Rotor Speed Heard as a Modulation Rate
Inside a rotary speaker cabinet nothing about the signal is being processed. A horn spins on top, a drum-shaped baffle spins underneath, and the swirl an organist hears is what happens when two loudspeaker mouths sweep past the ears several times a second. A horn turning at 400 revolutions per minute throws its beam past the listener 400 times a minute, and the wobble it produces carries the same figure as a 400 BPM pulse. Both units count events over one minute, so rotor speed and modulation rate are one number wearing two labels.
That equivalence bridges a service manual, which lists rotor speeds, and a plugin panel, which offers a rate control and a pair of speed buttons. With both rotors expressed in the same units as the music, it is obvious why a slow setting sits under a ballad and a fast one turns a held chord into a shimmer.
Two rotors, two jobs
The Treble Horn
The Bass Drum Baffle
The Half-Moon Switch
Dialling a Rotor Speed Into a Session
Whether you are matching a plugin to a cabinet you once recorded or writing a rate into a patch sheet, the workflow is the same short loop.
Enter the rotor speed from the manual
Type the figure the service documentation or a strobe measurement gives you into the revolutions field. It starts at 1 and recalculates as each digit lands, so horn and drum can be checked one after another.
Read the swirl as a pulse rate
The beats side gives the number a musician can judge by ear. Anything under about 60 reads as a slow drift, and anything past 300 reads as a continuous shimmer rather than countable pulses.
Restate it for the plugin's rate knob
Most rotary emulations label their controls in hertz. Pick hertz in the searchable menu on the output side and the rotor speed comes back in the units the panel expects.
Go the other way for an ear-first setting
If you tuned the effect by feel and want to know what mechanical speed you just invented, put the pulse rate in the beats field or press the swap arrows to lead with it.
Horn and Drum Speeds in Chorale and Tremolo
Figures vary with the cabinet model, the pulleys and how tired the belts are, but the classic layout sits close to the values below. The pattern matters more than the digits: the horn always outruns the drum, and fast is roughly eight to ten times slow.
| Rotor and setting | Rotor speed | Swirl rate | Plugin rate |
|---|---|---|---|
| Horn, chorale (slow) | 48 RPM | 48 BPM | 0.80 Hz |
| Horn, tremolo (fast) | 400 RPM | 400 BPM | 6.67 Hz |
| Drum, chorale (slow) | 40 RPM | 40 BPM | 0.67 Hz |
| Drum, tremolo (fast) | 342 RPM | 342 BPM | 5.70 Hz |
| Horn mid-ramp | 200 RPM | 200 BPM | 3.33 Hz |
| Brake engaged | 0 RPM | 0 BPM | 0 Hz |
The mid-ramp row is not a setting anybody selects; it is where the horn passes through on its way from chorale to tremolo, and it lasts perhaps half a second. Everything the effect is famous for lives in that transition rather than in the two endpoints.
Keep Horn and Drum on Separate Numbers
Each rotor gets its own pass, so a patch sheet carries two speeds instead of collapsing the cabinet into one average figure.
Lift a Rate Straight Into the Plugin
The copy button on either field yields the bare figure with no unit attached, which is what a rate parameter box wants pasted into it.
Search for the Unit the Panel Speaks
Type a few letters into either menu to jump between revolutions, beats and hertz as different manuals and emulations demand.
Questions About Rotary Cabinets and Their Speeds
How fast do the chorale and tremolo settings actually spin?
On a typical tone cabinet the horn idles near 48 RPM in chorale and reaches around 400 RPM in tremolo, while the bass drum sits near 40 RPM slow and around 342 RPM fast. Individual cabinets drift as belts stretch and motors age, and later models used different pulleys, so treat published numbers as a target to measure against rather than a specification.
Why do the horn and the bass rotor turn at different speeds and in opposite directions?
If both rotors ran at the same rate in the same direction their sweeps would coincide, and the result would be a single heavy throb. Offsetting the drum by roughly fifteen percent and reversing its rotation keeps the two beams permanently out of step, so highs and lows arrive from different places at different moments. That disagreement is what makes the sound feel three-dimensional.
How long does the cabinet take to change speed?
The light horn is up to tremolo in roughly a second and coasts back down over two or three, while the far heavier drum takes several seconds in each direction. Because each rotor accelerates at its own pace, the highs reach the new speed long before the lows do and the sound smears through a rising or falling swirl on the way. Emulations offering separate acceleration times per rotor are chasing exactly this behaviour.
What does a plugin's rate control correspond to on a real cabinet?
It is the steady-state rotor speed converted into cycles per second. A rate of 6.7 is the horn at full tremolo, a rate under 1 is chorale, and everything between corresponds to speeds a real motor only passes through. Plugins exposing separate horn and drum rates, or one rate with a fixed offset, are modelling the gearing in the table above.
Is the effect pitch movement or volume movement?
Both, arriving together at the rotor's rate. As the mouth swings toward the listener the source is momentarily approaching, which lifts the pitch slightly; as it swings away the pitch sags. Meanwhile the beam sweeps on and off axis so loudness rises and falls, and wall reflections add a third layer out of phase with the direct sound. Amplitude modulation alone sounds flat by comparison, which is why a simple tremolo never quite passes for a spinning cabinet.
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