THE words ``string quartet'' conjure up images of white ties, tails, and music by composers like Haydn and Beethoven. But some of today's most popular quartet ensembles - ignoring the tuxedoed tradition - have donned the funkier garb of pop, soul, blues, and rock, and started playing the music of the most adventurous contemporary composers - people like Philip Glass, Steve Reich, John Zorn, and Astor Piazzolla. The drift away from tradition started with the Kronos Quartet, when it released a 1986 album (``Kronos Quartet''), which included a rendition of Jimi Hendrix's ``Purple Haze.'' That track set fans of classical quartet music back on their heels. With two more albums, dedicated to the music of jazz pianists Thelonious Monk and Bill Evans, respectively, the Kronos made it pretty clear that Beethoven, Haydn, Stravinsky, Bartok, and Schoenberg weren't the only way for serious string players to go.
Now, 11 years after the formation of the Kronos, ensembles with the similar goal of playing just contemporary music are springing up. They include the Soldier String Quartet here, whose music reflects influences that range from vocals by African pygmies to the American pop of James Brown.
There's also New York's Black Swan String Quartet, which employs the unusual instrumentation of two cellos, a violin, and a bass (played by jazz bassist Reggie Workman).
And then there's the Turtle Island String Quartet, whose name, derived from an American Indian term for North America, signifies the group's commitment to all kinds of contemporary American music.
One aspect of the new approach to quartet music is the use of improvisation. The Turtle Island group emphasizes improvisation more than the others, which is natural enough, since all of its players are jazz musicians.
In a telephone interview, Irene Sazer, a violinist and singer who has also had extensive classical training and who plays viola for Turtle Island, said, ``Many of the classical composers themselves were improvisers.
``Today, string players are involved in folk music from around the world, but in terms of classical music, for some reason the improvising stopped,'' she added. ``But string players are now at the point where they've done a lot of classical music. ... Some of them are starting to get kind of itchy and want to get on to some other stuff.''
For Turtle Island, the other ``stuff'' means original compositions, along with pieces from the jazz repertoire, like Dizzy Gillespie's ``A Night in Tunisia,'' Lee Morgan's ``Sidewinder,'' and John Coltrane's ``Naima.''
During her early years of playing the violin ``I had never heard of jazz,'' said Ms. Sazer. ``Although I would sing, and I knew about improvisation in other ways, I never thought I could do it on the violin.'' Then she discovered the music of jazz violinists Stephane Grappelli, Joe Venuti, and Stuff Smith.
She had considered getting into a chamber ensemble to avoid being swallowed up in the anonymity of a symphony orchestra, but by the time an opportunity arose, she said, ``I was way into jazz, and I was so excited about improvisation, there was no way I was going to take a classical, traditional path. So I kind of chuckle about the fact that I got into a string quartet!''
Unlike the Turtle Island String Quartet, the Kronos Quartet, despite the players' interest in jazz, is not deeply involved in improvisation. The group tends to focus on the music by a variety of modern composers - people like Terry Riley, John Zorn, Steve Reich, Kevin Volans, Astor Piazzolla.
DAVID HARRINGTON, a violinist and the founder of Kronos, said in a telephone interview, ``There's a tremendous explosion of interest in string quartets now, not only on the part of audiences but also on the part of composers.''
By the time he formed Kronos in 1973, Mr. Harrington had already developed a clear idea of the feeling he wanted the group's music to convey. But, he said, ``There were so many composers that I didn't even know about at that point; so for me, it was only a sense that there were some things that were possible, and I hoped that there would be a way to do it.''
Not only did he find a way, but the experience opened up a vision to him of a whole new way of looking at string quartets. ``There are things going on in this field that are every bit as far-reaching as what was going on in Vienna in an earlier time,'' Harrington said. ``I've spent a lot of my life trying to move the center of quartets away from central Europe and to include the rest of the world.''
Part of this effort is reflected in the Kronos's involvement in ``world music,'' music that incorporates sounds and rhythms from cultures around the globe, and in bringing percussive elements into the string quartet, mostly by means of unusual bowing techniques. Turtle Island uses them too.
For example, one of the pieces Kronos performs, Jon Hassell's ``Pano da Costa,'' involves attaching caxixi (Brazilian handshakers) to the bows. The group has also been influenced by Terry Riley's interest in African music, and by South African composer Kevin Volans.
``Volans came to one of our rehearsals, and several of the images he brought up had to do with the way elephants or hyenas would walk,'' said Harrington.
``All of a sudden, I realized that I had never heard of or been involved in a musical situation where we were thinking of African animals! So the frame of reference all of a sudden just exploded open, and we had a whole other palette of colors to draw on.''
Irene Sazer said Turtle Island likes to think of itself as speaking ``with an American accent,'' while traditional quartets speak ``with a European accent.'' But she emphasized, ``One of the things about being an American is that almost all of the world cultures are present.'' And she added that the group is definitely interested in playing ethnic music.
Both Harrington and Sazer are encouraged by the number of young string players who are beginning to stretch the boundaries.
``I work with lots of kids on improvisation, and they have no problem improvising,'' said Sazer. ``I get them to compose, and they're just string players who wouldn't get to do this otherwise. I have one student who, I am positive, would quit if it weren't for doing this kind of thing.''
And David Harrington said, ``When we play at a place like Tanglewood or Aspen, the young players know all about what we're doing. I have a feeling that what's known as a string quartet, in the '90s, is going to be very exciting.''