Tonewheel
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A tonewheel is a simple electromechanical apparatus for generating electronic musical notes. The tonewheel assembly consists of a synchronous AC motor and an associated gearbox that drives a series of rotating disks. Each disk has a given number of smooth bumps at the rim; these generate a specific frequency as the disk rotates close to a pickup assembly that consists of a magnet and coil.[footnotes 1] The frequency depends on the speed of rotation of the disk and the number of bumps. A single fundamental frequency can be combined with one or more harmonics to produce complex sounds. A tonewheel was used in the Telharmonium and in the original Hammond organs.
Tonewheel leakage occurs in the Hammond organ and in similar situations, where the large number of tonewheels causes pickups to overhear tonewheels other than their own. This causes the organ to add chromatics to played notes. In some kinds of music this is undesirable, but in others it has become an important part of the Hammond sound. On some digital simulations of Hammond organs tonewheel leakage is a user-set parameter.
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- ^ This is electrically and magnetically similar to a guitar pickup, in that a permanent magnet is placed within the coil and the moving element is unmagnetized. Unlike most generators or dynamos, there is no external field applied through the moving part.
