A Leslie speaker is a specially designed speaker system commonly used with Hammond organs, though it can be used with other instruments as well. It’s known for its unique sound, which is created by the physical movement of sound within the unit. The Leslie speaker contains an amplifier and two sets of rotating speakers: a treble horn and a bass speaker. As these speakers rotate, they create a Doppler effect, giving the organ’s sound a distinct vibrato and tremolo effect. This movement results in a rich, three-dimensional sound that’s become a staple in various music genres, particularly in jazz, gospel, and rock. The Leslie speaker’s distinctive sound is integral to the characteristic tone of the Hammond organ.

The Leslie speaker was invented by Donald Leslie in the 1940s. Initially, Leslie sought to create a speaker system that mimicked the complex sounds of a pipe organ. He achieved this by developing a method of sound modulation using rotating speakers. The resulting Leslie speaker added a unique, swirling sound to the Hammond organ and quickly became popular among organists. Despite the initial resistance from Laurens Hammond, the inventor of the Hammond organ, the Leslie speaker’s distinct effect became synonymous with the Hammond organ’s sound in various music genres. Donald Leslie’s invention has since become a significant and iconic part of musical instrumentation history.