A Compressor is a versatile tool in all music producers' arsenals. Compression can be applied as a creative effect in certain contexts, or it can be used in a more engineering and technical setting, the variety of scenarios in which you'd use a compressor are numerous.

Chances are, every time you’re making a track, you’re using compression in some way or another, but if you're like many producers, you may not know the full breadth of its capabilities. Or, you may have an extensive background in compressors and have favourites and ones you hate, but all the same, hopefully there's something here for everyone.

So today with Top Music Arts, we will take you through what compression is, how you can use it creatively, and we'll also take a look at Ableton Live's built in models Compressor and Glue Compressor.

What is Compression?

First of all, let’s cover the basics.

Compression (or Dynamic Range Processing) is a process whereby audio signals are treated by either (or both) lowering the louder parts and amplifying the quieter parts. This therefore lowers the dynamic range of the audio signal, or the difference between the loudest and the quietest point.

There are two types of compression, Upward or Downward compression. As mentioned above, upward compression increases the loudness of sounds below a certain threshold, while leaving louder sounds unaffected, bringing the quieter ones up to the same level as everything else.

Downward compression, on the other hand, reduces loud sounds which are higher than a certain threshold, while the quieter sounds are unaffected. These are subtly different, but used in different ways.

A Limiter is also a type of compressor, traditionally with a high ratio, and usually a fast attack. A Limiter is mostly used when you want a track or piece of audio to avoid clipping over a certain volume level. It can be useful in a Mastering context as well as every day production scenarios.

How do they work?

So, compressors are dynamic processors, affecting sounds as we've just discussed.

But how exactly do they do it?

There are various controls of a compressor that you can edit as a user, let's take a look at a few of the most relevant and universal ones found in all models of compressor.

Threshold

This control dictates what portion of the audio signal is treated by the compressor. The Threshold is set in decibels (dB) and the basic concept is: the lower the threshold, the larger portion of the signal is treated by the Compressor. Anything over the threshold has its volume reduced, anything under that threshold remains untouched.

Ratio

The Ratio control on a Compressor dictates by how much the signal is reduced. A common starting Ratio is 4:1, which means, for every 4dB above the threshold, the input signal is reduced to 1dB above the threshold. It’s been reduced by 3dB. Obviously, a higher Ratio means more gain reduction, and the Ratio often goes all the way up to infinity:1. This is effectively a Limiter.

Attack and Release

These controls should be familiar, as they’re found in synthesis ASDR envelopes. In a compressor plug in, they dictate how quickly the Compressor kicks in and stops processing, respectively. Attack and Release are measured in milliseconds (ms) and can affect the sound of the Compressor in either subtle or very noticeable ways.

A fast Attack will instantly reduce the incoming signal, whereas a slow attack will have a more gradual effect in reducing the gain to the level set by the Threshold. Similarly, the ms control of Release affects how quickly or gradually the signal returns to normal after the gain reduction has been applied. You can get creative affects with a Compressor and how it affects the audio it’s processing by experimenting with Attack and Release. Try it out, a Fast Attack and Fast Release sounds very different to a Slow Attack and Fast Release, or a Fast Attack and a Slow Release, for example. There are many possibilities.

It’s worth noting that some Compressors don’t allow users to edit these Attack and Release parameters, as they can depend on the model or the input signal itself.

Knee

The Knee value on a compressor adjusts how quickly or slowly the compression occurs as the threshold is neared by the source signal.

A lower setting on the Knee means no compression is applied to signals below the value set by the Threshold, and full compression applied to those over or at the Threshold. This is called a Hard Knee, and with a higher Ratio it sounds very apparent.

A higher Knee setting means the compression begins more gradually as the threshold is approached, creating less of an abrupt sound. This is called a Soft Knee.

Make Up Gain

When a compressor is applied to reduce the gain of an input signal, an output gain control will be present, applying a fixed amount of make up gain to make sure the perceived volume of the sound isn’t lowered too much. This can be good when you want to shape the specific dynamics of a particular sound, but not necessarily have it be too loud or quiet with drastic change to the volume itself.

Uses of Compression.

So, we’ve discussed what the core controls of a standard Compressor are, so let’s take a look at what the functions and uses of Compression can be.

One is to increase the perceived loudness of an audio signal without actually affecting the overall volume. By increasing the quieter parts of a signal without affecting the louder parts, the overall perceived volume of the song or piece of audio seems louder. This can be done in public spaces such as shopping outlets or restaurants where music is played over the hustle and bustle of everyday background noise.

It’s also a core component of mixing a song or track when you’re done with the compositional side of it. Evening out the average volume of an instrumental part can help it sit better in a mix amongst other elements of a track.

You can also use Compressors creatively, one such example is side chaining, which is a popular technique in electronic dance music. The effect is that ‘pumping’ sound that is so common, whereby portions of a track are ‘side chained’ to the kick.

What this means is, a compressor may be placed on an instrument track, for example, and the Compressor’s Side Chain control is enabled. An audio source is then selected, so in our example of dance music, it would be the kick drum. This then tells the Compressor to apply gain reduction as usual, but only when triggered by the Side Chain's input signal. So every time the Kick drum plays, the signal of the instrument on which the side chain compression is applied is ‘ducked’ or reduced, creating that familiar pumping sound. You're using the signal of one track to affect the compression of another.

Ableton Live's Compressor

Ableton Live comes with two built in Compressors. The aptly titled Compressor and also Glue Compressor. We’ll take a look at each, and in particular we will take a look at why Glue Compressor is so great.

Live’s Compressor features all the standard controls you’d look for on a regular compression plug in that we’ve discussed above. It features a handy Gain Reduction meter, which shows how much the gain is being reduced at any given time by the parameters you’ve set on the compressor.

There’s also a Dry/Wet control, allowing you to blend the balance between the uncompressed signal and the compressed, where 100% only the compressed signal is heard, and at 0% none of the compression is actually heard, which essentially acts as bypassing the effect.

There are also options for an expanded or collapsed view, with the collapsed showing only the essential controls; Ratio, Attack, Release, Threshold, Gain Reduction, Output, and Dry/Wet. There are other buttons to toggle Peak and RMS, as well as showing the Knee (which controls how gradually of abruptly the compression occurs when the threshold level is approached)

The expanded view, on the other hand, shows the Transfer Curve, which helps when setting the Knee parameter, as it provides graphical feedback on a grid. You can also use the Activity Display to see the input, output and gain reduction referenced as waveforms.

There are further tweakable controls within Compressor, so refer to Live’s manual if you want a detailed look at them.

Where Ableton’s compression chops really step up, is with Glue Compressor.

Ableton Live’s Glue Compressor

Glue Compressor is not available in the Intro or Lite Editions of Live, so if you’re stuck with those versions, this section may not be of much use to you, but read on anyway, as it may inspire you to upgrade in order to unlock the power of Glue Compressor.

The Glue Compressor is -as stated by Ableton - an “analog-modeled compressor created in collaboration with Cytomic, and is based on the classic bus compressor from a famous 80’s mixing console.”

What this means is that the virtual circuitry in Glue Compressor is built to emulate an analog outboard bus compressor, giving a more distinct tone and character to the sound. There are a lot of these software compressors floating around, modelling some of the famous analog outboard compressors before software took over.

While it can still be used in the same way as a standard Compressor; controlling basic dynamics on individual instrument parts or tracks, where Glue Compressor really shines is when it’s applied to a Master or a Group Track, effectively..wait for it...gluing the multiple sound sources together. Get it?

The Threshold control works in a similar way to that of Compressor, effectively dictating when compression begins. However where Glue Compressor differs is that it doesn’t have a Knee that is adjustable by the user. The Knee instead becomes more sharp as the Ratio is increased.

The Attack and Release controls work much the same as Compressor, with the Release having an ‘Auto’ setting, which adjusts automatically based on the signal of the incoming audio. This is useful when Glue Compressor is applied to a wide range of audio signals, being able to apply compression more gently to the whole range.

What makes Glue Compressor so great?

So, we’ve covered the basics of both Ableton Live’s Compressor and Glue Compressor, so you should have a better understanding of how they’re applied in certain contexts if you didn’t before, or if you did, hopefully it’s been a good refresher. We briefly touched on this previously, but we wanted to take a little bit of a closer look at where Glue Compressor really shines.

Find a track you’re currently working on in Live, preferably one with a group of drum tracks to work with.

Now, apply the Glue Compressor to this group, and gently tweak the compression without overdoing it, and you should hear the elements of the drums starting to come together to form a more cohesive sound.

Glue Compressor’s emulation of old SSL style analog compression adds a warmth and color to your tracks that’s somewhat missing from Ableton’s built in Compressor, as an increase in the make up Gain also applies subtle saturation for that emulated analog sound.

Don’t overdo it though!

A good technique here is to also play with emulating parallel compression using Glue Compressors Dry/Wet control.

By applying heavy compression -

- and dialling back the Dry/Wet control, we achieve a compressed sound without losing the dynamics. How this works is, you apply heavy compression to the audio to bring out the punch, at the expense of the dynamics.

Reintroducing the original signal alongside this, though, brings back the dynamic range of the audio, so you have an end result that gives both punch to the drums and dynamic range too.

You can also use this to your advantage by sticking it on your Master track. There are a handful of presets within Glue Compressor that give you a good look at the various applications and scenarios it can be used in. If you’re not sure exactly what controls to use for certain scenarios, these presets can be a good place to start.

Conclusion

Hopefully you learned some useful tips regarding compression in this article, and there are a lot of other things you can do extra reading on if you’re curious! Parallel Compression, Analog Compressors, Creative Side Chaining, to name a few. Compression is a broad topic, and there are plenty of third party compression plug ins to check out as well.