User Guide

358 Chapter 22 EVB3
Tonewheel Sound Generation
Tonewheel sound generation resembles that of a siren. Of course, there’s no air being
blown through the holes of a revolving wheel. Rather, an electro-magnetic pickup,
much like a guitar pickup is used.
A notched metal wheel, called a tone wheel, 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, sent through the manuals, amplified, has
vibrato and expression applied to it, and is then amplified.
An AC synchronous motor drives a long drive shaft. 24 driving gears with 12 different
gear sizes are attached to the shaft. These gears drive the tone wheels. The frequency
depends on the gear ratios, and the number of notches in the wheels. The Hammond is
tuned to an almost exact well-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.
The Leslie
Don Leslie developed his rotor cabinets in 1937, and began marketing them in 1940.
Laurens Hammond wasn’t keen on the concept of rotating speakers at all! Leslie’s
approach was to simulate a variety of locations in the pipes, resulting in a new spatial
perception for every note. The rotor speaker cabinets could simulate this effect, and the
sense of space that they impart is incomparable, when placed side-by-side with any
fixed speaker. The periodic undulations in sound and volume, and the vibrato caused
by the doppler effect aren’t all there is to the Leslie sound—it’s the space effect, too!
The first Leslie, the model 30, had no chorale, just tremolo and stop. The Chorale idea
(which came much later) was borne of a desire to add a vibrato to the organ. Chorale
offers far more than a simple vibrato, and was first introduced to the market with the
122/147 models. At this time, Leslie also added the “Voice of the pipe organ” label to his
cabinets.
It wasn’t until 1980 that the two companies and brand names came together, six years
after the last tonewheel organ was built. Mechanical Leslie rotor cabinets are still being
built today, by the Hammond-Suzuki company. Even the newest digital B3 model is
combined with a real, mechanical, Leslie cabinet.