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.
https://en.wikipedia.org/wiki/Stereophonic_sound#Recording_methods describes standard stereo mic placements including issues with post-conversion to mono signal (e.g. playing on the radio):
- A-B technique: time-of-arrival stereophony
- X-Y technique: intensity stereophony
- M/S technique: mid/side stereophony
- Near-coincident technique: mixed stereophony
how those recommendations are decided on.
There are some standard patterns mentioned in this Wikipedia article for pairs of microphones; manufacturers of audio equipment might have some guides such as the one from Shure, but the general answer is that it depends.
For single microphones used to record sound from an instrument there are usually some "standard" positions which will capture a certain sound. By moving it closer or further away you might get a different sound, so it all comes down to what you want it to sound like. The microphones you have available and which tradeoffs you have to make with regard to the setting and surrounding environment is also a big factor. If you're working in live settings, there are other considerations, and you need to find a good balance between all of them.
In a studio you've (usually) got full control of the environment; the room tend to be quiet, so you have more flexibility in terms of where you want to place a microphone. In a live setting however, you might have wind or AC, a less-than-optimal room or stage and often a more limited choice in terms of which microphones to use.
(...) 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.
I currently only do live work, or at least used to before a certain pandemic, but generally it's a mix. Some musicians might have one or more preferred positions, others will leave it up to you. The positions themselves will vary depending on the setting and the desired sound, but often you can get a long way by listening to the instrument and seeing where you like the sound. If it sounds good and is doable within the setting, go for it.
When I'm uncertain I've found that it often pays to just ask. if they have any preferences or what they tend to do in other settings. The result might not be what you're after, but at least it tells you something.