Noise floors

That external noise floor is something I recently noticed is gone with my RAAL SR1a headphones. Which are really 2 speakers strapped to my ears. If you are a 2 channel person then you really need to hear these headphones. Drive them with a powerful 2 channel amp.

I went a little crazy a few weeks ago by setting up the RAAL system with the quietest gear I had. I used an Audience FrontRow speaker wire, 2 Benchmark AHB2 monos, and a CODA 07x preamp, with a couple of DACs that measured great (BM DAC3B and Gustard X26 Pro). By eliminating the external noise and reducing the internal noise to what seemed like 0, I heard some of the best sound from any system I have encountered.

My Benchmark gear is very quiet. So much so that I cannot hear any noise with my ear against the floor stander tweeter. The new Topping pre90 is a preamp I sometimes use when I want something quieter than the CODA 07x.

Speaking of vehicles, noise and internal combustion engines. I have 2021 Ducati Supersport S, Suzuki DRZ400 and 2011 Mustang GT modded to no end. They all make lots of noise, I may be guilty of imposing on someone's listening session. Still,I'm relatively tame when it comes to aftermarket exhausts, not obnoxiously loud.

As for electric vehicles, was up Northern MIchigan on Ducati, nephew on his Aprilia RSV4, got behind a Tesla X (1,020 hp) on a beautiful very little traveled winding and hilly road. The Tesla saw us behind him, decided to race, we got up to speeds well in excess of 100, fun times if you don't mind danger.  That Tesla is one torque monster, and that torque is always on tap. My nephew had to hit over 150mph to pass the Tesla, I'm not quite the risk taker my nephew is, I was happy to stay behind.
Don't laugh at electric vehicles, they will supplant the internal combustion engine, performance freaks need not fret. Sure, they don't make the sound and the diy mechanic (as I am) will likely not have much to work on. But they will outperform the internal combustion engine, offer more reliability, and be more durable. The present greatest defects, specifically range, charging time and available charging stations will be solved. I'm a gear head and not scared and or hateful of the electric car future. And then we'll have all that silence to behold, good for the listening room.

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.