Been to a lot of big games at ND. Michigan last year was every bit as loud to me as those other stadiums. Now, it's just my ears and I'm sure it could be measured differently by an instrument but I couldn't tell.
I think the difference in those fans/stadiums is they bring it more often throughout the season. Probably mostly because of conference rivals breed some hatred. ND plays random teams each year and the juice just isn't there.
For the record Seattle Seahwks stadium and the old Hoosier Dome are were the two loudest environments I have been in. I would assume the Saints stadium would be equal too or greater.