A cappella music: It's not just for college campuses anymore. Pentatonix seems to have a new viral video every couple of weeks, and 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. 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.