Philip Glass likes numbers. He does not mind telling you that he is now 66, three times the age at which David Henry Hwang wrote the two one-act plays he has just turned into chamber operas for the American Repertory Theatre. If counted separately, he says, they are operas #19 and #20 for him. He figures as many as 15,000 people might come to the ART to hear The Sound of a Voice in its five-week run, more than would see an opera of his in a brief run at a much bigger house. Of course, that’s nothing compared to the more than 10 million who saw The Hours, the Stephen Daldry film for which he wrote an Oscar-nominated score.

The Sound of a Voice, which begins previews this Saturday, is the third major collaboration between composer Glass and playwright/librettist Hwang. It’s hard to imagine two contemporary artists whose work reaches a broader spectrum of the American public, from the esoteric margins to the pop-culture mainstream and back again. They’re equally at home in Hollywood or at the Met, on Broadway or at BAM or in a regional theater in Seattle or Chicago. But both are creatures of the theater, and neither likes to stay away for too long. Glass began his career in the mid 1960s as a charter member of the experimental theater collective Mabou Mines. Hwang’s first plays — including the two one-acts that make up The Sound of a Voice — were produced in the early 1980s by Joseph Papp in New York. Glass and Hwang first teamed up in 1988 on the solo sci-fi spectacle 1000 Airplanes on the Roof. Then in 1992, Hwang provided the libretto for Glass’s grand opera The Voyage, a commission from the Metropolitan Opera for the Columbus Quincentenary.

Hwang, a Tony winner for M. Butterfly, first wrote the two short plays more than 20 years ago when he was a young Chinese-American — he says, in hindsight, " looking for a kind of East-West fusion æsthetic that was not coming from America. At that point, Chinese artists didn’t have enough freedom and hadn’t been opened to the West long enough to absorb a lot of influences in a deep way. It was Japan that managed both to hold onto its culture and to incorporate Western points of view. So I would read a lot of Japanese literature and go to a lot of Japanese movies. " The films of Masaki Koboyashi and Masahiro Shinoda and the stories of Yasunari Kawabata and Yukio Mishima were of particular inspiration to Hwang, and The Sound of a Voice amounts to his creative response to these sources.

Conceived as companion pieces, the two plays share themes and structures. Each presents a stark and ominous encounter between a man and a woman. " In both plays, " Hwang explains, " each of the characters has an agenda that he or she is hiding from the other. As they fall in love, they are struggling to figure out what is more important, the feelings that I am beginning to have for this person or my larger objective, which would either exploit or destroy the other person. " Looking back on his early work now, Hwang is " struck by the loneliness in the plays, the degree to which the characters seem to exist in this isolation. They are yearning to break out of it, but they don’t know how to do it. They don’t know how to connect with another person. "

If that condition characterized Hwang’s own life 20 years ago, he is a changed man at 45, happily married and living on the Upper West Side with his wife and two children, a successful playwright, screenwriter, and librettist with all the work he can handle. His new version of Rodgers & Hammerstein’s Flower Drum Song closed in March after six months on Broadway and an earlier, triumphant run in Los Angeles. Aida, the Disney musical for which he co-wrote the book, is still going strong, and Tarzan, with Hwang’s book, songs by Phil Collins, and an environmental set by Bob Crowley, is making its way down the Disney assembly line toward Broadway. In August, Tanglewood will present the world premiere of Ainadamar, composer Osvaldo Golijov’s opera about the final days of Spanish poet and playwright Federico García Lorca, with a libretto by Hwang. And he is working on a screen adaptation of last year’s Across the Nightingale Floor, a fantasy novel set in feudal Japan. His biggest regret of late, it seems, is that he has not had the time to finish a play since Golden Child more than five years ago.

If anything, Philip Glass is only busier. He presides over a small musical empire that includes a recording studio, a music-publishing company, a record label, a production company, an extensive Web site, a touring ensemble, and his non-stop writing of new music. Last June, his opera Galileo Galilei premiered at Chicago’s Goodman Theatre in a production directed by Mary Zimmerman. Then came the release of Naqoyqatsi, the third in Godfrey Reggio’s " qatsi " trilogy, and The Hours, both films with Glass scores. He is finishing up work on The Fog of War, Errol Morris’s documentary on former Secretary of Defense Robert McNamara. Soon, he’ll need to get down to work on a commission for the 2004 Olympics in Athens and then a piano concerto this summer and a symphony in the fall. " About this time next year, " he says, " I will be starting a new opera. Sounds like fun, doesn’t it? I have things to do. "

One of the things he has wanted to do for some time is compose music for Wu Man, a world-renowned pipa virtuoso from China who lives in the Boston area. Glass made the pipa, a four-stringed Chinese lute that dates back 2000 years, his point of departure for The Sound of a Voice. " I have an ensemble of four, " he explains, " starting with the pipa, and then I built around it what I needed to make the music work: a cello, a flute, and percussion. I needed a solid bass instrument to give a clear direction of tonality for the singers, so they can get the pitch right. I needed a melodic instrument to give me the long lines that are not provided in the text, which has a lot of short phrases back and forth. So I put the long lines in the flute. I needed the cello to establish the harmonic language, and I needed the pipa to give it the color. The percussion instruments in the first piece are different from the ones in the second, so it gives me a little bit of movement between the two. "

In choosing this instrumentation, Glass says, he did not realize at first the complexity of the task he had set himself. " I gave myself a tremendous problem because there are no instruments that make big chords. They are all linear instruments. The pipa can be strummed, but that is not the strongest aspect of the instrument. Writing a score that has the transparency and lightness of these instruments and still has all the emotional power that I want it to have was a big learning experience for me. " He composed the vocal parts specifically for the four singers who will perform The Sound of a Voice: Suzan Hanson (soprano), Herbert Perry (baritone), Janice Felty (mezzo-soprano), and Eugene Perry (baritone), all of whom he has worked with before. When Hwang first heard the music, Glass says, he found " a porousness to it that allows for a type of singing that is much more conversational than what you normally experience in opera. I think it walks a very interesting line between what we normally consider opera and what we normally consider musical theater. "

Throughout his career, Glass has pursued his own type of East-West or global fusion. He first worked with Ravi Shankar in the mid ’60s, and the cyclical and meditative quality of the music of India had a profound effect on his work. More recently, he has written for the Australian didjeridoo, the West African kora, and now the Chinese pipa. Hwang says, " I like the way his æsthetic works with my work because a lot of my work is about different worlds coming into contact in one way or another, and Philip’s music incorporates oppositions and also finds a way to blend them together in a way that they feel connected and transcend. An amazing example for that is his score for The Hours, which really pulled the film together and gave these different stories a unified character. "

Glass is just as appreciative of Hwang’s skills. " He has a veneer of being an ordinary person, and he can fool you — until you actually look at what the guy is doing and thinking and you understand how precocious and brilliant this man is. To have those two pieces carry through from the first scene of the first opera to the last scene of the second opera, to have that line going unerringly, without faltering, without hesitation, always knowing where it was going. And to do that when you are 22! My God! "

Although The Sound of a Voice is the first Hwang piece produced by the ART, Glass is no stranger to Loeb audiences. The company has produced three Glass operas — The Juniper Tree (1985), The Fall of the House of Usher (1988), and Orphée (1993) — as well as Beckett’s Endgame (1984) and a part of Robert Wilson’s the CIVIL warS (1985), both of which included original music by Glass. A few years ago, the composer and Robert Brustein began to discuss a new work, but Glass’s busy schedule dashed any hope of including it as part of Brustein’s farewell season last year. The new leadership team of Robert Woodruff, Robert Orchard, and Gideon Lester did not hesitate to pick up on the project. Woodruff himself will direct; this will mark, after Rinde Eckert’s Highway Ulysses, his second venture into new music theater this season and also his second collaboration with Glass. In the late ’80s at the Mark Taper Forum, he staged a version of Glass’s open-ended A Madrigal Opera, with a text by playwright Len Jenkin narrated by Kelsey Grammer. " It is always good to work with genius, " Woodruff says with ironic understatement. " It helps the collaborative process. When Philip calls and says, ‘Your new opera is done,’ you go, ‘Cool.’  "

The American Repertory Theatre presents The Sound of a Voice at the Loeb Drama Center, 64 Brattle Street in Harvard Square, in repertory May 24 through June 29. Tickets are $12 to $68; call (617) 547-8300.