Audio Compression For Dummies – A Simple Guide

Finally, a topic I’ve been wanting to cover for a while. I love talking about compression and I’m super excited to explain everything I know about it.

I know it can be confusing which is why I wrote Audio Compression For Dummies. This guide will help you understand exactly how to use a compressor to improve your mixdowns and get that pro sound.

There are so many different and creative ways you can use a compressor to manipulate audio.

Clean up mixes, make stuff punchier, add colour, shape sounds, design sounds, make sounds louder, quieter, shorter, louder, flatter, pump-i-er (wut?), the list goes on.

To be a better producer it is vital that you understand compression, and my hope is that by the end of this article, you will have learned everything you need to know to go forth and compress.

What is compression?

The best way to understand compression is to think of it as volume automation. I like to imagine a little man with ninja-like reflexes sitting with his finger on the volume fader, turning it up and down as he reacts to the signal either being too loud or too quiet.

So how does a compressor know when a signal is too loud? How does it know how much it should turn it down by and how does it know when it should turn it back up?

These factors are all determined by a few controls.

Threshold

The threshold is the level at which the compressor begins to compress. If your threshold is set to -10db, then the compressor will start to compress the signal the second goes over -10db. How does it know how much to compress it by? This is determined by the ratio.

Ratio

Ratio determines how much a compressor will reduce the gain of a signal when it crosses the threshold. If you have a 2:1 ratio, then for every 2db that the signal goes over the threshold it will allow 1db through. For example, if our threshold is -10db and the signal hits -4db then we have gone 6db over the threshold. With a ratio of 2:1, it will only allow 3db out of the total 6db through. You will, therefore, see a 3db gain reduction. An infinite ratio will cause the compressor to act as a limiter, not allowing any signal to cross the threshold.

Knee 

You will typically have a choice between hard and soft knee settings. The knee dictates how smooth the transition is as the compressor moves from un-compressed to compressed states. Soft is smoother and hard is more abrupt.

Attack

The attack, measured in ms, is the time it takes for the compressor to fully compress the signal once it crosses the threshold. If you have a longer attack, say 80ms this will cause the initial transient to cross the threshold uncompressed. Playing with attack time is usually how we shape transients. Longer attack time = punchier transient. Short attack times are used to smooth out signals and catch peaks.

Release

The amount of time it takes for the compressor to deactivate fully once the signal drops back below the threshold. Having a longer release can create a pumping effect, or can be used to clean up reverbs and squash down tails.

Makeup Gain

Generally the last parameter you adjust when setting up your compressor. If a compressor reduces the gain of a signal by 5db, you can use the makeup gain to turn it back up.

This is essentially just a volume knob at the end of your signal chain. This might seem counter-intuitive, I’ve had people asking “why would I compress the signal only to turn it back up?”

To answer this you need to understand that a compressor is changing the dynamics of a sound, not turning the entire thing down (unless it has a really long release and instant attack).

If you have a long attack and the transient is now more prominent, but the tail has been compressed down by 10db, you might want to increase makeup gain to really push that exaggerated transient up.  If you have used the compressor to smooth out a vocal but the resulting vocal is now 3db too quiet, you can increase the makeup gain to compensate.

Note that a lot of compressors have an automatic makeup gain toggle. I would recommend not using it, rather set the final level by ear than have the compressor constantly adjusting it automatically.

What is compression used for in electronic dance music?

In the previous section, I touched briefly on a few of the uses. I will now dive into more detail and give examples of how a compressor can be used in EDM production.

Dynamic Range Compression

Dynamic Range is the difference in volume between the loudest and quietest parts of a waveform.

Look at this 909 clap. If the transient peak is hitting at -3db and the tail is sitting around -15db then the dynamic range is 12db. We can increase the perceived loudness of this clap by reducing the dynamic range, and then turning the overall gain up of the clap.

How do we reduce the dynamic range? In our example of the clap we can do this by turning down the initial peak, but leaving the body and tail unaffected, and then increasing the overall gain of the clap.

We can see the peaks are all happening within the first 30ms of the clap, the first transient hits right at 0ms so I would set my compressor to 0ms.

Having a compressor set to 0ms can produce pops and clicks due to the sharp and instant gain reduction, so set your compressors attack to around 2ms lookahead to around 2ms. This will allow it to pre-empt the signal and therefore reduce pops and clicks.

Release – I would set this to around 15 to 20ms so that the compressor can fully release by the time it reaches the body/tail of the clap.

Threshold – I set this around -9db to catch only the peaks

Ratio – I set a ratio really high to almost limit the peaks – this resulted in a gain reduction of around 4db, squashing down the peaks and giving me the headroom to turn up the overall gain by 4db

Notice how the waveforms have changed in dynamics

Listen to how much louder the clap sounds.

Note that the peak volume of the clap has not changed – but it still sounds a lot louder.

We could also increase the dynamic range, if for example we didn’t like how loud the reverb tail was. Simply by having a long enough attack so that the initial transient doesn’t get compressed, and long release so that the body and tail get nice and squashed.

In the following video I use the compressor to increase the dynamic range to emphasize transients and squash down the body and reverb tail.

This is obviously exaggerated to show the effects of the compressor. 

Here’s one more example with a vocal. Look at the waveform and all its peaks that are sticking out. In EDM we don’t like these peaks as they eat up headroom and prevent us from pushing up loudness.

Notice how the peaks have been pushed down and the vocal looks more like a sausage.

Sidechain Compression

In EDM, Sidechain Compression is mostly used as a “ducking” effect. This effect is most noticeable in Eric Prydz – Call On Me where you can hear the vocal and instruments pumping around the kick drum “CALL ON MEEEEE-EEEE-EEEE-EEEE”.

This works by placing a compressor on the vocal channel, enabling the side chain feature, and selecting the kick as the input source.

The compressor then reacts to the audio signal of the kick drum instead of the vocal and will compress the vocal down every time the kick hits. You can control the intensity of this effect using attack, release, and threshold.

You can also use sidechain compression to clean up other elements in a mix, for example: Sidechaining the lead to the vocal so that when the vocal is playing the lead is reduced in volume, making more room for the vocal to stand out.

Shaping Transients

We’ve already touched on transient shaping but this is essentially either making transients more exaggerated to help them punch through the mix or to tame them and push them back in the mix.

To exaggerate a transient follow these steps:

Set a long attack, around 20-100ms depending on the sound.

Set the release so that the compressor resets completely before the next transient hits.

Bring your threshold down about 2-5 dB below the peak.

Set your ratio to taste and finally push up the makeup gain to make that transient louder.

Here we have a hat loop with the tails being squashed down the compressor and the transients being emphasized, its a very unnatural sound but once again has been exaggerated to illustrate the effect clearly. 

 

To smooth out transients and push them back in the mix:

Set a short attack, 0ms with a 2ms lookahead to prevent popping and clicking

Set a really quick release, around 5-20ms depending on how long the transient is.

Bring down the threshold 2-5db below the peak

Set ratio to taste

Makeup gain isn’t usually required unless the goal is to make the sound louder.

Notice how the loop sounds further away when we enable the compressor. This is how we can use transient shaping to push elements back in the mix.

Adding Colour

A lot of people get confused when they first hear about compressors “adding colour”.

This simply means that the compressor adds harmonics to the signal similar to how a saturation or distortion plugin would. The harmonics will increase depending on how hard you compress the signal.

Some compressors don’t add any harmonics, some add a lot of harmonics, some add more mid-range harmonics and some add more top end harmonics. This is one of the reasons many engineers will have preferred compressors for different types of instruments and sounds.

Some compressors even have a built-in saturation knob to allow you to control the amount of colour it imparts on the signal.

Look at the following comparison between Ableton’s Glue Compressor, TR5 White 2A (LA-2A emulation), Pro-C2 and Ozone 8 Vintage Compressor.

You can see they all add harmonics in different ways.

Types Of Compressors

Optical Compressors

Optical compressors work by converting the electricity from the audio signal into light. This light is detected by a sensor in a photosensitive cell. The brighter the light, the more the compressor reduces the gain. Due to the nature of this design, it leads to a very smooth release and a quick attack. These are typically good for legato type sounds like vocals.

The most famous optical compressor or “opto-compressor” is the LA-2A. A rather expensive compressor, if you want to buy the original hardware unit you would be looking at spending around £3500. However, there are many plugins modeled on this compressor that you can pick up for relatively cheap like the White 2A I mentioned earlier in the article, the CLA-2A by Waves or the VC-2A by Native Instruments.

Variable-Mu or Delta-Mu Compressors

These compressors are tube based and work by sending voltage to a vacuum tube which in-turn changes the tubes bias. Due to the reaction time of this style of compressor, we get a relatively smooth and organic kind of compression. Another characteristic of this kind of compressor is the ratio increases dynamically as the voltage increases – so the louder the transient, the more it gets compressed. These are great as a final touch on a mix, running an already well-mixed track through a Vari-Mu compressor will help smooth it out and glue the mix together.

The most well-known Variable-Mu compressor is Manley’s Vari-Mu, if you don’t have £3000-4000 lying around, try plugin emulations like the Fairchild 670 and 660, or the T-Racks Vintage Compressor.

FET Compressors

FET stands for Field Effect Transistor. These compressors use transistors instead of Tubes. Due to this design, we get a very quick and snappy compressor that adds warmth and is good for preserving transients. The first compressor made in this style was the UAD 1176. The fast attack times were the main selling point of this compressor. The recommended application for these compressors would be anything with quick and sharp transients like your drum bus or an arpeggiator.

The great thing is these compressors are now available in plugin form made by UAD themselves. Check out the 1176 Classic Limiter Collection by UAD.

VCA Compressors

VCA stands for Voltage Controlled Amplifier. They compress based on a control voltage and use solid state or integrated circuits. This allows for even more control over attack and release times and results in a cleaner, more transparent type of compression. The most well known VCA compressor is the SSL G-Series Bus Compressor. These compressors are best used for “gluing” mixes together, hence why it’s called a Bus Compressor.

For plugin emulations of the SSL G-Series Compressor check out the Waves SSL G-Master Buss Compressor.


Resources

I highly recommend FabFilters videos on compression, mastering and other topics so make sure to check them out!

iZotope also does some fantastic tutorials on compression, limiting and more.


That’s it for this article, I hope you found this interesting and now have a decent understanding of compression. Let me know what your favourite compressors are in the comments below!

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