Mastering Compression: Everything You Need to Know

The best mixing engineers and sound designers know their tools inside out.

Having a full understanding of the intricate details of a compressor will help you realize your sound goals when mastering.

A misunderstanding of mastering compression and therefore, bad settings, is where so many masters fall flat, and in the worst cases become completely ruined.

This blog will fill any gaps in your knowledge and help you get the best sound from your compressor in your next mastering session.

What is a Compressor?

In a nutshell, compressors reduce the difference between the loudest and quietest parts of the audio it's processing. They allow you to control, color, and manipulate the dynamics of audio. They’re powerful tools but using the wrong setting can suck the punch out of your music.

Balancing Audio with Compression

Using a compressor to balance out audio with a large dynamic variation will give a very uneven sound. The softer parts will sound unchanged and the louder parts will be squashed in an unnatural way.

Going back to the mix and automating a gain plugin is a much more natural approach to level out audio with large dynamic variations.

Once your channel has an even volume balance, you can use a compressor to thicken the sound and control the transients taking an artistic approach rather than a corrective approach.

Preserving Audio Character

Any compression you use should preserve the character of your audio’s transients. Even when you’re using a charismatic compressor with lots of color, your goal should be to musically bring out the natural dynamics of your signal.

As you’re tweaking your compression settings, think about what you’re trying to achieve. Are you adding character or adjusting the transients and dynamic range? How has the compression changed the relationship between the loud and soft parts of the audio? How have the transients changed? Do these settings slam the audio in too much of an obvious way?

Types of Compressors

Most mastering compressors employ an Opto circuit, which has slower movement characteristics and adds a nice pumping effect that beefs up the sound. As with all things audio, you can try different compressors and experiment with the different effects. But keep in mind that, often, tried and tested approaches are the go-to solution for many engineers for good reason.

Setting Compression Parameters

Threshold

Threshold sets the level at which the compressor will start reacting to the audio signal. If your audio is peaking at -10dB and the compressor threshold is set to -4dB, the compressor simply won’t react to your audio at all. If you wanted your compressor to only look at the peaks of your audio (using the -10dB example again) then you would set the threshold to around -13dB. If you wanted to heavily compress your audio then you would set the threshold to react to both the loudest and quietest parts of your audio.

Ratio

Ratio sets the amount of gain reduction. The higher the ratio, the more extreme the compression. To give you a ballpark idea, in the context of mastering, 1:1 is no compression, 2:1 is light compression, 3:1 is moderate, 4:1 will be substantial, 8:1 will be very noticeable, and anything above 10:1 can sound rather slammed unless used in tandem with a very light threshold. ∞:1 is limiting and nothing goes over the set volume.

This is how the ratio works: let’s take 3:1 as an example. For every 3dB of signal over the threshold, the compressor will only allow 1dB through. So if the signal is 6dB over the threshold, the compressor will reduce that to 2dB.

Attack and Release

Attack is the amount of time it takes the compressor to react to the incoming signal. If the attack is immediate or super fast, the compressor will catch the transients of your audio material. This can be really useful for when the transients sound a bit sharp. You can use a compressor to make the transient feel a bit blunter, and therefore a bit thicker. If you want to add some leveling to your audio but you want to leave the sharpness of the transients intact, go for a slower attack time.

A great starting point for the attack of your compressor is somewhere around 20-50 milliseconds. This leaves enough space for the short-sharp transients to poke through the mix without being so long that the compressor doesn’t react in time. You can tweak from this starting point and use your ears and reference tracks to help you dial in a setting that works well with your music.

Release is the amount of time it takes for the compressor to return to a non-compressing state. A fast release will mean the compressor will stop compressing quickly after the audio is no longer over the threshold. A slow release will mean it takes longer for the compressor to stop working once the audio is no longer surpassing the threshold. I like to set the release to work rhythmically with the audio that’s being fed into the compressor. I set the release to just long enough that the compressor returns to a neutral state before the audio re-triggers the compressor again. For a more obvious and pumping sound, you might choose to go for a longer release.

Knee

To fine-tune the characteristic of the dynamic reduction, you can look at adjusting the knee. Soft knee means the compression will be applied gradually as the signal approaches the threshold. Hard knee means the compression will be applied quickly as soon as the audio surpasses the threshold. Soft knees are usually more subtle and more appropriate for mastering.

Final Checks

As a final check of the compressor settings, preview how the master flows from the verse into the chorus and ensure it still has a positive dynamic impact. If the compressor clamps down too hard on that first beat of the chorus then you may need to increase the threshold to reduce the compression.

Avoiding Over-Compression

Over-compression is an incredibly fast way to ruin a mix. Squashing the transients can suck the dynamics and emotion out of a track and reduce the impact of a drop or chorus. To avoid this, dial in a low ratio of around 1.2:1 to 2:1 and a high threshold (relative to the signal) that gives only 2 or 3dB of gain reduction. By only tickling the signal in this way, the compression will be transparent and subtle. It will maintain the impact of the dynamic variation between the sections of your song. It will also help glue the sounds of your mix together, giving them a cohesive sound.

You can use the dynamic range analyzer in LEVELS to make sure your mix stays punchy. If the oscilloscope glows red then you may have over-compressed your track for your selected preset. Shooting for a lower ratio on your compressors can bring back some punch into your mix.

Advanced Mastering Techniques: Sidechain Input EQ with FabFilter Pro-C2

1. Targeted Frequency Compression

• Use the sidechain input EQ in FabFilter Pro-C2 to focus compression on specific frequency ranges. By isolating the kick drum's frequencies, you can apply compression that reacts primarily to the kick, preventing it from overwhelming the mix. This technique ensures the kick drum remains punchy without affecting the overall dynamics of the track.

2. Dynamic Control of Bass Elements

• Sidechain EQ allows you to compress bass frequencies dynamically in response to the kick drum. Set the sidechain to trigger the compressor whenever the kick hits, ensuring that the bass ducks slightly to make room for the kick. This results in a tighter low-end and a more balanced mix, with both the kick and bass maintaining clarity.

3. Transparent Midrange Compression

• Use the sidechain EQ to exclude midrange frequencies from triggering the compressor, focusing instead on the low-end and high-end. This prevents midrange elements such as vocals and guitars from being overly compressed, preserving their natural dynamics and ensuring a transparent and musical compression effect.

4. Precision in High-Frequency Management

• Apply sidechain EQ to manage high-frequency content, ensuring elements like cymbals and hi-hats do not trigger the compressor unnecessarily. By isolating the high frequencies in the sidechain input, you can prevent harshness and maintain a smooth and balanced top end in your master.

5. Enhanced Stereo Imaging

• Utilize the sidechain EQ to compress specific frequency ranges differently in the mid and side channels. This advanced technique allows you to control the stereo width dynamically, ensuring that certain frequencies remain centered while others are widened. This enhances the spatial quality of your mix without compromising its balance.

Using the sidechain input EQ feature in FabFilter Pro-C2 provides precise control over how different elements in your mix trigger the compressor. By strategically targeting specific frequencies, you can achieve a more polished, dynamic, and cohesive master, ensuring that key elements like the kick drum integrate seamlessly into the overall sound.

These advanced compression techniques can significantly enhance the quality and impact of your master, providing greater control and precision over the dynamic range and tonal balance of your mix. Experiment with these methods to achieve a more polished and professional sound.

Conclusion

Mastering compression requires a deep understanding of your tools and a precise approach to settings. By carefully balancing threshold, ratio, attack, release, and knee, you can enhance the dynamics and character of your music without compromising its integrity. Always reference your tracks and use tools like LEVELS to ensure you maintain the desired punch and clarity.