Apple MainStage 3 Instruments - MainStage 3 Instruments

The residual effect
The residual effect is a psychoacoustic phenomenon. Human beings can perceive the pitch of
a note, even when the fundamental tone is completely missing. If you pull out all registers of a
draw bar organ, except for the fundamental—16'—you’ll still perceive the same pitch. The sound
becomes thinner, with less bass and less warmth, but the pitch remains the same.
If human beings didn’t hear this way, it would make listening to music on a small transistor radio
impossible. The tiny speaker of a small radio can’t accurately play back the fundamental tone of
the bass line because this frequency is far below the range that the speaker can reproduce.
Setting draw bar registrations often involves this psychoacoustic phenomenon. In the lower
octaves, mixing the 8' and 5 1/3' sine draw bars creates the illusion of a 16' sound, although the
lower frequency is missing.
Old pipe organs also make use of the residual effect, by combining two smaller pipes, thus
eliminating the need for long, heavy, and expensive giant pipes. This tradition is continued in
modern organs and is the reason for arranging the 5 1/3' under 8': the 5 1/3' tends to create the
illusion of a pitch that is one octave lower than 8'.
Tonewheel sound generation
Tonewheel sound generation resembles that of an air horn, or a siren. Of course, there’s no air
being blown through the holes of a revolving wheel. Rather, an electromagnetic pickup, much
like a guitar pickup, is used to capture the sound.
A notched metal wheel, called a tonewheel, revolves at the end of a magnetized rod. The teeth of
the wheel cause variations in the magnetic field, inducing an electrical voltage. This voltage/tone
is then filtered, has vibrato and expression applied to it, and is then amplified.
An AC synchronous motor drives a long drive shaft. Twenty-four driving gears with 12 different
gear sizes are attached to the shaft. These gears drive the tonewheels. The frequency depends on
the gear ratios and the number of notches in the wheels. The Hammond is tuned to an (almost
exact) equal-tempered scale.
As with pipe organs that feature multiplexed registers, the Hammond organ uses certain
generators for more than one purpose. Some high frequency wheels serve as the fundamental
for high notes and provide harmonics for lower notes. This has a positive impact on the overall
organ sound, avoids detuning, and stabilizes levels between octaves.
Chapter 15 Vintage B3 429