When producing, mixing, and mastering a song, compression and EQ are the most used effects. Knowing the elements of a compressor, what they are used for and how to have them set correctly for each instrument is key to getting a mix to sound present and consistant. In this article I have labeled the standard compressor that comes with Ableton Live 11 and will describe each part's use.
Threshold - The threshold is the level at which the compressor will begin to act. This can be set anywhere, but on a properly gain staged track, will be somewhere in the -6 to -18dB range. Setting the threshold properly will be a consquence of the genre of music, the instrument being compressed, the actual audio recording itself, and taste of course.
Ratio - The Ratio is used to set how much compression will take place after the threshold. In the photo shown the compression is set to 4:1, a pretty standard moderate setting. This means that when the input sound level exceeds 0dB (the threshold setting shown), then the compressor will start reducing the signal in a 4:1 ratio. That means that if your output before you turned on the compressor was 8dB, the output would be reduced to 2dB. Ratios can range from 1:1(no compression) to infinity:1(limiting, the sound will never exceed the threshold) and anywhere in between.
Attack - is how quickly the compressor will act. A faster attack means that the compressor will start working on the input sound much quicker, in some cases almost immediately. A slow attack (50ms or higher) will allow the beginning of a sound to pass by untouched and will then activate and compress the rest of the sound. This can be dialed in to control the effect of the compressor on the transients of a performance.
Release - is the control that allows you to decide how long the compressor will continue to work after the level falls below the threshold. A long release creates sustain. I short release will allow the compressor to turn on and off, an action which may be referred to as "pumping". Some compressors also have an "auto" setting, which can be activated and the compressor will choose a release setting for you. The auto-release is an easy way to find a pleasing release setting.
Gain Reduction Display - The gain reduction display shows the amount of gain reduction that the compressor is causing. This is simply a tool to allow you to adjust the threshold, ratio, attack, and release and to see the effect that each is having. Gain reduction can be anywhere from 0 to -1.5dB when compressing a master track. Gain reduction will usually be higher on instruments such as bass and drums, which also usually receive a lower threshold and higher ratio.
Dry/Wet Knob - The dry/wet knob allows you to mix between a fully compressed signal and the original, unompressed signal.
Makeup Gain On/Off - This can be used to toggle whether Makeup gain is activated. Makeup gain is a gain stage control that will allow you to recover some of the lost volume you may experience from compression. Additionally it will allow you to attenuate the signal if you have picked up extra dB from the compression process.
Makeup Gain Output - The makeup gain output is where you can control the level of makeup gain. Some volume may be gained or lost due to compression and makeup gain is another place where you can control your gain staging. Utilize this to help keep your faders in a reasonable working range.
Compression is a critical tool to utilize when mixing and mastering a song. Understanding the components of the compressor is key to using your compressor correctly and getting your tracks to stand out in the mix.