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A Low Frequency Oscillator, or LFO for short, is a technique used in sound synthesis and sampling to create many different types of effect.
As the name suggests an LFO operates at low frequencies, anywhere from 0.01Hz up to around 200Hz, and unlike conventional oscillators is not intended to be a sound source per se, but rather a way of modulating other parameters.
For instance, if a tremolo (rhythmic changing of volume) effect was required, a LFO could modulate the amplifier section of a synth/sampler and the volume would change at a rate determined by the frequency of the LFO – for instance, a LFO rate of 10Hz would see the volume change at a rate of 10 times per second.
An LFO can be routed to many different parts of a synth or sampler to create many different effects. A rhythmic wah effect could be created by sending the LFO output to a filters cutoff control. On modern digital and software based synths and samplers, the routing possibilities are almost endless and virtual instruments such as z3ta+ by RGC:Audio push the boundaries of synthesis with their modulation matrixes.
Another common use of LFO is to modulate the pitch (creating vibrato) of an oscillator, but have the LFO rate dictated by a controller such as the modulation wheel – this allows keyboard players to inject some of the feel of an instrument such as a guitar into their playing.
The waveform of an LFO is often selectable from standard waveforms such as a sine, saw, triangle and square.
Below are some audio files showing the effect of an LFO on various parameters.
On all examples the LFO is set to aprox 12Hz and to a Triangle waveform: