Compression is one of the most important weapons in the sound engineering arsenal. Learning to wield it effectively is a vital skill for all producers.
In music production, compression reduces the amplitude (volume) of the loudest parts of a signal. Think of a compressor as an automatic channel fader that is turned down when the signal gets too loud and is returned to its previous position when the signal becomes quieter again. By doing this, a compressor reduces the dynamic range of an audio signal or recording, reducing the difference between silence and the signal’s loudest peaks.
By reducing the peak amplitude of a signal, compressors allow us to make a mix or instrument sound fuller and louder overall. This is because the loudest peaks in a signal restrict how much gain can be applied to an instrument before clipping occurs but the average signal level gives the impression of loudness. By reducing those peaks, then, we are able to increase the average signal level and therefore perceive the sound as louder.
When to compress
Compression can be applied at various points in the recording chain. In general, though, we want to process an entire signal with it. So, here, we use it as an insert effect, but there are some specialised compression techniques that use different patching and routing configurations (such as parallel compression and multiband compression).
When recording live sources, it can be useful to compress the input signal. This keeps loud peaks under control, allowing us to add more preamp gain to achieve a richer recorded signal. Be careful not to over-compress at this stage, though. Doing so will limit your options during the mixdown, and there is no way to remove bad or over-done compression.
This input conditioning is beneficial in analogue and digital recording but, in the latter case, the compression has to be applied before the signal is converted to digital. Standalone analogue compressors can do this and many mic preamps have built-in compressors too. There are even a few audio interfaces with such capabilities.
Come mixdown, compressors can still serve the function of levelling the dynamics of the elements in the mix, while also playing a secondary role in adding solidity, prominence and punch to parts that need it. Typically, the entire mix will also be treated to a dose of compression, whether during mixdown, mastering or both. The aim of full-mix compression is to create loudness and dynamic consistency within the mix.
Compression concepts
Despite their various operational and technical differences, all compressors work within the same core concepts and parameters. Keep in mind that while some compressors offer direct control over all these parameters, some only sport a few and others offer completely different settings altogether.
Remember: the concepts below apply generally to all compressors, even when the specifics of some implementations appear to be quite different.
Compression is essentially a process that modifies the relationship between the input signal’s amplitude and the output signal’s amplitude. This is expressed as a ratio. For example, a ratio of 3:1 would mean that for each increase of 3dB in the input signal’s amplitude, the output signal’s amplitude will only be raised by 1dB. ….
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