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LFO section of a modern synthesizer

The term low-frequency oscillation (LFO) is an audio signal usually below 20 Hz which creates a pulsating rhythm rather than an audible tone. LFO predominantly refers to an audio technique specifically used in the production of electronic music. The abbreviation is also very often used to refer to low-frequency oscillators themselves, which produce the effects explored in this article.

Low-frequency oscillation as a concept was first introduced in the modular synths of the 1960s and 70s. Often the LFO effect was accidental; so myriad were the number of configurations that could be 'patched' by the synth operator. LFOs have since appeared in some form on almost every synthesizer. More recently other electronic instruments, such as samplers and software synthesizers, have included LFOs to augment their sound alteration capabilities.

The primary oscillator circuits of a synthesizer are used to create the audio signals. An LFO is a secondary oscillator that operates at a significantly lower frequency (hence its name), typically around or below the threshold of human hearing (which is approximately 20 Hz). This lower frequency or control signal is used to modulate the audio signal, changing it without introducing another sound-signal source. Like a standard oscillator, this usually takes the form of a periodic waveform, such as a sine, sawtooth, triangle or square wave. Also like a standard oscillator, LFOs can incorporate any number of waveform types, including user-defined wavetables, rectified waves and random signals.

Using a low-frequency oscillation signal as a means of modulating another signal introduces complexities into the resulting sound, such that a variety of effects can be achieved. The specifics vary greatly depending on the type of modulation, the relative frequencies of the LFO signal and the signal being modulated, et cetera.

An LFO can be routed to control, for example, the frequency of the audio oscillator, its phase, stereo panning, filter frequency, or amplification. When routed to control pitch, an LFO creates vibrato. When an LFO modulates amplitude (volume), it creates tremolo. On most synthesizers and sound modules, LFOs feature several controllable parameters, which often include a variety of different waveforms, a rate control, routing options (as described above), a tempo sync feature, and an option to control how much the LFO will modulate the audio signal.

Electronic musicians use LFO for a variety of applications. They may be used to add simple vibrato or tremolo to a melody, or for more complex applications such as triggering gate envelopes, or controlling the rate of arpeggiation.

Differences between LFO rates also account for a number of commonly heard effects in modern music. A very low rate can be used to modulate a filter's cutoff frequency, thereby providing the characteristic gradual sensation of the sound becoming clearer or closer to the listener. Alternatively, a high rate can be used for bizarre 'rippling' sound effects (indeed, another important use of LFO would be for various sound effects used in films). Such effects are difficult to describe, and are more understandable when heard.

The British electronic music group LFO take their name directly from the concept of low-frequency oscillation. Its properties have been influential in their music production.

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