RAP music may come from the streets, but there is no reason it can't be heard in a concert hall.That's the message the Kronos Quartet delivered in a recent performance at New England Conservatory's Jordan Hall. They knocked their instruments with their knuckles, scratched their bows across the strings, and spun out minor-toned melodies in sync with a tape recording of two bantering rappers: "Pizzicato-pizzicato!" "Take it to the bridge!" "Funky violin, come on!" Classical enthusiasts may cringe, but the Kronos Quartet never aims to please the purists. Its mission is to perform the music of our time - whether it is minimalism, serialism, jazz, or rock - with the same respect given to Beethoven or Brahms. The group is a particular favorite with composers. The reason: The impressive number of new works commissioned for or by the Kronos Quartet - 12 to 14 a year - fuels creative resolve within a precarious profession. "Composers are very fond of the Kronos Quartet," says Scott Johnson, whose "Soliloquy" was performed by the group at the Jordan Hall concert. "That you can write works that will be played widely by a successful and dedicated group - this is a big plus." "If it hadn't been for a group like Kronos, I wouldn't have written 'Beat Boxer, says composer Michael Daugherty, referring to his rap music piece. "The Kronos has inspired a lot of composers to take risks.... There aren't many groups like them around." It has been 12 years since the Kronos Quartet began its unabashed assault on chamber music's stuffed-shirt reputation. Ditching tuxedos for colorful, punkish attire and adding mood lighting, the group created a theatrical stage presence that immediately began to draw younger audiences. Their repertoire - from Schonberg to Philip Glass to Thelonius Monk - has grown, even as it has embraced composers from nations such as Japan, Argentina, and the Soviet Union. Today Kronos performs between 100 and 200 concerts a year, boasts a lengthy discography, and enjoys growing popularity in Europe. (A European tour runs Dec. 1-20.) "Black Angels" is the group's best-selling record, staying on Billboard's Top 10 Classical Albums chart for more than a year. "Black Angels" reflects a strong antiwar sentiment with works by Shostakovich, Charles Ives, George Crumb, and others. It signals a trend toward theme-oriented albums. "As they've gotten more pieces in their repertoire, they've put together albums with more unifying concepts," says Carol Yaple, media director of the quartet's label, Elektra/Nonesuch. In January, Kronos will release "Pieces of Africa," spotlighting works of contemporary African composers, many of whom sing or perform with the group. The Kronos Quartet is "keeping their fingers in the traditional [contemporary] repertoire as well as in the new and unusual pieces," says Victoria Roth, program services director of Chamber Music America, a service association. The quartet's success, she says, has "broken the ice" for chamber ensembles who are trying unusual instrument combinations and musical styles. "I'm not at all surprised at the [group's] success," Mr. Johnson says. They are doing what ensembles historically have done: "In the 19th century, classical music was a living form, not an academic one," he explains. "It was part of the culture." By commissioning composers and paying attention to new music, "the Kronos clearly wants to take classical music out of the museum." Composer Daugherty hopes to continue working with the Kronos. "One idea I'm discussing with them is a piece about J. Edgar Hoover." One of the members suggested it to him, and "it struck me as a fascinating idea. Who would ever think of writing a piece on Hoover? And only a group like Kronos would be able to perform it."