To some degree what is "loud" depends entirely on one's hearing thresholds, as well as the frequency range of the noise. As some of you guys know, I'm an audiologist for my day job, so I test hearing (and some other diagonistic stuff of the ears and balance system) and I get plenty of folks in who's auditory thresholds start about 30-40 dB and nose dive from there, maybe not even being able to perceive 3 or 4 kHz until its at like 70+ dB
Those guys aren't going to hear any "hiss" from an amp or monitor speaker, aren't going to hear any fan noise etc. They also don't hear the beep of the microwave, have no idea what a soft spoken female waitress or cashier asks them etc etc. Its all kind of relative as far as hearing levels go, but also what people can get used to. Some people's brain is really good at "tuning out" a certain noise where they just aren't really aware of it, even if its fairly loud. Other people have kind of the opposite issue, misophonia, and certain sounds, while not very loud, just drive them crazy and produce a lot of emotional distress, which can turn to physical distress.
I don't have misophonia over fan noise, but I have had monitor speakers before who's "hiss" was just too distracting to me, didn't want to me around them doing anything because I felt like I couldn't not hear it, if that makes sense...
Point basically being, aware of sound and physical intensity levels of sound don't have to go hand and in. Think of it this way, people who live near a train track sleep through the train, even though the thing can be like 80 or 90dB, physically loud and its totally cool. At the same time, a very soft sound, barely audible, like 20 dB can be very annoying/disturbing/et al, if its like the sound of your parents have sex in the next room. Very soft, but you darn well probably don't want to be hearing that!
Auditory perception is an interesting field, but it keeps my job fun. Vision in comparison is pretty boring, 20/20 is the same for everyone. No one wants to have glasses that make things just a little fuzzy etc.
