SEATTLE — Having already handled the arctic chill of Green Bay and a cross-country flight to Carolina, the 49ers must now overcome the most inhospitable sports environment on earth in order to advance to the Super Bowl.

In Sunday’s NFC Championship game, they are up against not only the Seattle Seahawks’ rock stars like Russell Wilson and Marshawn Lynch but also their 68,000 screaming backup singers.

CenturyLink Field is the loudest sports venue in the world, a claim verified twice this season by the Guinness Book of World Records.

This is the place where communication goes to die, a challenge the 49ers understand all too well. They’ve been beaten like an eardrum their past two meetings in Seattle, losing by a combined score of 71-16 while committing seven turnovers.

The loudest-crowd distinction is more than a novelty. The stadium has coaxed more false-start penalties out of opponents than any team in the NFL since 2005. Offensive linemen simply can’t hear the quarterback barking signals and they jump at the wrong time.

What’s with all the racket?

“Whether it’s the stadium, whether it’s the crazy people in the Northwest, whether it’s gray skies that makes them want to come out and scream. I don’t know what it is,” Seattle coach Pete Carroll said.

Having been burned before, the 49ers vow to be better prepared this time. To communicate on offense, they spent the past week at practice refining hand gestures and other forms of nonverbal communication.

As usual, they also blared rock music during drills to simulate the wall of sound they’ll have to get past on Sunday.

“We are very familiar with going to a silent count, which is what we call it, on the road,” left tackle Joe Staley said. “That is something that we have not just practiced for one week, but we have practiced for the last three years.”

Kerry Carter, a former Stanford fullback who went on to play two NFL seasons in Seattle, hears confidence like Staley’s and smiles.

“What did Mike Tyson say? ‘Everybody has a plan until they get punched in the mouth?’,” Carter said. “Teams say, ‘We’re going to come in, we’re going to do this.’ And then you get here and you can’t.”

How loud is it? Seahawks fullback Derrick Coleman, who is legally deaf, said he gets goose bumps when he runs on the field for pre-game introduction.

“I feel it,” Coleman said. “I don’t exactly hear it. I don’t get pain in my ears like everybody else. … I can feel the vibrations. I know everybody is yelling. You have a lot of problems if you can’t feel that.”

It’s not his imagination. The ruckus at CenturyLink field has twice registered on the seismographs at the Pacific Northwest Seismic Network, including last week when Marshawn Lynch rumbled for a 31-yard touchdown. The crowd-generated tremor, known around here as a “Beast Quake” in honor of Lynch’s nickname, registered around a 2 on the Richter scale.

As much as this city embraces Lynch (who wears uniform No. 24) and quarterback Russell Wilson (who wears uniform No. 3), the most popular jersey around town is No. 12.

It symbolizes Seattle’s crowd — i.e. the “12th Man” — and the role it plays on game days. The Seahawks have a .688 winning percentage (66-30) since moving to this stadium in 2002.

“There is so much energy around our football team,” Wilson said. “There are so many people who love the Seahawks. That is the great thing about it. … There are not too many organizations that you feel that and you sense that.”

No. 12 is so sacred here that they raise a flag bearing that number before kickoff each week. Fans go into a frenzy as soon as the surprise local celebrity emerges in the south end zone to hoist the flag.

Detlef Schrempf, the former basketball star for the Seattle SuperSonics and University of Washington, raised the “12th Man” flag on Nov. 4, 2012.

“I’ve been in arenas all over the world and there’s no question to me that Seattle has the loudest,” Schrempf, 50, recalled by phone on Friday. “And it’s not just for a minute or two. It’s consistent, all the way through the fourth quarter.”

Ricky Watters, a former 49ers running back who finished his career in Seattle, raised the flag in 2008. Reached this week, he recalled the way the noise would disrupt opponents during his days with the Seahawks.

“If you’re a visiting lineman, at some point you’re going to jump,” Watters said, laughing. “You hear that noise and the louder it gets, the more revved up you get. Your body just has to move. You can’t help it.”

Considering the otherwise mellow — if caffeinated — vibe of the Pacific Northwest, it might seem odd that this is the yappiest place on earth.

Schrempf and Carter were among those who theorize that the cheers are fueled in part by “desperation” for success: The Seahawks have never won a Super Bowl. The Mariners have never so much as reached the World Series. The dearly departed SuperSonics won their only NBA title in 1979.

“There’s a very renewed sense of hope,” Carter said. “I’ve never been around the type of energy I’ve felt in this city. A (championship) is something that’s long overdue here and I think people sense that they’re closer to it.”

Architecture is also an important part of the equation. CenturyLink was designed to maximize crowd noise, as directed by owner Paul Allen. A story on Sports Illustrated’s website Friday explained how the Seahawks’ stadium covers 70 percent of all seats, in part to protect fans from rainy weather and in part to deflect fan-generated noise back toward the field.

The stadium’s big-roof parabolas also keep the sound from fading into the open air.

All that screaming has produced a few whispers: Some opposing players suspect that the place pipes in artificial noise — performance-enhancing decibels — to boost the home-field advantage.

“Those whispers come from other teams and other fan groups,” said Joe Tafoya, a former Seattle defensive end who now co-owns Volume 12, a company that caters to Seahawks fans. “And my response is that the NFL actually has rules against that and they come in and measure that. Those are just rumors.”

Tafoya, as much as anyone not in uniform, helped crank up the dial this season. He’s the one who offhandedly submitted an application to the Guinness Book of World Records over the summer, asking for a shot at the title of “Largest Crowd Roar at a Sports Stadium,” a mark previously held by a soccer crowd in Turkey.

The Seahawks set a new mark on Sept. 15 against the 49ers and did it again by reaching 137.6 decibels during a Monday night game against the New Orleans Saints on Dec. 2.

As Sunday’s game approaches, the town is already starting to rumble. At the innumerable Starbucks around town, baristas arrived in jerseys bearing the names “Wilson,” “Lynch” and “Sherman.” Bars post signs with slogans like, “The 12th Man Drinks Here.”

In the window of the Metropolitan Grill, a popular steakhouse, a red emergency sign features a “49ers Evacuation Route” with an arrow pointing toward the back.

The vibe reminds some of 2005, when the Seahawks beat the Carolina Panthers to advance to the Super Bowl. Since then, though, the city of Seattle and its “12th Man” seem to have raised their games right along with their football team. Now, the 49ers arrive just as the whole town is turned on its ear.

“It’s something unique and special that we have and you can’t ignore it,” said Carroll, the Seahawks coach. “You can’t say that it doesn’t factor in. It does.”

Follow Daniel Brown at Twitter.com/mercbrownie.