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What is an audio compressor

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You’ve certainly heard of compression before: you know it’s an essential effect for mixing, but at the same time, don’t you necessarily master every setting of your plugins?

This is normal: it is a somewhat complex subject. And if you don’t know exactly what effect each parameter has on the sound, you risk damaging your mixes rather than improving them.

Therefore, I advise you to take a few minutes to see what the different settings of your compressors correspond to, so that you can adjust them yourself: in fact, whether you use a compressor for mastering or an analog audio compressor, the settings they are generally still the same!

What is an audio compressor?

It is mainly an effect, as well as equalizers, reverbs, distortions, etc. It can take the form of an add-on or an external effects module.

In general, and although there are many possible ways to use a compressor, you can reduce the dynamic range of a recording or a complete mix. That is, reduce the gap between the loudest and weakest sounds on the track.

Hence the name, moreover: a compressor compresses the sound.An example of an external stereo compressor, the ART Pro Audio VLA II
For example, if we have a voice track with a significant level variation between the words, we can level the sound by attenuating the loudest parts.

Here is an example in pictures:

Compression example

In the image above, there is no compression: the signal (the singer’s voice, for example) alternates between significant peaks and less strong elements.

In the image below, compression was used to attenuate these spikes. In fact, they are now at a level closer to the rest of the recording. The dynamic range has therefore been reduced.

Compression threshold

The Threshold parameter is particularly important for successful compression.

It is simply the level in decibels (dBFS) from which the compressor begins to operate; in other words, it attenuates the signal.

For example, if your recording reaches a maximum of -12 dBFS and you set its threshold to -6 dBFS, the signal will not be compressed. In fact, the threshold is higher than the signal (-6 dBFS> -12 dBFS). Conversely, if you set it to -20 dBFS, the portion of the signal above this threshold can be compressed.

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The audio compressor, what is it for?

If there is an instrument in the audio field that says everything and the opposite of everything, and whose very function may be almost incomprehensible to novices, this is the compressor.

What is it and especially what is a compressor for?

Let’s try to get some clarity. A compressor is an instrument, analog or digital, hardware or software, that allows to intervene in the dynamics of the audio; The way it intervenes is regulated by a series of parameters that modify its operation.
In general, the use of a compressor aims to reduce the dynamic extension of the audio on which it acts, to subsequently increase its volume.
Let’s take any audio track as a reference.

What is meant by dynamics of an audio track?

The dynamics of an audio track defines the amplitude of the 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 environment, the volume of an audio track could vary, for example, between -50 dB (light background noise) and -5 dB (high volume): the dynamics (that is, the difference between the minimum value and the highest peak (highest) in this case would be 45 dB.
Track compression can reduce high peaks, for example by reducing them to -10 dB, with a decrease in overall dynamics to 40 dB: therefore, the dynamic spread decreases, i.e. by attenuating signal levels higher, we have limited the difference in volume of the same with the lower.
But why would you want to reduce the dynamics of a track?
Basically, because I lowered the highest peaks, I could now increase the overall volume of the entire track, causing audible sounds to be heard that were previously too low (or too hidden by too loud sounds).

How a compressor works

The compressor generally works on the basis of some user defined parameters:

– threshold: generally expressed in dB, it sets the volume level from which you want the compressor to start operating.
For example, by setting a threshold of -10 dB, the compressor will act on all sounds that exceed this threshold in volume.

– compression ratio (ratio): Sets how much the signal beyond the threshold should be compressed.
The ratio is expressed as a ratio: for example, a 2: 1 ratio means that a signal that exceeded the 10 dB threshold, after compression, will only exceed it by 5 dB.
That is, a 2: 1 ratio tells the compressor to reduce the signal overshoot beyond the threshold to 1/2.
When using very high compression ratios (over 10: 1), we are talking about limitation (and the compressor can be defined as a limiter), which is extreme compression that practically doesn’t allow anything or almost to cross the threshold.
When, on the other hand, we use an inverse compression ratio (for example, 1: 3), we talk about expander instead of compressor: the expander has an opposite action, that is, it tries to increase the dynamics of an audio track reducing the volume of the signals below a certain threshold. For example, an expander can be used as a noise reduction, zeroing signals below a very low threshold (that is, effectively eliminating background noise).