The process of attenuating (lowering) the level of a signal.
This is by far the most basic audio process, appearing in virtually all mixers and effects units. Using controlled fades is the most basic step in audio mixing, allowing more volume for prominent elements and less for secondary elements.
The process of adding volume to a signal.
Boosting is done using extremely slight amounts of amplification, enough to boost a signal without pushing it over the edge of a pre-amplified signal. Some volume control units may have the ability to boost and attenuate a signal.
The process of reducing the dynamic range, or the difference between the loudest and quietest parts of a signal.
This is done with an automatically controlled fader, which will reduce the volume of the signal after a user-adjustable threshold is reached. The ratio of reduction to gain above the threshold is often controllable as well, as well as the time it takes for the reduction to engage (attack) or release.
Most compressors will also apply a boost after gain reduction is responded to compensate for the quieter signal. Compression has many uses in the mixing process, from leveling vocal volume to enhancing drums.
The process of altering the balance of an audio signal between the left and right channels of a stereo signal.
The panning of a signal can be changed using a simple bi-directional pan control or an “auto panner” that continuously modulates and changes the panning of a signal.
Panning is often used in the mixing process to “arrange” track elements, simulating the placement of live bands.
Limiting is essentially an extremely harsh form of compression.
Rather than applying smooth reduction to the audio above the threshold, limiters forcibly “flatten” it, not allowing any signal above the threshold.
Many limiting units also have built-in compressors that reduce the amount of audio that actually crosses the threshold. Many limiters also use digital algorithms to “smooth” the harsh sound of limited audio, transforming the waveform rather than decapitating it entirely (removing part of the waveform entirely can lead to severe distortion and heavily altered tones).
Amounts of compression to create a more consistently loud track with fewer volume fluctuations, and harsher limiters can be used as distortion effects or emergency safety measures to prevent large speaker systems from blowing up. Many analog amplifiers are equipped with their own basic limiters to prevent high-voltage circuitry from overloading and exploding.
Dynamic expansion is essentially compression with an inverted threshold.
Any signal below a certain threshold is dynamically reduced while signals above the threshold remain untouched. Expansion is most commonly used to give volume to certain elements of the recordings, for example, the bass and snare of a recorded drum.
When a signal falls below a set threshold, a gate will reduce the gain until the output signal is forced below a certain level and continue to hold the gain at that level until the input rises above the threshold.