Compression is a tool used in mixing to even out the dynamics of certain instruments or tracks in a mix. This type of compression is called dynamic range compression. These types of compressors reduce the louder parts of an input signal, this is called downward compression, or the increase the quieter signal, this is called upward compression.

There are three controls on a compressor that are almost always standard. These controls are threshold, ratio, and makeup gain. You may also see controls for attack and release, as well as soft knee and hard knee controls.

Threshold is the control on a compressor that sets a decibel level that the audio will be reduced to. The ratio is the amount of reduction that your audio is going to receive. If your ratio is 2:1 then if your input audio is 2 decibels over your threshold then your output will be 1 decibel over your threshold level reducing your audio by 1 dB. If your threshold is -6 and your in put is -4 then the output audio will be -5dB. The audio now is quieter then before, to get the signal to the decibel level it was at the make up gain control will bring this back to the original decibel level. 

When compressing an audio signal we condensing the signal for a couple reasons, to make the notes or transients have a better consistency in the mix and to bring up any quite notes that may have been lost in the mix before.So while bringing the audio back up what has happened is that now you have an audio signal that has less dynamic range then before but has not lost any volume in the process.

There are other controls you may see on a compressor as well, attack and release control how fast or slow your compressor reduces that audio files or how fast or slow in release the audio over the threshold from reduction. For example if you have a drum mix and you want it to be a little more punchy you could have a faster attack on your stereo compressor. Soft knee and hard knee control how smooth your signal goes from uncompressed to compressed, a soft knees make this change slower than a hard knee. With higher ratios you may need to add a soft knee and adjust your attach and release so you don’t get a pumping or breathing effect on your audio.

Compressors also help in your mix to control your headroom or peak level. Many mastering engineers will use a compressor as a limiter, to act as a threshold point that wont let audio get any louder than that threshold. this let you turn the make up gain up and still have an even meter. This is so that they can get to the same perceived loudness as other commercial mixes.