When Judi Vinar takes the Atlanta Symphony Hall stage Friday night with the rest of Bobby McFerrin’s Gimme5 a cappella ensemble, her mind will be a blank slate. McFerrin and the four singers, who perform as part of WCLK/91.9’s fall benefit concert, will create an evening of spontaneous music. No song titles. No musical blueprints. Just improvisation.

After two decades of singing spur-of-the-moment music with McFerrin, Vinar still relishes the possibilities of the unknown.

“We try to empty our minds and come out on the stage so that we’re surrounded by whatever is in the present moment,” Vinar said.

McFerrin and his fellow singers — veteran improvisers Rhiannon, Joey Blake and David Worm round out the quintet — perform wholly improvised compositions called circle songs. These tunes typically begin with one vocalist singing an unplanned musical thought, slowly expanding it to a foundational brick of song, and building a tune by passing parts to other singers. In this fashion, the singers manufacture rich polyphony based in orchestral textures.

This unplanned music making has no distinct cutoff point. Some circle songs can go on for an hour, ranging through multiple rhythmic and harmonic permutations, with McFerrin conducting the audience or launching into his own improvisations. Other tunes rise and fall within minutes. Many are wordless vocalizations, but some singers offer up observational story songs or chestnuts from the standard jazz repertory. Or perhaps something more juvenile: During a recent tour of Europe, Vinar recalls McFerrin leading the audience in a rousing version of “The ABC Song.”

It’s been more than a decade since McFerrin performed in Atlanta. (He played at Chastain Park in 2005.)

Rhiannon has been singing with Bobby McFerrin for nearly four decades. CONTRIBUTED
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Vinar is the youngest member of the group, having spent only 20 years singing with McFerrin. Blake and Worm — who both improvise melodies and serve as a vocal rhythm section — have been performing with McFerrin for three decades. Rhiannon has been singing with McFerrin for 38 years.

“Because we’ve known each other for so long, at any point, one of us might change up the part,” Rhiannon said. “It becomes a group-led experience, which is very powerful. Rather than one person being in charge of the evening, which is one kind of improvisation, this is all of us able to reconstruct how the thing goes at any point.”

>> RELATED: Atlanta vocalist Virginia Schenck adds a chorus to the McFerrin sound

The singers started their improvisational journey with McFerrin as part of his 12-person Voicestra ensemble, with which he toured the world from the late 1980s. Before even stepping on stage, every member of the choir had to get in shape. That meant a year of weekly four-hour rehearsals with McFerrin at the helm.

“It was an amazing kind of graduate-level improvisation course,” Rhiannon remembers, but notes that nothing is lost with the smaller configuration. “The Voicestra group was, in the end, a very expensive proposition to travel around with. Gimme5 is much more portable.”

This immersive study and a dedication to finding new sounds for the voice make McFerrin’s music rise above any classification. It’s not simply jazz or classical or folk or blues music.

“Any good improviser has to know a lot of music,” Rhiannon said. “If you’re going to improvise, you’ve got to really understand chord progressions, you’ve got to understand diverse melodies, it helps if you understand a lot of world music, so you have this repertory of ideas inside to draw from when you’re improvising.”

In Atlanta, the group will bounce musical ideas off an ensemble of local vocalists and the Clark Atlanta Philharmonic Society Choir. McFerrin and the rest of his group will meet the majority of the musicians at soundcheck to go over some improvisation basics.

“There’s a really deep improvisational community there in Atlanta, and a bunch of them are in this choir,” Rhiannon said. “They come to this with a lot of knowledge, which makes it great for us to bring them on board.”

For this year’s winter fundraiser concert, WCLK is collaborating once again with the Atlanta Symphony Orchestra — the two organizations also worked together last year to bring Herbie Hancock to Symphony Hall. Station manager Wendy Williams said the station chose McFerrin for the concert because of his enduring popularity, but also his relative scarcity in the city.

“Atlanta is a vibrant concertgoing community. There’s something you can do every week in terms of seeing great music and great jazz,” Williams said, “so we also have to be careful that we choose someone who doesn’t get overplayed here.”

With Gimme5, each night is different, and the performance doesn’t stop at the edge of the stage. In these concerts, involving the paying public isn’t a hollow gesture toward audience participation. The singers frequently incorporate the audience into the performance.

“The coolest thing is when the whole room is singing,” Vinar said. “The room just kind of glows with this music.”

There’s a deep level of trust built into Gimme5, but that doesn’t mean McFerrin won’t switch things up in real time. When the improviser once went out on a limb by singing in another key from the vocal accompaniment, Vinar felt a flicker of panic, wondering if the circle song would disintegrate during the middle of the concert.

“The first time he did that with us, we were looking at each other going, ‘Huh?’” she said. “You know him well enough to trust whatever he’s doing. Now that’s become something he loves to do. He really does love to explore outside of the box.”

CONCERT PREVIEW

Bobby McFerrin

With Gimme5 and the Clark Atlanta Philharmonic Society Choir. 8 p.m. Friday, Nov. 30. $70-$135. Atlanta Symphony Hall, 1280 Peachtree St. NE, Atlanta. 404-733-4200, atlantasymphony.org.

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