The metric system. The difference between Shi’a and Sunni. Björk. All things that your average American citizen has a general awareness of but would be hard pressed to explain if asked. Volta (Atlantic) won’t help anyone remember how many kilometers are in a mile, but it is in some ways a “Björk Conversion Chart.” In a move that seems obvious in retrospect, in-demand superproducer Timbaland has contributed a handful of tracks to Björk’s new album, and while Volta is still far from “easily accessible,” this is probably as close as she’ll get.On the lead single (“Earth Intruders”) Timbaland’s gift with rhythm joins Björk’s affinity for tribal percussion to help create something that’s almost catchy in spite of itself. Long time fans shouldn’t be discouraged; it still has Björk’s usual atmosphere of a future where nature has regained control over man. It’s simply that Timbaland’s more focused approach translates the imagery more clearly than similar near-hits like “Jóga” and “Oceania.” Of the other Timbaland contributions, “Innocence” also brings relative structure to the dynamic concept, but “Hope” abandons the huge, deliberate percussion for a light mix of Eastern strings and finger drums.Tim’s fans who’ve tuned in out of curiosity won’t be completely lost with the remaining tracks, but there’s no question that Björk is still Björk. Her flare for theatrics is as strong as ever, especially on the striking “Vertebrae by Vertebrae” or “Wanderlust,” a reminder of her overlooked vocal ability. Most of the work maintains a consistent direction, going back and forth between the mechanical industrial arrangements and more delicate orchestral pieces. Her collaborations with Anthony Hegarty (“Dull Flame of Desire” and “My Juvenile”) will probably be the most off putting to the uninitiated. As a duet, they become an odd version of Serge Gainsbourgh and Jane Birkin, but despite the chemistry, Hegarty’s vocals are perhaps too far from the usual pop/rock style to have a wide appeal.By whatever standard, Björk is weird and she always will be. It would be a mistake, though, to let that be a barrier to giving in to curiosity and spending some time in her world. Volta has a refreshing depth in a musical community that’s decreasingly willing to take risks. Timbaland is a good reason to come, but Björk herself is definitely the reason to stay.