Whether you know it or not, 2007 is the Year of the Avett Brothers. The trio from Concord, North Carolina, released their fourth and most ambitious studio album, Emotionalism, in May; it debuted at number one on the Billboard Heatseekers chart and garnered rave reviews. One week later, they appeared on Late Night with Conan O'Brien. Since January, they've toured almost 10,000 miles, crisscrossing the country twice, earning frequent-flyer miles with longtime fans and picking up new followers with every show. The band—brothers Scott and Seth Avett, abetted by Bob Crawford on upright bass—have expanded in every possible way.

The one constraint the Avett Brothers initially abided, that launched them on the upward trajectory that's still rising as you read this, is playing Appalachian-style music as an acoustic trio. In other words, they're a bluegrass band. The raw-throated, barefooted, front-porch ease of their music, the guilelessness of it all, has, up until now, been their biggest selling point.

But with Emotionalism, all that's been tossed aside to reveal a band that's a lot more neurotic, sophisticated, and pop oriented than they first let on. "Bluegrass" no longer cuts it as a descriptor. The makeover seems like a headfake, one that might confuse casual fans and piss off the serious ones. But according to singer, banjo player, and kick-drummer Scott Avett, the band always had bigger things in mind than the front porch.

"The job of maintaining tradition or some authenticity, we've handed that off to the people that are good at it," says Avett. "That's just not our interest or our goal. We might've at one time thought we were gonna be tradition carriers, but we pretty much walked away from that. We had the ambitions from the start."

Those ambitions are there in the Avetts' earlier material. If you listen closely, you can hear a polished pop group and a raucous rock band hiding beneath their strummy, harmonized exterior. "Talk on Indolence," the opening salvo on their 2006 breakout, Four Thieves Gone, opens with a sort of punk-rock rap before settling into a hard-sung cacophony of anxiety. "I'm a little nervous/about what you might think/when you see me in my swimming trunks," Seth Avett sings. And then an electric guitar comes in, fuzzy and distorted, to close the song.

Those ambitions were there in Nemo, the rock band the brothers started almost a decade ago, before settling down with acoustic instruments. They continue in the side project Oh What a Nightmare, a sort-of roots metal band that harks back to the brothers' Southern hesher history.

"A band like Oh What a Nightmare, it's most definitely a release," Avett says. "We walked immediately away from the electric setup [of Nemo] to what we have now. We were playing with guys with two full guitar stacks, a big bass rig, a full drum kit—it was very loud. And we just cut that off and were done with it, but we're looking forward to doing it again. Right now with the Avett Brothers, we're diving into so many other types of expression with other instruments, and some electric here and there, and it kind of satisfies that appetite."

So they settled on whatever it is they do now—acoustic-electric modern-traditional grab-bag indie-pop miscellany. Bluegrass was the shortcut to their initial audience, and they appreciate the form, but it isn't what the Avetts are. Shaggy haired, bearded, hailing from rural Cackalacky—they certainly look the part. But purists couldn't handle their refusal to stay within its bounds. They weren't old-timey enough.

"You get guys coming out thinking it's gonna be a five-piece bluegrass band with a couple brothers in it," Avett says. "That really served us wrong."

By definition, pop and folk are opposed: Pop is for consumption, for entertainment, while folk attempts to perpetuate indigenous culture. But the Avetts blur that definition. They're barreling right down the middle, intent on giving as much as they get.

"We talk about that a lot," Avett says of the band's intentions. "We didn't in the beginning, when we were making and developing this, we just did what came natural and tried to maintain that. We got lucky that we've fallen into this groove. I don't think there's any evidence that we're heading one way or another."

And so it goes—the Avett Brothers keep explaining themselves one brilliant record and one sweat-drenched, string-busting, crowd-surfing show at a time.