In an instrumental representation of sadomasochism that recalls the opening of Nine Inch Nails’ album “The Downward Spiral,” a slowly repeated, violently struck tom punctuates “Buzz Saw.” The drum interrupts Jamie Stewart’s gently plaintive vocals, backed by wistful sustained piano chords, thirty seconds into the opener of Xiu Xiu’s “The Air Force.” But while Reznor’s 1994 classic is adolescent and testosterone-fueled, the violence and sexuality of Xiu Xiu’s music is characterized by dichotomy and ambiguity. Though “The Air Force” is aurally harsher and more percussive than Xiu Xiu’s previous work, the album is somehow more accessible, and remains as vulnerable as 2003’s spare “A Promise.” Initially somewhat uninviting — especially for those new to Xiu Xiu’s confrontational music and lyrics — “The Air Force” proves Xiu Xiu’s best and most accomplished work to date.

Much of “The Air Force” is marked by the musical complexity that distinguished 2005’s opaque “La Foret” from Xiu Xiu’s earlier work. Though the double bass is as much in use here as ever, its grounding melody is often buried deeper in the mix below a dense mesh of high-pitched electronic squeaks and metallic scratches. Further, the warm brass that marked songs like “Nieces Pieces” is completely gone. These changes make the music not only more complex, but also at times much sharper. Stewart’s already affected voice is sometimes pitched upward and abrasively distorted.

Xiu Xiu’s lyrics, meanwhile, are as transgressive as ever. Here the band explores rape, incest, gender ambiguity, sadism and molestation; the self-loathing and guilt of the songs’ subjects unify the album’s otherwise disparate tracks. Both Stewart’s and McElroy’s vocals, saturated with emotion and frailty, communicate this distress passionately and convincingly.

At the same time, “The Air Force” is more lyrically expansive than Xiu Xiu’s prior albums. Beyond simple developments like the disambiguation of song’s subjects from their narrators (as in “Vulture Piano” and “Bishop, CA”) the recurrent theme of childhood vs. adulthood lends itself to detailed explorations of power and powerlessness. “Buzz Saw” and “Boy Soprano,” for example, are among the first Xiu Xiu songs to depict a narrator in a position of authority. And even the reflection on suicide in “The Pineapple vs. the Watermelon” implores the deceased to “Say hello to Corey’s mom / Say hello to Freddy’s mom / Say hello to Ryan’s mom.”

The most striking example of the theme of childhood as manifested in “The Air Force” is “Hello from Eau Claire.” The most accessible song on the album, as well as the first sung entirely by McElroy, “Eau Claire” is sung from a naive, childish perspective, and backed with glockenspiel and toylike electronic chimes over a bouncy rhythm to hammer the point home. A naked confession of love and vulnerability, even as the narrator tries in vain to assert independence, the song is also the most overtly gender-ambiguous on the album.

Another standout track is “Save Me Save Me,” the second-shortest track on the album (after the wordless koto and birdcall song “Saint Pedro Glue Stick”). Featuring Nedelle on violin, creepily melodic instrumentation and multi-tracked vocals from Stewart at his most guttural, the song is simultaneously poppy and sinister, a balance no one achieves better than Xiu Xiu. The band’s maturing sensibilities and co-producer Greg Saunier’s (of Deerhoof) contributions only improve their capabilities in this regard.

Saunier and Nedelle’s contributions demonstrate Xiu Xiu’s willingness to collaborate with other artists, which has only enriched the band’s sound. Once practically an autobiographical solo project by Stewart, Xiu Xiu has developed into a more exploratory, ambitious group while remaining both personal and provocative. Closing track “The Wig Master” is based on a piece by bassist Devin Hoff (who plays on the track), yet the spoken lyrics are voiced (by Stewart) alternately naturally and through a chipmunky effect, to represent the song’s two voices. Set to sinusoidal strings punctuated by conspicuous silences, the song is both anticlimactic and, oddly, fitting.

Throughout “The Air Force,” Xiu Xiu challenges and engages with equal measure. Conceptually fascinating, bizarrely artistic and confoundingly melodic, the admittedly difficult album is one of the best of the year, and certainly the most ambitious — if not the best — of Xiu Xiu’s career.