AN Alexander Calder mobile will be conducting a musical group at the Pension Building this month.

As part of the wildly experimental "9th Street 1982" festival, the mobile, appropriately named "Chef d'Orchestre," will hang in the center of an open space, brightly spotlit, amid four percussionists. A player will strike it with a mallet, and the way the sculpture changes position will determine the way the performers play their music.

The music that emerges never sounds the same twice.

Other "9th Street" happenings -- they are sponsored by the Washington Performing Arts Society and are all over town this month with some episodes coming later -- include jazz from the Chicago Art Ensemble (Nov. 27) and Balinese gamelan music from the eclectic composer-performer Paul Dresher (Dec. 4). Besides Earle Brown's "Calder Piece: Chef d'Orchestre," there are also the Washington premiere of Steve Reich's new composition, "Tehillim" (Nov. 13) and a variety of tributes to John Cage, who has been shaking people's basic assumptions about music for nearly half a century.

The name of the game is diversity. Or maybe it's crossover: the breaking down of barriers between different kinds of music and art. The WPAS, which is best-known for bringing to town such established musicians as Itzhak Perlman, Vladimir Horowitz and the visiting symphony orchestras, had its basically conservative image shattered (not a moment too soon) when it sponsored the festival last year. It also discovered that the generations-old categories of "classical," "popular" and "jazz" that used to divide audiences are no longer as rigid as they once were.

This trend reflects the orientation of most of the festival's musicians, who elude the easily applied labels of the past. Even the basic division between musical and nonmusical art has blurred -- as you might expect in a festival where a Calder mobile will be serving as both conductor and percussion instrument for four percussionists. Last year, WPAS was not sure whether there was a large enough audience in Washington to support this kind of festival, but it found that concerts were selling out, with a clientele that ranged from the chamber music avant-garde to fans of New-Wave rock. Now, members of the mainstream classical audience, what WPAS staffers call "our Kennedy Center clientele," are showing interest. "Last year, they were a bit trepidatious, but now they're interested," says Deborah Hanzlik of WPAS.

John Cage, at 70, is recognized as the father-figure of experimental music in this country. He will be celebrated in two concerts, at the Terrace Theater and the Pension Building, as well as a reading at Washington Project for the Arts and a video tribute at the American Film Institute. Reich and Brown are both independent figures who do not see Cage's influence in their own work. But they do operate in an atmosphere of "anything goes" freedom that would be hard to imagine if Cage had not long ago begun changing the rules of the game.

His early work includes tightly organized serial compositions, but he has since developed many other forms: music for percussion orchestra, music based on chance or improvisation, music for prepared piano (in which the sound of the instrument is altered by putting foreign objects such as nuts and bolts between the strings), live electronic music and music as a form of mystical contemplation. He is probably best-known for "4'33" " -- a composition for any instrument or combination of instruments that consists of 4 minutes and 33 seconds of silence -- and demonstrates two of Cage's pet theories: that there is no such thing as silence and that any random sound can be accepted as music.

The five-hour Cage concert at the Pension Building (Nov. 20) will begin with "a sampling of his works from the earliest to the most recent," according to Nils Vigeland of the New York Bowery Ensemble, which will be performing. "We will do about one piece per decade, and some of the music will be having its first American performance." Two of Cage's major works will be the climactic points of the concert: the 1957-58 "Concert for Piano and Orchestra," and "Apartment House 1776," which he composed for the American bicentennial.

In the "Concert," the pianist and the members of the orchestra both have prescribed, written-out material to perform, but are free to choose when various things will be done and for how long, so that the musical interactions are unpredictable. "Apartment House 1776" can be played simultaneously with other pieces composed by Cage, but in this performance it will stand alone. All the materials in the work are music that could have been heard in this country in 1776, and its basic elements are performed (live or on tape) by four folksingers from the traditions of Sephardic Judaism, American Indians, black music and New England Protestant hymn-singing. These are the tenants of the "apartment house" that became a new nation in 1776; an image of that polyglot nation springs from the musical interactions as various kinds of music are played simultaneously. Each player does his own part with no way of knowing how it will fit into the whole.

IS STEVE Reich a popular or a classical composer? The answer is "yes"; he is both, or he is neither. In a sense, he is a medieval composer -- rooted in a time when such musical distinctions were not made. Whatever label is pinned on him (none fits comfortably), he is now moving out of a clearly defined and widely recognized style into something new. "Tehillim" (the Hebrew word for "Psalms") is the first composition using a text (sections of four psalms) that Reich has composed since his student days. Its melodic structures are reminiscent of medieval monody (solo vocal music), or perhaps of the Middle-Eastern music that strongly influenced medieval composers. They are freer and more expansive than the short melodic motifs he formerly used, and somewhat less tightly interlocked.

Reich avows influences from jazz figures like John Coltrane, with his repetition and elaboration of small motifs -- but the Renaissance is his strongest source. He is thinking of using one of the standard motifs of Renaissance music, the song "L'Homme Arme'," which was used by many composers as a theme for polyphonic treatment, in a new work that will have its premiere in Cologne next March. He calls this work "the biggest I have done in my life" and is writing it for full orchestra and 16-voice chorus. "It will last for over an hour," he says, "and will use techniques from all periods of my music; things that I have done before but not done for orchestra." It is one of five orchestral commissions that will be keeping Reich busy in the near future.

Brown's "Calder Piece: Chef d'Orchestre," which utilizes about 100 percussion instruments, will be the only non-Cage work played during the marathon Cage concert. Brown met Calder 30 years ago and considers his "kinetic art" a major influence on his own "open form" music. When a work for percussion quartet was commissioned in 1963 by French conductor and percussionist Diego Masson, Brown went to Calder and said he would like to write a piece using a mobile as a sort of conductor. Calder loved the idea and offered to custom-design a mobile specially for that purpose. "You can't just go around banging on someone else's mobile," he told Brown.

The result is that the music can only be performed with the mobile specially designed for it, which means that -- unlike other music -- there cannot be simultaneous performances. "I don't want it to be done apart from this mobile, which is integral to it," says Brown. "It will always be a piece for special events. I keep the mobile in storage in Rye, New York, where I live, and it is shipped to wherever the music will be played. I have to go along and teach the performers how to play it; there is quite a bit of choreography involved."

Choreography for performers of music is not limited to Brown's work. In one Cage piano piece, the Bowery ensemble has decided to use a percussionist rather than a pianist because the choreography of the player's movements, switching quickly from the keyboard to direct action on the piano strings, is more like the usual movements of a percussionist than a pianist.

With a mobile conducting and musicians performing like dancers, the music of the "9th Street" festival seems to be reaching out more and more into other arts. It's hardly a surprise at all when, asked who besides Calder has most influenced his music, Earle Brown comes up with the name of Jackson Pollock.