LFOs & Modulation

Static sounds get boring fast. Modulation means letting one thing move another thing automatically, so your sound breathes, wobbles, shimmers or pulses without you touching a knob. The LFO is the most common way to do it.

What is an LFO?

An LFO (Low Frequency Oscillator) is an oscillator that runs too slowly to hear as a tone, usually somewhere between 0.1 Hz and 20 Hz. Instead of sending it to your speakers, you connect it to a setting such as filter cutoff, pitch or volume. That setting then moves up and down in a repeating cycle.

In modulation, the thing doing the moving is the source (here, the LFO). The setting being moved is the destination or target.

Rate, Depth & Shape

Rate

Rate is how fast the LFO cycles. You can set it two ways:

• Free (Hz): a fixed speed in cycles per second, such as 5 Hz. It ignores the song tempo. Good for vibrato and subtle drift.
• Tempo-synced: a note length such as 1/4, 1/8 or 1/8T (an eighth-note triplet). The LFO locks to your project tempo and stays in time if you change the BPM. Use this for rhythmic effects like wobbles and gated pads.

Depth

Depth (also called amount or intensity) is how far the LFO moves the target. A small depth on pitch gives a gentle shimmer. A large depth gives a siren. Many synths let depth go negative, which flips the LFO upside down.

Shape

The shape (waveform) decides how the movement feels.

Common LFO Targets

The same LFO creates very different effects depending on where you point it.

Switch the demo below to pitch to hear vibrato. Start with a low depth: real vibrato is subtle.

Retrigger vs Free-Running

An LFO can start its cycle in two ways:

• Retrigger (also called key sync or note reset): the LFO restarts from the same point every time you play a note. Every note sounds the same. Use it for wobble basses and anything that must hit the same way on each note.
• Free-running: the LFO keeps cycling in the background no matter when you press a key. Each note catches it at a different point, so each one sounds slightly different. Use it for pads and textures that should feel organic.

Some LFOs also have a phase control (where in the cycle it starts), a fade-in or delay (so vibrato only kicks in on long notes, like a singer), and a one-shot mode that runs a single cycle and stops, which turns the LFO into an extra envelope.

The Modulation Matrix

A modulation matrix is a table inside a synth that lists every connection between sources and destinations. Each row is one route with an amount.

Synths like Serum and Vital (see Synthesis Types) also let you drag a source straight onto a knob. That creates a matrix row behind the scenes.

Other modulation sources

• Envelopes: one-time shapes per note (see Envelopes). Use them for movement that happens once, like a pluck's filter snap.
• Velocity: how hard each note is played. Great for brightness and level.
• Mod wheel (MIDI CC1): a wheel on your keyboard for live, hands-on control. Commonly mapped to vibrato depth or filter cutoff. See MIDI.
• Aftertouch: pressure on a key after it is pressed, on keyboards that support it.
• Macros: one knob that moves several settings at once. Turning a single "Energy" macro might open the filter, add distortion and raise reverb together.
• Key tracking: the note's pitch itself. Often used to open the filter more on higher notes so they don't sound dull.

How a Dubstep Wobble Bass Works

The wobble bass became a signature of dubstep in the late 2000s. Under the hood it is simple: a thick bass sound through a low-pass filter, with a tempo-synced LFO moving the cutoff. The rhythm comes from changing the LFO rate during the phrase.

LFOs Beyond the Synth

LFOs are inside many effects, too. A chorus, flanger and phaser each use an LFO to sweep a short delay or a filter. Auto-pan and tremolo plugins are just an LFO on pan or volume. When you push an LFO's rate up into the audio range (above about 20 Hz) it stops sounding like movement and starts creating new tones. That idea is the basis of FM synthesis.