The Hammond organ was invented by Laurens Hammond and John M. Hanert in 1934. The basic component sound of a Hammond organ is produced by a tonewheel. Each tonewheel rotates in front of an electromagnetic pickup. The variation in the magnetic field induces a small alternating current at a particular frequency, which represents a signal similar to a sine wave. When a key is pressed on the organ, it completes a circuit of nine electrical switches, which are linked to 38 sliders above the manuals, called drawbars. The position of the drawbars, combined with the switches selected by the key or pedal pressed, determines which tonewheels are allowed to sound. Every tonewheel is connected to a synchronous motor via a system of gears, which ensures that each note remains at a constant relative pitch to every other.
At your right you see a schematic representation of a typical tonewheel Hammond key and the underlying contacts. Solid arrows indicate the directions of the motion of the key, the contact pusher and the tip of the spring contact. (a) plastic key, (b) metal shaft, (c) contact pusher felt, (d) contact pusher, (e) insulated support, (f) foam, (g) pivotal point, (i) and (h) busbar, (j) and (k) contact spring, (l) and (m) support frame, (n) resistive wires from generator, (o) precious metal points, (p) adjusting tab.1
The combined signal from all activated keys and pedals is fed through to the vibrato system, which is driven by a metal scanner. As the scanner rotates around a set of pickups, it changes the pitch of the overall sound slightly. From here, the sound is sent to the main amplifier, and on to the Leslie speaker.2
The Leslie speaker consists of a 15” woofer and a horn, both equipped with rotors. With the organ’s half moon switch, the performer can make the rotors spin slowly (chorale), fast (tremolo) or sometimes stop them completely. Especially going from chorale to tremolo creates a huge rise in dynamic intensity.
The Hammond organ and the Leslie speaker go together like red beans and rice, like cream cheese and bagels, like champagne and oysters. That’s how we feel since Hammond organ music took off, but back in the day Laurens Hammond refused to license the speaker,3 invented by Don Leslie who was inspired by a truck driving past him creating the Doppler effect.4 Laurens Hammond intended his organ as a replacement for pipe organs in church and Hammond dealers weren’t even allowed to sell Leslie speakers.5
Back to the Hammond organ. When a key is released, the electrical circuit is cut off and therefore the sound. This means you will either have a sound, or no sound at all and the beginning and ending of each note is crucial for good timing. So, the Hammond organist has to be very conscious about where he starts his note, where he ends his note and if all the notes are played legato or slightly separated, because what is set on the machine is what will come out. The organist can not influence the tone by increasing or decreasing finger action, or using more or less arm weight like a pianist can do.
1 Giulio Moro, Andrew P. McPherson, Mark Brian Sandler, “Schematic representation of a typical tonewheel Hammond key,” ResearchGate, accessed May 9, 2021, https://www.researchgate.net/figure/Schematic-representation-of-a-typical-tonewheel-Hammond-key-and-the-underlying-contacts_fig3_320997402.
2 “Laurens Hammond, Inventor Of The Hammond Organ,” Key Man, accessed January 16, 2020, https://www.udiscovermusic.com/stories/man-who-invented-hammond.
3 Charles Richard Lester, “Laurens Hammond and Don Leslie,” The Story of the Leslie Speaker, accessed January 18, 2020, https://www.hammondclub.nl/nl/menu/Leslie/Donald-James-Leslie-uitvinder/The-Story-Of-The-Leslie-Speaker.
