If you're in downtown Minneapolis this week, don't be alarmed if you hear spontaneous singing in close harmony in a skyway, a restaurant or a hotel lobby.

It's only the invasion of the choral geeks.

The American Choral Directors Association, the world's largest association of choirs, is holding its national conference in Minneapolis starting Tuesday.

That means about 15,000 conductors, composers, teachers, students and performers, from a youth chorale from Lake Wobegon country to a choir from Inner Mongolia, will be in town. They'll spend 4½ days singing, listening to singing, talking about singing, even drinking beer while singing.

"Its arrival here is historic," said Brian Newhouse, managing director of classical programming at Minnesota Public Radio.

In the more than 40 years that the ACDA has been holding biannual national conferences, this is the first time it has come to the Twin Cities.

That puzzles just about everyone in the state.

"The region, and Minnesota in general, considers itself as the epicenter of choral music in the country," Newhouse said.

We're home to top college programs at St. Olaf College and Concordia College and premier groups like VocalEssence, Cantus, the Singers, the National Lutheran Choir and the Rose Ensemble. There are more than 4,000 choirs in the state, involving about half a million Minnesotans, said Philip Brunelle, artistic director and founder of Minneapolis-based VocalEssence.

"We've always said, 'You should be coming here,' " Brunelle said. "Singing is huge here."

The heart of the conference will be performances, more than 90 of them, running day and night at four venues: the Minneapolis Convention Center, Orchestra Hall, Central Lutheran Church and Westminster Presbyterian Church.

Performers, who were invited based on blind judging of audition recordings, include choirs from middle schools, high schools, colleges and professional organizations. That means glee clubs, barbershop quartets and church choirs. The Giving Voice Chorus, a local group that involves singers with dementia, will perform.

The repertoire will range from Bach and Brahms to the Beatles and Billy Joel, from jazz to folk to gospel.

The Navy is sending a choir to sing sea chanties. The Beer Choir Twin Cities will be singing drinking songs. The Minnesota Orchestra, Minnesota Boychoir, Minnesota Chorale, Minnesota Dance Theater and the University of Minnesota Chamber Singers will be performing "Carmina Burana."

There will even be Christmas carols — despite the fact that it's Lent. For the conference, the St. Olaf choirs will be doing a reprise performance of their famous Christmas festival.

"We need more Christmas in March," Brunelle said.

The event is attracting choral directors from Germany, Finland, China, South Korea and Mexico.

The Inner Mongolian Children's Choir will be singing "Praying for Rain" and "Mongolian Boots," while the 80-voice Orphei Drängar male choir from Sweden will be belting out "Five Ways to Kill a Man."

Educational seminars will take on topics like Appalachian folk songs, the choral music of Polynesia and how to recruit boys to middle and high school choirs. And, of course, there will be exhibitors peddling everything from robes to risers.

"This is a geek fest, and the cool thing about it is beautiful music will come out of it," Newhouse said.

The public can hear some of that beauty at a free "Welcome to Minneapolis" concert at 7:30 p.m. Tuesday at the Minneapolis Convention Center with a performance of "Te Deum," by Minnesota composer Dominick Argento and a singalong led by Garrison Keillor.

Composer and conductor Eric Whitacre, whom Newhouse describes as "the most famous and recognizable choral musician on the planet," will lead his singers in a public performance sponsored by Classical MPR on Thursday at 7:30 p.m. at the Cathedral of St. Paul.

Orphei Drängar will give a public performance Friday at 7:30 p.m. at Bethel University, and the German vocal ensemble Stuttgart Kammerchor will have a free concert Saturday at 2 p.m. at Plymouth Congregational Church.

Or you might get lucky and catch a song or two in any downtown locale.

"You'll hear barbershop quartets in elevators," said Tim Sharp, executive director of the ACDA. "They'll burst out in song in restaurants."

Newhouse agreed: "When you get that many together, it's kind of combustible."

Richard Chin • 612-673-1775