When you hear them, it sounds like magic — the incredible blending of unaccompanied vocal sonorities that have taken three outstanding L.A. area a cappella groups into the championship realm. The Scattertones, UCLA's coed a cappella ensemble; Chapman University's SoundCheck; and USC's premier a cappella group, the SoCal VoCals, are all virtuosi in an art form that originated in ancient times. Today, a cappella has branched out into all musical genres, and this weekend, at the Hollywood Bowl's ABBA Fest, these three friendly rivals — who all compete in the prestigious International Championship of Collegiate A Cappella — will compete for your vote in ABBA-Cappella, a show-tune-styled sing-off of ABBA classics. One of Us will win, but who? Does Your Mother Know? The Name of the Game is The Winner Takes It All, but Gimme Gimme Gimme a break — the judging will be by audience applause, a tough assignment, as each of these groups is so awesome that you'll have a heck of a time picking just one. The evening also features the great Swedish ABBA tribute band Waterloo, performing ABBA's greatest hits to an audience dance-along/sing-along. Thank You for the Music! Hollywood Bowl, 2301 N. Highland Ave., Hlywd.; Sun., Sept. 8, 7 p.m.; $15-$92. (323) 850-2000, hollywood​bowl.com.

Sun., Sept. 8, 7 p.m., 2013