The 'classic' Leslie cabinet has two speakers, treble and bass with a distinct crossover at 800hz separating the two. The crossover is important, for while the two speaker baffles rotate at roughly the same speed, they are not synchronised either in direction or speed, and are fed with discrete audio signals divided by the crossover. Furthermore, the ramp up and down between the two rotating speeds (fast and slow) and the stopped setting is much shorter for the frequencies above 800hz than for the lower frquencies, which produces a delicious further effect on the sound. Also, the crossover presents something of a gap in the freqency range, giving rise to a somewhat hollow sound, which is a particularly distinctive sound that emanates from the "classic" Leslie Cabinet. The classic Leslie amplifier is tube driven, which together with the design of the cabinet, thickens and warms the sound.
The tube driven amplifier can also be leaned on a little (or a lot) to produce overdrive "growl" which, unlike transistorised amplifiers, produces musical second harmonic distortion which is pleasing to the ear.
The effect on the sound produced by the rotating sound 'projectors' is extremely complex and unique, relying in part upon the doppler effect, whereby sound, in varying amount depending upon frequency, coming towards the listener produces a higher tone than sound moving away from the listener which gives rise to complex vibrato. The moving sound source also projects the sound out towards and away from the listener reflecting sound waves from the walls of the room where it is situated. This gives rise to both an undulating volume producing a three-dimensional amplitude modulation (tremelo) and also an alternating enhancement and diminution of different frequencies due to phase cancellation and fortification.
This 'character assassination' performed upon the basic organ tone cannot be fully reproduced by an effects box that relies entirely on electronics to produce the effect, if only because it cannot physically project sound towards alternating walls of a room in a regular moving pattern that gives a real Leslie its 3D sound quality. However, much of the distinctive Leslie sound can be reproduced electronically, often expensively when done properly, to the point where it becomes extremely difficult to distinguish on a recording or through a PA system whether or not a Leslie cabinet was used. Even room simulation can be incorporated into the design.
Thomas Adamson. UK.
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