THE electric whine of cicadas weaves in and out of the young tenor's aria. One side of the concert shed opens to the hillside and the late morning sun."You're singing it beautifully," the master-class teacher says after the applause. "But that isn't enough." "Now you have to move into the character. This is a very shallow, cruel man. He says my love might last forever ..., not our love. He offers her nothing permanent: This is a seduction aria. Let us decide whether we like him or not." The tenor, in the crook of the piano on the shed floor, starts Anatol's aria from Samuel Barber's "Vanessa" again. The teacher suddenly is in front of him. She massages his cheeks vigorously with her thumbs while he sings at full voice. "There's a little tightness in the corner of your mouth," she says. "It makes it sound a little precious." She repeats his lines: Do you know Paris, or Rome? Let her see the glass chandeliers, the marble stairways...." The next student, a tenor, sings Bach in Latin. He is reminded that Latin was once a spoken language; it is not just a form of instrumental sound. Standing behind a baritone, she pulls down on his shoulders to make him relax; she whispers to him as he sings. By turn during the 2 1/2-hour class, a dozen of today's most promising young singers are critiqued before an audience. The setting is the Tanglewood performance center in the Massachusetts Berkshires. The teacher is Phyllis Curtin, one of this country's great sopranos and, her students say, one of its finest performance coaches. "When I first came to Tanglewood in 1946," Ms. Curtin said in an interview afterward, "we all thought we would be hotshots in the musical world. I think the kids now are better educated musically than most of us were then. But what drives me crazy is that I don't know who wants them!" The musical field has changed. Opera in America once meant Italian opera performed by Italian singers. "The great age of American singers at the Met - Richard Tucker, Leonard Warren - was during World War II when the Europeans couldn't get here," Curtin says. "After the war, things went back to normal and most of the Americans were in the chorus, but very few were at all in the body of the company." "What we did have, which kids today don't have, were the organized concert audience tours," Curtin says. "A lot of us who were lucky enough to get management did pretty well in recital singing across the country. My generation also had much more oratorio experience. "Today, the recital audience is basically gone. The oratorio business is basically gone. The big visiting orchestras would do at least one big choral work. Now that's gone, largely because of cost. And choruses aren't around so much - amateur choruses - because music's out of the schools. "But what we do have, largely started with the National Endowment of the Arts, is a big lot of regional opera companies." "So, of course it's difficult," she concedes. "Parents every now and again ask me, 'Don't you think it's irresponsible to be teaching these people these things they'll never make a living from?' First, who knows? You may make a living at it. But you do it because of this yearning to be the best human being you can be." Curtin began her performing career in Boston. She went to nearby Wellesley College. She had been a violin student, and a dancer. "Somehow singing put it all together," she says. "It combined everything - the theater, poetry, literature, music. That was utterly captivating to me so I stayed on in Boston, and I just kept organizing my life more to study more." After a New York and worldwide career, she returned to Boston as dean of Boston University's School for the Arts. Leaving that post this summer, she will continue to teach a dozen young singers each semester. IN an electronic age, why study voice? "Because there is still a very vital and alive literature for the human voice," she answers. "In songs, in opera, in chamber music, they call on the voice just like they call on the violin. The voice is maybe replaced in one way or the other, or enhanced electronically. I put some notes about 10 years ago into one of the electronic labs at Stanford. I just sang two octaves in semi-tones on a variety of vowel sounds. And they were reproduced gloriously. "I asked a composer there, 'What would you do with that now that it's in there?' He said, 'I was just thinking, there's a component of that sound I like a lot, and I'm thinking of putting it with part of an oboe sound and part of a trumpet sound.' He's building up a whole new sounding out of a little bit of my voice. And it makes me mad! Every now and again I hear ads on TV, and I think, maybe that's me in all that mix. "Nonetheless, there is that particular experience, when you really get a direct hit from a live person, making a live sound to you. There isn't anything yet that can compare with that in terms of its emotional response and its intellectual response. That still is heart to heart. And if the singer's doing his job, it's from the composer and the poet to you through this particular instrument. "The lessons of singing apply to actors, to sculptors," Curtin says. "One day I was watching this young sculptor try his best to get a kind of curve he was working on in clay. I thought, 'If he would only think through the whole gesture, as a musical phrase, he'd have it.' There's just as much rhythm in sculpture as there is in almost anything else. "My daughter's a remarkable equestrian. Even as a very young girl she was great. Everybody gave her recalcitrant horses to break in. Of course it scared me to death. One day I said, 'Claudia, how did you learn how to do that?' "She said, 'Mom, I learned it in your class.' I said, 'What did you learn in class?' She said, 'Keep the thought ahead of the tone. That's exactly the same thing with riding. I know exactly the rhythm, and keeping my mind exactly ahead of the rhythm, it just goes through my legs into the horse. Earlier Curtin had told the shed audience: "When I say I'm not thinking of how I do it, that is because I am using a solid technique and allowing my imagination to run the technique - that is when the sound is supported by air so that there are no tensions and no holes in my throat or my jaw or my tongue or my face to prevent what I want to have happen. "Singing is not as casual as it seems. And it is in my singers' interest that they have all the skills technically to allow their minds, their hearts, their musical spirits to go anywhere, without impediment."