CAMBRIDGE -- Fifteen minutes into Xiu Xiu's 45-minute set at the Middle East Downstairs on Saturday afternoon, an equipment failure sent a small cloud of smoke through the purple lights at the back of the stage. The already-hushed crowd went still; it was possible to hear percussionist Ches Smith and frontman Jamie Stewart deliberating about how to proceed.
Then there was a shuffle of feet. A shortened drum roll. And Stewart was off, humming the introduction to ``Hives Hives," while lashing violently at a set of cymbals stationed at his side.
This is the atmosphere in which Xiu Xiu plays best -- stretched, vaguely uncomfortable silences, broken open in bursts by a kaleidoscopic orchestra of shrieks, cymbal clashes, and roughed-up synth fuzz. But Stewart and co-vocalist Caralee McElroy aren't out merely to shock their audience.
For Xiu Xiu, silence is every bit as important to a performance as the dark imagery of the music. Silence fills the long gaps between songs, and the breathless breaks between chorus and verse, bridge and chorus. And it allows Stewart the space to draw attention to the sharp, needling edges of his songs. There are few other acts playing today that do quiet so well.
Stewart and McElroy built Saturday's set around ``Air Force," which was released last month by indie imprint 5RC. ``Air Force" is bigger than anything Stewart has created before -- it's also more caustic -- and tracks like the gender-bending love letter ``Hello From Eau Claire" were issued with a practiced, self-conscious grace.
On ``Bishop, CA," Stewart was ``crying for the stupid world we share"; later, on ``PJ in the Streets," he whispered that ``it's just a fleeting thought, for there is nothing in our wondering."
The problem, though, with grandiosity on the stage is that it's often hard to know whether to cry, laugh, or to stand very, very still. For his part, Stewart seems uninterested in humor, or irony: He stared skyward for the majority of Saturday's concert, his brow furrowed in concentration, his voice throaty and grave. So when, in the final moments of the show, during a stripped-down, discordant rendition of Tracy Chapman's ``Fast Car," someone in the audience giggled -- and giggled loudly -- it was the most welcome sound on earth.