"This is not your mother's boychoir," John Romeri says.

Romeri, founder and artistic director of Cathedral Concerts and music director of the Roman Catholic Archdiocese of St. Louis, doesn't usually schedule concerts in the summer. He's making an exception for Libera.

Libera — the word means "free" in Latin — is an independent, not-for-profit South London-based boychoir. Since 2000, it's won gold records and other awards from Britain to Australia with a distinctive kind of crossover sound.

"They have people who really believe in them," Romeri says, noting that their worldwide following got their Facebook page more than 17,000 friends in just a few months. So far, Cathedral Concerts has sold tickets to people from 12 states — and France.

They perform in monkish, hooded, white robes, but with elaborate lighting designs, often accompanied by synthesizers. Famous for their pure high trebles, Libera's music-making is part spiritual, part showbiz.

Of the 40 boys age 7 to 16 who sing with the group, 24 will perform in the concert of sacred music at the Cathedral Basilica.

Like a traditional British boychoir, the singers chosen for Libera rehearse frequently, at least four times each week. Given the ephemeral nature of the boy soprano voice, the recruitment process is ongoing, with the music staff visiting assorted schools in South London to find likely candidates.

In an e-mail interview, their music director, Robert Prizeman, wrote, "The boys start with us usually around the age of 7 or 8, and mostly will not have sung much before. We…try to find youngsters who have some aptitude — though, of course, at that age, they will not have much experience. I have to assess their musical intelligence, as it were."

He doesn't care for the term "crossover."

"In record industry terms we are called crossover because we are not strictly classical, and we are certainly not pop-rock," Prizeman writes. "But it can seem a little derisive. I hope we manage to do something original with integrity."

The boys are a good-looking, engaging crew, as multiracial as London itself. Prizeman says their appearance is not a factor in their selection:

"It's hard enough to find talented singers without worrying about what they look like," Prizeman wrote. "However, when they perform, we do hope that they will show the enjoyment they have in the singing, and that their appearance will be engaging and, hopefully, charismatic."

This is their second U.S. tour. On their first, in April 2008, they sang at Pope Benedict XVI's Yankee Stadium mass, performed at the Crystal Cathedral in Los Angeles and sang at the Kennedy Center Honors.

This trip's a little more modest in its scope. Cathedral Concerts is one of five presenters in a short, mostly Midwestern tour that includes stops in in Dallas and Fort Worth, Texas; Nashville, Tenn.; and Atlanta.

The choir will sing with the cathedral organ in St. Louis, along with the Rosewood Ensemble, a St. Louis-based professional string ensemble, and their own synthesizer and other instrumental accompaniment.

"It's a pretty amazing sound, a very pure sound, very beautiful," Romeri says. "Libera is the Virgil Fox of boychoirs, when you throw in the synthesizer and the lights," adding showmanship and flair to well-done music.

"Libera has turned on thousands of people who would never touch a King's College Cambridge (traditional Anglican choir)."

Although the group doesn't yet have the kind of following in the United States that it has elsewhere in the world, Romeri thinks that to see and hear them is to be converted.

"The music is classical, and the sounds are pure and in tune and gorgeous," he says. "I think it's going to be kind of a cosmic experience."

You can learn more about Libera, hear them and see videos at www.libera.org.uk.