In the analog arsenal of modulation resources, low frequency oscillators – aka LFOs – are right up there with envelopes when it comes to usefulness and flexibility.
While the most common uses for LFOs are vibrato, tremolo and wah-wah effects, there’s a lot more that can be done with LFOs — if you’re crafty about it.
In this installment, we’ll cover the specifics, along with a few more exotic applications to get your juices flowing.
What’s an LFO?
The term “low frequency oscillator” is derived from the fact that audio oscillators operate within the range of human hearing, which is approximately 20 Hz to 20 kHz, whereas low frequency oscillators generally function below the frequency of 20 Hz.
While some LFOs can operate in ranges higher than that, we’re going to focus on the more traditional implementation described above.
Since they use slowly cycling waveforms, LFOs can be used to modulate the value of another parameter repeatedly.
The most common example of this type of modulation is vibrato: An undulation of pitch like that of an opera singer or violin.
By applying a sine or triangle wave LFO to the pitch of one or more oscillators, this “quivering” effect occurs.
In small amounts, the vibrato effect can be used to add a quasi-organic quality to accented notes in a sequence.
Larger amounts of pitch modulation can be used to emulate sirens or create wild electro sweeps.
Now that we’ve covered the essential concept, let’s move on to the specifics.
Common LFO parameters
This is the speed of the LFO modulation.
Some manufacturers measure the rate in Hertz (cycles per second), whereas others use arbitrary numbers.
Almost all software synths include an alternate mode – labeled “sync” – that allows the LFO to be synchronized to the sequence tempo at various note values.
This is the overall amount of LFO modulation.
Some synths include the destination (cutoff, pitch, amp, etc) in the LFO section.
Others locate the LFO amount in the section of the modulated parameter(s).
This selects the LFO waveform, which defines the overall character of the modulation.
If the waveform selection parameter is absent, this often means that the LFO will operate in sine or triangle mode only.
For example, Reason’s Subtractor includes an LFO – LFO 2 to be specific – that functions exclusively in sine/tri mode.
A list of common waveforms can be found later in this tutorial.
Delay – alternately referred to as “attack” – causes the LFO to start after a specified amount of time passes once the key is pressed.
This allows the unmodulated note to play for a time before the effect begins.
In practice, you can use this feature to allow staccato notes to play normally, while sustained notes receive the LFO treatment.
It’s also great for creating wiggly funk vibrato that engages after the note is held for a short time.
Sometimes abbreviated as “kbd” or “key”, keyboard tracking is a unique feature that allows the LFO speed to increase (or decrease, if negative amounts are implemented) as a user plays higher notes on the keyboard.
When used with slower LFO rates, keyboard tracking can yield somewhat idiosyncratic results, but if the LFO can function in the audio range, it’s useful for tuned ring modulation style effects.
Old school analog LFOs often cycled continuously, regardless of whether a key was pressed.
As synthesizer technology progressed, it became possible to reset the LFO cycle every time a new note was played.
This is called “retriggering”, “key trigger” or “key sync”, and is implemented as a simple on/off switch on some synths, including the Prophet 08, various Ableton devices, and Reason’s new Thor softsynth.
Offset is somewhat more exotic parameter that allows the user to define the point in the waveform that the LFO cycle begins, which is especially useful for refining the sound of tempo-synced rhythmic effects.
Some manufacturers refer to this parameter as “phase”, which is fine as it makes sense as well.
Common LFO waveforms
The waveform of an LFO determines the overall character of the repeating modulation.
As described above, vibrato relies on sine or triangle waves, so applying a different waveform will deliver very different results.
Here’s a list of common waveforms and practical applications for each.
Most of these waveforms (taken from Ableton’s Sampler) are pictured to the right of each description.
Sine waves [first image, right] are best suited for organic, classic effects.
In addition to vibrato, sine waves can be used for tremolo when applied to amplitude (loudness) or cycling wah-wah sounds when applied to filter cutoff.
In conjunction with slow rates and high modulation depths, sine waves can be used to create sirens and alarms.
Many LFOs – especially on older analog synths – rely on triangle waves [second image, right] instead of sine waves, since the end result is often indistinguishable.
Accordingly, either wave can be used for the applications described in the sine wave section.
Since the shape of a square wave [third image, right] consists of a jump between two specific values, square waves are used to create trill effects (an alternation between two notes, much like a UK police siren) when applied to pitch.
Applying a square wave LFO to amplitude or filter cutoff is a quick and easy way to create gated or pulsed effects, especially when synced to tempo.
Sawtooth LFO waveforms [fourth image, right] come in two different flavors: rising and falling.
Rising sawtooth waves, when applied to pitch, create “woop-woop” effects like a car alarm.
If you slow this effect down to the minimum possible rate, with a huge pitch depth setting, then process the synth output with a long delay, you can create those near-infinite pitch rises that are all the rage with the cool kids.
On the other hand, falling sawtooth waves applied to pitch are great for creating retro “booo-booo” sounds and laser zaps, if cheese is your weapon of choice.
Lactose-intolerant programmers will be more inclined to use falling sawtooths for rhythmic effects.
In fact, if you sync a falling saw LFO to eighth-notes and apply it to filter cutoff and/or amp volume, you can emulate the sound made popular by an artist such as Deadmau5 with minimal effort.
Random waves [fifth image, right] – sometimes referred to as “sample & hold” or “S&H” – shift randomly from value to value at the specified tempo.
Applying this waveform to amplitude adds a certain jitter to the volume.
When used on filter cutoff with a touch of resonance, it’s a resource for retro-style percolation.
Routed to pitch, especially with huge depth settings, random LFOs will create those hyper-goofy 1970s “computer” sounds from old sci-fi movies.
The secret to getting useful results lies in subtlety, creativity and avoidance of these clichés.
Some LFOs allow you to use white noise as a waveform.
Applied to pitch in small to medium amounts, noise modulation results in distortion-like effects.
Applied to highly resonant filters, noise is an excellent resource for both glitchy crackling effects and organic sounds like rain and sizzling bacon (really).
Experimentation is key with this waveform, as it has much potential as a timbral resource.
Working with multiple LFOs
If your synth includes more than one tempo-synced LFO, you’re in luck.
By combining different note-values and assigning each LFO to a timbre or amp related destination, you can create polyrhythmic patterns with a minimum of effort.
Here’s one possible configuration for a synth with three LFOs and key triggering set to “on” for all.
Assign a falling sawtooth LFO to filter cutoff using eighth-notes, then lower the overall cutoff frequency and increase the LFO depth to taste.
Assign the second LFO to oscillator pulse width, FM, or another waveform-related parameter, using a quarter-note triangle wave with fairly strong modulation depth.
Assign the final LFO to amplitude/volume with a sixteenth-note square wave.
From there, experiment with different rates and destinations for each LFO and use your creativity to develop a sound that fits your style.
One more lesson to go, then on to sound design tips and tricks!