What is an LFO?

LFOs in plain English

Every synth makes its sound with an oscillator, a circuit or piece of code that repeats a wave shape over and over. Play the A below middle C and the oscillator repeats its wave 220 times a second, fast enough that you hear the repeats as a pitch.

An LFO is the same kind of oscillator running far slower, usually a few times a second or less. That's too slow to hear as a tone, so its job is to turn another control up and down for you, smoothly and in time, for as long as the sound plays.

Say you wanted a pad to get darker and brighter every bar. You could hold the filter knob and turn it back and forth by hand for the whole track, or you could connect an LFO to the filter and let it do the turning. The LFO's wave becomes the movement of the knob, so a smooth sine gives a smooth sweep and a square flips between two positions.

Tremolo, vibrato, auto-pan, auto-filter, the wobble on dubstep bass, the trance gate and the moving delay times inside a chorus all come from that one idea. Each is an LFO connected to a different setting.

Reading the lab

The four cards at the top of the lab choose what the LFO moves: the volume, the pitch, the cutoff of a low-pass filter or the pan position. Below them, the big screen shows the whole loop. Its top lane is the setting the LFO is moving, recorded while the loop was processed, and its bottom lane is the sound that comes out, with the left channel above the line and the right below it. The faint shape behind is the original. Thin vertical lines mark the beats at 120 BPM, the tempo of the pack loops, and brighter ones mark the start of each bar.

Underneath, the left screen shows two cycles of the LFO itself on a scale from −1 to +1, with a dot at the moment you're hearing. Next to it is the setting right now, which might be a level, a pitch in cents, a filter curve over the sound's spectrum or a position between the speakers. On the right, a panel explains what your settings are doing. Its numbers are measured from the processed loop, so they change as you move a control.

Press A/B to switch the LFO's effect off. The lab plays both sides at the same average loudness, so a volume LFO can't win just by being louder or quieter overall. The sound picker has eight of the site's loops, or you can upload your own sound or record eight seconds from your microphone; nothing leaves your device.

The controls on an LFO

Synths and effects lay their LFOs out differently, but nearly all of them have the same handful of controls. Serum's LFOs and the one inside Ableton Live's Auto Filter look nothing alike, yet both come down to a shape, a rate you can sync to the tempo and an amount, and most of the differences are in the naming.

Shape

The shape is the waveform the LFO repeats, and it decides how the setting moves inside each cycle. Sine and triangle are smooth and even, a square jumps between two values, the two saws ramp one way and snap back, and random holds a new value for each cycle. The section on shapes below goes through what each one sounds like.

Rate

Rate is how many cycles the LFO makes per second, in hertz. At 0.5 Hz one cycle takes two seconds; at 5 Hz it takes 200 ms. Below about 1 Hz you hear a slow drift or sweep, from about 2 to 8 Hz a pulse or a wobble, and past 10 Hz the movement gets so quick that it blurs into a flutter. The lab's rate goes from 0.1 to 20 Hz.

Sync

With sync on, the rate is set as a note value in place of hertz and the LFO follows the tempo of your project. A 1/4 note is one cycle per beat and a 1/16 is four. Change the tempo and the LFO speeds up or slows down with it, so the movement always lands in the same place against the beat, which is what you want for anything rhythmic. The maths is in the section on rates and tempo below.

Depth

Depth, which some synths call amount or intensity, sets how far the LFO moves its setting. In the lab it reads in the units of whatever you're moving. On a filter it's octaves: ±1.5 octaves around a 1 kHz centre sends the cutoff from 354 Hz to 2.8 kHz. On pitch it's cents, where 100 cents is a semitone. On volume, 70% depth means the level drops to 30% of full at the bottom of each cycle, which is 20 × log10(0.3), about −10.5 dB.

Those examples also show the two ways an LFO can push. A bipolar LFO swings both ways around where the control sits, so the filter goes above and below its centre and the pitch goes sharp and flat. A unipolar one only pushes one way, which suits volume: the top of the wave is full volume and depth sets how far the level falls. Some synths have a switch between the two; in the lab the volume LFO is unipolar and the other three are bipolar.

Destination

The destination is the setting the LFO is connected to. On a simple effect such as a tremolo it's fixed. On a synth you choose it, sometimes from a list in a modulation matrix and sometimes by dragging the LFO onto a knob, as you do in Serum. One LFO can usually drive several destinations at once with a different depth for each.

Phase

Phase sets where in its cycle the LFO is when the beat starts. At 0° the lab's sine starts in the middle of its swing and rises; at 180° it starts halfway round, so every peak arrives half a cycle earlier. It matters most on synced LFOs. Put a square on the volume at a 1/4 note and the sound plays on each beat and stops halfway through it. Turn the phase to 180° and it goes silent on the beat and plays on the off-beat.

LFO shapes and what they sound like

Two LFOs with the same rate and depth sound nothing alike if one is a sine and the other a square, because one glides and the other switches.

A sine spends most of its time near the top and bottom of its swing, because that's where it slows down to turn round. On a filter that gives you long stretches of dark and bright with quick passes between them. A triangle crosses the middle at the same speed it reaches the ends, so the same sweep sounds more even.

The saws are the rhythmic shapes. Each cycle has one sudden jump and one steady ramp, so when they're synced the jump lands on the beat and gives the movement an accent. Saw down on a filter opens on each beat and closes. Saw up on the volume ducks on each beat and swells back, the pumping you'd normally get by sidechaining a compressor to the kick, and the sidechain vs volume shaping lesson compares the two ways of getting it.

Random is often labelled S&H, short for sample and hold, after the circuit on modular synths that took a reading of an input, usually a noise source, on every clock tick and held it until the next one. With the rate synced to 1/16 notes and the LFO on a resonant filter, it gives the bubbling, computer-like bleeps you hear on a lot of electronic records. Some synths also have a smooth random shape that glides between values, which is useful for small, natural-sounding drift.

Square waves on volume can click, because the level jumps in an instant and that sudden step has energy at every frequency. The lab rounds each jump over a few milliseconds to avoid it, and many plug-ins have a smoothing or slew control that does the same. If a gate you've made clicks, look for that control first.

LFO rates and tempo

Synced and free rates are the same thing measured two ways, and you can convert between them. One beat lasts 60 ÷ BPM seconds, so the rate in hertz is BPM ÷ 60 ÷ the number of beats one cycle lasts. At 120 BPM a beat is 0.5 seconds: a 1/4-note LFO makes 2 cycles per second, a 1/8 makes 4 and a 1/16 makes 8. At 140 BPM a 1/8 becomes 140 ÷ 60 ÷ 0.5, about 4.67 Hz.

Use sync whenever the movement is part of the rhythm: gates, pumps, wobbles, a filter that should open on every bar. A triplet or dotted value pulls against the straight beat and is a quick way to make a part swing. Leave sync off for vibrato and slow drift, where the rate should feel like a player's hand. In the lab, turn sync off and set about 1.3 Hz on the filter, and the coach tells you the peaks now fall in a different place in each bar.

What can an LFO modulate?

Anything with a number on it, in principle. These four are the ones you'll use most, and each has an effect named after it.

Volume

An LFO on the volume is a tremolo. A sine or triangle at 4 to 7 Hz gives the throb of a guitar amp, a square synced to 1/16 notes at full depth chops a pad into a gate, and a synced saw up gives pumping.

Pitch

On pitch it's vibrato. Keep the depth small, 10 to 40 cents, with a rate around 5 to 7 Hz, and leave sync off. A square on pitch flips between two notes, which is a trill: at ±50 cents they're a semitone apart, and at ±100 cents, the lab's maximum, a whole tone. To bend the pitch of a recording, the lab plays it through a delay whose length follows the LFO, which is how vibrato plug-ins work too, and the coach shows how long that delay gets.

Filter cutoff

This is the destination people mean most often when they talk about an LFO. The cutoff of a low-pass filter decides how bright the sound is, so moving it makes the tone sweep. A slow, smooth sweep gives you the evolving pad, and a fast one synced to the tempo on a bass gives you the wobble. With some resonance on the filter the sweep gets a vocal, wah-like edge, which is why the lab's filter has a little.

Pan

On pan it's auto-pan, the sound moving from speaker to speaker. Slow rates of a bar or two sound like movement; past about 4 Hz it turns into a stereo flutter. Check it in mono, where the two speakers are added together and the movement turns into a small change in level. With a −3 dB pan law like the lab's, that change is up to 3 dB.

Delay time and everything else

An LFO on a short delay time is what drives a chorus or a flanger, and an LFO moving the frequency of a chain of allpass filters is a phaser. The chorus vs flanger vs phaser lesson puts those side by side. On a synth you can also point an LFO at wavetable position, pulse width, the level of a second oscillator, the amount of distortion or the send to a reverb. If a part sounds static, an LFO with a small depth on one of those is often enough to make it feel played.

How does an LFO work?

Inside a synth or plug-in, the LFO produces a stream of numbers between −1 and +1 that follows its shape. Depth scales that stream, and the result is added to wherever the control is set. For the lab's filter, the cutoff at any moment is the centre × 2^{(3 × depth × LFO)}, which is why depth reads in octaves: at 50% depth with the LFO at its peak of +1, the cutoff is 1 kHz × 2^1.5, about 2.8 kHz.

The same stream can drive anything, which is why synths treat LFOs as modulation sources you can route freely. An envelope is the other common source; it produces one shape per note in place of a repeating one, and the filter envelopes lesson covers that side.

Retrigger, free-running and tempo sync

On a synth an LFO has to decide what to do when you play a note. With retrigger on (some synths call it key sync), it starts again from the same point every time you press a key, so a vibrato or a filter wobble always begins the same way. With it off, the LFO runs freely and each note catches it at a different point in its cycle. Free-running suits pads and chords that should drift and never repeat exactly. Retrigger suits leads and basses, where you want each note to move the same way.

Effect plug-ins usually lock a synced LFO to the song position in your DAW, so it lines up with the bar wherever you start playback. The lab works that way: its LFO is tied to the loop, and Phase sets where in the cycle it is on beat 1.

How to set up an LFO, step by step

This order works for most LFO jobs. The example is a synced filter wobble on a bass.

1. Set the control by hand firstTurn the filter cutoff to where the part sounds right with no movement. The LFO will swing around that point, so if it starts in a bad place it will sound bad half the time.
2. Connect the LFO with the depth at zeroChoose the cutoff as the destination, or drag the LFO onto the knob, and start from nothing so you hear the movement come in.
3. Choose the rateSync it for anything rhythmic and pick a note value: a 1/4 for a slow wobble, a 1/8 or 1/16 for a fast one. Leave it free for vibrato and slow drift.
4. Bring the depth up until you can hear it, then back it offThe right amount is usually a little under the point where it first becomes obvious, especially once the other parts are playing.
5. Pick the shapeStart on a sine. Try a saw if the movement needs to hit on the beat, or a square if it should switch.
6. Check retrigger and phaseDecide whether each note should start the LFO from the same point, and turn the phase until the peaks land where you want them against the beat.

Starting settings for common LFO effects

These are places to begin, at 120 BPM, and every one of them can be built in the lab.

Common LFO mistakes

Too much depth

An LFO is easy to hear when you solo the part to set it up, so it's tempting to keep turning it up. In the full mix that movement pulls attention away from everything else. Set the depth with the other parts playing, and expect to end up lower than you thought.

A free rate on a rhythmic part

A gate or a wobble at 3.8 Hz in a 120 BPM track drifts against the beat, landing in a slightly different place every bar, and it sounds like a mistake. If the movement belongs to the groove, turn sync on.

Forgetting about retrigger

If some notes of a lead have a vibrato that starts sharp and others start flat, the LFO is free-running, so turn retrigger on. If a pad repeats the same move on every chord, try turning it off.

Swinging around the wrong starting point

The LFO moves the control around wherever it's set. A filter sweep that sounds muddy at the bottom usually wants a higher cutoff and the same depth. Get the static setting right first, then add the movement.

Not checking in mono

Fast auto-pan and wide stereo LFOs can sound exciting on headphones and turn into a wobbling level on a phone speaker. Listen in mono before you commit.

Train your ears

The quiz under the lab connects the LFO to one of the four destinations with hidden settings, covers the screens and asks what's moving. Volume and filter are easy to mix up, because a darker sound also seems a little quieter, so listen for whether the tone changes as well as the level. Use headphones or two speakers so the pan rounds are fair. Eight rounds take a few minutes, and your best score is saved in your browser.

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