What Does an LFO Do in a Synth? Vibrato, Tremolo and Wobble

An LFO, short for low-frequency oscillator, is a slow repeating wave, usually under 20 Hz, that a synth uses to move one of its own settings up and down on a loop. Aim it at pitch and the note wavers (vibrato), aim it at the filter cutoff and the tone opens and closes into a wah or a wobble bass, and aim it at volume for tremolo. Rate sets how fast the movement repeats, and depth sets how far it goes.

What does LFO stand for in music?

LFO stands for low-frequency oscillator. An oscillator is the part of a synth that makes a repeating wave, and the main oscillators run fast enough to hear as notes: A4 is a wave repeating 440 times a second, or 440 Hz. An LFO makes the same wave shapes far more slowly, anywhere from one cycle every 20 seconds (0.05 Hz) or slower up to around 20 cycles a second. Below roughly 20 Hz your ear stops hearing a pitch and hears a pulse or a sweep.

On most synths you never hear the LFO itself. Its output is a control signal wired to another setting, so what you hear is that setting moving. Picture a hand turning one knob back and forth forever at a steady speed. On the modular synths of the 1960s, such as the Moog, that hand was a slow oscillator and a patch cable.

Most LFOs swing the target above and below wherever you left its knob. Set the filter cutoff to 800 Hz, add some LFO, and the cutoff swings around 800 Hz. The knob sets the center of the movement and the LFO amount sets its width.

What does an LFO do on a synth? Pitch, filter and volume

An LFO does nothing until you give it a target, also called a destination. The same slow wave gets a different name depending on where you send it:

Nearly every synth with an LFO can reach the first three rows. The rest depend on how the synth makes sound: an FM amount target only exists on an FM synth, and pulse width only matters when there is a pulse wave to change.

Vibrato and tremolo get swapped all the time, and guitar gear is partly to blame. Leo Fender called the 1954 Stratocaster’s pitch-bending bridge a “synchronized tremolo”, then two years later used the word vibrato for the volume-pulsing circuit in his Vibrolux amp. Guitarists still use the swapped names. On a synth the words are strict: vibrato moves pitch and tremolo moves volume.

The filter is the target with the widest range. A slow LFO with a small amount on a pad’s cutoff frequency keeps a held chord from sounding frozen. A faster LFO with more amount and some resonance, the peak at the cutoff that makes a filter sound vocal, gives the “wow-wow” of a wobble bass. If those knobs are new, see how cutoff and resonance work.

Gelato Synth, our synth for iPhone, has one LFO on a page of its own: you pick a target (pitch, filter, FM, shape or volume), a shape, a speed and an amount. With one LFO you always know what is moving the sound, which makes each target easy to learn by ear. Its Wobble preset, in the Bass category, is a good one to take apart: open the LFO page and move the speed slider while you hold a low note.

What do LFO rate and depth do?

Rate is speed: how many cycles the LFO completes each second, shown in hertz, or as a note value when it is synced to tempo. The rate decides which effect you hear, in rough zones:

• 0.05 to 0.5 Hz (one cycle every 2 to 20 seconds): slow drift, like a pad that breathes or a drone that changes color.
• 0.5 to 3 Hz: a sway you can nod along to, good for slow tremolo, auto-pan and a lazy wah.
• 3 to 8 Hz: vibrato, fast tremolo and busy wobbles. A singer’s natural vibrato usually runs at about 5 to 7 Hz, so a pitch LFO in that zone sounds human.
• 8 to 20 Hz: flutter and fast trills, which blur into a rough growl near the top.

Past 20 Hz the modulator is running at audio rate. Push it up into the hundreds of cycles a second and the movement turns into new overtones, which is how FM synthesis works.

Tempo sync sets the rate as a note value (1/4, 1/8, 1/8T for eighth-note triplets, 1/16), so the movement stays locked to the beat when the tempo changes. If your synth only shows hertz, convert by hand: Hz = BPM ÷ 60 × cycles per beat, counting a whole bar of 4/4 as 1/4, quarter notes as 1, eighths as 2, eighth-note triplets as 3 and sixteenths as 4.

Depth, also labeled Amount or Intensity, sets how far the target moves. On many synths the knob reads 0 to 100 or 0 to 127, so you set it by ear. Where pitch depth is shown in cents (100 to a semitone), 10 to 25 cents either side is a gentle synth vibrato, 50 cents is a wide, operatic one, and a full octave (1,200 cents) at a slow rate is a siren. On a filter, a large depth gives the dramatic open and close of a wobble, and a small one keeps a pad moving without drawing attention.

A habit that saves time: set the rate with the depth turned well up, where the speed is easy to hear, then pull the depth back until the effect sits inside the sound.

How does an LFO work? Shapes and retrigger

The shape is the path the LFO follows through each cycle. Keep the rate and depth the same, switch the shape, and the character of the movement changes:

A square LFO on pitch flips between two notes. One or two semitones apart that is a trill, and 12 apart it is an octave jump; set the depth so the gap is a whole number of semitones, or the pair sounds out of tune. A rising saw on volume, synced to quarter notes, starts every beat quiet and swells back, close to the pumping sound dance producers usually get from sidechain compression.

Two more settings show up on many LFOs:

• Retrigger (also key sync) restarts the LFO from the same point each time you press a key, so every note moves the same way. A free-running LFO keeps cycling in the background, so each note catches the wave somewhere different. Retrigger suits basses and rhythmic parts; free-running suits pads, where a little drift sounds natural.
• Delay or fade-in holds the LFO back for a moment after a note starts. Short notes stay straight, and long notes grow a vibrato the way a singer’s held note does.

Both an LFO and an envelope move a setting over time, but the envelope runs once per note and stops, while the LFO loops for as long as the sound plays.

LFO recipes: vibrato, tremolo and wobble bass

Start each recipe from a plain saw or sine sound with nothing else moving. Synths scale their knobs differently, so treat the numbers as starting points.

• Natural vibrato. Target pitch, sine shape, rate 5.5 Hz, depth 15 to 20 cents, and an LFO delay of about 400 ms if your synth has one. A long held note starts straight and begins to sing half a second in.
• Amp tremolo. Target volume, sine or triangle, rate 5 Hz, depth about 40 percent: the pulse of an old guitar amp, and a good fit for electric piano and organ. At 2 Hz it turns sleepy.
• Trance gate. Target volume, square shape, synced to 1/16, depth 100 percent, retrigger on. A held chord becomes a stutter locked to the beat.
• Breathing pad. Target filter cutoff, triangle shape, rate 0.2 Hz (one cycle every five seconds), cutoff near halfway, small depth, resonance around 20 percent. A chord held for 20 seconds swells and settles four times.

How to make a wobble bass

Dubstep, the home of the wobble bass, nearly always sits between 132 and 142 BPM, and 140 is the most common. Here is a wobble at 140 BPM:

1. Oscillator. One saw wave, or two saws detuned about 10 cents apart for a thicker tone. Play low, around F1 (about 44 Hz).
2. Filter. A low-pass filter, 24 dB per octave if you have the choice, with the cutoff low enough to muffle the note, resonance around 40 percent and a little drive for grit.
3. LFO. Sine shape, target filter cutoff, retrigger on, with enough amount that the top of each cycle opens the filter to roughly 2 kHz.
4. Rate. Sync to 1/8. Without sync, dial in 4.67 Hz from the table above.
5. Rhythm. Change the rate while the line plays: a bar of quarter notes, a bar of eighths, then triplets or sixteenths going into the next phrase. In a DAW you automate the rate; on hardware you turn the rate knob between synced values.

To keep the low end solid, layer a plain sine sub-bass on the same low F with no LFO on it, so the floor stays steady while the wobble moves on top. An octave below F1 would sit near 22 Hz, too low for most speakers to play. The guide to making a synth bass shows how to build that sub layer.