The raw, slamming sounds of girl Band A reverberate liked trapped thunder against white cinder-block walls.
Standing in a circle - rock-and-roll poses perfected, little-kid smiles wiped away - each girl adds her personal noise to the collective din, now growing loud enough to kill small animals.
At Girls Rock Philly day camp at Girard College this week, the first all-girl rock camp in the Philadelphia area, you don't ask if it's good. The questions are: Is it awesome? and Does it rock?
"Thank you, America!" cowboy-booted lead singer Emma Ragsdale, 10, of Merion, intones when the rumbling ceases and the windows stop rattling. She appears to be acknowledging applause heard only in her head.
"Of course, as you know," she goes on breathlessly, "I'm the lead singer. When I grow up, I'm going to be a famous singer and play all over the world and help people."
The girls look happy. And that's pretty much the whole point.
If children are to be seen and not heard, girls especially must remain forever silent.
At least, that's what too many people believe, according to Beth Warshaw, volunteer director of the $400-per-child, weeklong camp.
"The empowering thing is to make noise," says Warshaw, a producer at WXPN-FM (88.5), "which girls and women are discouraged from doing."
Thanks to donated Fender amplifiers and tough-skinned drum kits, these kids are, above all things, profoundly empowered.
"I love it," says Emma's pal, tiny blond guitarist Becky Dame, 11, also of Merion.
"I know, it's so awesome," Emma agrees.
Around 100 summer camps in the United States are specifically devoted to rock music, and about 5,000 camps incorporate some aspect of rock playing or instruction, according to Jeffrey Solomon, executive director of the National Camp Association.
"But a rock camp just for girls is unusual," he adds. Girls Rock Philly is one of a handful of regional girls-only rock camps that belong to the nonprofit Girls Rock Camp Alliance, based in Portland, Ore., and founded in 2000.
The alliance is probably the only girls' rock-camp organization in America, Solomon says.
These days, of course, rock is not so much a rebellion as an everyday activity. Fifty years of jangling guitars and shouted lyrics have universalized the art form.
Even your grandmother could pick Mick Jagger out of a lineup. She may, in fact, have dated him.
Still, there's an enduring mystique to rock, a kind of contrarian power. And anyone who straps on a guitar or sits at a drum set is afforded a certain measure of cool.
At the Philly camp, many of the 20 girls - ages 9 to 17 and divided into Bands A through E - have some musical experience. Quite a few, however, picked up an instrument for the first time this week.
While playing rock music isn't as difficult as, say, performing gall bladder surgery, not just anyone can do it. And it's safe to say that most of the Philly rock girls won't be signing any recording contracts in the foreseeable future.
Still, rock rewards energy, and the high-wattage Philly girls qualify as an alternate power source.
"Oh, it's really cool to have a band," Becky assures a visitor. "But there's some drama in it. Like, some people get in a bad mood or get annoyed."
Of course, playing music without boys around is its own joy.
"There's not much pressure with girls," says Becky, who is barely taller than the purple electric guitar she twangs. "Boys make fun of girls: 'Oh, you're not good.' Boys think they're better. But girls are more supportive than boys."
Perhaps, but rock is a boys' club. Even the brightest musical cognoscenti are hard-pressed to list more than a few top-drawing, mainstream bands that consist of women only.
There is a dearth of role models for the kids: The Donnas. Sleater-Kinney. Years ago, there were the Runaways, the Bangles and the Go-Go's.
Most female artists are singers with male backing bands, Sara Sherr tells the Philly rock girls in a lecture one afternoon.
"A lot of women are told off the bat, 'No, you can't play,' " says Sherr, who writes about alternative music in Philadelphia. "When Rolling Stone puts women on the cover, they're usually half-naked."
None of that seems to matter to the eager girls of Band A. The fact that their musical heroines - the Dixie Chicks, Pink, Avril Lavigne, Kelly Clarkson - play with male musicians makes little difference to them.
"I'm here to express feelings," says drummer Kellie Gordon, 12, of Sewell, Gloucester County.
Practicing an earnest little song that Emma wrote entitled "I'm Not There," the band creates riffs and grooves on the fly.
"Just start jamming to find the chorus," suggests Kathryn Doherty-Chapman, an East Kensington drummer who works in community development and volunteers as a band coach.
At the end of the week, all the girls will appear onstage at Girard in a showcase. Band A, which plans to rename itself the Consequences, is psyched - although perhaps not quite ready - for the performance.
During an end-of-the-day assembly at camp this week, a girl asks Warshaw whether there will be "strong men around to help us move our equipment."
"There are going to be strong women," Warshaw corrects. "You'll move the gear. The men will be here just to watch you play."
To see video of the girls at rock camp, go to http://go.philly.com/girlsrock EndText