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The two most important processors that the sound engineer uses when mixing are EQs and compressors.
In this article, we are going to introduce the concept of audio compression, look at why audio compression is necessary, and why we need it and then finish off by seeing what compressors do to our mixes with how compressors work.
This is not a how-to audio compression guide.
If you already have a decent idea of what compression is and are looking for a step-by-step guide, this is audio compression 101.
Let’s get started.
What is audio compression?
Audio compression is the process of reducing a signal’s dynamic range. That’s why it’s also known as dynamic range compression.
Compressing a track is very different from simply lowering and raising the volume.
Compressors are one of the more important tools an audio engineer has.
During a typical recording session, some degree of compression may be used on nearly every channel of audio as well as on the main output.
Having a good grasp of how compression works will help immensely in creating better mixes.
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What is the Dynamic Range? What are Audio Dynamics?
When recording sound, some parts will be louder, other parts will be quieter. This is called sound dynamics.
And often these differences should be there, like when the drummer hits a little harder during the chorus to give it more power.
The dynamic range is the difference between the loudest and quietest part of an audio signal.
Sometimes, dynamics are not intended.
Picture a guitar player strumming those strings a little too hard or a singer getting a little too close to the mic during a vocal performance.
This is where compression comes in.
What is audio compression used for? Why do we use audio compressors?
*Dynamic range compression is used so that both the louder and quieter parts of your audio signal can be heard clearly.
We use compression to even out the level of audio tracks.
Ideally, you need to reduce the dynamic range of most audio signals for them to sound natural when recorded.
Imagine a whisper and a scream on the same audio track.
If they were the same difference in loudness as they are in real life. 🙉 Unlistenable doesn’t even begin to describe it.
An uncompressed track sounds natural and unprocessed like you would hear if you were standing in front of the drum kit.
A compressor’s main job is to even out the level of an audio track.
Think of how a singer performs a song.
Sometimes they can sing loud, and sometimes really soft. When you mix the vocal track with other tracks in the band, it can be very difficult to hear the loud parts and the soft parts equally.
If you turn the volume up to hear the soft parts, then the loud parts will be too loud.
And if you turn the volume down, you won’t hear the soft parts.
If you’ve ever been to a concert and you couldn’t hear the singer, this is where a good compressor can save the day.
Compression makes mixing easier
Compression is the art of controlling dynamics over time.
With good control over dynamics, mixing will become much easier.
Everything from getting the vocals to balancing the bass guitar with the kick drum becomes much easier.
A compressor gives a track more punch
A compressor brings out all the details and nuances of an audio track bringing more life and energy to it, or as we say more punch.
The tone comes out more clearly cutting through the instruments.
In the context of the full mix, this matters a lot.
How audio compressors work
Compressors work by attenuating the loudest parts of your signals and then boosting the result.
A compressor will take the loudest parts of the audio signal and lower them to a pleasant loudness. It doesn’t lower the entire signal the way adjusting the volume would.
It only lowers the parts of a signal that are too loud.
Of course, the audio engineer decides what too loud is.
After compression, the quieter parts of the signal are more apparent, since the dynamic range has been reduced.
To understand compression, we have to talk about transient and dynamic signals.
Transient and dynamic signals
Transient signals are the initial high energy bursts at the beginning of an audio track.
Transients give the listener a lot of information about a track’s quality.
Dynamic signals are a mixture of transient signals and their decay.
We saw that compression is used so that both the louder and quieter parts of your audio signal can be heard clearly.
When you are dialing in a compressor you have to listen for the signal’s dynamic, not its timbre.
You focus on your transient and dynamic range.
The 5 basic compressor controls: Threshold, release, attack, ratio, and gain
The five basic controls on a compressor are threshold, release, attack, ratio, and gain.
Let’s start with the first four, seeing how each affects the audio signal, and then finish off with makeup gain.
These controls are the key to getting musical sounding compression.
The threshold sets the signal level where the compressor’s gain reduction starts working.
A lower threshold will apply a gain reduction to a greater portion of your signal. A higher threshold will affect only the most aggressive peaks leaving the rest untouched.
Anything below the threshold stays unprocessed, anything above the threshold will be compressed.
That means when the threshold is set to zero, everything will pass through unprocessed.
The more you lower the threshold, the sooner the compressor kicks in.
Attack and release are the timings of the gain reduction.
The attack control determines how fast the compressor reaches its full range of reduction when the signal passes the threshold.
In other words, the attack is the speed at which the compressor reacts once a threshold has been exceeded.
A shorter attack will make the signal reduction happen quicker. A longer attack will have a more gradual increase in the reduction until the level specified by the ratio is reached.
Slowing the attack
The attack needs to be slow enough so that the compressor doesn’t crash the transients.
If we attenuate them too much, they will lose their impact.
The release sets how quickly the gain reduction stops after the signal drops below the threshold.
It’s pretty much the opposite of the attack.
It controls how slowly the level returns to uncompressed once the signal falls back below the threshold.
The ratio determines how much gain reduction your compressor applies when the signal goes above the threshold.
The ratio controls the strength of the compression.
It’s called the ratio because it is expressed in comparison to the uncompressed signal.
A ratio of 4:1 means that if the input level is 4 decibels over threshold output, the signal will be compressed to 1 decibel over the threshold.
A ratio of 1:1 is the same as no compression since the input level is always the same as the output level.
On the other hand, a ratio of 8:1 would be considered a lot of compression since every 8 decibels over the threshold would be reduced to only one decibel over the threshold.
The higher the first number of the ratio, the greater the factor by which the gain is reduced.
You use the ratio to tailor the amount of compression so that it’s effect isn’t obvious or distracting.
The audio engineer decides how much they want to compress the loud part by adjusting the ratio.
#5: The makeup gain and level-matching
Another thing that’s important when adjusting your compression settings is the makeup gain.
The makeup gain is the amount of level you need to add to the entire track after compressing it.
This is because if we are compressing the louder parts to be lower and more even with the soft parts, then our entire track is going to be lower in volume after compressing it.
The gain is simply an output volume control.
Just like we saw in what is mixing and what is mastering, it’s important to level-match your work before and after.
In addition to these controls, in most compressors, you’ll usually find some form of gain reduction meter, which shows you much you are attenuating your signal.
That should give you a pretty good idea of what compression is and how it works.
Compression is a powerful tool that can easily do more harm than good.
It’s a huge part of any great mix, and knowing when and how to apply it properly will help your music tremendously.
Keep in mind that there is no right or wrong setting, whatever achieves the result you are seeking is the right setting.
Obviously, there is a lot more to learn on compression than what I’ve introduced here such as chaining and choosing different types of compressors and much much more.
This is just an introduction and there’s more awesome detailed content to coming on this.
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In the meantime, if you want to learn more about compression and mixing, feel free to jump back to the mixing blog.
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That’s it for this article.