It's time for Mozart and Mendelssohn to move over. A wave of new music is hitting the classical airwaves, from Elliott Carter to Duke Ellington. Audiences are applauding, and radio progammers are taking a fresh look at the old formats and formulas.
The wave is originating from WNYC-FM, a major public-radio outlet. What began there two years ago as a trickle of contemporary music - a couple of hours per day - has grown to a flood, bolstered by enthusiastic listener response. About 85 percent of all records now played on the station contain works written since 1900, by composers as diverse as Aaron Copland and John Philip Sousa, with occasional visits from the likes of Art Tatum and Rodgers & Hart.
Despite some controversy, audience interest has remained strong, according to WNYC director John Beck. Plans for expanding the programming include construction of new studios and a recital hall for live concerts. Also coming up is the station's annual ''American Music Festival'' on Feb. 12-22, between the birthdays of Lincoln and Washington, stressing contemporary pieces of all sorts.
Meanwhile, WNYC influence is beginning to spread nationwide, through the American Public Radio Network, which includes about 85 percent of all National Public Radio stations. So far, WNYC contributions to this recently formed collective have leaned toward traditional styles. But the commitment to contemporary and American works has begun to make itself felt, and plans are afoot to beam out a new-music series soon, perhaps by next autumn.
''There is a lot of interest among programmers,'' Mr. Beck says. ''They're watching us because we're the first station in the public radio establishment - with a large audience, comparatively - to try this. If it works well for us, others will follow, though probably in a cautious way at first.''
It all represents a dramatic shift from the conservatism that often marks programming at classical radio stations, where occasional dollops of Shostakovich or Stravinsky are considered boldly modern. Even WNYC dropped its contemporary-oriented ''American Music Festival'' in the early '70s for budget reasons and concentrated on traditional fare.
The current experiment in heavy new-music programming began with an afternoon show two years ago, hosted by Tim Page. Although the station's management was anxious about listener response, Page pushed beyond the merely contemporary to the truly radical work of such mavericks as Philip Glass and Steve Reich - and received a torrent of audience reaction, nearly all of it positive. Page's show remains the cutting edge of WNYC programming, presenting an unpredictable flow of music and interviews with composers as diverse as Milton Babbitt and John Cage. ''The happy mail completely swamps the occasional hate letter,'' says Mr. Page, who receives notes from homemakers as well as musicologists.
The station's next step was reviving its annual festival of recent American music, which now incorporates live broadcasts, taped concerts, and a daylong ''Americathon'' show with distinguished composers and performers. Finally, the daily programming of contemporary pieces was gradually increased. Audience response remained ''surprisingly positive,'' says Beck, even as contemporary sounds took over a predominant share of the broadcasting day - indicating a new receptivity by radio listeners to the new and untried.
Two factors led WNYC to try the experiment. One was ''a sense of mission,'' Beck says. ''New York is a major music center in the world, but a sort of logjam has been stopping the flow of contemporary work to audiences. Institutions like the Metropolitan Opera and the New York Philharmonic can't shift their repertoire as easily as a radio station can. So we decided to try and break the logjam ourselves.''
And there was a ''strategy'' incentive, too. ''We originated classical record-playing on radio in 1927,'' Beck says. ''But now other stations do the same job, and reach more people, though they have commercials and we don't. We'd like to do something unique again, and lead the way into an unfamiliar area. Also, we'd like to play the development role - helping pieces get heard and create a demand - that the commercial stations do for commercial recordings.''
Plowing new territory presents new problems, as Beck and his colleagues are finding. For one, station personnel are having to ''digest'' and ''categorize'' the 20th-century repertoire, which includes many works they have only a passing familiarity with.
''We may know the William Schuman symphonies,'' says Beck, ''but we may not know which are the strongest, or which will sound best right after some other work. We have to learn this music so we can do a better job of programming it. And we want to reintroduce earlier music, too, in a way that's compatible and complementary. We want our new material to keep a continuity with the classics of the past, which means emphasizing the more familiar forms of contemporary music much of the time. We want to bring our audience along, not leave the more traditional listeners behind.''
The trick is to demonstrate that ''music isn't centered completely in Vienna of the 1830s, but is flourishing here and now, as well.'' In this way Beck hopes not only to freshen the airwaves, but to attract listeners ''who don't think they like classical music.'' People who have grown up with rock or jazz sounds may respond to some contemporary ''serious'' music - by Leonard Bernstein, say - more readily than to maestros of the past.
''Remember that 95 percent of radio listeners don't tune into classical or jazz at all,'' says Beck, who feels a more ''now'' approach may well succeed in reaching out to them.