2009

Table Of Contents
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 drawbar 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, as this frequency is far below the range that the speaker
can reproduce.
Setting drawbar registrations often involves this psychoacoustic phenomenon. In the
lower octaves, mixing the 8' and 5 1/3' sine drawbars 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 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.
180 Chapter 7 EVB3