 The Leslie system is an integrated speaker/amplifier combination in which sound is emitted by a rotating horn over a stationary treble compression driver, and a rotating baffle beneath a stationary bass woofer.This creates a characteristic sound because of the constantly changing pitch shifts that result from the Doppler effect created by the moving sound sources.The Leslie was originally designed to mimic the complex tones and constantly shifting sources of sound emanating from a large group of ranks in a pipe organ.