What is an LFO? Learn with Practical Examples
An LFO or Low-Frequency Oscillator is a section that you will find in pretty much any synthesizer that uses a waveform (a low-frequency waveform) to control or modulate the synth’s parameter over time.
In music production, LFOs are kind of like envelopes. Both are modulation sources. The main difference is that, while envelopes are shaped by ADSR and retrigger themselves once every time you press a key (see envelopes), LFOs are shaped by a waveform that is indefinitely and periodically oscillating.
This waveform, as the name says, has a very low frequency that is below the audible range for humans (below 20 Hz).
In most synthesizers, an LFO can modulate parameters such as amplitude, pitch, and filter cutoff (we’ll see some examples of these later). Other synths such as Serum will let you modulate virtually any parameter of the synth (you can even modulate an LFO parameter with another LFO or envelope which is crazy).
Some of the typical effects created by LFOs are vibrato (subtle pitch modulation) and tremolo (amplitude modulation).
Let’s take a listen to these effects. Here is an example of a completely clean sound, followed by the same sound but with vibrato, and then with tremolo:
Although these are subtle effects, you can use LFOs more experimentally and make extreme modulations (which is sometimes good for SFX —sound effects— or for genres that rely more on sound design than in harmony and melodies, like Bass House or Dubstep).
This tool is also useful for creating small subtle variations (mostly not-noticeable variations) over time so that every note we play is different. This can make the sound more natural, as if we played an instrument (a real instrument plays a subtly different sound every time: we talk about it here).
In this article, we’ll go through the main parameters of LFOs (the ones that you’ll probably find in every synth) along with some examples for each of them. These are the rate, shape and amount or depth.
The rate, frequency or speed of an LFO determines how fast it will oscillate. It’s usually a knob and determines the frequency of the oscillation, which, as I said earlier, is usually very low.
Although the rate of an LFO is measured in Hz, most synthesizers allow you to synchronize the oscillating speed with the tempo of the song. For example, you can set it to repeat every 1/4 note, 1/8 note, etc. This is particularly useful for rhythmic waveforms such as the Saw wave or the Square (as we’ll see later).
The range of frequencies in which you can set the rate varies from synth to synth. To give you some examples; Serum’s rate can be set between 0 Hz and 100 Hz; Ableton Operator’s is between 0.02 Hz and 30.9 Hz.
Of course, most of the time you will choose rate frequencies of under 20 Hz. If this frequency is higher than that, you will be entering new grounds such as FM or Frequency Modulation (in terms of pitch modulation), or Ring Modulation (in terms of amplitude). These two methods of synthesis are characterized by basically using LFOs but at a really high rate; above 20 Hz.
The rate of an LFO can also be modulated. This can be either automating the knob in your DAW, or —in some plugins such as serum— you can use an envelope to modulate it or even another (or the same!) LFO.
This is what the rate sounds like when making it decrease through time (here I’m modulating pitch and amplitude):
Note: if you are going to automate the rate of the LFO through time, I recommend to un-link it from the tempo of the song, unless you want to achieve a specific rhythmic pattern.
LFO Shape: The Basic Waveforms
The shape of the waveform of the LFO defines exactly how a given parameter will be modulated over time.
As we said earlier, most synthesizers (with the exception of Serum, Phase Plant, and some others) will let you choose between the basic waveforms, usually using a switch or button.
We’ve already covered these basic shapes in our article about sound waves, but let’s now see them applied to LFOs, along with some examples.
Triangle Wave in Serum’s LFO.
In terms of modulation, the triangle wave is the simplest of all waveforms (unlike sound-wise, where the most simple is the sine wave). This is because it is completely linear and symmetric from the minimum to the maximum.
This waveform is commonly used to control the pitch and create a vibrato effect (for the vibrato and tremolo examples showed before, I used a triangle wave). Another classic use is to pan the signal left and right with a triangle wave (which is called autopan).
Sine Wave in Serum’s LFO.
The sine wave in terms of LFO is very similar to the triangle. Is very smooth and fluid, but the modulation is not linear from the minimum to the maximum.
Nevertheless, the difference between these two is very subtle and some of the times you won’t even notice the difference between one or another.
Let’s take a listen to the sine wave in action:
Square Wave in Serum’s LFO.
The square wave, in terms of modulation, just jumps back and forth between two values. For example: if you are modulating the amplitude from 0 to 1, the square will just jump from 0 to 1, and vice versa (the triangle and sine, for example, would smoothly transition from 0 to 1, going through all the intermediate values).
This modulation sounds very unnatural and, therefore, electronic and synthetic. It’s commonly used in sequencers to create tonal differences between notes, but not often used to modulate pitch (instead, filters or other parameters).
Let’s take a listen to the square wave in action:
“Ramp up” Saw Wave in Serum’s LFO.
The saw wave in an LFO is a modulation signal that rises smoothly and then drops abruptly. You will often find two versions of the saw wave: rising (“ramp up”) and falling (“ramp down” — reverse saw)
This wave is very effective for creating rhythmic modulations, especially when modulating the amplitude. This is because it has a similar shape to every percussive sound (goes up very fast and goes down not that fast).
Let’s take a listen to the saw wave in action (first ramp up and then ramp down):
S&H or Random (step)
S&H LFO in Ableton’s Wavetable.
S&H (Sample & Hold) it’s not a waveform. In this case, the LFO generates random values and transitions abruptly between them. The speed of change between these is adjustable with the rate of the LFO.
Because it changes abruptly, such as the square, the resulting modulation is very synthetic and electronic, which may be exactly what you are looking for.
Let’s take a listen to S&H in action:
Noise or Random (fluent)
Noise LFO in Ableton’s Tension.
This is the same as Sample & Hold, with the difference that it does not transition abruptly between values, but smoothly. It’s like a noise oscillator.
This shape is very useful for giving a natural or analog feel to a synthetic audio. This is because, if you use it discreetly, it will make the sound vary very subtly through time, as real instruments do.
LFO Amount or Depth
Most of the time, the intensity with which the LFO affects a given parameter is controlled by the LFO Amount, Depth or Intensity.
This determines whether the modulation causes subtle or aggressive changes in the sound. In other words, it controls the amplitude of the LFO waveshape.
Nevertheless, each synthesizer has a different way of controlling this intensity. You have to familiarize yourself with the way your synthesizer works. Anyway, most modern synthesizers now work using a matrix, where you can see and adjust all the parameters that are being modulated by LFOs or envelopes.
The matrix in Serum. The source is the modulator (e.g. an LFO, envelope, etc.) and the destination is the parameter being modulated (in this case, pretty much any parameter of the synthesizer: amplitude, pitch, filter cutoff, etc). Between these two there is the amount control. The other columns are more complex concepts that deserve their own dedicated article.
Apart from the most common parameters in an LFO which are the rate, shape, and amount, some synthesizers also include other controls such as key sync, phase, and delay.
The Key Sync (also called Trigger, Retrigger, Key Trigger, or similar) is usually a switch or a button that determines whether the LFO oscillates freely or its waveform starts again every time a key is pressed.
Activating this switch is useful for aggressive modulations that have a very rhythmic sound (for example, when using a Saw wave on the LFO, although it can be useful for any waveform). I recommend to deactivate it if you are using a smooth waveform (triangle, sine, or noise) that is not that aggressive or it has a very fast rate (for example, tremolo or vibrato).
Here is an example of an audio, first with key sync activated and then deactivated (I’m modulating pitch in this case because it’s the most obvious example):
If you listen with enough attention, you’ll see how in the second example every note starts at a different point of the LFO. In the first audio, all of them start at a consistent point.
The same as for the phase in waveforms, in LFOs this control determines at which point the LFO waveshape starts when a key is pressed (only works if Key Sync is on).
This process is called phase shift because it moves the starting point forward; it shifts it. It’s especially useful when you don’t want the LFO to start at 0 (to start at the equilibrium point).
Some synths have a Delay control within the LFO module, which also works only when Key Sync is active. This parameter determines how long it will take the LFO to start affecting the signal (it’s as if we had an attack parameter within the LFO).
Applying a single LFO to a sound can make such a huge difference. While adding movement through time, it can give more dynamism and life to the music. A song without the use of LFOs can sound lifeless and boring.
Additionally, there are some electronic music genres based on sound design that, I think, need the use of LFOs. Some of these are: dubstep, glitch hop, riddim, bass house, hybrid trap, future bass, among others. If you are into one of these genres (or another similar one) I recommend you to focus a lot on LFOs and master them since they are a key element of your craft.
If you have the possibility to access a synthesizer like Serum, I recommend you to experiment and apply the LFOs to any parameter you can imagine. You can get really creative doing this and you might find some kind of modulation that practically no one uses.
Apart from LFOs, there are plenty of modules and controls in a synthesizer that you should know if you want to take your sound design skills to the next level.
If so, you can read our article about the basics of synthesis and sound design, where we go through the most important concepts about synthesis and even have a dedicated article for each of them.