The Leslie speaker is a specially constructe amplifier/loudspeaker used to create special audio effects utilizing the Doppler effect. Named after its inventor, Donald Leslie, it is particularly associated with the Hammond organ. Separate Leslie speakers were a "must have" accessory for all Hammond owners, particularly after its characteristic sound was popularised by such acts as Procol Harum on "A Whiter Shade of Pale," by Lee Michaels on "Do You Know What I Mean", or the Spencer Davis Group on "Gimme Some Lovin'," or on much of the '60s output of Steppenwolf.