An LFO, or low frequency oscillator, is an oscillator that runs too slowly to hear. Instead of making a tone you listen to, it moves another control up and down over time, so a sound can waver, pulse or sweep on its own. It is one of the two main ways a synth adds movement, and once you see what it points at, most of its uses fall into place.
The idea applies to almost any synth, sampler or plugin, in any DAW or a browser instrument. Rate, depth, shape and destination mean the same thing wherever you meet them. None of this is tied to one product.
An LFO, or low frequency oscillator, is an oscillator that runs below the range of hearing and is used to move another control over time rather than to be heard as a pitch. A normal oscillator in a synth cycles fast enough to produce a tone. An LFO cycles so slowly that you never hear it as sound at all. Instead, its slow rise and fall is wired to a parameter, and that parameter follows it up and down for as long as the LFO runs.
Human hearing starts at roughly 20 Hz, and an LFO sits well below that. A typical rate runs from a fraction of a hertz up to a few hertz, meaning anything from one slow cycle every several seconds to a few cycles a second. Slow settings give a gentle drift you barely notice; faster ones give an obvious wobble or flutter. The point is that the LFO is always too slow to be a note, so its job is to shape something else rather than to be heard directly.
Using one control to move another automatically like this is called modulation, and the LFO is one of the two everyday sources of it, alongside the envelope. If you are still getting a feel for how the parts of a synth fit together, our guide on what a synthesizer is sets the scene.
An LFO is not something you hear on its own. It is a slow, steady motion you assign to a knob. Point it at pitch and the pitch wavers; point it at volume and the volume pulses; point it at the filter and the tone sweeps back and forth. The LFO itself stays silent. Everything you hear is the parameter it is moving.
The main LFO controls are rate, depth, shape and destination, usually with a phase setting and a tempo sync option. Rate sets how fast it moves, depth sets how far, shape sets the path it takes, and destination sets what it moves. Here is what each one does.
Rate, or speed. How fast the LFO cycles, set in hertz or, when synced, as a musical note value. Low is a slow drift over seconds; high is a fast flutter several times a second. This is the control that decides whether the movement reads as a slow evolution or a rapid wobble.
Depth, or amount. How far the LFO pushes its target away from where you set it. A little depth is a subtle shimmer; a lot is a dramatic swing. This is usually the first control you reach for, because it decides whether the movement is barely felt or plainly obvious.
Shape, or waveform. The path the LFO traces as it moves. A sine or triangle glides smoothly up and down; a square jumps between two values with nothing in between; a sawtooth ramps one way then snaps back; and a sample and hold setting steps to a new random value each cycle for a jittery, unpredictable movement. The shape changes the feel of the effect as much as the rate does.
Destination, or target. The parameter the LFO moves. This is the choice that decides what the LFO actually does, and the common targets are pitch, volume, the filter cutoff and pan. On many synths a single LFO can be routed to more than one target at once.
Phase. Where in its cycle the LFO begins. Setting the start phase decides whether a note starts at the top of the movement, the bottom or somewhere in between. It matters most when the LFO restarts with each note, so that every note begins from the same point.
Tempo sync. Instead of a free rate in hertz, most LFOs can lock to the song tempo and cycle in note values, such as a half note, a quarter, an eighth, or dotted and triplet versions of those. Synced this way, the movement lines up with the beat, so a filter wobble or an auto-pan sits in time with the track rather than drifting against it.
You use an LFO to add movement to a sound by pointing it at a parameter, and each classic use comes from a different destination. Send it to pitch for vibrato, to volume for tremolo, to the filter cutoff for a wobble, to pan for auto-pan, and to the pulse width of a pulse wave for PWM. The table shows the common ones.
| Destination | Effect |
|---|---|
| Pitch | Vibrato, a gentle and regular waver in pitch, the same effect a singer or a violinist adds by hand. |
| Amplitude (volume) | Tremolo, the volume pulsing in and out, from a soft throb up to a hard, rhythmic chop. |
| Filter cutoff | A wah or wobble as the tone sweeps brighter and darker, a staple of bass and electronic sounds. |
| Pan | Auto-pan, the sound drifting or bouncing between the left and right speakers. |
| Pulse width | PWM, or pulse width modulation, a shifting, shimmering thickness in a pulse wave. |
How strong each of these feels comes down to depth and rate. Keep the depth low and the effect is a subtle life added under the sound; push it up and the same routing becomes an obvious, rhythmic effect. Sync the rate to the tempo and the movement locks to the beat, which is what turns a filter wobble or an auto-pan into something that grooves with the track rather than fighting it.
Most of the time an LFO works best when you barely notice it. A small amount of pitch or filter movement, too slight to name, is often what separates a sound that feels alive from one that sits flat. Save the large, obvious settings for when you want the movement itself to be the effect, such as a hard tremolo chop or a deep filter wobble.
An LFO repeats forever, while an envelope runs once per note. An LFO cycles up and down over and over for as long as the sound plays, giving steady, repeating movement. An envelope moves through its stages a single time each time you press a key, giving a one-time shape with a beginning and an end. That is the core difference, and it decides which one you reach for.
An envelope is the other main modulator in a synth. It runs from the moment you press a key, through its stages, to some point after you let go, and then it is done until the next note. The classic version has four stages, attack, decay, sustain and release, covered in our guide on what an ADSR envelope is. Because it happens once, an envelope is what you use for the shape of a note: the way it fades in, holds and dies away.
An LFO, by contrast, has no natural end. It keeps cycling at its rate, so it is what you use for anything that should repeat: a constant vibrato, an ongoing wobble, a steady pulse. Where an envelope is a single gesture, an LFO is a loop.
The line between the two blurs a little because of two settings. Many LFOs offer a one-shot mode that runs a single cycle and then stops, which makes the LFO behave rather like a simple envelope. And an LFO can usually be set to retrigger, restarting its cycle from the beginning with every new note, or to run free, cycling continuously in the background so each note catches it at a different point. Retriggering makes the movement identical on every note; free running gives a looser, less predictable feel.
You rarely choose one or the other. A typical patch uses an envelope to shape the note and an LFO to add ongoing movement on top, often pointed at different targets. The envelope gives the sound its arc, from the attack to the fade; the LFO gives it life while it plays. Learn where each one belongs and most of a synth's movement stops being a mystery.
Modulation is far easier to understand by ear than on paper. The quickest way to get it is to listen to a sound that is moving on its own and notice what is changing underneath: the pitch, the volume, the tone.
ARGISH is a self-playing synthesizer. Its sound evolves and moves over time on its own, so you can hear modulation at work, the slow shifts in tone and level that this guide describes, rather than only read about them. Set a mood and a key, build a progression on the chord wheel, and it composes and drifts in the background while the sound keeps changing under it. It runs in your browser, or as a plugin in your DAW: an Audio Unit, a VST3, an AAX for Pro Tools and a standalone app.
You can hear it free in your browser first, no install and no account, then decide.
Everything on this page also works by hand on almost any synth, which is rather the point of writing it down.
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