Mixing and Mastering/Sidechaining
Sidechaining is an often misunderstood tool in the mixers bag that can be used to fix a variety of sonic challenges in a mix, and to shape sounds creatively as well. Technically, sidechaining is simply using a compressor on a track, but instead of the compressor listening to the input, the compressor is 'listening' to a different audio source altogether. There are many situations where it can be useful to control the volume of one track by responding to the volume changes of another.
Ducking - Ducking is often used in producing commercial spots. The compressor listens to the announcers track and 'ducks' the background music underneath the announcer so that the volume of the music bed is raised when there is silent pauses in the announcers performance. If set properly, the effect is that the music is loud but is pushed under the voice keeping the words out in front.
Ducking can also be used to keep the low end from 'piling up' in heavy dance music. The bass track 'listens' to the kick track so that the volume of the bass is lowered momentarily to allow the kick to dominate the bottom on the kick drum's attack.
Another popular trick is to put several rhythm guitar tracks on a bus, then set that bus compressor to 'listen' to another rhythm guitar track. This will subtly 'tighten' all the rhythm guitars together.
Sidechaining can also be used to tame a problem frequency that only occasionally pops out of a track without gutting that frequency from the entire track. A more advanced trick is to add an eq in before the sidechained compressor and exaggerate a problem frequency. A copy of the track with the problem is made and given a separate track. When the two are sidechained together the track will play normally until the 'problem' pops up in the drum or vocal part. When the annoying frequency happens, the track is lowered momentarily without having to pull that frequency out of the entire track. This is often used on vocal tracks to tame the sibilance (high frequency content in 's'es and 't's). This is also called 'de-essing'.