I will split this explanation up as I believe it will be easier to tell you about everything involved, so let's get started!
Mixing: The process of creating sonic balance and harmony by:
- Gain Staging (levels)
- Compression (taming instruments)
- Equalization (shaping instruments to live in their own space)
- Automation (used for automatic, real-time changes)
- Effects (creating spaces and more)
Gain staging is the first process involved with mixing after cleaning up any silence on tracks. What this accomplishes is two things:
- Establishes proper headroom in this digital domain
- Starts the mix
Headroom is significant in mixing due to the fact that there is so much room dynamically that you do not need to record or mix very hot as one might do using tape machines and big consoles for pleasant saturation. If levels are too hot in the digital realm, you risk clipping and destroying the integrity of the audio. You really only want to reach clipping with older consoles that saturate nicely, otherwise, avoid clipping at all costs other than specific creative purposes.
Here, you can see the levels of all the tracks which are hovering around -18dB, this particular level offers the most benefit while mixing as it leaves plenty of headroom for the mastering stage and establishes proper gains for the instruments to avoid clipping and distorting.
Essentially, the goal of gain staging is to begin the “rough mix” which is just a term for general mix balance. Once established, compression and EQ come into play to really begin shaping the sound field. Mixing is really the art of balance because as engineers we are constantly going back and forth between various EQ plug-ins, fine tuning levels and effects to achieve the best spectral balance we can possibly with the material given to us.
There is not too much more to say with get staging except that it is necessary in a lot of cases, especially in those cases involving tracks that come in around -5dB all over. That's just way too hot and completely unnecessary in today's production world. That being said, all of us engineers mix completely different from one another and have different views, a lot of audio is subjective but a lot is also generally agreed upon.
In short, compression simply tames loud spikes to get a more even and smooth level response. It can also be used to bring lower information UP as well.