Music Producer TIPS: HOW AUDIO COMPRESSOR WORKS

If there is a tool in the audio field about which everything and the opposite of everything is said, and whose very role can be almost incomprehensible to newbies, this is the compressor.
What is a compressor and, above all, what is it used for?
Let’s try to make a little clarity. A compressor is an instrument, analog or digital, hardware or software, which allows to intervene on the audio dynamics; the way it intervenes is regulated by a series of parameters, which modify its functioning.
Generally, the use of a compressor aims to reduce the dynamic range of the audio on which it acts, in order to subsequently increase its volume.
Let’s take any audio track as a reference.
What is the dynamic range of an audio track?
The dynamics of an audio track defines the amplitude of variation, in terms of volume, of the track itself: in practice, the difference between the maximum and minimum volume.
Let’s take an example.
Considering that we are in the digital domain, the volume of an audio track could for example vary between -50 dB (slight background noise) and -5 dB (high volume): the dynamic (i.e. the difference between the minimum value and the highest peak) in this case would be 45 dB.
Compression of the track can lower the high peaks by reducing them to -10 dB, for example, with a lowering of the overall dynamic to 40 dB: the dynamic range is therefore decreased, that is, by attenuating the higher signal levels we have limited the difference in volume of the same with the lower ones.
But why would I want to reduce the dynamic range of a track?
Basically, because having lowered the highest peaks, I could now increase the overall volume of the entire track, making sounds audible that were previously too low (or too hidden by louder sounds) to be heard.

HOW AN AUDIO COMPRESSOR WORKS

The compressor generally works on the basis of a few user-defined parameters:

• Threshold: generally expressed in dB, it establishes the volume level at which you want the compressor to start acting.
  For example, if you set a threshold of -10 dB, the compressor will act on all sounds that are louder than this threshold.
• Compression ratio: determines how much the signal exceeding the threshold should be compressed.
  The ratio is expressed by a ratio: for example, a ratio of 2:1 means that a signal that exceeded the threshold by 10 dB will exceed it by only 5 dB after compression.
  That is, a ratio of 2:1 tells the compressor to reduce the amount by which the signal exceeds the threshold to 1/2.
  When very high compression ratios are used (over 10:1), we talk about limiting (and the compressor can then be defined as a limiter), that is an extreme compression that in practice does not allow anything or almost anything to go beyond the threshold.
  On the other hand, when an inverse compression ratio is used (e.g. 1:3), we speak of expander instead of compressor: the expander has an opposite action, that is, it tries to increase the dynamics of an audio track by reducing the volume of signals below a certain threshold. For example, an expander can be used as a noise reduction, zeroing signals below a very low threshold (i.e. zeroing the noise floor).

• Attack time: generally expressed in milliseconds (ms), this determines how fast the compressor should start acting when the signal exceeds the threshold.
  A medium or long attack generally makes the compressor’s action more natural and smooth, but on the other hand it will probably leave time for some signal to “splash” over the threshold before being compressed.
  On the other hand, a very short attack will make the compression action almost immediate, but risks closing the attack of some instruments a little too much.
• Release time: expressed in ms or seconds, generally slower than the attack, it determines how long the compressor should take to finish its action.
  A very short release time causes the compressor to faithfully follow the variations in volume of the signal, risking an effect of distortion or continuous oscillation of the volume itself; a longer release time is generally more natural, but can end up prolonging the action of the compressor even when the signal has already returned below the threshold (thus acting even on sounds that you would not want to compress).
• Gain (or output): establishes how much gain to apply to the compressed signal at the output, to compensate for the loss of volume due to compression.

Sometimes, there are also other “minor” parameters, such as knee, which when set to “soft” or “hard” modifies the softness or hardness of the compressor action.
Obviously, there are no absolute right or wrong settings, not even for a certain instrument: it depends a lot on what you want to achieve, as well as the initial sound… most software compressors offer a whole series of presets for various instruments, which can be a good starting point to start experimenting and understanding how compression works.

USE AN AUDIO COMPRESSOR

The use of an audio compressor can have many purposes.
Generally speaking, we have already said that the purpose is often to reduce the dynamics of a track in order to be able to increase its overall volume… but, in addition to a question of volume, it is the sound of the track itself that is modified.
Typically, drums and bass are instruments that are very often compressed: a well compressed drum kit can be sharper, more powerful… a bass track can become stronger, maybe even sound better played.
Even vocals are sometimes compressed, although, at least in my opinion, more caution is needed in this regard (it also depends a lot on taste and artistic choices, of course).
We have already seen how a dynamics processor can also be used as an expander (also called a gate in this case) to eliminate background noise.

To deepen the use of a compressor in practice, I highly recommend reading the series of articles on mixing, in which the use of compression is addressed several times, providing valuable advice from a long experience in the studio.

Compression is also a fundamental step in mastering: in this case you work on an entire mix, and usually use multi-band compressors (i.e., with the ability to act independently on different frequencies), obviously more complex tools.

I hope with this first article on Music Producer Tips to have clarified some points on what is a compressor and how to use it. This article was written by Andrea Basagni for HomeRecording.it