), and the higher octaves 8', 4', 2' and 1'
(white drawbars) can be freely mixed, in any combination. 16' is commonly described as
the “sub-octave”. When we’re regarding this register as the fundamental, the octave
above 8' is the second partial, 4' the fourth, 2' the eighth and 1' the sixteenth partial.
With the 5 1/3' register—the second brown drawbar—you can add the third partial.
This is the fifth above the 8'. Basically, the drawbars are arranged by pitch, but there is,
however, an exception. The second drawbar (5 1/3') is sounding a fifth higher than the
third drawbar. See the “Residual Effect” on page 482, for an explanation.
2 2/3' gives the sixth, 1 3/5' the tenth and 1 1/3' the twelfth partial. So the
electromechanical tone-wheel organ gives you the partials 1 (16'), 2 (8'), 3 (5 1/3'), 4 (4'),
6 (2 2/3'), 8 (2'), 10 (1 3/5'), 12 (1 1/3') and 16 (1'). As you can see, the harmonic spectrum
is nowhere near “complete”. That’s the reason why overdrive distortion effects are so
popular with electromechanical tone-wheel organs—they enrich the harmonic spectra
by generating more partials.
Note: The term “partial” is basically the same as “harmonic”, but they are counted in a
slightly different way. The fundamental is counted as the first partial. its octave, twice
the frequency, is the second partial, but is known as the first harmonic. The fifth partial
oscillates at five times the frequency of the fundamental. The fifth partial is known as
the fourth harmonic, because with harmonics, the fundamental is not counted (which
makes the term “harmonic” less practical to use).