Amateurish try to better understand derivation of the Overall Sound-Pressure-Level, A-weighted SPL, and Narrowbands and one-third octave bands.
Output data from a Aero-acoustic project I did with a colleague 1.5 years back is used here. It is a broadband spectrum (dB vs Hz narrowband) obtained after FFT was applied on a CFD-CAA Hybrid LES run.