Bumped from the diaries; I'm a sucker for footnotes.
In my opinion, from reading the few articles I could find on stadium noise, the lack of noise in Michigan Stadium is because of the fans.
First of all, from scanning the internet, many articles claim noise levels, but do not describe how they were measured. The best I could find on other stadiums was quotes of "the ESPN Crew measured noise levels of XdB. But at least for the Big House, the Michigan Engineering department stepped in for something scientific.
"A Louder Big House", The Michigan Daily
First the bottom line is that the noise measured 100 dB. Which changes to the stadium they estimate an improvement to 110 dB. Unfortunately this is still far behind the Oregon claim of 127.2 dB. Now to the best of my memory, 3dB is a doubling of noise level. so with Autzen (Ducks) Stadium at 27dB higher, that's 9 Doublings of noise level!! Or 2 to the 9th power for the computer engineers. Looking at the pictures of Autzen stadium I can't see how the shape can possibly be responsible for all that, although admittedly I'm not a sound engineer, and don't have any experience making these measurements in different shaped enclosures.
Autzen Stadium from Wikipedia
But check out the images at that page yourself. Here's the quote from the section on crowd noise. Notice that when anyone wants a measuring stick to prove how "awesome" they are, it is frequently Michigan that serves as that stick.
"Autzen is known for its crowd noise. On October 27, 2007, during a 24-17 defeat of the USC Trojans, a record crowd of 59,277 fans was recorded at 127.2 decibels. A similarly-loud 31-27 upset of third-ranked Michigan in 2003 prompted a Michigan Daily columnist to write
||Autzen's 59,000 strong make the Big House sound like a pathetic whimper. It's louder than ... The Swamp at Florida, The Shoe in Columbus, and Death Valley at Louisiana State. Autzen Stadium is where great teams go to die.
Michigan coach Lloyd Carr later said that Autzen Stadium was the loudest stadium he'd ever been in.
In 2006, a Sporting News columnist named Autzen the most intimidating college football stadium in the nation.
Lee Corso of ESPN College Gameday frequently says that; "Per person Autzen Stadium is the loudest stadium that I have ever been in my entire life!" "
Autzen Stadium seats just under 60,000 fans.
And here's the quote from the Daily article when the Professor measured the Big House.
"Crowd participation was almost entirely located in the student section. If all 109,840 individuals had yelled at the same intensity, Navvab said the measurement would have increased to 102 or 103 decibels - a significant sound increase."
In conclusion, I think the Big House will definitely look more impressive when the construction is completed, but I don't think it's going to come close to sounding imtimidating.
So finally here is a link to Top 15 Stadiums that provides good pictures of the "conventional wisdom" on most intimidating stadium.
Top 15 College Football Stadiums