What's actually going on with the Leslie effect?

Question

Hammond organs often use the Leslie effect, which is produced by a rotating baffle and horn. They speed up and slow down at different rates, and have become the 'Hammond sound'.

But what's actually happening?

There's certainly a tremolo effect, where the volume changes as the sound is moved around. Could there also be a vibrato effect, whereby the Doppler effect is produced by the sound being moved at different speeds? (The Doppler effect is heard when an ambulance or fire engine passes, and its siren seems to change pitch). Is it a phase change? All of the above, and/or something else?

I understand what's happening inside the speaker cabinet, so am not asking about that, just the physics behind the unique effect, not possible with phaser, flanger or chorus.

Answer 1

The rotating baffles change the distance the sound takes to travel to the listener, so there also is frequency modulation due to Doppler effect. The sound is also reflected in the enclosure that has multiple exit slots (louvres) so there are numerous changing sound paths to the listener. The horn baffles (only one is active, the other is there for balance) carry deflectors that change the balance between frequency and amplitude modulation and change the directionality of the horns and the influence of rotation speed on the frequency response.

A large part of the effect is that the resulting sound field is complex and moving so that the listener's head movements give an impression of the complexity of the sound field that is completely flattened when recording and replaying from static speakers: consequently Leslie simulators are close to perfect (and less noisy) in reproducing the sound of a recorded Leslie speaker but do not capture the ambience of one being played at a live venue.

However, this loss of spatiality also occurs when miking and reproducing the Leslie through a P.A., so this loss is only apparent with small to mid-size venues where no P.A. system is interspersed and players just use stage amps (that then include the Leslie which does not come in kilowatts versions).

Answer 2

Basically “All of the above” and even more. As user86458 said, it has a lot to do with how the sound travels both inside and outside of the cabinet, which makes both the time- and frequency response, and complex phase response, dependent on the rotation-position and thus on modulation time. What makes it even more complex is that the horn and the woofer rotate separately, and take different times to speed up and slow down. (Though many Leslies have the woofer rotation just completely disabled.)

I found that a Leslie can be modelled quite well as eight sets of impulse-response recordings (two for each microphone and speaker, in orthogonal positions of the movement, respectively). Each of these is like an IR sample you could make of a guitar amp. But then they are mixed together with both time-dependent amplitude and delay, both modulated in sync for each speaker but not in sync between the speakers. All this together creates the rich combination of tremolo, chorus, phasing and resonance/reverberation elements, without however losing the sonically cohesive character of it all working together as one effect instead of separate units for each stacked together. I implemented a Hammond VST instrument whose Leslie works this way – I wouldn't claim it's perfect or as close as commercial Leslie-simulator, not sure what exact tricks these use. But I would say it has unmistakeably Leslie character. That VST can be heard on this recording.

Simpler combinations, like only syncing together the LFOs of a chorus, tremolo and filter or multiple short reverbs with tremolos, do not give results that I found convincing; it really requires that both amplitudes and phase delays are modulated in the complex way that impulse responses provide.