It's from this paper https://www.aes.org/e-lib/browse.cfm?elib=19436 , so one may take a look there, I personally wouldn't know exactly why. The general point is that if you are far (i.e. 20+db like crin) below or above ~80db you are essentially not listening to the sound as intended and may miss either bass or highs. The itu standard (ITU-R BS.1116-3) states 78 dBA, maybe they have a better (open) explanation? https://www.itu.int/rec/R-REC-BS.1116/en