In a parking lot on the Minnesota State Fairgrounds, a group of men crowds together to lean against an old Dodge minivan, their hands pressing against the closed passenger window.

Another group sits on top of the car, weighing down the roof, as if they’re trying to keep a powerful monster from trying to escape.

Standing safely outside the car, the vehicle’s owner presses a button on a remote control, and, suddenly, there’s a mighty, deep “BLEERGGGH!” that shakes the sheet metal and makes the windshield quiver. The men exchange grins as a sound wave as loud as a space shuttle launch rattles through their chest cavities.

This is what it sounds like when guys — and it’s almost all guys — compete to make the most noise with a car stereo.

The battle to build the loudest sound system on four wheels was part of the Street Heat car show, held last month at the fairgrounds. Run by Midwest SPL (which stands for Sound Pressure Level), the contest was a season opener of sorts for the competitive, crank-it-to-11 car stereo nuts in the state.

The competitors, who were stuck in a corner of the car show parking lot, looked a little out of place among the shiny rows of hot rods, lowriders, muscle cars, exotic supercars and pristine show cars on display. The extreme car stereo vehicles were ordinary SUVs, utility vans and economy compacts, some with chipped and faded paint jobs badly in need of a wash.

But earsplitting sound is more than sheet-metal deep.

The humble exteriors of these cars hide thousands of dollars and hundreds of hours of work creating bass-heavy sound blasters: They are stuffed with extra capacitors, amplifiers, alternators and arrays of speakers that fill up the trunk and sometimes even the interior, all the way to the front seats.

Older is louder

You can’t judge the sound a car can make by its looks. Older cars are sometimes louder, said Mike Larson, a Brooklyn Park car audio shop owner who judges Midwest SPL competitions in Minnesota.

“This is one of the loudest cars in the U.S.,” Larson said of his 1989 Honda Civic with 265,000 miles on the odometer and two 35,000-watt amplifiers in the trunk.

Something about the shape and geometry of older Honda Civics makes them good resonators for ultra-loud sound systems, said Larson, whose car is capable of producing more than 161 decibels of noise. (For comparison, a jet engine at 100 feet away creates about 140 decibels of sound.)

Dodge Caravan minivans from the 1980s and 1990s also are prized for their potential loudness, said Dan Horner, a world champion car audio builder from New Ulm, Minn.

“We take a $400 van and stick in about $30,000 in the sound system,” he said.

Horner, who owns a car accessory and audio business, competed in this spring’s Street Heat with a 1993 Dodge Caravan with 288,000 miles and packing tape holding a side view mirror together. Duct tape covered the windshield, which shattered when he was testing the audio system.

“I’m hoping for 166 [decibels]-plus,” he said.

Dan Libor, a drywall installer from St. Cloud, brought a cargo version of the Dodge Caravan, a 1988 Dodge Mini Ram Van, that he picked up in North Carolina. It’s one of three vans he has turned into a rolling boom box. He also has about 120 trophies and several national records for car audio competitions.

“I like it loud,” he said.

Jesse Dise, an arborist from Anoka, came to compete with a 2006 Subaru Outback. It had a child’s stroller strapped to the roof rack because there wasn’t room for it inside with all the speakers and amplifiers.

He said that he bought the car as a family vehicle for his wife but that she thought it was too slow. It turned out the car came with an extra 1,000 pounds of sound-damping material.

“I unknowingly bought a car with an extensive history in SPL,” said Dise, who decided to restore the car to its noisy glory days.

“I enjoyed listening to loud music since I was young,” he said. “It’s like sitting in a car in a parking lot and having a rock concert brought to you.”

Burps, yes; profanity, no

The car noise competition involves dozens of cars lining up throughout the day for their turn in a judging lane.

When a car is in park, a sound pressure sensor is attached inside the windshield. Competitors then play 90 seconds of music or just a single tone or “burp” of sound, depending on the category of competition. The winner is the one that registers the highest “peak,” or average decibels.

The sound waves from these stereo systems are so powerful they can literally raise the roof, causing the metal car bodies to visibly flex and vibrate and the windshield wipers to bounce off the glass. Competitors sometimes recruit helpers to sit on the car or lean against the windows to help keep the sound’s energy from dissipating. Some competitors even put concrete inside vehicle interiors.

While the sound can be obnoxiously loud, the music that competitors play can’t be offensive. At Midwest SPL’s summer events, rules forbid competitors from using music with foul language, profanity or lewd and suggestive lyrics.

The owners of the loudest vehicles typically stand outside of their cars and trigger the speakers with a control panel on an extension cable.

But some competitors stay in their vehicles, without ear protection. (They claim that the low frequency, bass-heavy music and burps blaring out of their subwoofers don’t damage the ears like higher-frequency noise.)

Dise admits that being in a car that loud makes it “hard to breathe,” but explained that it’s “like a roller coaster. You go for the thrill of it.”

“I sit in the vehicle when I do it,” said Tanner Burden, a competitor from New Ulm who built a competition sound system in his Chevy Blazer. “It definitely hits you in the chest. I myself don’t worry about it.”

Hard on ears and cars

Dr. Tina Huang disagrees.

Huang, an ear specialist and assistant professor at the University of Minnesota Medical School, advises ear protection and limited exposure to that kind of noise. While loud, low-frequency noise may not make your ears ring like the higher frequencies, research has shown that even short exposure to low-frequency noise affects the inner ear.

“Your ears, even though they may not hurt, there’s something changing in the inner ear,” Huang said.

Extreme noise can be hard on cars, as well.

Dise said he’s had to replace the hatch on his Subaru because his powerful sound system has warped the hinges.

“It will tear sheet metal,” Larson said. “None of this stuff is designed to do what we do with it.”

Several of the audio competitors admit they listen to music loud on the highway, but insist that they keep it down when in residential areas.

“I try to be respectful of the neighbors,” said Jimmy Zukauska, a machinist in Cambridge, Minn., who has a 2002 GMC Yukon Denali crammed with 32 speakers and powered by four alternators.

To avoid noise complaints, Horner said, he moved his New Ulm audio shop to three locations, finally settling in an industrial zone.

“Our main focus is making people loud,” he said.

And the reward for being loud?

“You get a little trophy, a little neck ribbon,” Horner said.

The real payoff, he said, is “just the challenge of it.”

The rules of loud

How do you figure out who has the loudest auto sound system? Even in an extreme event, there have to be some guidelines. Here are some competition rules used by the Midwest SPL (Sound Pressure Level) organization to judge which vehicle is the noisiest.

Vehicles must have front seats.

All window and door/hatch openings need to function normally unless otherwise noted by class division.

People are allowed to hold or sit on the vehicle while it is being metered, provided no other rules are broken.

Competitors can open their doors, windows and other openings of the vehicle during the metering process, but doors and other openings are not to be slammed during this time.

For safety reasons, if a window is broken or shattered, it may be taped to prevent further breakage.

Competitors may use any audio format they choose to achieve their highest sound pressure level, including test tones, sine sweeps, MP3s, commercially available CDs and burned CDs.

Competitors cannot use any audible music with foul language, profanity or lewd and suggestive lyrics at any time or place during the event.