An interesting, useful lecture on perceived loudness

http://www.physics.mcgill.ca/~guymoore/ph224/notes/lecture13.pdf


Also this interesting lecture.
http://musicweb.ucsd.edu/~trsmyth/loudness175/loudness175_4up.pdf


(A basic corollary of the Fletcher-Muson curves is that increasing the acoustic intensity of a recording increases bass heaviness. Hence the need of low cut or high pass wave filters on production, see Pleiades filters.)