A cappella music: It’s not just for college campuses anymore.

The art of unaccompanied singing has become a pop cultural sensation. Pentatonix seems to have a new viral video every couple of weeks (most recently a cover of "Little Drummer Boy"), and the show that launched them into the spotlight, NBC’s "The Sing-Off," just recently started its fourth season last Monday. Then there’s "Pitch Perfect," the surprise comedy hit of last year with a sequel coming in 2015.

Helping lead the charge for many of these groups is Straight No Chaser, who will bring their instrument-free innovation to the Riverside Tuesday night.

The all-male a cappella crew, one of the first to hit it big in this new wave of a cappella popularity, was founded back in 1996 at Indiana University. The guys were in a larger choir called Singing Hoosiers when they decided to break off and form their own group.

"We just thought it would be fun to get together a group where we could do our own thing, choose our own music, and primarily sing for girls and free food," said Randy Stine, one of the founding members of Straight No Chaser.

The boys grew in popularity, first on campus and eventually across the country. While other students relaxed over their summer vacations, the guys of Straight No Chaser moved over to Chicago for the season to perform at Cubs games, Sox games, Navy Pier and for anyone who would listen.

Their big breakthrough, however, wouldn’t come until nearly a decade later, thanks to the magic of YouTube. Back in 2006, their cleverly arranged cover of "The 12 Days of Christmas" from 1998 became a viral hit, catching the eye of Atlantic Records. Seeing potential, the company signed the ten original, now graduated members of Straight No Chaser to a recording contract. The ensuing albums – most recently "Under the Influence" released earlier this year – turned Straight No Chaser into the kings of a cappella, helping to start the current craze.

"I think there’s this resurgence of respect for people who can actually sing and who aren’t processed," Stine noted. "While I’m not a big fan of reality shows in general, I’m glad shows – like "The Voice" and "American Idol" and "The Sing-Off" – are putting the magnifying glass on people who actually can sing. They’re showing that realness to music that’s coming back. I think that’s why a cappella is coming back. People are sick of the Autotuned, overproduced songs where these artists have become interchangeable because their voices have been so tuned by computers."

Things have obviously changed a lot for the guys of Straight No Chaser since they started the group in school. They’re now scattered across the country, in places from Las Vegas to West Palm Beach to Chicago, with families and lives of their own. But they still find a way to bring their tunes together.

"The process is usually sparked by somebody in the group having an idea for a song, or multiple guys having an idea for a song," Stine said. "There have been multiple conversations this summer, for example, of songs we wanted to do on this fall tour. There was talk of doing "Get Lucky" by Daft Punk. There was talk of doing "Blurred Lines" by Robin Thicke. Then on top of that, there are older songs that we thought would be fun to cover, like some Prince or another Michael Jackson song."

Various members write the vocal arrangements, and from there, the tunes morph and transform. Songs are tweaked or perhaps combined with other songs to create mash-ups. Eventually, the whole group is sent the final arrangement, where they must learn and perfect their particular part on their own. Then, they gather together, usually with a little less than a week before the start of a tour, to bring the parts together and meld them into a finished song.

"We love the fact that, when we do have time off, we get to spend it with our families in our individual hometowns," Stine said.

"We’re in a rhythm where we know our own tastes, our own tempo and our own habits, and we know we’ll be a certain way along before we even get together with guys pre-rehearsing. Back when we first started, it was a really intense six-month period where almost every weekend, we were in New York rehearsing to begin recording of the first album. We hadn’t done it before, and we wanted to make sure there was nothing possibly missed."

Over the years, a few members have dropped out due to the strains and time needed for touring. When that happens, however, the group calls upon the college edition of Straight No Chaser back at Indiana University – now called Another Round in order to avoid confusion – to serve as "their farm team." The group then stays together, pleasing crowds and hopefully creating some a cappella converts in the crowd.

"After each show, we do a signing line," Stine said. "People come through on a regular basis come up and say, ‘You know, I didn’t want to come to this. I was dragged here by my girlfriend or wife, and they bought me a ticket, and I didn’t want to come.’ And then they come through the line, and they bought three albums and a T-shirt."

One of the misconceptions some people have about a cappella music is that the songs are merely covers, just with vocal parts in the place of instruments. However, the best groups, the ones that make it big, are those whose arrangements take a beloved song, and make it fresh and new.

"I think you have to have your own unique take on it," Stine said. "If you don’t do anything differently from the original artist and it’s just a blueprinted copy down to the tempo and key and everything, then you’re not giving a reason to listen to yours rather than the original. If you cover it exactly the same, I’m probably still going to listen to the original."

For Straight No Chaser, sometimes that’s taking a song, like "Wonderwall," and slowing it down into a ballad, or doing the reverse, taking "Under the Bridge" by the Red Hot Chili Peppers and cranking up the tempo. Other times, it’s simply putting a male soloist on a traditionally female song, such as Adele’s "Rolling In The Deep."

Mash-ups are also a popular way to bring a twist to beloved songs, whether the combinations are obvious (linking Fun.’s "We Are Young" with "Some Nights") or a bit trickier (a mix of "Billy Jean" and Bel Biv DeVoe’s "Poison"). Either way, it’s about taking something popular and adding a dash of unexpected surprise.

And if there’s one thing the guys of Straight No Chaser know about, it’s unexpected surprises. After all, they transformed from a crew of college guys singing for food and female attention into one of the signature acts in a continually growing music genre.

They’re even in talks with Dreamworks to potentially turn their story into a movie. And while they haven’t appeared on "The Sing-Off" (and likely won’t – at least as competitors – considering the show is produced by Sony, a rival label), their influence is surely felt.

"They talk about us on the show sometimes as us being the goal of what the groups want to do," Stine said. "So to get to tour and have a life performing like we’re doing right now, we consider ourselves very lucky."