Everyone has a different process when it comes to mixing. Each individual producer or engineer will use different tools and plugins to achieve what sounds to them as a balanced and cohesive mix. What makes a good mix is always subjective, and will differ from engineer to engineer. At its core, a good mix is when each instrument has its own presence, but is not clashing with any other instruments. Everything should be balanced throughout the stereo field, but should also sound clear when in mono. Finally, everything should work together harmonically.
Mixing is a long process that can be hard on the ears, and for that reason can take a long time to complete. In fact, sometimes it’s quite hard to know when a mix is actually done. Every song will have a different mixing process, and with advancements in technology, the options can sometimes be overwhelming. When you think that your mix is done, or at a point where it can be shared, it is always best to get a second set of ears to listen to the mix. Listening to it for too long on your own can not only accustom your brain to the sound, but can also damage your ears.
Mastering is the final step of the mixing process, but most of engineers see it as its own process, completely separate from mixing. Mastering should not be used to fix a mix – it’s only after perfecting the mix that the mastering process starts. If you’re trying to fix your mix with a quick master, then your mix still needs work. On the other hand, if your mix is perfect, then you probably don’t need to master at all.
The purpose of mastering is to optimize a track for all sound systems and streaming services. Sometimes a track may have multiple masters for different streaming services such as Spotify or Soundcloud, both of which process your audio differently upon uploading. Mastering can add loudness and can also enhance a song’s stereo image to make it wider across the frequency spectrum, using tools such as maximizers, compressors, and imagers.
With mastering, it’s very easy to overdo it. Some engineers swear by the “keep it under 0dB” rule, but I think it depends on the track. Just make sure that your track isn’t distorting, and use your ears! You can get very technical when it comes to mixing and mastering, but at the end of the day, if it sounds good then it sounds good.