You could read a book on it.
And another freshman answer. Correct but incomplete. Want to try again?
I don't think disdain is the right tone to employ when asking a question.
it's a vast domain and you can't make one answer that could cover it all. I don't really see how having a described part of the brain would enlighten anyone about our limit of perception.
and about the "good enough for human ears" part, it depends on a lot of parameters, most of which you could google. sometimes a sound can be completely masked by another sound just 20 or 25db louder.
under other circumstances we can notice one sound, and later, another sound 100db louder without a problem. but as soon as music is playing, we lose that ability as the brain prioritize the data.
all in all I think it is widely accepted that while listening to music, anything manifesting at least 80db under the loudest played sound is considered unnoticed.
so that means noises and distortion at -80db (or 0.01%) are considered inaudible while music is playing.
as a joke, the industry counts headphones with under 1% of distortion as ok. (it show how bad headphones are compared to the rest of the audio system).
about objective arguments, you give 2 samples of music with a difference at -40db and see if people can accurately discriminate the samples. you do the same with differences at -50db etc.
it has been done. they tried to bait giant squids with glowing ping pong balls, so you can imagine that scientists and doctors were already long bored with what humans can and cannot do.
it was also demonstrated that some kind of noises or distortions are more noticeable than others, same for some frequencies over others.
all this is well documented, but that's medicine more than headfi.