Pulsing synth lines, frequency sweeping pads, earth-shaking wobble basses. These are a couple of examples of LFO at work. LFO or Low Frequency Oscillator is one of those mystical acronyms in synthesizer jargon that can be somewhat intimidating to new synth users. But at its heart, the LFO is a pretty straightforward concept to understand. In this post, I would like to break it down.
Sound is produced as a series of waves and the frequency, or rate of wave cycles occurring in a given span of time, is what determines the pitch (note value) of a sound. This is measured in Hertz (Hz). The average human can hear sounds that fall between 20 Hz and 20,000 Hz.
Those booming low sub basses we hear in hop hop and drum & bass usually fall somewhere in the range between 20 and 100 Hz. These are the kinds of basses you “feel” more than hear. Anything below 20 Hz is too low for human hearing. So why would we care about using waves in these ranges?
This is the beauty of LFO. LFO allows you to use its waveform to modulate (control) a parameter of another sound. For example, if we use an LFO sine wave to control the pitch of a different sine wave that we can hear (oscillating at an audible frequency), the pitch of the sine wave we hear will rise and fall steadily. This audible sine wave is now being “shaped” by the lower frequency sine wave, which is the LFO. Modulating pitch in this manner is how siren sounds are produced. Here is an audio example:
Other attributes besides pitch can be controlled in this manner. Here is an example of amplitude, or volume, being controlled by an LFO sine wave. This gives us that swelling or pulsing effect:
These are two of the most fundamental uses. However when used in combination with other synthesizer modulation tools such as envelopes or filters… well, here is where the full power of an LFO can really be unleashed. Here is an example of an LFO modulating a Low Pass filter, on a saw wave (which is richer in harmonic content than a sine wave). This gives us that classic sweeping, phasing effect we all love:
All of the above examples use a sine wav as the LFO. A sine wave has a very smooth up and down shape and thus produces a very smooth up and down sound. But we are not limited to only using a sine wave. Other wavefroms can be used, each with their own characteristic shapes, resulting in their own characteristic sounds. By looking at the shape of the waveform, one can get an idea of what the resultant sound would be.
A triangle wave has a linear up and down sound. This results in a smooth up and down progression, even smoother than the Sine when used as an LFO.
A sawtooth features a rise and then a sudden drop to silence.
A square is merely on or off, kind of like a binary function applied to sound. This switches the sound on and switches the sound off.
Finally, by matching the LFO frequency to the tempo of our song, we can get incredible timed patterns, that almost make our LFO act as a sort of sequencer. Most modern synths or synth software have a sync function that allows you to easily input the timing of the LFO rate. In other words, do you want the cycles to occur on quarter notes, eighth notes or through a whole measure?
So we have seen how LFO is used to alter the attributes of a given sound and how the waveform chosen for our LFO offers different sonic options. By combining multiple LFOs or having the same LFO control multiple synth parameters, some incredibly crazy sounds can be dished up. The LFO is an incredibly versatile and effective tool to liven audio productions. It’s amazing how something we can’t even hear can offer us so much power. Kind of like our imaginations…