<font color='#000000'>If I may be so bold as to toss in my 2 cents here, I do have some background in the acoustics area of engineering and will try to offer somewhat of a "layman's" explanation of A-weighting.
First, it's important to realize that the human ear is really a filter of sorts. That is, a 70dB sound at 500 Hz will not sound as loud to a human listener as a 70dB sound at 2000 Hz. So the ear mechanism conveys sounds (to the brain) differently depending on the frequency of the sound being heard.
So, some 50 years ago (or more? I can't recall exactly without looking it up in my reference material), a listening study was conducted that attempted to define this filter effect of the human ear. A large number of quite young men (I believe late teens to early twenties, if memory serves, and no I don't recall that women were included) were tested to determine their threshold (in dB) of hearing over the entire audible frequency spectrum.
I mention that youngsters were the subjects of the study because the expectation was that their hearing would still be excellent at their young age, thus providing as accurate of a measure of human hearing as possible. (Nowadays, they might need to solicit the involvement of even younger people to ensure that their hearing would still be considered excellent!)
The resulting graph of average threshold dB became the A-weight filter, which can be considered a fair approximation of the human ear "filter". Thus, an A-weighted dB level should more accurately describe how a human ear would perceive the "loudness" of a given sound than an unweighted, "straight" dB level.
Subsequently, many other studies have been conducted with the resulting dB-B, dB-C and other weightings, but that old dB-A weighting is still in very wide use today.</font>
