Sweep an LFO modulating a VCA from 5 Hz to 200 Hz and watch tremolo blur into ring modulation, the same Leslie-speaker effect turned sideband generator.
In the world
A Leslie speaker spins a horn around a vertical axis so the sound source physically moves toward and away from the listener, producing the amplitude wobble at the heart of every Hammond-organ-through-Leslie record.
Patch an LFO into a VCA's CV input and the VCA gain oscillates. At 5 Hz you get tremolo, a rhythmic wobble. As you increase the LFO rate toward audio frequencies something remarkable happens: around 20 Hz the wobble fuses into a buzz, and by 200 Hz you are doing ring modulation, which creates sum and difference frequencies that are not even in the original signal.
There is no hard boundary between tremolo and ring mod; they are the same operation at different speeds. Your ear draws the line wherever the modulator enters the audio band.
Did you know?
The Leslie speaker cabinet weighs over 70 kg and spins a horn at up to 340 RPM. Don Lesie invented it in 1941 specifically for the Hammond organ, but Hammond's founder hated it so much he refused to sell it in his showrooms. Organists bought Leslies anyway.
Explore
A lighthouse beacon rotates on the right. Move your cursor up/down to control the LFO rate from 1 Hz to 400 Hz. At slow rates the beam sweeps smoothly. Speed it up past 20 Hz and it starts to strobe. Push it into audio range and phantom beams appear between the real sweeps - you are seeing ring modulation sidebands made visible as ghost light.