IT'S EXHILARATING to have an avant-garde that doesn't scare one away with obscurity or intellectual preachments. One hesitates to call it a movement - it isn't that conscious or unified - but there is a drift in the arts that has been developing slowly over the past decade. It lies outside the mainstream, and it is intent on breaking new ground. But unlike some of the more belligerently esoteric "isms" of the '60s - serialist composer Milton Babbitt once penned a notorious essay entitled "Who Cares If You Listen" - this new, offbeat art courts appreciation by a large, unspecialized public. And it occasionally succeeds in getting it.

Take "Einstein on the Beach," for instance - the highly unconventional "opera" by Robert Wilson and Philip Glass, as distinguished an example of the new trend as has thus far appeared. A panorama of visual and sonic imagery emerging with the slowness of the tides, it lasted four and a half hours. Yet it had its premiere at France's prestigious Avignon Festival in a production subsidized by the French government to the tune of $140,000, played major European opera houses, and then drew 9,000 people to two performances in that bastion of conservatism, the Metropolitan Opera House.

Glass, the composer of "Einstein," presented a concert of his music at Lincoln Center Plaza last summer which attracted 6,000 listeners.

"We want terribly to communicate," Glass put it in a recent interview." Not necessarily with a Madison Square Garden mob scene or a Hollywood Bowl rock crowd, but with an audience with lots of staying power and the potential to grow."

Now Glass will join fellow composers John Cage and Steve Reich to represent the "classical" side of an unusual Washington event that will also offer the work of such inventive jazz artists as Sam Rivers, Cecil Taylor and Anthony Braxton, among others. Organized by Earl Bateman, and called - rather confusingly, since it embraces three days as well as two evenings - "Two Nights of New Music," the festival will present evening-long concerts Friday and Sunday nights at Constitution Hall, as well as workshops, discussions and performances at four downtown sites Friday, Saturday and Sunday.

Glass emphasizes that the new "pulse music," as it has sometimes been called (also "minimal music," "phase music," "trance music," and "modular music," among other terms) is eminently listenable in ways that the serial electronic and other "advanced" musics of the '50s and '60s were not.

"One thing we're finding out about the new music scene is that there is actually a popular constituency out there, like there used to be for 'serious music' in the old days," Glass observes. "And it's not by any means just the arts people, but the curious and adventurous of all backgrounds."

This is part of the rationale for the New Music festival here, which hopes to attract a large "crossover" turnout of enthusiasts for all kinds of music and performance. Such heterodox festivals have been a standard feature of European concert life for some years, which is why composers like Glass and Reich have been also to build sizable reputations abroad while remaining relatively marginal figures in this country.

The approachability of "pulse music" has largely to do with its simple surface features, which may often cover an underlying complexity, of technique, but which in any case elicit a direct, visceral response in the listener. The serial music which dominated the '60s, developing out of the theories and procedures of Arnold Schoenberg and Anton Webern, elaborated intricate formal schemes that were impressive on paper, but were nearly impossible to perceive aurally even for initiates.

Glass and Reich and others of their persuasion have rejected the whole principle of unheard structure. "I am interested in perceptible processes," Reich has written. "I want to be able to hear the process happening throughout the sounding music."

Pulse music, as the name implies, restores a steady, immutable rhythmic pulsation to musical texture, in total opposition to serial music which obliterated it with highly asymmetrical rhythmic discourse. Pulse music has a beat, in the sense that Bach and Beethoven and rock and disco have a beat, and it remains in the foreground of the listening sensation.

Glass identifies other components of his music as "repetition, very simple tonal units, and a purposely elemental harmonic structure." "There's no dynamic inflection," he notes, "it starts loud, says loud and ends loud. All this is to center attention on the rhythmic structure, which is based on an accumulative, additive principle, in contrast to the narrative or developmental principles of conventional music. People talk about the effect being trance-like, but I'm wary of the word trance. People associate it with drugs and so forth. You don't have to get high to listen to my music. On the other hand, I don't think it's an accident that people who like it are also often interested in other 'non-ordinary' experiences. I like to think of my music as relating to trance, or meditation, in the best sense, in the sense of waking you up, of alerting you to new possibilities of awareness."

Both Glass and Reich came to their allied, but differing personal formulations of pulse music after contact with the music of non-Western cultures. Both were well schooled in the classical tradition, and both traveled extensively in Africa and the Far East.

Glass, born in Baltimore, was studying flute at Peabody at the age of 8, then zippejd through the UNiversity of Chicago as something of a prodigy (he finished at 19) and began composing there.

"My models at the time were Webern, Schoenberg and Ives," he recalls. "My first piece was a serial string quartet with tone-rows and the whole bit, can you believe it?" There followed further studies at Juilliard, two years in Pittsburgh in the Ford Foundation's Composers in Schools project ("I was writing middle-of-the-road Coplandiana at the time), and finally, in 1964, two years in Paris with Nadia Boulanger. In Paris, however, he also met Ravi Shankar.

"I was feeling," he says, "that the music I knew, serial, electronic, Americana, was a total dead end. In the rhythmic organization of Shankar's music I saw a powerful set of ideas on a fundamental level. It was a clue to me as to how to discover my own direction. I also realize that all we were taught about music being either Western or primitive was a shocking form of colonialism, and that we desperately needed to start thinking in terms of world music."

The repetition, additive structure and slow, cyclical evolution which characterize pulse music are to be found not just in the works of Glass and Reich, but also the music of Terry Riley and La Monte Young; the theater pieces of Robert Wilson, Richard Foreman and the Mabou Mines group; the dance works of Laura Dean, who has worked with Reich, Andy de Groat, the choreographer for "Einstein on the Beach," Lucinda Childs, Kei Takei, Meredith Monk and others, and in the films of Michael Snow and other cinema "structuralists."

The phenomenon can be traced back at least as far as Erik Satie, who may be the godfather of the whole thing with his deadpan "furniture music," and there are links as well with punk rock, space rock and such disparate jazz artists as McCoy Tyner and Keith Jarrett.

Within the coming New Music Festival, the coordination with jazz seems more a matter of basic rapport than esthetic ties. Glass says: "It makes sense. Many of these jazzmen, like Rivers and Taylor, were nurtured in the New York underground, just as were Cage and Reich and myself. We've all been rejected by the establishment, we're all involved in frontiers, and we share a progressive spirit - we're all trying to re-invent a language. I feel comfortable in a format like this, where I'd feel extremely uncomfortable on a program, say, with Charles Wuorinen or Elliott Carter."

Among other things were diverse creations have in common is a different kind of time sense than in most conventional Western art - a feeling, not of driving hard toward a known destination, but rather of contemplative drift or rotation. Most symphonic music, for example, whether by Beethoven or Franck or Stravinsky, compels a sense of action, of going forward, of a dramatic succession of tensions and resolutions. In pulse music, the feeling is more one of floating, and time seems measured on a cosmic scale. Listening to a Glass or Reich composition is something like sitting in a train and becoming conscious of the clicking of wheels against the rails, and slowly beginning to discern shifting patterns of accents within the unvarying clicks. It's also like looking up at a night sky with its swarm of stars in seeming random confusion, and gradually picking out groupings and shapes and recognizable outlines.

Glass' music for "Einstein on the Beach" duplicates this process. One instrument intones a simple melodic figure with a sharply defined rhythm, and repeatr it, again and again. After a while - minutes perhaps - another joins in, with the same rhythm, but now elongated, so that the two overlap in arresting ways. Gradually the whole ensemble joins in, setting up layers of steady pulsation which slowly shift in relation to one another. Nothing detracts from the hypnotic power of the repeating cycles - the harmonies are soothing, stable, the volume level constant.

It sounds boring in description, and there are times in listening when one's mind wanders and you lose track of the patterns. But once you're "hooked" into the calming undulation of it all, it's like floating on a raft in an open sea and surrendering oneself to the spectacle of cloud formations, as they merge and separate, shrink and swell.

The stage picture in "Einstein" is of the same character - repetitious action and dialogue, slow changes, and a cumulatively mesmerizing effect. A toy plane strung along a wire stretched the whole diagonal length of the Met stage begins to move upward from floor towards ceiling, but the motion is imperceptible at any one moment - it takes maybe a quarter of an hour before you realize the plane is further up the wire than it was before. Meanwhile, other things are happening at different, but equally temperate rates. You begin to feel you are tuned in to the rhythms of the universe.

The same feeling of mystical, but seductive, ritual pervades other manifestations of the pulse principle. In Laura Dean's dances, the dancers spin in place, arms slowly rising or lowering, each one starting at a different interval and changing direction of the spin at a different moment. In Meredith Monk's "Venice," a women "gondolier" bobs ceaselessly up and down in a crouch, pushing an invisible pole through imagined canal waters.

Glass will be represented on the Saturday night concert with excerpts from "North Star," a score for a film about sculptor Mark di Suvero recently shown at AFI, and with two segments from the "Einstein on the Beach" score, performed by himself and his ensemble of amplified keyboard instruments, winds and voice. On Sunday evening's program, Reich will present "Music for 18 Musicians," "Clapping Music," and "Muisc for Mallet Instruments, Voices and Organ."

Also on Sunday, John Cage will perform his "Branches" complete for the first time, a work utilizing dried cacti whose tines have been tuned to various pitches, the tones beging elicited by plucking and blowing.

Cage doesn't compose pulse music, but as the archiconoclast of the American avant-garde, the inventor of happenings and chance music, he is scarcely out of place in this company.

To call pulse music, and its kindred manifestations in other arts, the wave of the future would be brazenly premature. What can be said with a measure of confidence is that this is the closest thing on the horizon to a coherent esthetic tendency of the times, and that from its accessibility, its ecumenical breadth and its immediacy of impact, a rich artistic outpouring may soon flow.