One of the most important tools available to a music producer while working on a song inside a studio is compression. Compression is used to control the level of dynamics within a song. 

A compressor engages itself and brings down the volume/level of the audio signal when it exceeds a set threshold value.  There are various controls to a compressor such as threshold, ratio, attack, hold, release, gain and bypass using which one can control how the compressor behaves.

In many cases, dynamics within the song is generally considered to be a good thing but taming and controlling the difference between the loudest and the quietest parts in a song may lead to increased presence and more clarity.

This article gives you an introduction to compression and its various controls. 

http://www.songstuff.com/recording/article/compression/

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