What Are LFOs And How Do They Work? (w/ Illustrations & Audio Demos)

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  • What does LFO stand for?
  • How do we use LFOs in synthesis and effects?
  • We explore the differences between LFOs and VCOs.

LFOs, or low frequency oscillators, are used in synthesizers for modulation – changing the pitch, filter cutoff, volume, and other parameters that affect the sound. They are very similar to regular oscillators (VCOs or DCOs on analog synths) except they generally oscillate much slower and exist primarily as a modulation source rather than as a musical tone.

If you’ve ever played with tremolo, vibrato, flange, or phaser effects, you’ve heard an LFO in action. But these are just simple applications – how deep can you really go with LFOs? We’ll find out…

What Is An LFO?

LFO stands for low frequency oscillator. They are important components in synthesizers that are vital for creating movement and interest inside any synth patch. This is because they affect parameters inside the patch, creating sweeps, flutters, gating effects, and even bizarre inharmonic noises depending on how they are used.

To better understand LFOs, it’s important to get a feel for modulation.

This is different from musical modulation, so we’re not talking about transposition and key changes. Instead, we want to examine the modulation of sound – including its pitch, volume, and other properties.

What Is Modulation? (Sources and Destinations Explained)

Modulation is used in synth patches to create movement. Without it, our patches would sound like static tones most of the time.

Modulation in synthesis refers to the interaction between a source and a destination. Modulation creates movement and character by allowing LFOs to change the way oscillators, filters, and other synth modules behave. But LFOs are just one potential modulation source.

A source is any signal or value that can be used to modulate a parameter in a synth patch. This parameter is known as the destination – a property of the sound that can be varied depending on the level of the source and the amount of modulation applied.

Common modulation sources include:

  • LFOs
  • Envelopes (ADSR, etc) and envelope followers
  • Velocity, note pitch, modwheel, CC, and other MIDI data
  • MPE data
  • Macro controls
  • Busses that include multiple sources mixed together
  • Randomly generated values

Generally, you will find even more destinations than sources on your synth. This is because really any parameter on your synth can be a destination. It just depends on whether or not the manufacturer/developer enabled modulation for that parameter.

Having said this, not every parameter will respond to modulation in a desirable way, but you should definitely experiment to get a feel for how different sources work with different destinations.

Some common modulation destinations include:

  • Oscillator pitch, volume, shape, panning
  • Filter cutoff, resonance, drive
  • Envelope times
  • LFO rates and shapes
  • Effect parameters like delay feedback, distortion level, wet / dry mix.

As you can see, it is entirely possible to use one LFO to change the behavior of another. For example, the output from one LFO can be used to control the speed of another LFO. The relationship between sources and destinations can even be a bit blurry in this sense. In a broad sense, destinations are inputs and sources are outputs, but considering many sources have destinations attached to them, you can make some very complex webs of modulation.

LFO Controls

LFOs have a number of controls that change how they act upon the sound. Although this will be different for every synthesizer, there are a few common controls you will want to become familiar with here.

Rate

This is perhaps the most important control. This sets the speed of the LFO and may be expressed in Hertz or Hz. You can think of this as the rate of modulation, so if the LFO is tied to the oscillator pitch, the rate control will change how fast the pitch changes. At high settings, the LFO will have a very noticeable effect on the tone, creating inharmonic effects similar to ring modulation.

Slower rate settings will create more gentle “sweeps” suitable for synth pads and risers.

You may see a few different terms used for this control depending on the synth. Common alternative labels include Speed, Freq, and Hz.

Shape / Waveform

This is the overall wave shape of the LFO. Most often you will be presented with a menu of static waveforms such as saw, square, pulse, triangle, and sine waves. This is very similar to how standard oscillators work in many analog and subtractive synths. What’s interesting here is that inside an “audio” oscillator, the shape control changes the tone color in ways that can be difficult to describe, but with LFOs, it’s much easier to imagine what the different shapes do just by looking at them.

More advanced synthesizers will let you customize the wave shape to be anything you want. For example, Serum lets you draw in your own shapes with a variety of tools. Due to the level of specificity here, the LFO section on Serum can easily double as a step sequencer.

You can even modulate the breakpoints with other modulation sources to create complex LFO shapes that evolve over time, as you can see in this video:

Level / Depth

This will change the overall output of the LFO, and will greatly affect how much modulation is occurring. However, most often you will want different modulation levels for different sources, so it’s rare to just have one control for LFO depth. Really this control is more common on hardware synths that have only a limited number of modulation routings, but don’t expect to see a single “depth” control on a softsynth.

Instead, you will likely find individual depth controls at each destination or only deal with LFO depth in the context of a modulation matrix. It really depends on how complex your synthesizer is.

So setting the LFO depth is different for every synth. For example, a Roland Juno 106 has an LFO level fader next to the filter controls instead of being grouped together with the other LFO controls. The Korg Polysix is an example of a synthesizer that does have a single depth control for the LFO, except Korg doesn’t use the term LFO here, opting instead for MG (modulation generator).

In Serum, the LFO depth appears as a colored band around whatever control is being affected. You can also change this in the Matrix tab with the Amount fader.

A modulation matrix is a screen that shows all your modulation side-by-side, listing mod sources, destinations, and the level of each connection. You will only find this on digital synths, very few analog synths can make this work.

PS This is what Serum looks like when you disable the font cache service in Windows as I did.
The Matrix tab in Serum is an example of a modulation matrix.

Fade / Delay

This controls the fade-in time of the LFO, allowing it to increase in intensity as the note is held. Some other synths will also have a fade-out time for the LFO as well.

Sometimes this control is labeled “Delay”, which can be confusing. In Serum, this will properly delay the onset of the LFO, ensuring that it kicks in at full depth after a given time. You can instead use the Rise control to apply a gradual fade-in after the delay time. But on a Juno 106, the Delay control is just a fade.

LFOs Vs Oscillators (VCOs)

So far you might have noticed many similarities between LFOs and synth oscillators. The oscillator in an LFO, if patched directly to your speakers or the master track in your DAW, would sound like a standard synth oscillator if you increased the rate beyond 20 Hz. Really an LFO and a synth oscillator are the same ideas applied differently.

Here are the key differences between an LFO and a synth oscillator (or VCO):

  • LFOs are never directly heard and exist primarily for modulation.
  • LFOs are almost always much lower in frequency than other synth oscillators.
  • The frequency of a synth oscillator is influenced by MIDI pitch, but an LFO is typically independent of key pitch.

Having said this, in the world of modular synthesis, the distinction is less clear. It’s beneficial for hardware manufacturers to provide VCOs that can double as LFOs and vice-versa. After all, in modular synthesis, audio signals are also used as modulation sources, and we’ve already established that the oscillators used in LFOs are practically identical to VCOs.

(For those looking to dive into the world of modular synthesis, we’ve got you covered with Modular Synthesis Basics (How It Works and How to Dive In))

For a more detailed example of “audio-rate modulation” where the lines between a low frequency oscillator and a synth oscillator are blurred, check out our final example below…

LFOs vs Envelopes

What if I told you that LFOs and envelopes are also so similar they can effectively replace each other?

This is at least true for any envelope that has a “loop” function, or any LFO that has an “Env” or “one-shot” function. Think about it – if an envelope loops from the sustain point back to the beginning, and you set the sustain point to 0, you can use the attack and decay controls to change the “LFO” speed.

Conversely, an LFO that plays just once is effectively an envelope. For example, Serum has an Env mode for the LFOs, and I personally find it more convenient to use an LFO as an envelope in many situations.

In the world of modular synthesis, modules aka “function generators” can work as both LFOs and envelopes.

Examples – LFOs In Action!

Now let’s listen to some examples so you can hear LFOs in action. I’ve also described my process for each scenario, so you can try recreating them yourself. I’m using Vital, which is a free VST synth you should definitely have in your collection. It’s a great way to get started with synthesis despite being a very complex synthesizer in its own right.

Example 1: Basic Modulation

In this example, I am using the default LFO speed to change the pitch of a sine wave. First I am using a triangle LFO shape, then a square shape. This gives you an idea of how different LFO shapes will drastically influence the character of the modulation.

To set this patch up, simply drag the LFO1 tab over to the coarse pitch control for OSC1 (the left “Pitch” numbox). Then change OSC1 to a sine wave, which you’ll find in the Basic Shapes wavetable in Vital.

Vital gives users very precise control over the LFO shape, and you can save these custom shapes as presets for later. I encourage you to spend a bit of time familiarising yourself with common shapes like square and saw waves, so you know when to reach for them next time you are creating a patch.

Example 2: Slower Modulation

So far, the rate of our LFO has been relatively brisk and rhythmic, so let’s slow things down. Here we are repeating the process of modulating the pitch and changing the shape, but with an LFO speed of 2.639 seconds.

To set the LFO rate in seconds, click the music note above the Frequency label in LFO1. Usually, LFO rates are set in Hz but Vital uses seconds instead.

Example 3: Filter Modulation

So far, we have only been modulating the pitch, but there are so many other things we can change with LFOs as well. For this example, I’ve changed the oscillator waveform to a square wave so we have more harmonics to filter out.

If you’re looking for the square wave in Vital, move the Frame slider to just past halfway on the Basic Shapes wavetable. The Frame slider is the same as the WT Pos control in Serum, and you can find it just to the right of the waveform display. Remember that here we are changing OSC1, not LFO1.

After changing the oscillator waveform, I activated Filter1 and set it to Ladder: 24dB. I reduced the filter cutoff a bit then set LFO1 to modulate this control. (The cutoff control is the horizontal slider just below the filter graph.)

In this example, I switch between medium and slow LFO rates, and the LFO shape is a triangle in both cases. With this type of modulation, it’s very easy to hear how classic dubstep “wobbles” basses are created. Slow and gentle filter modulation also works wonders with synth pads, adding a subtle “sweeping” feeling.

Example 4: Wavetable Modulation

Wavetables are basically made for modulation, and LFOs are a perfect choice for this. Having said this, so far we’ve been using the Basic Shapes wavetable and you will hear that if you move the Frame control, the waveform changes quite suddenly. So this may not be the best wavetable for this example.

Instead, I’ve switched to AnaSawPWM, which can be found in our free Producer Hive Synthwave Wavetables pack. I also set the unison to 6 voices for a thicker sound and added a lowpass filter with Filter 1.

This example starts with no modulation to the wavetable position (the Frame slider). So far, the movement in the sound comes largely from the unison effect. The next two sounds you hear have LFO1 modulating the Frame slider. At first, the modulation speed is fast and ravey, and then I slow it down for the last part.

The effect this has will depend entirely on the wavetable you are using – try switching to other wavetables and you will hear how varied the results can be. This is a good thing – it leads to way more possibilities, and when you are also using different warp modes and filtering, you really appreciate how powerful wavetable synthesis is.

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Example 5: Fast Modulation

I’m putting this one last as the results here tend to be less musical and more suited for sound effects and experimental noises. So far, the LFO speeds we have used are well below 20 Hz and most of the time this is where you will be working. However, there is nothing stopping you from completely ignoring the L in LFO and pushing things into new sonic territory.

If you’ve ever played around with a ring modulator, you might notice some similarities in the results. That’s because ring modulators use oscillators that are fixed to a set frequency, and this creates all sorts of inharmonic activity when mixed with a sound source with varying pitch.

If you tie an LFO to the pitch of an oscillator and increase the LFO speed beyond a certain point, you will hear strange tonal effects as the speed of the modulation approaches the lower limits of human hearing. FM synthesis works on this principle, except the oscillators typically scale with the keyboard pitch, making the harmonic structure much easier to control.

In this final example, I increase the LFO speed each time so you can really hear how faster modulation drastically changes the sound. The LFO is tied to the pitch, then the volume, then the filter cutoff.

Wrapping Up

If you’re new to synthesis, it can seem intimidating getting to grips with the terminology, and this can make things seem too complicated. But I have always maintained that synthesis is quite easy to break down into simple parts, it’s just that how those parts interact can be more complex.

LFOs are an important piece of the puzzle when it comes to creating synth patches with movement. The more you investigate LFOs, the more you will realize just how similar they are to synth oscillators. This is particularly true in the world of modular synthesis, where there is little distinction between a modulation source and an audio generator.

LFOs have been a part of synthesis since the very early days. Although they have changed a lot over the years and can be found in more and more complex forms, they have never gone away. We’ll always need some source of movement for our synth patches, and LFOs fit the bill perfectly.

If you really want to master LFOs, as usual, I suggest playing around with them in your DAW and hearing how they work for yourself. Try attaching an LFO to oscillator pitch, volume, and filter cutoff for starters. Then you can try more complex routings, like using one LFO to change the speed of another one. Like so many things in synthesis, the possibilities are endless, and no matter how familiar you get with LFOs, the sounds they can make will still surprise you.