Chelsea Wolfe gained an audience for her dark dirges with her 2011 breakthrough album Apokalypsis, but Unknown Rooms: A Collection of Acoustic Songs isn't exactly a logical follow-up -- which makes it all the more interesting. Wolfe reteamed with several of the players who helped her make Apokalypsis as fittingly doomy as it was, including Ben Chisholm, but this time she and her crew focus on the fragility and versatility of her voice, a couple of things that occasionally got lost in her previous album's brooding, lo-fi din. Strings feature heavily on Unknown Rooms, and on songs such as "The Way We Used To," they complement her voice perfectly, to the point where the delicacy and fluidity of her singing suggests a violin, and vice-versa. Perhaps even more so than on Apokalypsis, there's a genuine sense of mystery floating around these songs, which didn't fit on either of Wolfe's previous albums but create an introspective dream-world together. Wolfe doesn't stray far from her usual lyrical territory -- love, death, loneliness, terror -- but she takes a slightly different perspective here than she has before. On the most striking songs, she finds different dimensions to her intensity: "Flatlands" overlays an innocent-seeming folk melody with complicated strings, and "Spinning Centers" is a piece of surreal, stream-of-consciousness nightmare folk in which Wolfe evokes her beloved in a whispery soprano "when the face of death is spinning after me." Yet on "Boyfriend," she's equally terrifying and devoted, from the way she whispers/growls "boyyyyfriend" at the beginning of the song to the way she sings lyrics like "you call this passion but I call it a cancer" so slowly it's like they're being pulled out of her. Meanwhile, the album closer "Sunstorm" makes an odd keyboard and overlapping vocals sound like a duet between conflicting memories, and it's just as vivid as her louder work. Elsewhere, Wolfe and company use the album's limited palette and roomy intimacy to explore more theatrical realms like "Appalachia"'s mix of alluring strings and rough-edged guitars, "Our Work Was Good"'s spaghetti western flair, and eerie vignettes such as "I Died with You" and "Hyper Oz." At times reminiscent of the mercurial dreaminess of Cat Power's The Covers Record or the ghostly rawness of PJ Harvey's later work, Unknown Rooms is spare, gorgeous, and haunting, offering surprises for her established fans and likely winning her new ones in the process.