My room is the only finished/occupied room in the basement level of my home, is on a concrete foundation with insulation beneath the foundation, carpet and decent undercarpet padding on top of the concrete, two layers of 5/8 drywall on the ceilings and the two walls that face inside the house, one layer of 5/8 on the two walls that face the concrete foundation walls, a solid-core entry door, a double-paned window, and good (though not high-end) insulation behind the walls and above the ceiling. Even with that, it doesn't get below about 25dB A-weighted, and 27-32dB B- and C-weighted - and of course it's higher when there's a lot of activity on the 1st floor or loud trucks and such outside (I live in a city).
I could have spent more on insulation I guess, and I could put weather-stripping on the door jamb to block a little more sound, but I doubt that would make much difference (and the window is required by code for emergency egress from the basement). It's just the reality of residential living, and the room is still quiet enough that it's often possible for me to hear the quiet, approx 32-35dB (A-weighted) fan from my iMac several feet away from the listening position, in the back corner of the room. It is what it is.
Honestly, it makes hi-fi more affordable and less stressful, because I'm not going to worry about getting an amp with SINAD of -125 vs one with -112, or a DAC with SINAD of -122 vs one with -115, if the price differences are huge, given that my ambient room noise is -25dB and I don't listen to music beyond a 95dB peak level.
Finally, I'd make a plug for Decibel X, a free/inexpensive-paid SPL app for iOS (and for all I know, Android too). I don't have a pro-level reference, but I do have a dedicated Radio Shack SPL meter, and my iPhone with Decibel and the Radio Shack meter measure close enough to each other so as to be interchangeable. Both could be inaccurate of course, but even if they are, it seems strange that both would be inaccurate by the same amount in the same direction.
So I'd say that phone apps can certainly be "good enough" for many home applications even if they are not great.