It's hard to say which aspect of Purity Ring is more powerful: the duo's rib-shaking bass and drums, which bear Atlanta hip-hop's stamp, or Megan James' heart-stopping voice, as bracing as a breath of winter air. “Fineshrine,” from the Alberta duo's 2012 debut, pairs skittering hi-hats with an unforgettable vocal hook; “Begin Again” doubles down on icy trance synths while proving the group's effortless way with melody. With “Bodyache,” they play with tropical keys and half-time beats, bobbing like an iceberg on the equator.