You know, the place where I last worked wanted to use our reading room as backdrop for a filmed interview (we had a very pretty reading room). On the day the film crew was there, the audio guy came over to my desk which was at the edge of the space and said "Look, you can keep working 'cus you're not doing anything too loud, but in a minute I'm going to go over there and call for silence for 10-20 seconds, and during that time I need you to not make any noise." And I went "lol sure" but he clearly felt a little uncomfortable telling me to not move at my own desk so he explained; the purpose of those 20 seconds is to record the silence in the room.
It's so they have a patch they can edit "silence" over some extraneous background noise later (the phone ringing, me getting an email, the toilet flushing in the bathroom next door, the elevator coming and going, noisy student group, etc), but the point was that they can't just slap any old "silence" over a recording done in a certain room. They have to use the "silence" *from that room* or it will be jarring on a subliminal level to the people listening. Because silence has a sound, and it's a little different everywhere you hear it.
That's what water tastes like.
