Air/Sia, Somerset House, London

Fresh from a well-received turn at this year's Big Chill festival, the support act Sia shuffled on stage, clearly nervous in the grand surroundings. Their sassy Australian-born singer, Sia Furler, shot to prominence when she added jazz-inflected vocals to tracks on Zero 7's lush 2001 album Simple Things.

Furler's soulful voice has been compared to heavyweights like Janis Joplin and Joe Cocker, and there are traces of their hefty screeches in her performance. However, too often in this open-air venue she strayed perilously close to the dirge- ridden warblings of a pub singer or worse, Texas's Sharleen Spiteri. A tad harsh, perhaps, as she was clearly tense - a fact betrayed by her feeble mutterings between songs. But overall, Sia's music failed to ignite, plodding along while the restless crowd chatted among themselves. In fact, apart from a blast of flute from the keyboardist, the set was nondescript.

So, the scene was set for the main draw, Air, to lift everyone's mood. But any thoughts that this static French duo intended to ape their countryman Jean Michel Jarre with a trip through the light fantastic were banished early on. For "Venus", the opening track from their latest album, Talkie Walkie, the light show was restrained, much like the band. They're the kind of act you could picture Alan Partridge relating to while tapping his fingers on the dashboard of his robust company car.

The magisterial Somerset House on a warm summer's evening should have ideally suited Air's slices of cosmic, ambient pop. However, standing on the cobbles listening to "La Femme d'Argent" induced a feeling akin to motionless sickness. Air's raison d'être is to produce languid grooves punctuated by atmospheric whoops and bleeps - not music to pump weights to in the gym.

Despite this, there was something dispiriting about the dearth of movement in the young crowd. Some managed to summon a sideways shuffle to "Sexy Boy" and "Kelly Watch the Stars", but on the whole the music lacked soul. Air played as though their lives didn't depend on it, seemingly safe in the knowledge that the lucre they accrued from their gorgeous 1998 album Moon Safari, plus the endless use of their tracks for advertisements, could buy them vast tracts of Provence.

The romantic slush of Talkie Walkie was thoroughly mined. "Cherry Blossom Girl", "Alpha Beta Gaga" and "Run" breezed by innocuously. But eventually the lack of stage presence and a shocking scarcity of va-va voom began to grate; the only distractions being singer Nicolas Godin's David Cassidy-style hairdo and the megawatt presence of the actress Scarlett Johansson. But, clearly, neither of these could save a concert devoid of charisma. You don't expect hero worship, air guitar or lighter-waving ecstasy at an Air gig, and there is no danger of any stage diving, but this was just mediocre. Come back Jean Michel Jarre, all is forgiven. Even the laser beams.