The sound of an empty room
Every room has a sound even when nobody is talking: air conditioning, the hum of lights, distant traffic through a window, the acoustics of the walls. That background is called room tone. Recording engineers capture a minute of it on every set so editors can patch gaps, and sound designers use it to place a scene.
Choosing one
Match the size and character of the room: a small carpeted office, a tiled kitchen and a concert hall each sound different. Steady tones without clicks, voices or passing cars are the most useful, since they can be looped for as long as you need. Keep the level very low, just above silence.
Room tones are rarely contested for rights and many are released as CC0. Each card shows the licence and the credit line when one is required.