HOW TO USE COMPRESSION (free Vst Plugins included)

Hello! Today we're going to talk about a fundamental effect for mixing and mastering, and take a look to some practical tips on how to use it.
Compression can be seen as an automatic volume control, with the volume being turned down very quickly every time a loud sound occurs, and the volume of the quieter sounds being turned up in order to match the louder parts. The result is a smaller difference between loud and quiet sounds, making the signal sound ‘compressed’.

First off let's take a look to the main controls featured in most of the compression processors:

Threshold: A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. This value is commonly set in dB, where a lower threshold (e.g. -50 dB) means a larger portion of the signal will be treated (compared to a higher threshold of −5 dB).


Attack: The 'attack phase' is the period when the compressor is decreasing gain to reach the level that is determined by the ratio. The shorter is the time, the faster the compression will engage.


Release: The 'release phase' is the period when the compressor is increasing gain to the level determined by the ratio, or, to zero dB, once the level has fallen below the threshold. The longer the time, the more the compressor will keep applying the effect before stopping.


Ratio:  This allows the user to control the proportion (usually in decibels) by which signal exceeding the threshold will be reduced. This basically means that the output volume will increase of 1db for each value set on the ratio: the higher the value (es. 20:1), the more the volume differencies between low and high will be normalized.
The highest ratio of ∞:1 is often known as 'limiting'. It is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (except briefly after a sudden increase in input loudness, known as an "attack").


Make up Gain: Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of make-up gain at the output is usually provided so that an optimum level can be used.


Soft/Hard knee: Hard knee compression reduces gain abruptly on any signal exceeding the threshold (and not at all on signals below it). Soft knee compression introduces gain reduction gradually on signals approaching the threshold, and progressively (up to the ratio set by the user) as they exceed it. Most compressors with an "auto" function apply soft knee compression when "auto" is selected.

When to use compression: Compression is a great effect to be applied both on single instruments (on the mixing phase), and the whole track, on the mastering phase. The instruments that usually needs to be compressed in order to sound tight are usually (as we have already seen) bass, vocals and drums; guitars don't need always to be compressed, it really depends on the music style (for example funky or other clean styles, like the acoustic guitar, may need it, while heavier genres often stick to gain, which is a natural compressor, and use a tip of compressor only if needed).
In order to see some tips about how to compress these single instruments, click on the links to go to the dedicated article.
On the mastering phase, bus compressor is a valuable ally, since it helps to "glue" the instruments together and give to the mix the punch it needs to sound properly. Today there are many vst bus compressors that emulates the solid state ones that made the history of music, and between them we can suggest some free ones: Density and Antress.

How to start compressing a sound from scratch: Here's the basics to start using a compressor (for example the free ReaComp) on a single instrument, for example the drum kick.
First off, lower the Threshold as far as it will go, and increase the ratio all the way. You should hear that the kick sound is extremely over compressed, now lower the attack time to the shortest setting possible.

Now, If you slowly rise the attack control, you will hear the sound start to ‘click’ – when the click becomes fairly pronounced you know that the transient ‘attack’ on the drum is coming through, and that your attack setting is correct.

Now that the attack is set correctly we can return the threshold and ratio controls to something more realistic, for example a ratio of 5:1. Now raise the threshold to a level where you feel the drum start to come back to life, and keep it there. In this phase you can keep the makeup gain on auto, but if you're not satisfied with the result, you can always turn it off and set the gain manually in order to match the volume of the compressed drum kick with its uncompressed version (hit "bypass" to check).

Setting the release time is also important, as if the compressor has not ‘switched off’ before the next drum hits you will have wasted your time setting the attack control. 

As a general rule you’re going to want to have the release control set on a "not too long" time, but you can get some strange effects if you lower it too far. A good guideline is around 200ms, but it’s a good idea to check that your compressor’s gain reduction meter has returned to zero (or near) before the next drum hit sounds, if you want to be sure not to have any problem.

Addictional awesomeness: Remember that after compression you may also need to EQ a little bass back into your sounds, as compression often affects lower frequencies. Also, if your drums are going to be compressed again after this initial compression (for example with parallel compression, or bus compression), then you may need to be more gentle with your gain reduction, in order to avoid a final result excessively "squeezed" (over compression).
Plus, if you have some compressor that colours the sound on a particular way that you need for your mix, you can also try SERIAL COMPRESSION, which basically consists in using more compressors on the same chain, one after the other, set very low to not oversqueeze the sound. But beware, you really have to be careful, otherwise the obtained sound will be a total disaster!

Here are the Advanced Compression techniques:


PARALLEL COMPRESSION

MULTIBAND COMPRESSION

SIDECHAIN COMPRESSION

BUSS COMPRESSION

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