What is an LFO?

The short answer

An LFO, short for low-frequency oscillator, is a slow wave that you never hear on its own. A normal oscillator, the kind that makes a tone, swings back and forth hundreds or thousands of times a second, fast enough for your ear to read it as a pitch. An LFO swings the same way but far slower, from a fraction of a cycle up to about ten cycles per second, which is below the range you hear as sound. So instead of pushing your speakers, it does something else: it moves a control.

Picture a hand slowly turning a knob back and forth, over and over. That is an LFO. It does not add a sound of its own; it takes some setting inside an effect, a delay time, a filter frequency, a pitch, a volume, and gently rocks it up and down. You do not hear the wave. You hear what the wave does to the thing it is connected to: a pitch that wavers, a tone that sweeps, a level that pulses. The LFO is a control signal, not an audio signal.

Rate and depth: the only two dials that matter

Because an LFO is just a wave that swings, two numbers describe it completely, and they are the two controls you will meet again and again on every effect that uses one.

• Rate. How fast the wave cycles, measured in cycles per second, or hertz. A low rate is a slow, seasick drift that takes a couple of seconds to come around. Turn it up and the same wave becomes a quick wobble. Rate is the speed of the movement, nothing more.
• Depth. How far the wave swings away from where the control was resting. A small depth nudges the parameter a hair in each direction, for a subtle, barely-there motion. A large depth throws it wide, for an obvious, dramatic sweep. Depth is the size of the movement.

That is genuinely the whole model. In one line, an LFO takes a resting value and adds a slow wave to it:

Set rate and you have fixed how often it moves; set depth and you have fixed how far. Everything else is a question of what shape that wave is, and what control you have wired it to.

The shapes an LFO can take

The wave does not have to be a smooth curve. The shape of an LFO decides how the parameter travels between its high and low points, and a handful of shapes cover almost everything you will meet.

• Sine. A smooth, rounded swing that eases through the top and bottom. It is the gentlest option and the default for anything that should feel natural, like the waver of a singer's vibrato.
• Triangle. Straight ramps up and down at a constant speed, with a sharp turnaround at each end. Close to a sine but a touch more even and mechanical through the middle of the sweep.
• Square. Jumps straight to the top, holds, then jumps straight to the bottom. There is no in-between, so the parameter flips between two values rather than gliding, which gives a stepped, on-off motion.
• Random, or sample and hold. Picks a new level every so often and holds it flat until the next jump, so the parameter wanders unpredictably. This is the shape behind unsteady, human, slightly drunk movement rather than a tidy repeat.

Every modulation effect is an LFO sweeping something

Here is why the LFO is worth understanding on its own: the whole family of modulation effects is really one idea wearing different hats. Each one takes an LFO and points it at a different control. Learn to hear the slow wave and you can hear what every one of them is doing.

• Chorus sweeps a short delay time. An LFO slowly lengthens and shortens the delay on a copy of your voice, which continuously detunes it a little, so the copy drifts in and out of tune with the original and sounds like several voices at once.
• Flanger sweeps a very short delay. The same idea but with a much shorter delay and some feedback, which stacks up a row of evenly spaced notches. The LFO slides them across the sound for the classic jet-plane whoosh.
• Phaser sweeps a set of notch frequencies. An LFO moves a few notches through the sound, spaced differently from the flanger's comb, for a smoother, more liquid swirl.
• Vibrato sweeps the pitch. Point the LFO straight at the pitch and it rocks the note sharp and flat, the tremor you hear in a held vocal note.

The one cousin that is not driven by an LFO is the ring modulator, whose oscillator runs up at audio rate rather than slowly, which is why it sounds metallic instead of wavering. Everything else in the modulation family is some version of an LFO nudging a control, which is exactly why the same two dials, rate and depth, keep showing up.

The LFO in patchd

In patchd there is no box labelled LFO, and that is on purpose. There is no standalone or routable LFO, and no modulation matrix where you patch one oscillator into arbitrary parameters. Instead the LFO lives inside each modulation node, and you meet it through the two controls this page is about. A node's Rate is its LFO speed, and its Depth is its LFO amount. Learn those two and you have learned the moving part of every modulation effect in the rack.

The modulation nodes live in the same Studio voice-fx rack as reverb and delay, and each is a DSP node you add and order like any other. The Chorus, Flanger, and Phaser nodes each expose a Rate and a Depth, plus their own character controls; the Ring Modulator trades its LFO for an audio-rate carrier; and the Vibrato node points its wave straight at pitch, with a Shape control that runs from a smooth even sine at zero to a random, unsteady warble at full. Vibrato is documented at zero latency, and the real-time engine as a whole adds only single-digit milliseconds of engine-internal latency at a small ASIO buffer, so the movement stays live under your voice.

The takeaway

An LFO is a low-frequency oscillator: a slow wave, well below the range you hear, that does not make a sound but moves a control up and down over time. Two dials describe it completely, rate for how fast it cycles and depth for how far it swings, and a small set of shapes, sine, triangle, square, and random, decides how it travels between the two. Once you can picture that slow wave, chorus, flanger, phaser, and vibrato stop being separate tricks and become one idea: an LFO sweeping something.

patchd is a pre-launch Windows audio mixer with a Studio voice-fx rack whose modulation nodes each put that slow wave under a Rate and a Depth control. If you want a mixer that makes modulation this legible, join the waitlist and get notified when it ships. There are two tiers, Free and Studio; Studio is $39.99 per year, and the real-time engine is identical on both.