There are a few things that are a little different about Autzen, first, not noise related, but the field runs East West rather than North South like most fields. Second, it's dug into the ground, I believe the playing surface is lower than the parking lot. Third, it was built for football only so the fans are much closer to the field and they are elevated so the front row is probably 8-10 feet above the playing surface. This puts the fans about 10-12 feet from the opposing bench. The East and West Endzones have large video screens and scoreboards, the North side has a wall of vertical sky boxes and on the South side you have the roof. I suspect all of these to some degree (except the East West field) impact keeping the sound inside the stadium and focused on the playing field.
Also, we don't have a lot of fans sitting on their butts during games.