Noise floors

I live 4-5 miles from Nellis AFB. When they are doing exercises some of the jets make enough sound to rattle my windows.

I love it.

As I understand it, "noise floor" does not really refer to what the OP was asking about, which might better be described as "ambient noise." Millercarbon's comment is relevant to the concept of "noise floor," which refers to the capacity of one's system for resolving very subtle sounds--not just quiet sounds, but subtle sounds. The decay of a resonant acoustic after the performers stop playing. The difference in timbre between an oboe and an English horn. These subtleties are easier to perceive in a quiet environment, but a good system will resolve them in thrillingly audible ways even when there is a plenty of measurable ambient noise (for instance, surface noise from vinyl). 

In fact, there is a theory in psychoacoustics that a certain amount of low-level noise actually enables the brain to distinguish subtle differences better than complete silence would. I'm not at home at the moment, and can't tell you the name of this theory (I don't have my main laptop with me here in Dalmatia), but I think it's a plausible explanation for why so many people find vinyl to sound "better" than a digital signal that measures better.

That being said, the OP's question is still extremely important to happy audiophilia, IMO. Before we moved to our current house, which is on a mountaintop in the country and has a listening room with gorgeous acoustics (and ambient noise in the low 20db range), we lived in a house with a "great room" which shared the same space between the living room, where the stereo was, and the kitchen, where the refrigerator was. I eventually went shopping for a new fridge--it's being quiet the only important criterion. The salespeople were amused to be told that replacing my fridge would be the single best improvement I could make to my stereo system. And even now, I sometimes turn the HVAC system off when seriously listening. We have a very good, and very quiet HVAC system, but I still notice every time it comes on, and I can be distracted by it during quiet passages of music.

Of course ambient noises are distracting! Of course one would wish to limit them as much as possible. Serious lovers of "classical" music disdain the Hollywood Bowl, even though it's such a crowd pleasing venue: distant freeway noise, planes randomly flying overhead, etc. etc. There's no way to control an outdoor acoustic in a big city. Even the Ojai Music Festival, as wonderful as it is, has to deal with that problem, and is inevitably compromised by it. This, it seems to me, is what the OP is asking about. And yes, it makes perfect sense to control the extraneous noises one can control in one's environment. Hurray for quiet refrigerators! 

Ambient noise only becomes an issue above a certain level, although some noise is more invasive than other.

If you have an ambient noise problem arising either from your home or from outside, first big steer is to listen at night when noise is way down.

Second, choose the location of your room if you can.  Mine is in the basement of a stone built house.  With the system down my room is very very quiet.

If you have a/c, switch it off while listening, as in many theatres and concert halls.

Remember also that room treatments absorb sound coming from outside the same as sound coming from inside.

I do not understand all this, apart from identifying that if there is much snow on the ground outside, then the sound inside is great

The neighborhood had a power outage two days ago. The silence was deafening.

Frank

Others have said the same thing.  My view is you don't get many power outages!  I live in an area where they are common, and I assure you, after five minutes, every generator within listening distance fires up, including my own. It's like a symphony of bad lawn mowers.