Editing audio can be a fairly mind-numbing experience. But there is one rather abstract procedure which gives me a curious satisfaction: creating room tone.
Many recording sessions start or end with the engineer asking for a few seconds of silence while he records some—well—“silence”. This silence is technically known as “room tone” or “presence”. Here's why we do it...
The first thing to know is that every location has a distinct sound, or presence, even when nobody is making a sound.
Due to the way recording sessions take place, it's not necessarily a given that all stretches of silence will have been performed. Or even if they were, they might not be quiet (or smooth) enough. I'm mostly referring to those hushed moments between movements in classical music, but basically anywhere in a single work, recorded in the same space, where there are breaks or cæsuræ in the music. In such cases where there is no recorded silence or the silence is not silent enough, it must be somehow approximated during the editing phase.
It's not enough to simply 'fade to black' as that has a tendency to jarr, sounding like equipment failure. Rather, one must allow the decay of the instruments and room reverb to naturally disappear into the room tone; apply a cross-fade from there into our purposefully recorded section of room tone; and follow that with a cross-fade just before the next sound.
While this is standard practice in classical editing, I also like to liberally apply the technique to any acoustic music where the tracks on the album were recorded in the same space. For overdubbed, multi-tracked studio recordings, the rules are slightly different, but I still like to experiement with cutting in mic presence; an approach quite common in film audio dialogue editing and one which can really help maintain realism in vocal edits, for example.
It's such a curious little process, and you find yourself listening intently to silence, creating these little loops of nothingness, trying to ensure the silence is as smooth as possible. It's pretty trippy.
As an ironic addendum to this: the only time I ever blew a speaker was listening to silence. I had the gain up really high checking the smoothness of the silence when the computer unexpectedly crashed causing a spike in the audio output, and—BANG. All that lingered was the smell of toasted woofer.