Design
Unlike a high-fidelity loudspeaker, the Leslie is specifically designed, via reproduction of the Doppler effect, to alter or modify sound. Although there have been many variations over the years, the classic Leslie speaker consists of two driver units - a stationary treble unit with spinning horns, and a stationary woofer and spinning rotor, with a crossover, that divides the frequencies between the horn and the woofer. The key feature is that both the horns (in reality one working horn with a dummy to counterbalance it) and a sound baffle or scoop for the bass are electromechanically rotated to create Doppler effect–based vibrato, tremolo and chorus effects. The rotating elements can be stopped, switched between slow (chorale) and fast (tremolo), or transitioned between the two settings. To stop a Leslie's rotor, a special brake circuit was added to the Leslie motor controls, that incorporated a tube relay, which sent the rotors into chorale before cutting power from the rotors to bring them to a quicker stop. Solid state relays now make this possible. A three-position switch must be used to allow the rotors to "brake" on a two-speed Leslie. Some other model Leslies had no slow motor and were basically one speed, "fast" or "off."
Much of the Leslie's unique tone is due to the fact that the system is at least partially enclosed, whereby linear louvres along the sides and front of the unit can vent the sound from within the box after the sound has bounced around inside, mellowing it. The tone is also affected by the wood used. Tone differences, due to cost cutting using particle board for speaker and rotor shelves instead of the previous plywood, are evident in the Leslie's sound. The thinner ply of the top of the cabinet adds a certain resonance as well. Like an acoustic instrument, a Leslie's tone is uniquely defined by its cabinet design and construction, the amp and speakers used, and the motors—not merely by the spinning of rotors.
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