That dull roar still rumbling between your ears a day later is not your imagination.

It’s the echo from the 49ers-Seahawks game Sunday night, when an already notoriously loud hometown crowd outdid itself. How?

Start with CenturyLink Field, a U-shaped stadium with cantilevered roofs extending over most of the 67,000 seats in the grandstands, a configuration designed to bounce back sound. Then throw in some fans presumably hopped up on espresso and, thanks to a later starting time, some more who stopped at Safeco Field on the way over to quaff 24-oz. beers offered through a promotion at a mere $4.50 each.

Next, mix in their dislike for a nasty NFC West rival and especially coach Jim Harbaugh, who smacked the Washington Huskies every chance he got when he was at Stanford and has been tormenting Seahawks coach Pete Carroll ever since.

Finally, throw in that early, unexpected lead and — voila! —  a near-perfect sound storm.

Just know it could have been worse.

“Obviously, they were jacked up last night,” said Fred Gaudelli, the innovative producer of “Sunday Night Football” on NBC. “But in my mind, it’s one of the underrated sports towns in America. Actually, the special challenge there is always to convey how loud it actually is.

“We knew that going in, plus we knew the 49ers were the team their fans hate the most. So at Wednesday’s regular ‘brainstorming session,’ we turn to our head audio engineer and said, ‘How do we make viewers understand you can’t hear the person next to you most of the time, even if he’s yelling?’ We wanted to be ready.”

Gaudelli knows what can happen to a team that ventures into Seattle without preparing for the wall of noise.

In 2005, the visiting New York Giants collected 11 false-start penalties in a single game, the start of a five-year span when opponents piled up league-leading totals, averaging twice as many there as the Seahawks. The Carolina Panthers once practiced for a game there by dragging loudspeakers down to the practice field and simulating the sound of a jet engine. If that sounds over the top, it is, by about 18 decibels. Jets are routinely measured at around 130, Century Link’s best is only 112.

Gaudelli and his crew hatched a plan to demonstrate that by having sideline reporter Michelle Tafoya speak into a microphone as the sound reverberated, then take a step back and try again. When they ran through it before the game, he had a stadium staffer simulate the crowd noise over the PA system. At the point Tafoya’s words were drowned out the system was cranked to 50 percent of volume.

“So I asked the guy, is it really going to be that loud? He looked at me,” Gaudelli chuckled into the phone, “and said, ‘Double it.”’

The guy was right. That much was apparent at the start of the broadcast, when Tafoya interviewed Carroll — remember, the game hadn’t even begun — and didn’t dare stand anywhere but uncomfortably close.

Experts have been arguing over the worth of home-field advantage for decades. Most concluded that in those places where it’s statistically significant, it’s usually because of a number of factors and not just one, such as noise. Since CenturyLink opened up in 2002, Seattle is 58-29 at home, a 67 percent winning clip that ranks the Seahawks sixth in the NFL over that span.

Try this: After a 2001 earthquake shook a viaduct that runs along the water and near the stadium, the University of Washington set up a lab to track future “seismic events.” One of them actually occurred during Marshawn Lynch’s thundering, winning, 67-yard touchdown run in a memorable upset of the then-defending Super Bowl champion New Orleans Saints on Jan. 9, 2011.

Yet while we know how Seattle fans make so much noise, why remains the subject of much speculation. Gaudelli, like a lot of people, blames coffee. But I’m going with a theory advanced Sunday night by announcer Al Michaels, who suggested the locals roar non-stop because showcase games gives them a rare chance to remind the rest of the country they’re there.

“For media people on the East Coast,” he said half in jest, “Seattle might as well be Bulgaria.”

———

Jim Litke writes for the Associated Press.