Anyone given to despair over the state of so-called "classical" music should have spent Sunday evening with the improvising (and enterprising) duo of American violinist Hilary Hahn and German pianist Hauschka (a k a Volker Bertelmann) at Aria in Minneapolis. Not because the music was invariably wonderful, but because it showed just how exhilarating it can be when energy flows freely from performers to audience and back again, unimpeded by a score. Imagine: 80 minutes of newly birthed music with nary a written note!

The sold-out event, co-presented by the Schubert Club's Mix and the SPCO's Liquid Music series, built on the duo's experience with "Silfra," their improvised 2012 CD, recorded in Iceland. The collaborators wisely resisted the impulse to regurgitate their album — there's nothing so stale as yesterday's improvisation — and chose on Sunday to develop new material, much of it striking.

The thirty-something Hahn's command of the violin is astonishing. She could play the same handful of concertos from now till doomsday. But she has a questing spirit, and is reluctant to be confined by her gifts or her training. She's worked with singer/songwriters and progressive rockers. Collaboration clearly suits her; she seems content to be the junior partner if she learns something in the process.

Hauschka is less virtuosic, more inventive. Using digital technology, he has carried the technique of the prepared piano — a conventional piano into which sundry objects (bottlecaps, foil, duct tape, floss) are inserted to expand or subvert its sonic palette — far beyond that of John Cage, who christened the instrument in 1938.

Hauschka conjures an entire percussion section, if not an entire orchestra, from his gadgetry. He and Hahn seem an odd pair. Yet somehow they click.

Late in Sunday's performance came an oddly poignant minute of theater. As Hahn vamped quietly, Hauschka visibly "de-prepared" his piano, plucking all manner of sound-modifying items from its innards and tossing them on the floor. It was as if the hard-working Steinway, after a series of brilliant impersonations, threw off its mask and revealed its true face in a moment of unexpected depth.

Two quibbles. From where I sat, Hahn's violin seemed over-amplified and over-reverbed. And the type in the printed program was illegibly condensed. We may all be hip, but we're not all young.

Larry Fuchsberg writes about music and theater.