Compression, the bane of any young producer’s existence. Hundreds of videos and articles written about the subject and you still can’t quite grasp it. Don’t worry, you’re not alone. We’re here for you to help clear the air.
DYNAMICS
Before we get into the crux of compression, we need to discuss dynamics in audio, or changes in the signal volume over time. You’ve probably heard the term before. Another useful term for our purposes is dynamic range, the difference between the loudest and quietest parts of an audio signal. You’ve probably heard of the loudness war, this is where dynamics and dynamic range are crucial.
Nevertheless, dynamics are vital to the characteristics of any sound source, and the ability to manipulate the dynamics of a sound is one of the key tools in an engineer’s tool belt.
WHAT IS COMPRESSION?
So, what is compression? Well, to put it simply, compression is the process of reducing an audio signal’s dynamic range. What that means is a compressor, when dialed in properly, will tame the loudest parts of a signal while boosting the quietest.
Think of a kick drum.
A kick drum will have a loud initial transient in the waveform followed by a relatively fast decay. Using a compressor, we can tame that initial transient of the kick, while also boosting the signal of the sustain. So, we go from a kick drum with a broad dynamic range to one with a fuller body and less dynamic range.
However, you will walk a fine line. A track with too many transients and not enough body won’t be interesting to the ear, and on the other side of the coin, a song with no dynamics is lifeless and fatiguing. Your goal with compressing any audio signal should be finding the perfect balance between good dynamic range and a full body.
COMPRESSOR SETTINGS
Most compressors will have some variation of these basic controls: Threshold, Ratio, Attack, and Release.
THRESHOLD
The compression threshold is the level (in dB) in which the compressor begins to act upon the audio signal. So, if our threshold is set to -18 dB, this means the compressor will begin compressing any audio signal that goes above the -18dB limit we’ve put in place. The amount of signal that gets compressed is determined by our next parameter, Ratio.
RATIO
Nearly every compressor comes equipped with a ratio setting; and luckily, it’s pretty self-explanatory. We’ll begin with a common ratio setting, 4:1. Remember our threshold setting from before? -18dB. Well, if our audio signal rises above -18dB, thus activating our compressor, the signal above our threshold is now being compressed by a ratio of 4:1. Or, in other words, any signal that exceeds 4dB above the threshold will be reduced down to a measly 1dB.
Typical ratios you’ll see will be 1:1, also known as no compression (you would probably use this setting to get the tone or warmth of the compressor), 2:1, 4:1, 8:1, 10:1, and inf:1. Practically any ratio that is above a ratio of 10:1 is getting into the territory of limiting, and inf:1 is called “brickwall” limiting, meaning that no amount of signal will ever go above the threshold.
A quick note on knees. Some compressors come with a “knee” setting. A knee is the smoothness with which the compressor reacts. A “soft knee” is typically more subtle and behaves more realistically than a “hard knee” setting, but experiment with your audio signal to find what best fits your taste!
Now that you’ve set your threshold and ratio, how do you dial it in to fit the audio signal perfectly? The answer, and the most challenging aspect of getting compression correct, lies in the Attack and Release controls.
ATTACK AND RELEASE
The attack and release controls both affect the timing of your compression. Attack controls how fast your compressor reacts to its audio signal, while the release setting controls when the compressor stops acting on your signal.
You can see by the above illustration the difference between a fast and slow attack time. So, let’s go back to our kick drum. Imagine if our drummer played a little inconsistently throughout their performance. By setting your compressor to have a fast attack time, we are squashing those initial transients that the drummer hit too hard and helping to level the signal so we’re not overpowered by that sudden burst of air, detracting from the track.
On the opposite side of that, we can look at our release time. A fast release time means the compressor will stop compressing the signal quickly once the audio is no longer over threshold, while a slow release time means the compressor will take longer to return to a non-compressed state once the signal is no longer over threshold. A popular method for setting release times is to work musically, watch your VU meter to have your compressor release fully just as your next transient is about to hit. This way the compressor is working rhythmically within your track.
FINAL THOUGHTS
Compression is not a one-size-fits-all process. Every track, every instrument, is different. Often, you can better do the task of a compressor by automating the volume of a particular section.