Quietly -- or rather, noisily -- a revolution is happening in popular music.
It hasn't much affected the commercial mainstream -- the ''top 40'' hits or the radio ''play-lists.'' And its boldest experiments may never pick up a really wide following outside the adventurous clubs and theaters willing to take a chance on a new thing.
But sounds of change are in the air, and their influence is sure to grow. Tired of the usual formulas, inventive young musicians are following their instincts into uncharted territories, where the beat of the drum and the twang of the electric guitar may be the only recognizable landmarks.
Some of the latest experiments are so radical that the terms ''rock'' and ''pop'' scarcely apply, though these come closer than other labels. The traditions of rock -- heavy amplification, a strong pulse, aggressive instrumentation - have been transformed into a brave new noise that thrives on risk and energy.
The stars of the new scene are a varied lot. Many have years of classical training behind them. Others spring from the jazz or rock-and-roll traditions. Some, such as Laurie Anderson and the group Polyrock, have made inroads on the commercial pop scene. Others, such as guitarist Glenn Branca and percussionist David van Tieghem, explore more esoteric paths. And a few, such as composer-performers Philip Glass and Steve Reich, are really classical musicians whose experiments have proved so accessible that casual ''pop'' fans are attracted along with more specialized audiences.
What all these artists have in common is a disdain for ''proper'' and ''approved'' approaches. They travel wherever their ears lead them, often finding themselves in the wide-open spaces between established musical categories.
Broadly speaking, three tendencies mark much of the new music:
* A celebration of noise. Several current musicians, especially the post-''punk'' and post-''new wave'' rockers, go beyond the usual limits of music to a sound that might be called ''creative noise.'' Melody, harmony, and even rhythm may vanish into a metallic texture that shifts and develops according to its own rules. Or noise may be used as a rhythmic element in more traditional rock.
* Theatricality. Many musicians like to mix their media, adding dance or video elements, or cultivating distinctive gestures and postures. Costumes and extravagant performing styles are back in vogue.
* Blurred boundaries between popular and classical idioms. Some of the most provocative musicians on the art-rock scene come from sophisticated classical backgrounds, or have collaborated with academically trained composers. By the same token, some classical musicians have performed their works successfully in clubs and discos that normally resound to pure pop sounds. Such music isn't rock at all, properly speaking, but it appeals to rock audiences through propulsive rhythms and forceful textures.
Put it all together, and you have a totally unpredictable scene -- a healthy thing for any art. Music cheered at the Metropolitan Opera House is cheered again in a rock club. Complex technological gear is used not just to amplify, but also to generate ''electronic music,'' bend voices and instrumental sounds into new shapes, and provide complicated rhythm tracks. A percussionist may stop playing and begin to dance; a singer may accompany herself with movies or videotapes; jazz and folk elements may find new life in rock-and-roll settings. Anything can happen. And if it clicks, it will keep on happening.
For listeners not softened up by exposure to conventional rock, the most radical sonic experiments of the new music may be jolting. Such musicians as Glenn Branca, Zev, the Bush Tetras, DNA, and the Feelies have limited interest in traditional musical concerns, even the direct and comparatively primitive concerns of rock-and-roll. They are explorers of sheer sound.
Glenn Branca, for example, has dreamed up new roles for the trusty electric guitar -- furnishing it with wires instead of strings, for example, or mounting it horizontally so a single player can command the equivalent of several instruments at once. When eight or ten of these devices play simultaneously, heroically amplified, the effect is not soothing. Yet it's enormously exciting, channeled through Branca's carefully woven structures of rhythm and texture.
Theatricality is also rampant among experimental popular musicians. David van Tieghem, for example, begins his solo percussion act by banging on everything except drums, aiming his sticks and mallets at a table covered with cans, bowls, bottles, toys, and other doodads. After a bit of this, he switches from sound to light, abandoning sticks in favor of glowing rods. Finally he leaves his paraphernalia altogether, and launches into an amusingly robotic dance.
Leading the pack, in terms of current popularity, is Laurie Anderson, who will premiere the complete version of her multimedia show ''U.S.A.'' this fall at the Brooklyn Academy of Music. A true technologist, she often spices her music with electronic effects and recording techniques aimed at bending her voice into whole new sounds. One of the most successful new musicians, she reached the top of the British pop-record charts with her elegant song ''O Superman,'' and recently released a strong American album called ''Big Science'' (Warner Bros., BSK 3674) that demonstrates her versatility and ingenuity.Other performers concentrate more on music, leaving out the multimedia extras, but cultivating distinctive stage personalities and mannerisms.
The most dramatic example of cross-fertilization between rock and classical idioms may be composer Philip Glass and the ''new wave'' group Polyrock. Glass is a serious composer with a strong academic background, including years at the Juilliard School and studies with Nadia Boulanger in Paris. Sometimes using his Philip Glass Ensemble, he has developed his own style of stripped-down ''minimal'' music, which has found favor with diverse audiences.
The appeal of Glass's music to rock-conditioned listeners is no mystery. It is highly amplified, and its textures are forcefully woven by assertive instruments, including electric organs and closely miked saxophones. Moreover, his pieces usually have a solid beat.
Whence come these qualities in music by a classical composer? A longtime jazz aficionado, Glass also admires segments of the rock scene, regarding ''punk'' and ''new wave'' as ''the most interesting'' of current popular trends. Speaking of such favorite groups as the Bush Tetras and the Raybeats, he has remarked that ''they are trying to strip popular music to very basic ingredients. It's similar to what I did in classical music, in a way.''
He also notes ''an integrity to the new wave,'' as well as ''a seriousness of purpose'' and ''a liveliness'' -- though he also recognizes the ephemerality of the scene.
Matching his actions to his enthusiasms, Glass has produced two albums by the group Polyrock, an ''art-rock'' band from New York. The latest, ''Changing Hearts,'' came out recently on the RCA label (AFL1-4043). He also continues to play his own classical music -- including parts of ''Einstein on the Beach,'' which has been performed at the Met and in rock clubs and discos.
Glass knows this is unconventional, but enjoys the idea of breaking new ground. ''There are hidden, unspoken rules of behavior,'' he said in an interview with the Monitor, referring to traditional musical boundary lines. ''I simply decided I was going to ignore them.''
He isn't alone. Another ''minimalist'' composer, Steve Reich, has brought his music to such New York rock clubs as the Bottom Line and the Bond International Casino, with notable success. So have the Laura Dean Musicians, whose artfully repetitive and percussive piano music seems equally at home in the Bond rock club (where it was part of a massive benefit concert several months ago) and the staid Brooklyn Academy.
Another blender of classical sophistication and ''pop'' energy is Rhys Chatham, who has earned a growing reputation with his ''drastic classical music'' for electric instruments. Drastic is certainly the word for a piece like ''Stairway to Heaven,'' which has a melody built on a single pitch.
With a very different style, singer-composer Meredith Monk has also performed her dazzlingly eclectic music in rock-club as well as concert-hall settings. Tomorrow: Diverse groups with a common theme.