Donald Leslie (1913–2004) bought a Hammond organ in 1937, as a substitute for a pipe organ. But at home in a small room, it could not reproduce the grand sound of an organ. Since the pipe organ has different locations for each pipe, he designed a moving loudspeaker.
The Leslie speaker uses an electric motor to move an acoustic horn in a circle around a loudspeaker. Thus we have a moving sound source and a stationary listener, which is a well-known situation that produces the Doppler effect.
It exploits the Doppler effect to produce frequency modulation. The classic Leslie speaker has a crossover that divides the low and high frequencies. It consists of a fixed treble unit with spinning horns, a fixed woofer and spinning rotor. Both the horns (actually, one horn and a dummy used as a counterbalance) and a bass sound baffle rotate, thus creating vibrato due to the changing velocity in the direction of the listener, and tremolo due to the changing distance. The rotating elements can move at varied speeds, or stopped completely. Furthermore, the system is partially enclosed and it uses a rotating speaker port. So the listener hears multiple reflections at different Doppler shifts to produce a chorus-like effect.
The Leslie speaker has been widely used in popular music, especially when the Hammond B-3 organ was played out through a Leslie speaker. This combination can be heard on many classic and progressive rock songs, including hits by Boston, Santana, Steppenwolf, Deep Purple and The Doors. And the Leslie speaker has also found extensive use in modifying guitar and vocal sounds.
Ironically, Donald Leslie had originally tried to license his loudspeaker to the Hammond company, and even gave the Hammond company a special demonstration. But at the time, Laurens Hammond (founder of the Hammond organ company) did not like the concept at all.