What Is Compression?
A compressor is an audio dynamics processor that reduces the dynamic range of audio signals passing through it.
Dynamic range refers to the difference between the quietest and loudest part of a signal. The most basic use of a compressor is to reduce the dynamic range by turning down the loud parts in relation to the quiet parts. This has the effect of smoothing out the signal level throughout the performance.
The action of compressing a signal alone will reduce the overall signal level. But when you boost the signal after compressing it, you’ll be left with a louder signal than you started with. That’s because you’ve reduced the loud sections in relation to the quiet sections and you’ve turned everything up together.
How To Use A Compressor
Let’s take a look at the controls you’ll commonly find on a compressor and how to use them.
In the video above, I use one of my favorite compressors to demonstrate compression – the Fabfilter Pro-C 2. The reason I love this compressor is because it sounds great, it’s very flexible, and the metering is unmatched.
The threshold is the point at which the compressor will start compressing.
I want to take a moment to mention that some compressors don’t have a threshold adjustment, but instead have a fixed threshold. This is the case with the Universal Audio 1176. Instead of bringing the threshold down to the signal level, with these compressors you’ll boost the signal level up to the threshold using the input gain knob.
The ratio determines how much compression will occur when the signal level exceeds the threshold.
Gain Reduction Meter
The gain reduction meter tells us how much the compressor is actually reducing the signal level.
We’ll see more gain reduction anytime we lower the threshold or increase the ratio.
Using the Threshold and Ratio controls alone just turns down the signal. You might be thinking, “I thought compressors are used to make things louder.” This is where the makeup gain control comes into play.
The makeup gain is used to turn the overall signal level up to make up for the gain reduction we’ve applied with the compressor.
I find it helpful to think about it this way… If we want to make the quiet parts louder, we could just turn the overall signal level up. But then the louder parts would exceed 0 dB, which would cause clipping in a fixed-point system or distortion in an analog system. This is why we use the compressor to turn the loud hits down and use the makeup gain to turn everything up.
The attack knob determines how quickly the compressor will begin compressing once the signal level exceeds the threshold.
With a fast attack time, the compression will begin almost immediately after the threshold is exceeded. With a slow attack time, there will be a slight delay before the compression kicks in.
Sometimes you might want to let the transient through and only compress the tonal part of the signal. The transient is the high-amplitude, short-duration sound at the beginning of the waveform. The tonal part of a sound is what comes after the transient.
Using the attack on a compressor, you can decide if the compressor will quickly clamp down on the transient or if the compressor will let the transient through and only kick in during the tonal part of the sound.
The release knob determines the time the compressor will take to recover after compressing.
A fast release time will recover more quickly, while a slow release time will take longer to recover.
The hold adjustment controls how long peaks in gain reduction will be sustained.
While the release usually begins directly after the peak in gain reduction, the hold time can push that back so that the gain reduction is prolonged before the release begins.
The knee setting can be used to soften or harden the compression around the threshold.
A softer knee will create a more gradual and transparent compression, generally speaking.
Wet / Dry
Many plugins have a wet/dry knob for parallel compression. The wet signal is the signal after compression while the dry signal is the signal before compression.
You can sometimes achieve a more realistic product by blending the two together – preserving the transient of the dry signal and the thickness of the compressed wet signal. This is called parallel compression – I’ll go into more detail in a future video.