Sound level meters, at least the good professional ones have weighting networks that adjust the sound pressure levels to correspond to what the human ear "hears".
There are usually three different weighting networks provided, used for different applications, A, B, and C.
The most commonly used is the A frequency weighting network, and sound pressure levels are then quoted as dBA. The human ear is highly non linear in both frequency response and amplitude, so it is very difficult to give precise defined figures, because they vary so much.
Back in the 30's Bell telephone Labs did a lot of pioneering work work on audio measurements.
Interestingly the Bell, and decibel are named after Bell labs.
Fletcher and Munson also published their work on loudness curves:
**broken link removed**