An electronic organ, designed in the 1930s, that uses a unique electromechanical sound synthesis method divised by its inventor and namesake, Laurens Hammond. To produce the tones, classic Hammond models use a “tonewheel” system.A tonewheel is a metal plate, cut into the shape of a polyhedron or a mechanical gear, and rotating on a shaft. A magnetic pickup is mounted near the edge of the rotating plate, and the plate induces a waveform in the pickup as the bumps and variations of the plate pass by. The tonewheels are all mounted on a common shaft which is driven by a rather peculiar motor arrangement; most have to be “started” in a manner rather like starting a car engine. The keyboard is wired to a complex matrix of switches that selects the proper wheels for each key. The running motor is synchronized to the incoming power line frequency, which provides for stable tuning, but also necessitates having different versions for North American (60 Hz) and European (50 Hz) power.
The Hammond organ can be thought of as a primitive additive synthesis machine. Sounds are made of a mix of a fundamental frequency plus harmonics up to the 9th harmonic, plus the second and third subharmonics (signals that are 1/2 and 1/3 the frequency of the fundemental). On most Hammonds sounds can be created with a set of “drawbars”, which are simply slider-type controls that are mounted so that they pull out or push into a panel, rather than sliding back and forth across the panel like conventional slider knobs. Each drawbar controls the level of one individual harmonic. A typical Hammond model has about 90 tonewheels from which all of the harmonics for any given note can be formed. (However, the lowest and highest notes don’t always have all of their tones; sometimes a tone an octave lower or higher is used for some of the harmonics of the highest and lowest notes. This is known as “foldback”, and it is part of the Hammond sound.)
Hammond produced a number of models over the years, but the most desired models by keyboard players and synthesists are the B3, C3, and A100 models. The B3 and C3 are "console" designs originally intended to be installed in churches which already had their own sound systems. They each have two manuals and a pedalboard. They each have 12 keys per manual which can be used to select patches. For each manual, there are two sets of drawbars which are selected by two of the patch keys; the other 10 can be changed by moving connections on a terminal block inside the case. The sound of the pedalboard is controlled by two drawbars. These models have "percussion", a feature which provides a short attack, fast decay "ping" at the beginning of each note (not when playing legato). The percussion is an important part of the Hammond sound. These models also have chorus and vibrato settings (provided by an electromechanical scanner), and a spring reverb. The A100, a "spinet" model intended for home use, has all of the above plus a tube-type power amp and built-in speakers. (The B3 and C3 differ only in the cabinetry.)
Hammonds have a classic sound commonly used in jazz and progressive rock, but the electromechanical models are not used as much as they were at one time due to their bulk, and weight (most weigh over 300 lbs.), and relative fragility. However, the design was groundbreaking in the pre-WWII era, and it predated the emergence of “true” additive synthesis by decades. See also Leslie and Novachord.