Room tone: the sound you only notice when it stops
Every room has a hum. Recording it well is harder than it sounds.
Room tone is the quiet signature of a space — air, walls, distant traffic, the fridge two rooms away. Films use it to hide edits; we use it because silence with texture is far easier to rest in than true silence.
Recording it means being still for a long time and resisting the urge to fix anything. The temptation is to clean the noise up until it is sterile, which removes exactly what made the room feel like a room.
A good tone is boring on purpose. If you can listen for ten minutes and remember nothing about it afterwards, it has done its job.