LFO stands for low frequency oscillator. LFOs oscillate at a frequency so low that the human ear cannot actually hear them. This speed is perfect however for creating noticeable change over time (time being determined by you).
The most obvious example of an LFO at work is when it is used to modulate the pitch of a siren (police, EMS etc.). The siren’s pitch slowly rises, and then falls. Another widely used example of an LFO is modulating the cutoff of a lowpass filter to create “wobbles” and “womps” (dude, bro, this dubstep is totally melting my face off).
Although it is very important to note that not all modulation of lowpass filters will create a “womp”. A great example of this is in a new release called Feeling by Meramek (an alias of Oliver Smith signed to Anjunadeep). Listen to the swelling pad at the beginning and how it sounds like it is making a “wow” sound with an eighth note rhythm.
The most common and basic parameters on an LFO are rate, and waveform. LFOs use the same geometric waveforms that normal oscillators use, allowing for different modulation shapes and curves. Lets look at the waveforms again to refresh our memory.
The sine wave is used quite frequently. It slows down as it approaches the top or bottom, making for an interesting sounding curve. This is the waveform used to generate the rise and fall in pitch of a siren. Producers love using siren like sounds in their productions. There is a great example at around 2:15 of this preview for the track Girl on Girl by Wolfgang Gartner (follow Wolfgang on Twitter here). The rate of the LFO starts off extremely fast, is slowly turned down and then slowly increased again.
Square waves are nice for creating tremolo and dramatic stepwise motion in parameters. Sawtooth waves are great for creating slow gradual rises over however long of a period you choose (determined by rate). What is nice about rate is that it can be synced to the tempo of your song. By default the rate of an LFO is measured in hertz. If you sync an LFO to the master tempo, you can tell it to modulate based on the beat which can be extremely useful. It is important to note that sometimes having an LFO not synched to the beat can definitely be more interesting depending on the circumstances. (Learn more about LFOs inside Abelton at our music production classes)
This brings up a very important point about modulators. Modulators will not work their magic on anything unless you specifically tell them to do so. Many synths have an entire section dedicated to modulation routing. A great example of this is the DCAM Synth Squad by FXpansion.
So change overtime is a very good thing. But an LFO is not the proper tool with which to simulate quick, 1 time events such as the pluck of a guitar string. For that we need envelope generators. Next we will look at envelope generators and how they shape one time events and provide modulation in our sounds.
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