10.6

656Logic Pro Instruments
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.
A brief Hammond history
Three inventions inspired Laurens Hammond (1895–1973), a manufacturer of electric
clocks, to construct and market a compact electromechanical organ with tonewheel sound
generation. The Telharmonium by Thaddeus Cahill was the musical inspiration; Henry
Ford’s mass production methods and the domestic synchron clock motor were the other
factors.
The Telharmonium (built around 1900) was the first musical instrument that made use of
electromechanical sound generation techniques. Its immense tonewheel generators filled
a two-story building in New York. For a short period around this time, subscribers could
order Telharmonium music over the New York telephone network (the streaming audio
system of the time). The only amplification tool was the telephone mechanical diaphragm
because a proper tube amplifier and acceptable speakers had not yet been invented.
The Telharmonium was a commercial flop, but its historical status as the predecessor of
modern electronic musical instruments is undeniable. The Telharmonium also introduced
the principles of electronic additive synthesis. See Additive synthesis with draw bars.
Laurens Hammond began producing organs in 1935 in Chicago, Illinois, making use of the
same sound generation method. However, he used much smaller tone generators and fewer
registers. The patent for his model A organ dates from 1934.
Hammond also holds the patent for the electromechanical spring reverb, still found in
countless guitar amplifiers today.