When it was announced that the bluegrass songbird Alison Krauss was working on an album with the king of classic cock rock, Robert Plant, many people anticipated a fire and ice result. Krauss is simply the most gifted, ethereal female vocalist of her generation; whereas Plant’s vocal transmissions have always been aggressively sexual. But Raising Sand defies all expectations. On this collection of duets, Krauss is Krauss (i.e., wonderful), but Plant is a revelation. Only on the codas of a handful of tracks does he revert to Zeppelinesque strings of scratchily falsetto grunts and ahs. The rest of the time, he all but matches Krauss’ technical perfection, especially on “Killing the Blues,” “Polly Come Home,” “Your Long Journey,” and “Please Read the Letter.” While Plant’s Un-Leded projects have always hinted at a softer side, never has he sounded more gorgeous and relaxed than he does with Krauss. Hence, their live show should be nothing short of spectacular.

Wed., Oct. 1, 8 p.m., 2008