Most information I can get on the internet about mic placement is on specific placements for certain recorded instruments or sounds but I was wondering how those recommendations are decided on. Is it purely based off how it subjectively sounds or is there more acoustic physics and deliberate sound design behind the decision. Im just wondering so I know whether it is more effective to understand how to mic anything, or better to build a catalogue of instrument specific mic placements in my head to refer to when recording something. If you could link any helpful sources on mic placement that would also be very helpful.
This is a bit subjective but I would say that mic placement is a mix of physical laws (how a given instrument or source acoustically radiates, what is it's environment, what is your mic directivity) and subjective rules (what is the desired sonic output, the experience and taste of the sound recorder).
So learning a few standard patterns is always useful, but nothing matches experience and try and listen as far as mic placement is involved.