The acronym LFO stands for "Low Frequency Oscillator." As the name suggests, this is an oscillator just like we talked about early in this series, but with an important difference - it plays at frequencies that are below the range of our hearing. So what good is an oscillator you can't hear? LFO's aren't used as an audio signal. Instead, they're used as a control signal.
An LFO is like that robot assistant I mentioned a couple posts back. It smoothly and repeatedly turns the value of whatever parameter you assign it to up and down automatically. Instead of using programmed ADSR values to determine the 'shape' of the modulation, an LFO takes the shape of the waveform it's producing and uses that to create the modulation. The LFO waveforms you'll find on almost any synth are the same 'classic' analog waveforms we discussed at the beginning of this series. Thus, the upward slope of a triangle wave increases the value of the modulation, while the downward slope decreases it. A sinewave does the same thing, but the peaks and valleys are a bit more smoothed out. A sawtooth will result in modulation that continuously rises and then drops off sharply over and over. A squarewave will produce a modulation that goes back and forth between two values. These modulations will repeat for as long as you hold the note.
Some synths will have an additional option among the LFO waveforms called 'SAMPLE & HOLD'. Fuller-featured synths might even have a dedicated Sample & Hold Generator. Sample and hold generates random values (the 'sampling' part) and holds them for a period of time before changing to yet another random value. If you assign it to modulate your filter cutoff, you open the door to complex, changing burbling effects like this:
Assign it to modulate the pitch of one of your audio oscillators, and you get R2-D2/sci-fi type sound effects like this:
Like an audio oscillator, all LFOs have a frequency control. On an audio oscillator, the frequency value changes the pitch, but on an LFO, it controls the speed of the modulation. This allows you to create filter sweeps that take place over several bars at slow speeds, or to create vibrato effects at higher speeds and everything in between. Most modern synths allow you to sync your LFO to the tempo of your host/sequencer, so you can have changes in the sound take place over musically coherent periods. Older synths, however, generally lack this capability, so getting synced LFO effects is a little trickier with them.
Another parameter you might run into on an LFO is DELAY. The delay parameter does just what it says. It waits a pre-determined amount of time before allowing the LFO to kick in. Think of a cello player who plays a note and as it sustains gently adds in some vibrato. Delay allows you to replicate that.
If this all seems a bit confusing, here are some practical examples that'll hopefully clear things up a bit. In this first example, an LFO with a TRIANGLE wave modulates the filter cutoff. It starts with a low frequency value, and gradually increases the frequency. The triangle wave is probably the one you'll use the most when it comes to modulation:
The most common use of LFOs is to add vibrato. You do this by assigning it to modulate the pitch of your oscillator(s). For these types of effects, you generally want to keep the modulation amount pretty conservative, otherwise you end up with something that sounds like a siren. Here is an LFO set to the TRIANGLE wave modulating the pitch of a sawtooth oscillator:
If you've got a stereo synth, you can even use an LFO to modulate the pan position of a sound for instant, auto-panning . Here, I'm using a sine wave to cause the sound to pan back and forth in the stereo field:
LFOs are great for creating pulsing, rhythmic effects, too. Here, I am using an LFO set to a squarewave to modulate the amplitude, resulting in what sounds like a simple sequenced part playing quarter notes, but is actually produced from sustained whole notes:
As you can see, LFOs are extremely useful and have a wide number of possible applications all of which can add a sense of movement to your sounds. What's better, most modern synths offer you several LFOs which can all be assigned to different waveforms/values to modulate separate synth parameters. Which is a good thing, because once you get used to using LFOs to modulate sounds, it can get addictive.