Can a completely unpredictable rock band do something to surprise you? No one took The Mars Volta guitarist and producer Omar Rodriguez-López that seriously when he dropped hints that his band’s next album would be “acoustic.” The group’s relentless sonic assaults are legendary, with lonnnng prog-ish jams punctuated by manic drumming, explosive guitar runs, and the Robert Plant–like screeching of vocalist Cedric Bixler-Zavala. The Mars Volta, in other words, will go acoustic the day Taylor Swift records a jazz album (don’t go getting any ideas, Taylor). But that’s not to say that Octahedron ( WB) doesn’t boast some quieter, folkish interludes. Rodriguez-López says he’s been listening to a lot of Nick Drake and Leonard Cohen, but this ain’t that: If at first you’re reminded of songs as mawkish as Kansas’s “Dust in the Wind,” it’s the influence of Pink Floyd co-founder Syd Barrett that soon becomes evident. Though this wildly original band can hardly be compared to any other, imagining a frenzied version of Barrett’s “See Emily Play” or “Interstellar Overdrive” (which The Mars Volta has actually covered) might get you close. When Rodriguez-López and Bixler-Zavala left the El Paso group At the Drive-In, in 2001, they took along its fury but left behind its brevity. The Mars Volta have indulged in the squarest, most retro of rock genres—the prog concept album—and managed at times to make it sound fresh. By adding in some breathing room, they’ve made their most listenable album to date.