Call it "strings gone wild."
While continuing to uphold the legacy of Beethoven and Brahms, innovative string quartets and trios are doing what jazz and rock bands have done since the early 20th century - improvising, adding a jazz riff here and a pop reference there - even incorporating straight-up rock songs by U2 and Metallica.
On its Grammy-nominated 2007 album, "Whirled Chamber Music," Quartet San Francisco - a traditional quartet of two violins, viola, and cello - plays a moody, agitated version of "Pick up the Pieces" by the Average White Band, as well as works by Tower of Power, Chick Corea, and Duke Ellington. The New York-based quartet Ethel performs contemporary chamber music with the raucous energy of a bar band. Locally, the Boston String Quartet is finishing a new CD that mixes classical, Celtic, Latin, pop, and New Age music, and wraps up with a Beatles finale.
"String players are playing the music they like to listen to," said Mimi Rabson, an assistant professor in the string department of Berklee College of Music. Put it another way: "The violin is my vehicle," she said, "but I drive on all roads."
Many musicians credit innovations in string playing to California's Turtle Island Quartet, founded in 1985, one of the first string quartets to win commercial success by incorporating improvisation, jazz, and popular music. An additional impetus, Rabson said, is music students' increasing exposure to new forms of music from around the world.
Berklee violin student Aaron Colverson practices his classical etudes, then kicks back to blues, Steely Dan, and Jimi Hendrix. "Walking around the practice rooms, you can hear five, six, 10 genres of music, " he said. "Then the players go home and listen to something completely opposite."
Growing up, Cohen loved rock, pop, and jazz - the music of his generation - but didn't have a way to express it with his violin. So Cohen has made it his life's work to change that. The result: "We don't sound like a nerd string quartet trying to play funky music," he said of Quartet San Francisco. Rather, his group performs an eclectic crossover repertoire within the framework of traditional chamber music.
The division between classical music and popular music itself is artificial, argued Ralph Farris, one of the Juilliard-trained composers and musicians who founded Ethel, a self-described "musically omnivorous" quartet that performs tomorrow night at Sanders Theatre. "The fact we're even calling it 'classical' music is silly," Farris said. "Verdi was the Andrew Lloyd Webber of his time. Brahms played in brothels."
Ethel's Cambridge concert will be a survey of the group's 10-year history of genre-defying works, including Rodney Yazzie's "Obsessed by Obscurity" - "heavy metal for strings," as Farris calls it.
Farris, who was raised in Dover and attended New England Conservatory's preparatory division and the Longy School, said Ethel's journey is not unlike that of "Star Trek" captain James T. Kirk: "You could say we're exploring strange new worlds."
Worlds are virtually colliding in the repertoire of some string-based groups. Take the recent concert playlist of the Boston-based International String Trio. It starts with the "The Swan," by C. Saint-Saëns and moves on to Handel, "When the Saints Go Marching In," and "Georgia on My Mind."
"We're trying to push the boundaries a bit," said trio member Slava Tolstoy, a 2002 Berklee graduate and guitarist who founded the group in 1999.
Violinist Christopher Vuk of the Boston String Quartet thinks that string quartets with a contemporary sound may encourage younger folk, accustomed to three-minute pop songs, to "digest" longer and more musically challenging classical fare. "Let's meet them on their level," he said.
Vuk's quartet is "mixing styles like crazy" for its upcoming third album, tentatively called "Xibus," which will reflect Latin, flamenco, New Age, and Celtic influences. "We're going to cap it off with 'Hey Jude,' " Vuk said.
It's not news that the classical music market is suffering. According to the Recording Industry Association of America, classical music accounted for only 1.9 percent of consumer music sales in 2006. Ironically, there are probably more classical musicians now than there have ever been, according to Vuk, but the jobs are becoming fewer and fewer.
"Orchestras have to reinvent themselves to keep themselves financially viable," Farris said.
Many people today lack a context for understanding classical chamber music because they don't experience it in their daily lives, Cohen noted. But when Quartet San Francisco played its calypso version of "Under the Sea" from "The Little Mermaid" for an audience of children, "They freaked out," he said. "They screamed at the beginning of it. They screamed at the end of it."
The quartet's "Whirled Chamber Music" mines the musical depths of works with recognizable hooks, like "Gee Officer Krupke" from "West Side Story" and "The Toy Trumpet" by Raymond Scott, whose music was used in Warner Bros. cartoons.
But it's creativity, not marketing pressure, that motivates musicians to head into new territory, said Rabson, who teaches an Alternative Chamber Music Ensemble class at Berklee. "It's to find an individual, unique voice, not to be a slave to genres."
For example, one of Rabson's students, Liz Maxfield, a member of the Folk Arts Quartet, has been applying a "beautiful fiddle style" to her cello playing. The result is a mix of traditional, Celtic, and classical, something more in the "new grass" genre.
"I had a classical teacher who didn't quite approve of what I'm doing," Maxfield said. "But as far as audiences go, they really like hearing something new."
Quartet San Francisco plays Saturday at the Center for the Arts in Natick (8 p.m. Tickets $28. 14 Summer St., Natick. 508-647-0097. natickarts.org) and Sunday at the Regent Theatre (7 p.m. Tickets $27.50. 7 Medford St., Arlington. 781-646-4849. regenttheatre.com).
Ethel plays tomorrow at 8 p.m. $25-$38. Sanders Theatre, 45 Quincy St., Cambridge. 617-496-2222. celebrityseries.org