Arguably one of the most difficult techniques in mixing to master is the art of compression. Compression in its simplest form is used to get a more uniform average volume of a sound in order to maximize headroom in a mix, which will allow you to make your mixes louder in the long run. Used correctly, compression will make your track sound fat and powerful while still keeping the overall mix tight. Used incorrectly, a compressor can crush the life of your track into a dull, soulless cacophony. There are 4 main elements that all compressors share: the Threshold, Ratio, Attack, and Release. The Threshold is simply the volume level at which the compressor kicks into action. Whenever an audio signal passes above the threshold level, it tells the compressor to begin compressing. The ratio determines how hard the compressor actually compresses the signal. A ratio of 2:1 means that for every 2 dbs of audio that pass over the threshold, only 1 db will be outputted after compression. The attack tells the compressor how soon to begin compressing the signal after it passes over the threshold setting. A fast attack setting helps you clamp down on aggressive transients while a slower attack setting can allow you to maintain the initial punch of a sound while still compressing the rest of the signal. Lastly, the release is how fast the compressor resets to 0 dbs of compression after it is done compressing the last piece of the audio signal. Compression is an art that takes a long time to learn, but once you master it, your mixes will be tighter, fatter, and overall better sounding than ever before.