How Does Audio Compression Work – Music Production 101

How Does Audio Compression WorkIf you’ve ever wanted to know how audio compression works in music production, and how it is used by audio engineers and producers in their mixes, then this is the post for you!

In very simple terms, you will understand everything you need to know about audio compression.

We’ll also discuss how it is used, and give you a few little fun exercises to work on while you read.

Let’s get started!

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What is a Compressor in Music?

A compressor is a device used to apply audio compression on the dynamic range of an audio signal. This device could be either a software plugin (called a VST) or hardware plugin. In either case, they work the same way.

Of course, to the uninitiated, this definition may mean practically nothing. I’ve simply thrown some jargon at you. Hence, to go further, we need to understand what we mean when we use the terms “dynamic range” and “audio signal”.

How Audio Compression Works

The Dynamic Range & Audio Signal

Each piece of audio you make or record varies in loudness. We can call this amplitude. This loudness, or amplitude, is measured in decibels, or dB for short.

In an audio signal, we can have a varying degree of loudness amplitude passing through our audio cables. It can be a super loud train station, or it can be a quiet wind on a winter morning. It is this variation in this loudness and softness that we define as the dynamic range of an audio signal. An audio signal being that which carries the sound data (electrical or digital) of these sounds.

Let’s say you plug your guitar, violin, microphone, or whatever, into your computer DAW via audio interface. From that audio source, you will get a range of sound varying between quiet and loud whenever you play your instrument or speak or sing into the microphone. This is the “dynamic range”. It is dynamic because it is always changing over time – in other words, it is not static.

What is Audio Compression?

Now that you know that any audio signal that transmits over a wire will have a dynamic range in loudness (amplitude) measured as dB, we can understand that there are cases in which we would like to control how wide that dynamic range is. In other words, how much we can change the difference of loudness and softness of that audio.

We can do this by raising the quietest sounds or reducing the loudest sounds. Or we can do both at the same time. Either way, this causes us to “compress” the audio signal range from wide to narrow.

This is the very basic function of a compressor in music production. Let’s say you’ve decided to add a compressor plugin to a track in your song. You do this because you want to, for valid reasons you will come across in your production and engineering, reduce the highest sounds at their peak, in order to raise the lowest sounds at their quietest. Usually, this reason would be that you wanted to change the loudness perception of an instrument or track, where otherwise the instrument may lack ‘presence’ in your mix.

But that, of course, is an oversimplification of a compressor’s function. To really know what is a compressor, you’d have to get acquainted with its basic functioning and some terminologies, as all audio compressors will be able to do a few things that are very necessary for music producers and audio engineers.

What all Audio Compressors Do

We’ve up to this point discussed the reduction of loudness and raising of quietness. The result of doing this “compresses” the dynamic range from wide to narrow, hence the name. To get a better idea of what it does, let’s examine the basic functions you’ll find in any audio compressor…

The Audio Compressor – Basic Functions & Terminologies

Each audio compressor has a few sets of controls which you can use. These will be knobs, switches, slides, or some other fancy way of controlling them.

Here’s a task for you to do: Open up your digital audio workstation, look up a basic compressor plugin (you can just choose the one that comes with your DAW, that should work fine).

Now, look for these words and their corresponding controls on your compressor plugin:

  1. Threshold
  2. Ratio
  3. Attack
  4. Release
  5. Knee
  6. Make-Up Gain
  7. Output

Since you’ve done that, we’ll take a look at what each does, one by one.

1 – Threshold

Threshold determines how loud your audio signal must be before your compressor is allowed to reduce its volume. This means that your compressor will not do anything to your audio signal until your signal’s amplitude reaches the threshold you’ve set. Sound, in other words, needs to get loud enough for the compressor to kick in. You determine what “loud enough” is.

2 – Ratio

This control determines how much compression will be applied to the audio signal once that loudness threshold is passed. In other words, how much reduction in gain (loudness) will be applied to the audio signal once it gets “loud enough.”

Graph of various compression ratios on an audio compressor
Various compression ratios on a compressor

For example, if you use a ratio of 4:1, the audio signal would have to go 4 dB above the loudness threshold which you set in order to increase the amplitude by only 1 dB. This means the audio will be “compressed” downward by 3 dB at its loudest points. Think of it like someone’s at the volume knob turning down the stereo every time it gets too loud.

With a 2:1 setting, 2 dB becomes 1 dB. With 4:1, 4 dB becomes 1 dB. With 6:1, 6 dB becomes 1 dB. You get the idea…

3 – Attack

Attack is how fast the compressor will reduce loudness. On a lot of processors, this is measured in milliseconds. But it’s best to just think of it in terms of fast and slow, and use your ears.

Try this

  1. Load up a track in your DAW and set it to loop. It can be some vocals, an instrument, or a song by your favourite singer, doesn’t matter. But this works best if it’s a track that has a lot of variations in a short space of time between loud and soft.
  2. Place a compressor plugin on the track.
  3. Now set your threshold (the point at which your compressor kicks in).
  4. Set your ratio (how much compression is applied).
  5. Set your attack to as fast as possible, and then as slow as possible…
The attack and release phase graph on an audio compressor
The attack and release phase for fast and slow

What you will begin to hear is how fast your compressor will be at reducing the loudest parts of your audio signal. That is the attack setting.

In our example of the person operating the stereo knob, let’s imagine that on a fast attack setting, he’s turning down the volume very quickly as soon as the music gets loud enough. But if the setting is slow, once the volume gets loud enough, he will take his time to turn it down gently.

4 – Release

This is like attack but it works the other way around. Your signal is not going to be above threshold all the time. When it dips in loudness below the threshold point, the compressor needs to know if it needs to stop compressing that audio signal immediately or gently.

Fast release will give your audio a very quick jump back in volume when your track gets quieter (or goes below the threshold point).

A slow release will give your audio a gentle return to its normal volume.

Continuing with the guy at the stereo knob analogy, imagine that a fast release setting simply means that he turns up the volume back up quickly as soon as is gets quiet. But on the slow release setting, he takes his time to turn up the volume slowly and very gingerly.

Either setting works great depending on the style of music you’re working with.

Continue to experiment with the release settings on your compressor.

5 – Knee

This is similar to the attack and release settings. This smooths out the ratio that you set the threshold to.

Once the audio signal begins to approach the threshold at which the volume is to be reduced or returned to normal, a “soft knee” will begin to gently increase or reduce the volume based on your ratio setting. If it is a hard knee, compression attack or release on the loudness will be implemented immediately, once loudness reaches the threshold point.

This control works well with attack and release settings. If you set your attack time to slow, for instance, you’ll notice that when there is a peak in loudness approaching, it make take some time for the compressor to begin reducing. But with a soft knee, the audio will gradually begin lowering the audio as the audio signal amplitude approaches the threshold. This makes the change in volume less noticeable to the listener.

Graph showing a soft knee and a hard knee on a compressor's threshold
Example of a soft knee and a hard knee on a compressor’s threshold

Try playing with various settings between soft and hard knee, fast and slow release and attack times to get a feel for how it works.

Take again our example at the stereo knob. Using a “soft knee” as an example (for the purposes of explanation), when the guy at the stereo starts to hear the music getting louder or softer, instead of waiting for the volume to reach a certain threshold in loudness to turn up or down the knob, he keeps his hands on the knob, and makes little adjustments to the volume based how close the music gets to that threshold point, so that every change is nice and smooth.

6 – Make-Up Gain

Since the role of the compressor is to, well, compress, the audio signal, you will naturally end up with an overall quieter sound. To fix this, a make-up gain control is used to “make-up” the amount of gain within the compressor that has been lost from all that amplitude attenuation.

7 – Output

Similar to the make-up gain, this simply allows you to increase or decrease the sound coming out of the compressor to a desired level.

Bonus Feature – the ‘Look Ahead’

This is found on many compressors. Working with live audio isn’t perfect. You may have the perfect setting for you attack, release, and knee. But you may find a transient or sudden dip will always appear in your source audio, which means you’d need to “ride the fader” if you’re in one of these situations.

The look-ahead feature simply creates another setting to catch when these things happen. But in order to do this, the signal would have to be delayed just a little in order to perceive the change. Hence, it doesn’t so much “look ahead” but sort of looks back to make adjustments from creating a delay to keep it ahead of the output signal.

The Uses of an Audio Compressor in Music Production

Basic Usage

Ironically, despite the fact that a compressor’s purpose is the “reduce” the dynamic range, therefore reducing the volume before make-up and output, compressors are actually used to increase the ‘perceived loudness’ of an audio signal.

This is done by reducing the loudest parts of a track so that you can increase the overall volume of the audio, this allows for the quieter dynamics to come through, giving you an overall louder signal. The best example of this use is in mixing drums and getting a bass under control so they can be heard.

In music production, this is used to give audio signals a more consistent volume range in a mix. A voice, for instance, may require a compression plugin in order for it to stand out and be consistently noticeable, no matter how loud or soft the music gets. A good compressor that pairs well with vocals, along with the right settings to use while recording and mixing, will help a lot with that, as well as provide well needed dynamic “control.”

A compressor may also be used on instruments that don’t have a very long sustain. A drum kick or snare, for instance, is loud but it doesn’t last long. A guitar pick is similar, as well as the higher notes on the piano. A compressor will reduce to loudest part (the instant a note is struck), so that the volume of the tail end of the note can be increased, giving it a longer, fuller sound.

Many producers love to use this in their snares to get that nice, fat snare sound, or on kicks to give the kick more heaviness and roundness.

Sidechain Compression

Another very popular technique used by producers, which you’ve probably heard of, is called sidechain compression, or simply, “sidechaining”.

If you listen to a lot of electronic music, especially house music, you will notice an effect where there is a consistent pumping going on; a “dipping” or “sucking out” of sound on the beat. This is achieved by putting a compressor on a bass synth or an instrument bus, then routing the track of a kick, snare, or some other interval instrument with a high transient and fast decay into the compressor on the bass synth track or instrument bus.

The compressor then acts like it is compressing the drum or snare, but in actuality it is compressing the bass synth or instruments, causing it to “duck” with each drum or snare hit.

Listen to this track by Minnesota called ‘To The Floor’ (turn down your volume). At the 45 second mark, you’ll hear the kick squeezing out the rest of the sounds in the track. That is called sidechaining.

The original purpose for which audio engineers applied sidechaining to tracks was to keep the kick and the bass frequencies from interfering with each other. Since they process similar frequencies electrical audio signal, they can result in an unpleasant muddy sound that doesn’t naturally occur in the real work when they play together.

To remedy this, either the bass or the kick would be forced to duck under each other whenever they both played together. But now, this technique has become an electronic and dance music standard, as it creates the pumping throbbing effect.

Final Thoughts

And that is what an audio compressor in music production does.

In this post, you learned that a compressor alters the dynamic range of an audio signal by attenuating (reducing) the amplitude (loudness) of the loudest section of an audio signal. In doing so, a producer or engineer can increase the gain and output volume in order to bring the quietest part of an audio signal up to a more desirable volume.

You learned that, with audio compression, a producer can alter the “perceived loudness” of an audio signal by making the audio seem louder. Hence, you understood that it could be used to give an audio track more presence or attention in a mix, as well as “thicken” an instrument’s sound by increasing the volume its decay.

You’ve familiarized yourself with the various important terminologies and controls of the compressor plugin in music production: threshold, ratio, attack, release, knee, make-up gain, output, and look-ahead, and you also learned how to used them.

You also learned of a technique called “sidechain compression” or simply, sidechaining, which you can use to cause a track, or group of tracks on a bus channel, to “duck” under the sound coming from another track, usually, a snare or drum hit.

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