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. If you want to have authority over the compressors you use and understand how to use compression to make your mixes sound better, then this post is for you.
In a nutshell, compressors reduce the difference between the loudest and quietest parts of the audio its processing. They allow you to control, colour and manipulate the dynamics of audio. They’re powerful tools but using the wrong setting can suck the punch out of your music. So when should you use a compressor and how should you tweak the parameters to work with the material?
The first step is to decide whether the compressor the right tool for the job. 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. 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.
Any compression you use should preserve the character of your audios transients. Even when you’re using a charismatic compressor with lots of colour or applying intense settings for obvious effect, 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 dynamics 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?
Think about the vibe and purpose of what you’re trying to achieve. The table below will give you an idea of which compressor is the right tool for the job.
Compressor Parameters
What does threshold mean in a compressor?
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 (-10dB 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.
What does ratio mean in a compressor?
Ratio sets the amount of gain reduction. The higher the ratio, the more extreme the compression. To give you a ballpark idea, 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. 
You can use the dynamic range analyser 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.
What does attack mean in a compressor?
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 levelling to your audio but you want to leave the sharpness of the transients intact, go for a slower attack time.
What does release mean in a compressor?
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. For example, If I’m compressing a kick, I will 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.
What does Knee mean in a compressor?
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.
Input / Output or Gain: The compressor is effectively turning down the louder parts of the audio and leaving the quieter parts untouched, thus reducing the difference between the louder and quieter signals. So this will turn down the overall volume of that channel. So you’ll most likely want to use the output or makeup gain to bring the level back up so it works well in the mix. I like to bypass the plugin on and off to make sure the channel is still well balanced in the mix.
Fast attack and release times can sometimes cause low frequencies to distort. Low frequencies have long waves so the fast attack causes the gain reduction to begin working within one cycle of the wave causing it to clip. The problem is, sometimes you need the fast attack time to attenuate the transient of the audio. My preferred fix for this is to increase the ‘lookahead’ time. This means the compressor begins the gain reduction a few milliseconds before the transient, allowing the low frequencies to pass through undistorted.
Conclusion
Compression is not a simple process, but it’s made much easier when you understand the fundamental principles of the controls. If you found this post useful then you’ll love my 138 page eBook on songwriting, mixing and mastering. Click below to find out more.
