Compressors control the dynamic range of a signal, changing the relationship between the loudest part and the quietest, it is a very useful and essential tool when used right, it helps create clear and punchier mixes and can also be used on many other individual parts including drums, vocals, synths, bass, aux, mastering/ full mixes etc.
A compressor's main function is to reduce the dynamic range of a signal by reducing the volume of its loudest point/ transient.
Compressors are really effective whilst grouping sounds together, a example would be on a return buss/ Aux buss, they help glue parts together into a cohesive block, for example a drum buss, taking the loudest peaks and reducing them whilst giving the quietest sounds and making them louder, this works well to get the tails of snares, hats etc.
One compressor doesn't fit all, this is so true when looking at compressors, there are not all made to equal each other as some compressors use 2 compressors in series to achieve a unique blend of compression as the Shadow Hills Master Comp shows below
The Shadow Hill Compressor has 2 compressors in series with each other, 1st is a gentle optical section with a set ratio, attack and release (Optical) & then a more aggressive and precise VCA (voltage controlled amplifiers) section (Discrete) each section can be bypass, you can combine these 3 ways, The output allows you to choose from 3 distinctive transformers Nickle (clean), Iron (coloured) & Steel (dirty), Sidechain give us a high-pass filter letting anything below 90hz through.
Every compressor has its own signature, configuration or algorithm which sets them all apart. Software emulations like the UAD-2 version compared to the hardware version is near enough exact.
The pictures below show iconic classic compressors that have shaped our music every since the 60's.
Each one of the above is a emulation of a classic hardware compressor, there are many companies who have emulated these & done a really good job in doing so, but again non sound the same as each other very similar but not the same. If you would like to know more about the setting of the compressor click HERE to be taken to the effects page where you can learn more and about other effects.