Sidechain processing is using an external signal to trigger devices, for instance the sidechain compressor. When sidechain is engaged, you may route another tracks output to the compressor that is located on a different track, once this external track is played, the compressor will engage causing the sound to “duck” beneath the other. Once the externAl track has stopped playing sound, the compressor will disengage, allowing the tracks full volume to be present. Sidechain compression is best utilized on sounds using very similar frequency bands that cause masking when played together.
A common case of sidechain compression comes from the kick and the bass relationship. Both of these instruments dominate the low frequency ranges and often cause problems when combined and played over top of one another. These problems include frequency masking, a surplus of amplitude or the phase cancellation of similar frequencies rendering a loss in sound to varying degrees.
The instrument or track that has a shorter existence should be the external signal that triggers the compressor, thus the compressor shall go on the other, opposing track. In the kick and bass example, the kick drum is a very short, intermittent sound while basslines tend to sustain for far longer, therefore, the compressor will be used on the bass track while the kick drum is used to trigger the compressor. In theory, every time the kick drum occurs, the compressor will bite down on the bass track for that duration (based on attack/release times too) and once the kick drum has passed, the compressor will disengage allowing the bass track to come to life. This style of sidechain compression should be practically unnoticeable unless the desire for a big-room pumping effect is there, in which case having the compressor linger around for an extra few milliseconds can create an exaggerated pumping feel.
Dynamic EQ is relatively similar to sidechain compression but typically these devices mirror more of a multiband compressor, allowing you to attenuate a specific band of frequencies as opposed to the entire spectrum. A standard compressor can be referred to as a single band compressor.