I've been to games at the old Pitt stadium, at Clemson, at Auburn, at LSU, at Georgia, and in the Dome prior to its current configuration.
Clemson and Auburn and LSU and Georgia were loud. It's a different loud, though. It's deep, wide, weighty. The decibel level is not frightening. It sounds like 70K+ people screaming, outside.
The Dome's noise is frightening, even before the new roof was put on. It sounds like 40K people screaming right into your ears. The sound shakes your innards in a way that outdoor stadiums don't.
The loudest outdoor sound I have ever heard was in the Iron Bowl in Birmingham for the 1996 Olympics, sold out, USA vs Mexico, we scored a goal in the first minute off the opening face off. It was thunderous. And inferior to any Dome outburst I have experienced.
The loudest thing I have ever heard, and which I think probably damaged my hearing permanently, was a concert at the War Memorial in 1982, Judas Priest and Iron Maiden. I didn't have earplugs. Afterward, the sound of footfalls on the sidewalk outside sounded like high-frequency swishing strikes on a cymbal.