Not many groups could update their repertoire by adding a set of tunes from the 19th and 20th centuries, but that is exactly what the acclaimed a cappella women's quartet Anonymous 4 have done with their most recent recording. Gloryland, a collection of traditional American folk songs, spirituals and gospel hymns, builds on the approach introduced two years earlier on the group's American Angels, but with one significant difference: For the first time in the group's 20-year career, they are not alone.

"When the idea come up for doing this second set of American music, I knew that I wanted to have instrumentalists," says Marsha Genensky, who served as the group's music director on both Americana collections. She recruited two well-known Bay Area acoustic string players, Darol Anger (fiddle and mandolins) and Mike Marshall (mandolins and guitar) to augment the vocal harmonies in recording sessions at Marin County's Skywalker Ranch.

It was a comfortable association, Genensky reflects; the two founding members of the Turtle Island String Quartet and members of the Anonymous 4 had hit it off long ago when they met backstage at a taping of A Prairie Home Companion.

"These guys are so great," Genensky says. "They start with the tradition that we're working with, and then they move on out on to all kinds of wonderful planets of music. It's just fabulous."

The expanded sonic palette will be on display when Anonymous 4 return to the North Bay on Jan. 13, accompanied at the Wells Fargo Center by Anger with guitarist Scott Nygaard standing in for Marshall, whose schedule could not accommodate the tour. (Nygaard regularly accompanies Anger in the Republic of Strings.) Genensky says the substitution was not a difficult adjustment. "They're definitely different personalities, but we've come to absolutely love both of them."

The music of both American Angels and Gloryland is predominantly religious, primarily folk hymns and melodies culled from very old church songbooks, including a few familiar songs, such as "Shall We Gather by the River" and "Wayfaring Stranger."

This gives it a central commonality with the Medieval music that Anonymous 4 are more closely associated with, even though the styles are quite different. But Genensky says that as singers, they approach it in much the same way.

"Really, it's all about singing together and listening," she explains. "It's just a matter of singing different styles. Singing chant is deceptively difficult. Many people think, 'Oh, that's easy. You don't have to sing in parts. You all sing together, its fine.' But it actually takes the most concentration to sing in unison. That's really different than singing traditional music from America, which tends not to be quite as wide-ranging, tends not to require the same level of exactitude, although you would be surprised. Preciseness is still good; it's just not the same kind of preciseness.

"I actually think some of the most precise singing comes from bluegrass signers," she continues. "They're very, very tight. You would think it would be a loosey-goosey kind of thing, but beneath that is extreme precision."

Despite their name (which is an inside joke for musicologists, a reference to the unknown author of an important Medieval treatise on the composers of that era), Genensky and colleagues Susan Hellauer, Jacqueline Horner and Johanna Maria Rose are readily identifiable. In a departure from their historic process, which was intensely collaborative and consensual, Genensky took an individual lead role in the preparation for Gloryland, primarily because she spent the past year as a visiting scholar at Stanford, while the other three singers remained in New York. "We tweaked it together, but it basically was how it was by the time I got to everybody." And, Genensky adds, she derived considerable pleasure from hunting down the old songs.

"If you ask me in the moment where I'm sticking my nose through thousands of pages of 19th-century tune books at UCLA special collections or at Yale or one of the other places where I go, I'd say, 'Oh, this is my favorite thing to do. This is so great, I can't stand it.'" She laughs. "But then when we're actually singing, I'd say, 'This is so great, I can't stand it.'"

Anonymous 4, joined by Darol Anger and Scott Nygaard, perform 'Long Time Traveling,' a collection of Americana songs, on Saturday, Jan. 13, at the Wells Fargo Center. 50 Mark West Springs Road, Santa Rosa. 8pm. $20-$49. 707.546.3600.

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