Using the human voice as a competitive sports device has reached dangerous levels of overexposure. Sure, magnificent vocalists have come from television's retinue of singing battle shows: Kelly Clarkson, Adam Lambert, Susan Boyle, Fantasia Barrino. Philadelphia's Ali Wadsworth, a current competitor on The Voice, is another. But there are too many televised bouts of vocal brio to contend with, too many losing singers to balm or ignore after defeat.

No show is more bloodthirsty than the Gladiator-like bombast of The Sing-Off. Singing a cappella is like boxing without gloves. Yet, its focus on the purity and the ferocious art of crooning without benefit of a band or backing music makes The Sing-Off more rewarding in its victory.

No strictly vocal act sounds more ecstatic in triumph than Pentatonix, the Texas-based winners of The Sing-Off's third season that sold out Union Transfer on Monday. Euphoric best describes the joyful, self-described "futuristic a cappella" quintet who played before a wildly enthusiastic Philly audience.

Individually, smooth baritone Scott Hoying, ace alto Kristie Maldonado, high-pitched Mitch Grassi, resonating bass singer Avi Kaplan and beat-boxer Kevin Olusola were swell during solos. In particular, Kaplan's fluid low notes and Olusola's arsenal of percussive tones, and their rhythmic clicks, whirrs, bleats, and screeches were delicious.

This wasn't your granddad's doo-wop, though there was sweet street-corner serenading heard throughout. Slick, aggressive vocalese coos and scats made the group's jazzy cover of Usher's "OMG" pop in a way that would have made Lambert, Hendricks & Ross roar with approval. The sputtering engine noises and mechanical oh-ah-ohs of The Buggles' "Video Killed the Radio Star" worked in tandem with their robot-dance choreography.

Steppenwolf's "Born to be Wild" took on a trip-hop feel with a Middle Eastern swerve to its vocal melody. And the group's version of Florence and the Machine's nagging "Dog Days are Over" took a warmly emotive turn.

Happily, Pentatonix even had equally energetic, cool and corny originals ("Show You How to Love") to go with its covers, so that on some tunes, there was no need to compare different takes and decide whose version of the hits were better.