The Leslie speaker consists of two driver units - a treble unit with horns, and a bass unit. The key feature is that the horns of the treble unit, and a sound baffle for the bass unit, are rapidly rotated using electric motors to create vibrato, tremelo and chorus effects. The resulting sound is instantly identifiable as that of the Hammond organ, frequently heard on psychedelic and rock music of the 1960s and 1970s. Unlike a high fidelity loudspeaker, the Leslie is specifically designed to alter or modify sound, not reproduce it, and so faithful reproduction has never been part of its appeal.
While normally used with the Hammond, because it is a separate unit, any musical source, such as an electric guitar, can be played back through it, creating a wide range of surprising and dramatic effects. The classic Leslie is still made and sold to this day, though in modern times similar effects can be obtained digitally.