What is a rotary speaker?

A rotary speaker in plain English

A rotary speaker is a cabinet with speakers that spin. Sound comes out of a horn at the top and a drum at the bottom, and both turn round and round on motors. When the horn points at you it's loud and bright; as it swings away it gets quieter and duller, and then it comes back. While it moves towards you the pitch goes up a little, and while it moves away the pitch drops, the same thing you hear when an ambulance passes.

All of that happens at once, several times a second, and differently at each ear, because the horn is facing one side of the room while it faces away from the other. Your ears hear the result as a swirl: a sound that moves around the room and shimmers without ever going out of tune.

The best-known rotary speaker is the Leslie, named after Don Leslie, who started building them in the late 1930s and sold the first ones in 1941. It became the usual partner for the Hammond organ, and a lot of the organ you hear on rock, soul and gospel records is a Hammond played through a Leslie. Today most people get the sound from a plug-in or a pedal that models one. This page covers what the spinning does to a sound and how to set the controls on those plug-ins. For the wider family of effects that move a sound with an LFO, start with the chorus lesson.

Inside a Leslie cabinet

Inside the cabinet, an amplifier feeds a crossover, a filter that splits the sound in two. In a Leslie 122, the classic partner for a Hammond B-3, the split is at 800 Hz. Everything above goes to a treble driver that fires up into the rotating horn, and everything below goes to a woofer that fires down into the rotating drum. The two halves spin on separate motors and belts, at different speeds.

The horn

The treble rotor looks like two horns joined back to back, and only one of them makes sound; the other is blocked off and is there as a counterweight, so the whole thing spins smoothly. Sound leaves through one narrow mouth, which throws it in one direction like a torch beam sweeping round the room. That's why the treble swings so far in level and why most of the shimmer in a Leslie comes from the top.

The drum

The woofer doesn't move. It fires down into a cylinder, the drum, which has a scoop cut into it that throws the sound out of one side. When the drum turns, the opening sweeps round the cabinet. Low frequencies spread out more than high ones, so the drum's movement is gentler than the horn's: a softer swell in level and a smaller bend in pitch. It gives the effect its body, and it is easy to lose in a plug-in that has the balance tipped towards the horn.

Two speeds and the ramp between them

A Leslie has two speeds. Leslie called the slow one chorale and the fast one tremolo, and on most organs and plug-ins they're just labelled slow and fast. The figures usually quoted for a Leslie 122 are around 40 to 50 revolutions a minute on slow and 340 to 400 on fast, with the horn a little quicker than the drum. Every cabinet differs a bit, because the speeds depend on motors and belts. The lab uses 0.8 and 6.7 turns a second for the horn, and 0.67 and 5.7 for the drum.

The rotors have weight, so they can't change speed instantly. When you switch from slow to fast, the light horn reaches its new speed in about a second and the heavy drum takes several. For those few seconds the two are turning at different, changing speeds, and you hear the swirl wind up as they go. Organists use the switch as part of the performance, often with a half-moon switch on the organ or a foot switch, flipping to fast to lift a chorus and back to slow afterwards.

The Doppler shift

A sound coming towards you arrives with its waves squashed slightly closer together, so you hear it a little higher; moving away, the waves are stretched and you hear it a little lower. That's the Doppler effect, and a spinning horn makes it happen on purpose. Each time the mouth of the horn swings towards a microphone the pitch rises, and as it swings away the pitch falls.

How far the pitch moves depends on how fast the mouth of the horn is travelling. In the lab the mouth sits 15 cm from the centre. On fast, at 6.7 turns a second, it covers 2π × 0.15 × 6.7, which is about 6.3 metres a second. Sound travels at about 343 metres a second, so the pitch rises by up to 6.3 ÷ 343, about 1.8%, which is roughly 32 cents. A cent is a hundredth of a semitone, so that's a third of a semitone either way. On slow the horn moves at 0.8 turns a second and the shift drops to about 4 cents, too small to hear as pitch, so slow sounds like a gentle turning in level and tone.

The level swing stays the same at both speeds, because it depends only on which way the rotor is pointing. The pitch shift grows with speed. So on fast the pitch and the level are both moving hard, several times a second, and they're tied together: the pitch is highest a quarter of a turn before the horn faces a mic, and the level is highest when it does. Those two movements, plus the change in brightness and the difference between the left and right mics, are what separate a rotary speaker from a tremolo, which only moves the level, or a vibrato, which only moves the pitch.

Reading the lab

The cards along the top pick the speed: Dry for the original sound, Brake to stop both rotors, and Slow and Fast. Once you've clicked inside the lab, the keys 1 to 4 do the same. When you switch while the loop is playing, the rotors ramp to the new speed the way a real cabinet does, so it takes a few seconds for the drum to settle.

The screen at the top left shows the cabinet from above, with the horn on the left and the drum on the right, turning at the speed the audio is using. The bright end of the horn is the one that makes sound, and the dots around each rotor are the microphones. Next to it, the speed screen draws how fast each rotor has been turning over the last ten seconds, which is where you can see the drum lag behind the horn.

The wide screen underneath follows one full turn of each rotor. Its top lane is the pitch shift at each mic in cents, at the speed the rotor is turning right now, so it grows as the rotors speed up; its bottom lane is the level. The thin vertical lines show where each rotor is in its turn. The Right now panel reads out the pitch shift, the level swing and the correlation between left and right, measured on the loop as the lab renders it through the same model you're hearing.

Every option plays at the same loudness as the dry sound, so the effect can't win by being louder. You can upload your own sound or record eight seconds of one, and nothing leaves your device. Try the chords loop: less than 1% of its energy is above 800 Hz, so nearly all of it goes to the drum and the horn has almost nothing to play. The guitar splits close to half and half, which is why the lab starts on it.

The controls on a rotary speaker plug-in

Rotary plug-ins and pedals vary, but most have the same handful of controls. Logic Pro's Rotor Cabinet, Arturia's Rotary CLS-222 and hardware pedals such as the Strymon Lex and the Neo Instruments Ventilator all give you a speed switch, and most add some of the rest.

Speed

Slow, fast and often brake, which stops the rotors. Some plug-ins let you set the slow and fast speeds of each rotor. The defaults are usually close to a real cabinet, so leave them there until you know what you want to change: the gap between the horn and drum speeds is part of what makes it sound like a Leslie and unlike a tremolo.

Acceleration, ramp or inertia

How long the rotors take to change speed. Short ramps make the switch sound sudden and electronic; long ones give you a slow, dramatic wind-up. In the lab the Ramp knob sets the horn's time and the drum always takes four and a half times as long.

Horn and drum balance

The level of the treble rotor against the bass rotor. More horn sounds brighter and moves more; more drum sounds darker and fuller. A real cabinet's balance is set by its amplifier and speakers, and engineers adjust it further with the microphones.

Microphones

A Leslie is a mono speaker, so a stereo rotary sound comes from microphones placed at different points around the horn, usually with one more on the drum. Plug-ins call the control mic angle, mic distance, spread or width. Wider placements make the sound swing further between the speakers; closer ones keep it near the middle. In the lab, Mic width is the angle between the two horn mics.

Drive

The amplifier in a Leslie 122 runs on valves, and pushing it adds saturation: the peaks get rounded over and harmonics are added. A lot of rock organ sounds are as much about that grit as about the spinning. Drive comes before the rotors, so the harmonics it adds get spun along with everything else. The tube saturation lesson covers that part of the sound in detail.

Starting settings for common sounds

These are places to begin. They assume the plug-in's own slow and fast speeds and a balance that starts in the middle.

How to use a rotary speaker in a mix

1. Put it on the track as an insertA rotary speaker replaces the sound with the spinning version, the way a guitar amp does, so it belongs directly on the track. On a send, the dry sound underneath would comb filter against the moving copy, which sounds closer to a chorus.
2. Start on slowPlay the part through it on slow and set the horn and drum balance first. If the sound feels thin, bring up the drum; if the movement feels buried, bring up the horn.
3. Set the micsWiden the mic angle until the sound moves across the speakers by the amount you want, then check the mix in mono. If the part drops away in mono, narrow the mics.
4. Add drive if the part needs itFor rock organ, push the drive until the chords start to growl on the loudest notes. For a clean pad or a vocal, leave it off.
5. Automate the speedSwitch to fast where you want the part to lift and back to slow where it should settle. Put the switch a beat or two early, because the drum takes a few seconds to reach full speed.
6. Compare at matched loudnessBypass the plug-in and match the levels, then decide whether the movement is helping the part sit or just pulling attention.

Stereo from a mono cabinet

On a real cabinet, the left and right channels come from two microphones on the horn, so the stereo picture depends entirely on where they are. With the mics on opposite sides, one hears the horn coming while the other hears it going, and the sound swings hard from speaker to speaker. In the lab, with the mics 180° apart, the correlation between the channels of the guitar loop falls from 0.94 to 0.59 at either speed, because the angle sets the width and the speed only sets how fast it moves.

That width is lost when the mix is played in mono, which happens on phone speakers and plenty of club systems. The movement survives, because it's there in each mic on its own, but the part can get quieter or thinner when the two channels add together. If you're relying on a rotary sound to fill out the sides, check it in mono and read the mono compatibility lesson for what to listen for.

Rotary speaker vs chorus, tremolo and vibrato

A rotary speaker gets compared with the effects that each copy one part of it. A tremolo moves only the volume, and two of them half a cycle apart make an auto-pan, which gets you the side-to-side movement without the shimmer. Vibrato does the opposite and bends the pitch with the level held still. A chorus mixes moving copies with the original, which thickens a sound; a rotary speaker has no dry signal underneath, so it moves the sound without thickening it.

The Uni-Vibe, the pedal Jimi Hendrix used, was designed by Fumio Mieda in the 1960s as an attempt to copy a rotating speaker. It works more like a phaser, with a lamp and light sensors moving a set of filters, and it ended up with a sound of its own, which is why it's still made and copied today.

Common rotary speaker mistakes

Leaving it on fast

Fast is exciting for a bar or two and tiring for a whole song. The contrast between slow and fast is what makes the switch feel like a lift, so use fast where something should happen and go back to slow afterwards.

Switching on the beat you want it fast

The drum needs several seconds to get up to speed. If the chorus starts on bar 17, flip the switch a beat or two before, so the rotors are mostly there when it lands.

Turning the drum down too far

A rotary speaker with the drum turned down sounds thin and fizzy, and it loses the low-end swell that holds the effect together. Start with the balance even and move it from there.

Using it as a send effect

Blending a rotary plug-in under the dry track mixes a moving copy with a still one, so you get comb filtering and a chorus-like blur in place of the swirl. If you want less of the effect, use slow, narrow the mics or turn the balance towards the drum.

Train your ears

The quiz under the lab plays the rotary speaker on fast with the pitch movement, the volume movement or both switched on, and covers the screens while you say which parts are moving. It stays on fast because on slow the pitch moves by about 4 cents, which nobody can pick out. Volume only is the easiest to spot once you know it, because the notes hold still. Eight rounds take a few minutes, and your best score is saved in your browser.

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