Jeff Austin, mandolin player and one of the vocalists in Yonder Mountain String Band, wants you to know that he is not in a jam band. In fact, he doesn't even believe such a thing exists.

"[Jam band] is the worst term ever given to music in the history of music. Aren't all bands jam bands? Don't all bands learn to play by jamming? Just because our audience might be younger with longer hair and smoking more pot doesn't mean we're a jam band," he said in a recent phone interview, calling from his house tucked away in the wooded foothills of Colorado.

Strong words coming from a guy whose band of over a decade plays what, at first listen, seems to be one of the lightest and bounciest genres in music: traditional bluegrass. But make no mistake; as Austin says (and his playing backs him up), though other music may have heavier beats or louder guitars, Yonder Mountain String Band's unique take on American roots music kicks more grass than you might ever expect from this pack of laid back, nature-loving dudes.

And while today Austin's love of bluegrass is anchored by the music's fast, spontaneous and improvisational nature, its raw energy and its pure fun, it was the music's vocals that first drew him in -- the very, very high vocals.

"When I was young, I had this incredibly high voice that didn't deepen until I was in my 20s. I was coming from a musical theater background, where if you're a guy with a high voice, you sing all the girls' parts," Austin said. "But bluegrass allowed me a huge catalog to work with; the voices are high, but you sure aren't a girly singer. If you told the Stanley Brothers [of recent "O Brother, Where Art Thou?" fame] they were girly, you'd end up in a field somewhere."

After dropping out of University of Cincinnati College's Conservatory of Music after less than a year ["I'd call my mom at 3 in the morning and worry that school wasn't for me. She'd say, 'Look, you've called four nights in a row. I think you already know the answer' "] Austin was invited to join the subtly named Bluegrassholes -- before he'd even learned the mandolin. He picked up the instrument "not as a quick study, but as a panicked study," and by the time the band dissolved, Austin relocated to Colorado to start fresh. Along with his college buddy, banjoist Dave Johnston, Austin met like-minded bassist Ben Kaufmann and guitarist Adam Aijala, and Yonder Mountain String Band took shape.

"Initially, we wanted to play like the greats. But then, that's someone else's style -- and they play it great, so why wouldn't we be ourselves?" said Austin.

While traditional bluegrass still thrives in many parts of the country, the band's decision to stray from standard tunes and add modern deviations both distinguished them as a band, not merely musicians, and allowed the pack of twenty-somethings to leave a mark on a genre that'd been most popular decades before they were born. In that way, Yonder Mountain String Band straddle a cultural divide; they are revivalists but pioneers, wild children with old souls.

As the band plays Hartwood, only its second show of the summer, the boys have just released "Mountain Tracks: Volume 5," the latest in a series of live albums. And while many a band releases a live album, few put out more live records than studio records.

"It's five for four right now -- we'll release the next studio album next year and tie it up again," Austin said.

The statistic is a true testament to one undeniable truth: Yonder Mountain, more than even a -- excuse the term -- jam band like Dave Matthews Band, are a live band at heart. "Volume 5," a double-disc, is nothing less than a party record, with furiously fast, catchy tunes, lyrics about drivin' south and leavin' town, wild, whirling solos and bumpy, walking bass lines that make stillness a near-impossibility. The music is heavy and intense but uplifting and soaring, real-life ideas shrouded in Saturday night carelessness ("I never really trusted folks who have to talk so loud," from "Things You're Selling").

And these country boys sure can shred. But don't psych out Austin for his solos.

"Soloing just isn't how my mind works. I get in my head too often and in turn my body locks up. I'll be playing during a song and know my solo is approaching, just thinking 'Here it comes, are you ready? No, you're not ready. Damnit!' " he said.

Maybe it's this skip-the-dramatics and pour-on-the-fun attitude that gets Yonder Mountain lumped in with the so-called jam band collective (see: moe., The Disco Biscuits, Keller Williams). Or maybe it's that Yonder Mountain fans often follow the band throughout an entire tour (see: The Grateful Dead, Phish). Or maybe it's that, well, most tunes have long, complex instrumentals (see: all of the above).

But to Austin, it's just music. And speaking from Colorado, where he spends his days palling around with his dog Soyer, attempting to build his own park benches and spying on the moose couple that roam his five acres, and night after night inspiring crowds of the Yonder faithful to their feet, the music just feels right.

"We're less and less a bluegrass band everyday, it seems. We're just ourselves."