The Leslie speaker is a combined amplifier and loudspeaker that projects the signal from an electric instrument and modifies the sound by rotating a baffle chamber ("drum") in front of the loudspeakers.
In the late 1930s, the Leslie speaker was born, designed and invented by Donald J. Leslie.
The Leslie speakers for example the 2101 MK2 - 2103 MK3 - 3300WP - 3300P - 122XP is a specially designed amplifier / speaker that is used to make special sound effects by utilizing the Doppler effect, which makes sound behave as if it's moving past you.
The speaker is particularly associated with the Hammond organ. Separate Leslie speakers were a necessary accessory for all Hammond owners, especially after its distinctive sound had become popular thanks to e.g. Procol Harum on "A Whiter Shade of Pale," Lee Michaels '(Do You Know What I Mean) and Spencer Davis Group in "Gimme Some Lovin'." Other users of the speakers include the '60s band Steppenwolf.