How does one compose a symphony?

Judd Greenstein takes long walks through his Brooklyn neighborhood, listens closely to his thoughts and when a great melody line runs through his head he might sing it into his phone.

"If it's really a good one, it'll stick in my head," he said.

But this is all just preparation. When Greenstein sits down at his piano, he waits for that precious moment when the opening notes of a composition come to him.

"Everything is a response to the opening," Greenstein said. "It gives you the texture and melody."

This week, Twin Cities audiences can sample the fruit of Greenstein's walks and ruminations. On Thursday, New Amsterdam Records presents work by Greenstein and fellow composers Nico Muhly, Missy Mazzoli and Nadia Sirota, among others, at the Bryant-Lake Bowl in Minneapolis. Greenstein founded the record label in 2008 to distribute his work.

Then comes the world premiere of "Acadia," a 30-minute symphony to be performed by the Minnesota Orchestra on Friday and Saturday. The work results from a micro-commission by the orchestra. Rather than a single individual or entity funding a new work, this micro-commission has drawn on smaller donations from more than 400 people. It is the orchestra's first venture in grass-roots funding.

Greenstein will talk more about his method and how "Acadia" came together at the orchestra performances, which are part of an "Inside the Classics" program. During the first half, he will chat about the work with conductor Sarah Hicks and host/violist Sam Bergman, while the orchestra plays a few passages here and there. Then, after intermission, the musicians will play the work in its entirety.

Greenstein, 32, is part of the "indie classical" community in New York. He has had work performed at the Tanglewood Festival of Contemporary Music and the Bang on a Can Marathon. In addition to the Minnesota premiere, he has received recent commissions from Carnegie Hall and Ethel, a New York-based string quartet.

He curates concerts, produces, writes commissions and makes some royalties.

"It's a living," he said.

A really big deal

Greenstein was in Minneapolis the week before the Super Bowl -- which is only noteworthy for the New York Giants sweater he wore at Orchestra Hall. He was listening to a quartet of flute, violin, cello and clarinet playing his 14-minute piece "At the End of a Really Great Day." It was scheduled for the finale of a concert that weekend.

He followed on his computer screen, scrolling through the score and occasionally reacting to the instruments.

"You sound beautiful," Greenstein said at the conclusion of rehearsal and then went off to talk about "Acadia" and his thoughts on this coming weekend. The micro-commission is a "really big deal," he said, by two important measures. It's the longest in duration and the largest in terms of instruments that he's ever written. A 2004 orchestra commission was eight minutes and written for a youth orchestra.

"To say they put their confidence in me is an understatement," Greenstein said of the Minnesota Orchestra.

We often don't consider the composer's plight among artists. A painter can go through many canvases, discarding what doesn't work until the right image appears. A playwright has the privilege of staged readings, to hear actors' voices clarify what works and doesn't. Greenstein himself usually writes chamber music, which permits a small group of musicians to workshop a piece and the composer to listen and mold. But tinkering and listening won't work with a 90-piece orchestra.

"You can't stop and tell 15 violins to 'let it ride through this section,'" Greenstein said. "It's a very unfamiliar position."

Indeed, the symphony needs to arrive in complete form before the writer gets a chance to hear how it will sound at the first rehearsal on Tuesday.

"It's pretty terrifying," Greenstein said. "I'll just sit there and try not to have a heart attack."

But perhaps he'll get some useful feedback from the musicians, a few helpful questions?

"Those aren't always a good thing," he said. "If it's about tempi, OK. But if it's 'What were you thinking?' or "How did you get this commission?' then not so much."

What's the worst that can happen, though?

Greenstein, a supremely affable and articulate fellow, laughed at that thought, admitting that the stakes are low. If it all falls apart, no one is going to lose their life.

"On the high side, you can change someone's life," he said. "The worst that can happen is they don't like it."

If so, he can walk away, learn something about himself and his work, and write again.