I do live recording almost exclusively. To get room tone or atmosphere there are many ways to go about it depending on the basic nature of the recording technique.
If what you are trying to do is simply capture the performance and the room it was recorded in, in stereo use a stereo mic setup. This can be done in a few ways.
1. X-Y micing with cardiods.
2. Left and right stage omnis
3. Mid-Side recording (this is how stereo mics work)
4. Binaural (can really only be played back properly on headphones)
By far the best of these is the Left and right stage omnis mic setup. The problem is, that to do this properly, you need a matched stereo pair of very good mics. $600-$3000 is about what you will have to spend for a pair of new well matched omnis. (You can pay less and you can pay more, but this is the average range)
The X-Y setup also uses a pair of matched mics, but you use cardiod type. Basically you criss-cross the mics so that they form an "X" and aim the pair at the stage. The problem with this is cardiods have funky off-axis response and depending on the microphones chosen, this can muddy up the sound and or the stereo image.
Mid side requires out board processing unless you use a simple stereo mic. I have a few recordings I have made with
my AT 822 stereo mic and overall the sound was OK, but not great.
Binaural recording can create extremely realistic recordings at the expense of only being able to listen to them properly only on headphones. I have made some very good "environmental" recordings with binarual mics that still blow me away some times. Many years ago I wandered around Harvard Square in Cambridge MA with a Sony Walkman Pro and my binaural setup recording the various street musicians.
If you are more interested in getting a nice clean recording with close mics, but still want the natural sound of the room there are also a few ways to do this.
1. Close mic everything and place a stereo mic somewhere over the audience facing the back wall of the performance hall.
2. Make an impulse of the performance hall and apply it to the finished mix.
Both methods work better if you are multi-tracking.
With the stereo mic, you mix some of it back into the main mix until you are happy with the ambience.
With an impulse, you need a sofisticated setup where you can record a frequency sweep being played back on a flat monitor on stage and record it with a flat omni microphone. You then process the recording in software to make a "print" of the room which you can then apply to the mix as if it were a reverb. This is not for the faint hearthed.
