Once you have an optimized PA system and your mics selected and placed, it’s time to mix all those signals together.
How does one know in what ratio the channels sound be mixed?
Experience and studying the mixes of others. Given the cnoice, a large part of a day for me will include listening to music. Preferably music that has been mixed well. Interestingly, in general, I think human nature leads us to roughly the same place regarding “mixing”. No one would consider that having a vocalist completely buried in a mix is acceptable. No one would expect to see band members on a stage but not hear them play anything. Buford Jones recently stated during his Mixing Workshop that given the choice, you’re better off to hear everything you see at a show than to have a better mix but be missing things. I agree.
Let’s go with the “hear what we see” concept.
With a simple 3 piece band (drums, bass and guitar), the roles are well defined. It is as you start getting more complex in instrumentation that the lines get blurry. With multiple guitars, keyboards, background vocals, percussion, etc…how do all those signals blend with that original 3 piece band to become something coherent and well balanced?
I look at mixing like putting a puzzle together. It’s my job to figure out where a certain piece goes. Once I get a specific channel adjusted with EQ so that the signal is band limited to it’s natural frequency range and solved any imbalance due to mic selection / mic placement / etc…, it’s time to carve out a sonic space for that signal.
This can be done with panning. It can be done with level. It can be done with effects. It can be done with dynamic adjustments. You can’t fit every single signal into the sonic landscape all the time. This is one reason why we still need mix engineers. If the process could be automated, we’d all be out of work but that situation is likely never to arise.
There will always need to be a pair of well trained ears between the mixing console and the speakers.