This post focuses on capturing ambience by playing back and re-recording sounds in acoustic spaces.
The idea of piping sounds into a space and retracking it isn’t new by any stretch of the imagination. Before the advent of reverb processors it was common practice to route audio to speakers in reverb chambers to achieve ambient effects. Now it seems rather quaint to do so, but there are a good number of engineers who simply don’t like using artificial ambience. And real rooms offer a very different, tangible process as opposed to the knob twiddling of ‘verb processors.
The sort of speaker you’ll need depends largely on the kind of space you’ll be recording. The smaller the space the more the speaker quality matters, as you’ll be recording more of the direct sound. The main thing to look for are speakers that have a full frequency response. For larger spaces a PA system should be adequate. For smaller spaces, a studio monitor probably will work better (mainly for the lower distortion). If you don’t need the high-end of the reverberation, a raw speaker without a tweeter will work great (I like mellow ambience, personally). It’s also good if your speaker can accurately recreate the volume of the instrument you’re playing back. Meaning, playback of a drumset will require more SPL output (and more headroom for the transients) than playback of an oboe.
Place the speaker in the room where you would typically seat a performer. If you have a favorite room mic (or mics) and placement, go ahead and try that spot first. If not, wander around the room while the sound plays and try to find the spot that has the smoothest reverberation and the quality of ambience that you’re trying to achieve.
After that, just play back the sound and simultaneously record the mic signal onto a track. That’s really about it. Feel free to shift the ambient signal earlier or later to taste. On ‘vocal’ instruments (and vocals…) I like my ‘verb with a healthy predelay, so I often shift later.
Don’t just limit yourself to your studio space, either. A good sized boombox in a cathedral with a portable DAW can be a whole lot of fun, too.