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A rotary or Leslie-style speaker uses moving parts, such as a rotating horn or drum, to physically move the sound around the room. As the sound source spins, it repeatedly moves towards and away from the listener, creating a blend of level modulation, subtle EQ shifts, and Doppler pitch wobble. Many guitar and organ pedals imitate this behaviour digitally.

Classic rotary cabinets often offer two speeds. The slower setting, Chorale, gives a gentle, slow swirl. The faster setting, Tremolo, produces a more intense, fast wobble. This calculator lets you plug in your slow and fast RPM values so you can see the equivalent modulation rates and pitch movement.

Modulation rate is how many times per second the effect cycles. For a rotating speaker it’s how often the horn or drum completes a full turn past the listener. If you’re designing a pedal or plugin, this is roughly the LFO rate you’d use to match a given RPM.

The tool uses the classical Doppler formula for a moving sound source and a stationary listener. It estimates how far the pitch shifts when the horn is moving towards you compared with when it is moving away, and expresses that in cents and semitones. In realistic rotary setups the total range is usually a fraction of a semitone — enough to sound alive without becoming seasick.

You don’t need exact measurements. Use a reasonable estimate of the distance from the centre of rotation to the part that throws sound outwards. For pedal design work the ballpark values matter more than millimetre accuracy.

Yes. The maths is general: any source rotating in air at the given speed and radius will follow the same basic Doppler relationships. Just bear in mind that the tool assumes a stationary listener and a simplified model of the speed of sound.