What Is an LFO? Rates, Shapes, Sync and Classic Synth Uses

What is an LFO in a synth?

An LFO is a low-frequency oscillator: a second oscillator that runs too slowly to hear, used only to move other controls up and down in a repeating cycle. Apple's Logic Pro guide describes it as a modulation source found on nearly all synthesizers that adds vibrato, filter sweeps and similar movement without producing an audible signal of its own.^[1]

Think of an LFO as a hand that keeps turning one knob back and forth forever. The waveform is the path the hand follows, the rate is how fast it moves, and the depth (also called amount) is how far it turns the knob. An envelope, by contrast, moves a knob once per note; see ADSR envelopes explained for that side of modulation.

Ranges vary by instrument. Ableton's Simpler, for instance, runs its LFO freely between 0.01 and 30 Hz or synced to divisions of the Set's tempo, and applies it to each played note individually.^[2]

Which LFO waveform should you use?

Pick the LFO waveform by the motion you want: sine and triangle for smooth back-and-forth, saw for ramps that snap back, square for switching between two values and random for unpredictable steps. Apple singles out triangle waves for slow filter sweeps and square waves for rapid switches between two pitches.^[1]

Many synths go beyond the basic list. Ableton's Drift offers nine LFO shapes, including Sample & Hold, a smoothed random shape called Wander and two one-shot envelope shapes, while Serum's LFOs are drawn as editable graphs with point, ramp and step tools.^[2][3]

LFO rate, depth and tempo sync explained

Rate is the speed of the cycle and depth is its size; tempo sync swaps the Hz readout for note values so the movement lands on the beat. Logic's Sampler, for example, lets each LFO run freely or lock to the project tempo in values from 32 bars down to 1/128th triplets.^[4]

Use sync for anything rhythmic: wobbles, gating, pumping and sweeps that must restart on the downbeat. Leave it off for vibrato and slow drift, where a rate that ignores the grid sounds more human. If you need to translate between the two, a quarter-note LFO runs at your BPM divided by 60 in Hz, and every halving of the note value doubles the rate.

Depth deserves as much attention as rate. The same 1/8 LFO sounds like a gentle pulse at 10% on cutoff and like a full dubstep wobble at 80%. Set depth with the loop playing and pull it back until the effect supports the part instead of taking it over.

What does retrigger do on an LFO?

Retrigger restarts the LFO from the same point in its cycle every time you play a note, so each note gets identical movement. With retrigger off, the LFO free-runs and each note catches the cycle wherever it happens to be. In Ableton's Analog, enabling Retrig restarts the LFO at the same phase position on each note, and an Offset control sets that start phase.^[2]

• Free-running The cycle ignores note timing. Serum's FREE mode follows the host clock, and in FL Studio's sampler the Global option keeps an LFO running instead of restarting it with each note.^[3][5] Best for pads, slow sweeps and anything that should keep evolving across notes.
• Retrigger (key sync) Every note starts the cycle at the same phase. Logic calls this Key Trigger; Serum calls it RETRIG.^[4] Best for wobble basses, plucks and any part where each hit must sound the same.
• Envelope or one-shot The LFO plays its shape once and stops, so a drawn curve acts like a custom envelope. Serum's ENVELOPE mode works this way.^[3]
• Mono vs poly LFO A poly LFO runs separately for each voice, so chord notes move out of step. A mono LFO moves all voices together. Logic's Sampler offers both, and Serum has a MONO switch per LFO.^[4]

Classic LFO uses: vibrato, tremolo, wobble, sweeps and auto-pan

The five classic LFO effects come from pointing the same oscillator at different destinations: pitch gives vibrato, volume gives tremolo, filter cutoff gives wobble or sweeps and pan gives auto-pan. The settings below are starting points to adjust by ear.

For vibrato, singers are a good reference: a study of ten singers performing Schubert's Ave Maria measured a mean vibrato rate of 6.1 Hz.^[6] Rates far above that start to sound nervous, and far below it the pitch audibly wanders.

Keep auto-pan and heavy stereo LFOs off anything below about 120 Hz; bass that swings left and right collapses on mono speakers. Tremolo can also live on an effect insert instead of the synth, and free tremolo plugins cover that route.

How to make a wobble bass with an LFO

A wobble bass is a low-pass filter whose cutoff is swept by a tempo-synced, retriggered LFO, with the rhythm written by changing the LFO rate. The steps below work in any synth with a filter and an assignable LFO. For deeper patches, read wobble bass sound design and the broader dubstep production guide.

1. Start from a saw
   Load an init patch with one or two saw oscillators, set the synth to mono and keep unison low so the low end stays solid.
2. Close the filter
   Use a 24 dB low-pass, cutoff around 150-300 Hz, resonance 20-35%. The bass should sound dull and round.
3. Assign the LFO
   Route LFO 1 to cutoff with a large positive amount so each cycle opens the filter to a few kHz.
4. Sync and retrigger
   Turn on tempo sync at 1/8 and enable retrigger so every note starts its wobble at the same point.
5. Write rhythm with rate
   Automate the LFO rate per beat, for example 1/4, 1/8, 1/8 triplet, 1/16. This automation is the groove.
6. Finish the tone
   Add distortion after the filter for grit, and keep a clean sine sub on its own layer so the wobble never thins out the low end.

LFO delay, fade-in and LFO sidechain tricks

LFO delay and fade-in hold the modulation back at the start of a note and bring it in gradually, which is how real players add vibrato. Apple's guide gives the example of a string sound where vibrato arrives a second or two into the sustained note.^[1]

The controls go by different names. Ableton's Analog uses Delay and Attack, Serum uses DELAY and RISE,^[3] Logic's Sampler uses a Fade In ramp,^[4] and a community Vital user guide lists Delay plus a Fade In option on each LFO.^[2][7] Start with a 200-400 ms delay and a similar rise time for leads.

A synced LFO on volume can also fake sidechain compression. Draw a shape that drops to zero on each beat and recovers over the quarter note, retrigger it with the kick pattern, and the pad pumps without a compressor or a sidechain route. Dedicated tools like Xfer LFOTool do the same with a drawn curve. If you want the real thing, sidechain compression explained covers when a compressor is the better choice.