The primary purpose of a compressor in music production is to control dynamic range. By reducing the difference between the loudest and softest parts of a piece of audio, a greater average recording level can be used without the risk of clipping the signal.
Side-chain compression is a very widely used technique amongst music producers and sound engineers alike. Instead of using the compressor’s Threshold setting to trigger activation, audio is fed into its side-chain input. When the signal feeding the side-chain becomes active, the compressor starts to compress whatever audio source it is strapped across.
A good example of this is at a Radio station. Whenever the DJ speaks into a microphone over any music being played, the volume of the music is lowered so that focus is given to his/her voice. When they finish speaking, the volume of the music returns to normal. This is achieved by running a compressor over the music track, and feeding the microphone into its side-chain input. Via the side-chain, any signal from the microphone (eg. the DJ speaking) triggers the compressor to reduce the gain of the music. The amount of gain reduction is configured using the compressor’s Ratio control. The higher the ratio, the harder the compressor will work to lower the volume.
A more practical application of side-chain compression in music production is to create space in a mix to fit a key sound such as vocals. A typical example is where a vocal track occupies many of the same frequencies as a lead instrument. When mixed together, the shared frequencies may mask each other, essentially ruining the clarity of either part. To resolve this issue, a compressor could be strapped onto the lead, with the vocal track being fed into the side-chain inputs. When the performer on the vocal track sings, the compressor is activated and lowers the volume of the lead instrument. This creates enough frequency space in the mix for the vocal to really cut through.radio strap
Another common use of side-chaining in dance music production is to prevent a bassline and kick drum from sharing the same frequency space. If the arrangement calls for them to play on the same beat, the crossover in the low frequencies could cause massive problems in the mix. Side-chain compression can help to overcome this problem by routing the kick drum signal into the side-chain inputs of the compressor on the bassline channel. Whenever the kick drum fires, the compressor will be activated to duck the bassline out of the way.
Many producers even create a secondary kick drum track to feed the side-chain inputs of various compressors. The output of this track is not routed to the Master Bus, as they only require the audio to feed the side-chains. By using this method, the producer can perform any automated EQ or volume adjustments to their main kick drum track during the course of their arrangement, whilst maintaining consistent side-chain compression on other parts triggered by the secondary kick track.