Microphone placement is very important. Getting the microphone in the just right place can allow for a great sounding mix. Get it wrong and you can cause unnecessary problems for yourself.
Say for example you are miking a guitar amp and for this exercise you are using two SM57s one positioned in the middle of the speaker and the other positioned at the edge. Each one will sound slightly different from the other due to the fact that they are picking up different parts of the sound being produced by the amp.
The distance from the sound source is also a factor to take into consideration. Generally speaking if you are ‘close’ miking instruments then you are looking at no more than about two or three inches away from the source. The thing to bear in mind hear is that the further away it is from the desired source, the more likely you are to pick up unwanted background (ambient) noise from other instruments. In a loud live music environment distant miking isn’t really recommended as you open yourself up to problems like feedback. In a recording environment if you may wish to have an ambient microphone (usually a condenser mic) as well as a close mic so that you can add some depth to your final mix. It all depends on what you are looking for in your mix. If for example you are miking a choir on a stage you would be better to use ‘shotgun’ microphones or hanging microphones so that you pick up the choir as a whole rather than close miking them as individuals. But if you are on a rock gig, instruments would need to be individually close miked as you would need more control over the sound. For close miking in a live environment it is recommended to use dynamic microphones, such as an SM58.
It’a really up to you where you place the microphone to get the right sound from the sound source but it is worth experimenting with mic placement as different positions produce different audio qualities.