There is no way 77 is a max shout. You can easily scream much louder than that.
I have often been accused of yelling or talking too loud when I think I am not loud at all. A volume meter revealed I was regularly in the mid-to-high-70s. My goal was under 72 - preferably in the 60s - for normal volume. That was what a speech therapist told me. Of course, I was never able to stay in the 60s all the time, so I still have to put up with the “hearing aids will make you talk softer” crap ignorant people repeatedly insist. I doubt it because part of the speech problem is psychological.