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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.

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  • @Dom - thanks for the edit - I felt Hammond and Lesie were apposite - generic 'organ' doesn't do it - it only refers to Hammonds, not other makes - and it was also used by guitarists. So, delete organ, re-instate Hammond and Leslie tags??
    – Tim
    Commented Apr 20, 2022 at 9:34
  • Do these tags represent a group of questions on the site though? We've been through this before when we combined pipe organ and organ music.meta.stackexchange.com/questions/3393/…. Eventually the split may be needed, but if we do let's make sure we group other questions that need it.
    – Dom
    Commented Apr 20, 2022 at 15:13
  • @Dom - if they aren't representative tags , what's the pont in having them available? This question refers particularly to Hammond and Leslie, not any old organ. Thus, my choice of tags. And it is aimed more at the speaker rather than the organ itself, which, to me, puts 'organ' as too general. For example, someone with a Farfisa question would have no help from answers to this.
    – Tim
    Commented Apr 20, 2022 at 15:26
  • Tim if you feel like there's a pattern feel free to add the tags, but they should be added when they are needed and we should check if this pattern persists in other questions. As the meta points out, we have other Hammond questions, but not a ton of organ questions in general hence why the organ tags got merged in the first place. I will point out we don't have separation in concepts like keys either. Major, minor, C major, ect all go under one category. We may need the hammond tag eventually and maybe that's now, but we need to make sure we take care of any introduced tags
    – Dom
    Commented Apr 20, 2022 at 15:50
  • @Tim Farfisa with Leslie here: youtube.com/watch?v=HzqF2NnFCzE. Guitar and Moog: youtube.com/watch?v=hNeZx5eYbEs. I know some recordings where I'm quite sure it's used on vocals but can't find sources to confirm it. As far as I know, Leslie was originally a third party product and for a long time Hammond did not approve of it.
    – ojs
    Commented Apr 22, 2022 at 8:37

2 Answers 2

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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).

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  • 2
    I saw Georgie Fame as a teenager, and he got over the stage problem by using TWO Leslie cabs - one at each side of the stage. What a sound!
    – Tim
    Commented Apr 19, 2022 at 7:48
  • @Tim Couldn't we overcome this problem by placing two mics, one at each side of the cabinet?
    – Marcel
    Commented Apr 19, 2022 at 15:45
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    @Marcel - it gets close, but it's not the same, even when the two mics are panned hard left and right. The speakers and rotating baffles swirl the sound sequencially round, through the full 360 degrees, and the mics would only pick up part of that spaciality. I use the Leslie effect on stage, with stereo speakers, and, while it gets some way there, it's just not the same. Would maybe work with more mics and surround sound amps/speakers, but now it's getting complicated.
    – Tim
    Commented Apr 19, 2022 at 16:00
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    Is this in any way similar to how the sound is made in old-school police/fire truck/ambulance sirens? Not the new ones, those are mostly electronic speakers. But the old ones just blew air through a spinning element, which seems similar to what some organs do, just, less musical of course. Commented Apr 19, 2022 at 17:27
  • No, it is in no way similar. Commented Apr 20, 2022 at 5:33
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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.

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    Thanks. I wonder whether the disabling of the woofer is merely that the motor stopped working years ago, and - so what? That movement of sound is still part and parcel of the Leslei effect, and probably poorer for the disablement, although the horn will always be more audible, partly due to its location at ear height, and high pitches are more directional anyway.
    – Tim
    Commented Apr 19, 2022 at 16:23
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    I never heard of organ players disabling their Leslie woofer or its motor. Instead, there’s some more portable models where only the tweeter horn rotated while the woofer rotation is simulated. Commented Apr 19, 2022 at 21:22

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