What makes Century Link so loud is the design of the roof. The angle of it make the sound reflections converge on the field. So not only are you getting direct sound from the fans, but you're also hearing the reflected sound which is designed to be loudest at the player level.

This article talks about it and other reasons that make it so loud. It's an interesting read.

http://mynorthwest.com/874/2358028/Built-To-Be-Loud-A-Chat-With-CenturyLink-Fields-Architect