How many vocal ensembles stay together long enough to celebrate 30 years of sweet harmony?

Not many these days. But longevity is not the only thing that sets apart Sweet Honey in the Rock, the Grammy-winning, African American women's vocal group that has been going strong since 1973.

With a unique, robust sound steeped in "roots" musical idioms, a catalog of close to 20 albums, and a vibrant stage presence, this powerful vocal aggregation has established its own bold niche in the world-music scene.

Whether the Sweet Honey sisters are singing an old spiritual such as "Balm in Gilead," rousing protest tunes, a Caribbean ode ("By the Waters of Babylon"), a blues tune such as "See See Rider," traditional West African numbers, or even rap songs ("Women! Should be a Priority!"), their entwining harmonies, melded vocal tones, and peppery rhythms (created with voices and with handheld drums and gourds) are rich and potent. And, amazingly, they have become international headliners while concertizing part-time, so they could also raise families and pursue other careers.

That's one of the reasons, says current Sweet Honey in the Rock leader Ysaye Barnwell, that the ensemble is still at it. "Maybe because we're women, all working women, we've designed this group to meet our own needs." Barnwell is a teacher and composer who works on outside commissions for orchestras and dance companies when she's not singing with Sweet Honey.

"Most musical groups go out and tour for months on end, and it's devastating in terms of relationships and family. We go out two weekends a month, do a special Black History tour each year and international tours every year or two. That makes it manageable. We accept that many women's lives are susceptible to change, and we accommodate those changes."

In fact, Sweet Honey has allowed the comings and goings of 22 members during its three-decade history. The current six-singer combination, for instance, includes Barnwell (a member since 1979), Aisha Kahlil (who joined two years later), Aisha's sister Nitanju Bolade Casel, Arnaé, and original members Carol Maillard and Louise Robinson‹the latter of whom has returned to the fold after a 27-year absence while she worked as an actor. (Sign-language interpreter Shirley Childress Saxton is a non-vocalizing member of the group.)

A testament to the group's staying power is their endurance after the 2003 retirement of a member who seemed irreplaceable: Sweet Honey in the Rock founder and longtime artistic director Bernice Johnson Reagon. "Much of what Sweet Honey became is due to Bernice's leadership," says Barnwell. "I'm hoping we have learned enough from her to continue a long time in her stead."

Reagon, the daughter of a minister, grew up in rural Georgia and sang every Sunday in her father's congregation at Mt. Early Baptist Church. Later she became active in the southern Civil Rights Movement, performing with the famed activist musical group the Freedom Singers. And in 1973, while living in Washington, D.C., she became vocal director of the D.C. Black Repertory Company and created a vocal workshop at the theater.

Out of that workshop came Sweet Honey in the Rock, named for an old gospel quartet song Reagon remembered from childhood. In the book We Who Believe in Freedom, a collective history of Sweet Honey's first 20 years, Reagon recalls asking her father, Rev. Jessie Johnson, about the song's meaning. "He told me it was based on a religious parable, but that it was not found in the Bible," she writes. "This parable described a land that was so rich that when you cracked the rocks, honey would flow from them. From the beginning, the phrase‹with sweetness and strength in it‹resonated in a deeply personal way with me."

For a short while, because the ensemble recorded for Holly Near's Redwood Records and had performed at Sisterfire and other female-oriented music festivals, their albums were classified as "women's music." But, Barnwell notes, the label never quite fit. "Most people feel compelled to pick a genre they fit into, but we really resist that urge," she explains. "We've always felt that who we were, and who we were singing to, had a universality. We're open to a mixed audience‹in fact that's critical for us. Even though it can be hard for record stores to decide what section our discs should go in!"

Sweet Honey makes decisions collectively and welcomes original songs by its members, as well as improvisation and experiments with the ensemble's vocal mix. "We don't catalog our voices precisely as alto, tenor, or whatever, because everybody sings everything," Barnwell remarks with a laugh. "I have the highest voice in the group, and the lowest voice. Sometimes I'm singing the bass part, and sometimes I'm singing the lead."

At Carnegie Hall this month, Barnwell promises some fresh treatments of well-known Sweet Honey songs ("We've often had five people, rather than six, so we've done new arrangements"). And there will likely be a suite of tunes in tribute to Reagon, who is concentrating now on individual artistic pursuits and her work as a curator at the Smithsonian Institution.

As this program went to press, however, Barnwell couldn't say for sure what the group would sing at Carnegie Hall. She estimates Sweet Honey's repertoire has "hundreds" of songs (currently they're being cataloged for the group's website), "and most are still fair game. They can be brought to the stage or called into rehearsal at any time. The nature of the group allows for a lot of improvisation."

Can Sweet Honey in the Rock keep on keeping on for another three decades? Barnwell makes no promises. But she does say, "Right now we're going to keep doing what we've done all along‹singing about issues that matter to us, singing songs we believe in. That's just who we are."

Misha Berson is a theater and arts critic for the Seattle Times.