It wasn't so long ago that string quartets stuck to a time-honored script: black formal dress, an elegant concert hall and well-aged composers such as Haydn, Mozart or Beethoven.

The quartet called Brooklyn Rider is among a growing and increasingly adventurous group of "modern classical" musicians methodically dismantling that stereotype. The Kronos Quartet is an inescapable influence, as is Yo-Yo Ma's Silk Road Ensemble, which counts every person in Brooklyn Rider as a member.

But the quartet, formed a little more than a decade ago, has made its own impact. The Los Angeles Times called its versatility "one of the wonders of contemporary music" and a writer for Strings Magazine gushed, "I've seen the future of chamber music and it is Brooklyn Rider."

The four members dress casually for performances, as if they were going to work at Kinko's. Their latest album is devoted to the string quartets of minimalist composer Philip Glass, which will be the focus of a concert Sunday at Cedar Cultural Center.

But perhaps the most unorthodox aspect of Brooklyn Rider is how the group varies the chambers where they play their chamber music.

They have dovetailed their violins, viola and cello at Joe's Pub in New York and the Todai-ji Temple in Japan, at the South by Southwest indie-rock conclave in Austin, Texas, and the U.S. Open tennis tournament. And next Thursday, for the sixth consecutive year, they will perform at the Washington County Historic Courthouse in Stillwater.

The concert is part of the annual Stillwater Music Festival, founded by Brooklyn Rider in 2006 as a forum in which to experiment in an intimate and relaxed setting close to their roots. Violist Nicholas Cords was born in White Bear Lake. The grandmother of two other members -- brothers Colin Jacobsen (violin) and Eric Jacobsen (cello) -- was a Stillwater resident and Brooklyn Rider's biggest fan until she died last year at age 102.

She had a cabin on Square Lake that became an especially peaceful and productive rehearsal space whenever the Jacobsens and Cords were back visiting their families, with fourth member Johnny Gandelman in tow.

"It is always so nice to go back to Stillwater," said violinist Gandelman by phone from New York. "I was the one without a connection there, and now there is the cabin, a coffee shop in Stillwater, the Minnesota State Fair, as well as the places we play, that feel like home.

"The first year of the festival, we didn't know what to expect or if anyone would show up. But the family connections were strong -- especially Colin and Eric's grandma -- the people were incredibly supportive and word spread fast. A huge portion of the audience has been very loyal and we've had sold-out shows at the courthouse pretty much every year. It's just a really nice setting for us to experiment."

Trial by fire at the courthouse

In previous years, Brooklyn Rider has used Stillwater to foster intrepid collaborations with folks like Irish fiddler Martin Hayes or Japanese flutist Kojiro Umezaki. This year, the experiment is from within. For the first time, all four members are concocting a truly group composition that will receive its rough premiere at the courthouse.

"The idea has been with us for a while, but it is a new step, and for me, a little bit terrifying," Gandelman said. "But there is a lot of friendship and musical trust between us. The sketches we have for the piece now will probably be very different after a week in Minnesota. This is definitely the right place to try it."

The courthouse concert will also include Brooklyn Rider's ingenious interpretation of Beethoven's renowned String Quartet in C-sharp minor, Opus 131, which they will reprise when they make their Carnegie Hall debut on Halloween night. There will be fresh arrangements of folk music from Brazil and the Indian state of Rajasthan.

Breaking down the barriers

On Tuesday, the group will perform another staple of the Stillwater fest, a free concert for children at the Stillwater Public Library. But the kickoff event is Sunday in Minneapolis, with the Glass string quartets and a rare public rendition -- the first in Minnesota -- of a suite of music that Glass composed for the film "Bent."

"A few years ago, I was listening to a lot of Glass, and just loved this Kronos recording of the Third Quartet, which is what we'll be opening with at the Cedar," said Nicholas Cords, in a separate phone conversation. "So I brought it to the group and we began playing it and this guy from Orange Mountain Music, which is Glass' music label, heard it and proposed the project. That's very exciting but also a huge thing -- it took us years to do it."

Meanwhile, all the Brooklyn Rider members keep their talents engaged in various projects in addition to their work in the Silk Road Ensemble. The Jacobsens are part of a chamber orchestra known as the Knights, which will be the subject of a PBS documentary in September. Gandelman runs a record label called In a Circle and promotes events linking music and visual art presentations. And Cords is on the faculty at New York's Stony Brook University.

"Throughout history, musicians and other creative artists have been busy reinventing what has been done, so I want to be careful about saying what is happening now is anything that special," Cords said. "But there does feel like a groundswell.

"Some of it is the opportunity we have. With computers, you can create a whole orchestra in your bedroom and communicate with people all over the world. We're finally getting to a place where the barriers are being broken down, where the alternative world and the classical world and jazz world and world world are talking with each other about collaborations. And they -- we -- have profound ways to make it happen."