Michael Cunio credits the success of his fledgling vocal group to what he called the principle of 1+1+1+1=5.

Michael Cunio credits the success of his fledgling vocal group to what he called the principle of 1+1+1+1=5.

No, not fuzzy math. More like harmonic convergence. As in, the convergence of four voices in harmony adding that "little bit of magic," as Cunio called it.

There is definitely a wave of renewed interest in singing and the human voice, and Under the Streetlamp has the talent to get noticed.

Cunio and his mates -- Michael Ingersoll, Christopher Kale Jones and Shonn Wiley -- met while members of the Chicago cast of the hit musical Jersey Boys.

During the show's run, Cunio said, the quartet had the opportunity to perform outside the show for festivals and charity events.

"Those shows were really the jumping-off point," Cunio explained.

"We were having such a good time that when Jersey Boys closed, we didn't want it to end.

"We were still getting offers to sing, so we decided we'd stick together for another six months or so," Cunio said.

"We didn't know how much this music would resonate with people, but we're so fortunate and thrilled that it has."

The music Cunio references is formative and early rock 'n' roll, songs from the '40s, '50s and '60s that Cunio said "audiences are hungry to hear."

Of course, the music of Frankie Valli and the Four Seasons finds its way into Under the Streetlamp's repertoire, but developing a set list happened kind of organically.

"When we started doing our own shows, we would just sit down and each of the guys would come up with songs they thought we should do. When we looked at the songs, it was sort of a history of classic rock 'n' roll.

"We of course wanted to harmonize, so this music proved to be kind of a natural fit."

The repertoire means audiences of all ages can find common ground, Cunio said.

"My mom likes ABBA, and I like Guns N' Roses. But we can both agree on The Beatles," he said. "These songs are called 'oldies,' but everyone knows all the words."

Besides the voices and the repertoire, Cunio said Streetlamp shows offer "uncomplicated entertainment."

"I like to think of the show as more of a party, with us as the hosts," he said. "The four of us have a real friendship, and we have a lot of fun and a lot of freedom to let that come through. You've got choreography, jokes and the music. It's kind of like a variety show."

Indeed, Streetlamp shows have been likened in feel to those of the Rat Pack for the interaction among the performers and between performers and audience.

"There is no fourth wall," Cunio said.

Given the season, Under the Streetlamp is working holiday songs into its set as well -- a selection of tunes from a Christmas album recorded over the summer that includes classic holiday rock 'n' roll plus the group's first original tune, co-written by Cunio.

"We wanted to do some Christmas songs, but within the context of our show," he said.

"There are some songs and certain other things, moments, that people who've seen our show either in person or on PBS sort of expect.

"We are rewarded by the audience's enthusiasm."