If you've never heard of Alice Babs, don't feel bad. You're not alone. Most Americans haven't.

She's just one of the best-known singers in Europe. Her mentor, Duke Ellington, once described her as "a performer beyond measure. She can sing anything she hears -- opera, Bach or jazz."

Washingtonians will get their first chance ever to hear the Swedish soprano in person when she performs tonight at the Wolf Trap International Jazz Festival. And she has only been in the United States once in the last 10 years.

"But she almost didn't accept the Wolf Trap engagement.

"I'm trying to wind up my career, "she says. "I got a call from [festival coordinator] J. Foster [at her home in Spain] and I said no. But I couldn't sleep that night. So I called back and said yes.

"I couldn't turn him and [festival artistic director] John Lewis down. I think so much of John. He's like Ellington to me."

"Nevertheless, she's still going to call it quits. After 40 years of performing, Babs, 56, says, "I'll perform one more time -- for the king and queen of Sweden in a private church concert at a castle -- and record an album of Negro spirituals and Ellington religious music." During the last 40 years, she has recorded about 800 songs, including jazz, classicial and European popular music.

"You live so tense for so long," she says in impeccable English. I've been performing for 40 years and I'am tired of living out of suitcases. I want to relax and enjoy life with my husband." She's also concerned about her health. Six years ago she moved from Stockholm to Quadalmina, Spain (near Seville) for a bronchial condition she'd developed. Living in a dry climate has eliminated the condition, but she still worries. "I'm not so strong in my chest," she says. "I've always got to watch people to see if they've got a cold."

I don't want to have to limit my repertoire because I can't reach certian notes or sing for very long."

Nevertheless, Babs shows no sign of vocal decline yet. At yesterday's rehearsal her voice was as clear as a raindrop. In her range of 3 1/2 octaves, she can still jump instantaneously from deep notes to stratospheric ones. Her timber is seraphic.

But Babs says she's done everything she wants to do as a singer.

She's performed -- and recorded -- Elizabethan love songs, Debussy, Ravel, Mozart, Chopin, Bach, Swedish pop tunes, Israeli folk songs and Danish melodies.

Babs started out at age 12, giving her first concert that even featured her yodeling. But jazz soon became an attraction. She first listened to Louis Armstrong and Ellington records. Her father, an orchestra leader and pianist who played for silent films, encouraged her, as did her mother, also a singer. Later on she studied in Stockholm.

No matter what turns a conversation with Babs takes, it usually ends up somewhere with Duke Ellington.

"He's been my maestro my whole life," she says. "I started buying his records when I was 15. And he played the first jazz concert I went to in my life -- in 1939. It was on his birthday -- his 40th. And I was one of the kids who sang to him that night.

"I met him later on, but I didn't work with him until 1963. We were doing a big Ellington concert in Stockholm with ballet. He was also performing. When he heard me, he said, 'I want that girl.' And a few days later when he was in Paris, he sent for me to record some of his songs. Ellington also asked me to tour with him, but I couldn't because I had teen-age children, and I didn't want to leave them at a sensitive age."

Said Ellington of her. "Alice Babs is a composer's dream, for with her he can forget all the limitations and just write his heart out."

So he ended up sending for her whenever he wanted a special singer for a special performance. She came to this country in 1968 to appear in Ellington's second Sacred Concert to critical raves and made her last U.S. appearance at the Newport Jazz Festival in 1973, with Ellington.

But now she is a mother of three and grandmother of eight, and she says she wants to concentrate on her golf game, daily swimming and drawing lessons. Also, she wants to spend more time with her husband of 36 years, Nils Sjoblom, who's a business agent for pigment factories in Spain selling to Swedish buyers.

"I may do an occasional television show," she says with a smile. "I don't want to leave any doors closed."