The gig-goer arriving at the Serpentine Sessions might be forgiven for feeling a litte deflated. The name conjures up an open-air concert on the lake's banks, with genteel laughter tinkling on the evening air. As it turns out, the Serpentine Sessions take place in a fenced-off compound, with a big, sweaty tent in the middle. The shadow of the huge stage where Blur will shortly appear compounds the feeling that you're attending an afterthought rather than an event.

Yet all the sense of event anyone might require is on offer inside the tent. There's 3,000 people here, testament to how Bon Iver's unassuming debut album For Emma, Forever Ago has wormed its way under people's skin. Introverted Americana, recorded alone in a remote Wisconsin log cabin by frontman Justin Vernon, doesn't really scream "bring glowsticks and prepare to punch the air", but Vernon's band have expanded, and his songs beefed up.

For meditations on loneliness delivered in a keening falsetto, they sound astonishingly muscular and primally thrilling. It's partly helped by the audience, who even listen to b-side Babys – an abstract collage of high harmonies, icy keyboards and crashing cymbals – in attentive silence. But it's mostly down to Vernon, who seems to have cracked how to deliver his music to a mass audience. For The Wolves, he suggests the audience sing along with increasing volume, then, as its song reaches its climax, scream. They're happy to oblige – frankly, if Vernon suggested they nip out and get him a Zinger Tower Burger, they'd climb over each other to reach KFC – and the effect is as stunning as anything live music currently has to offer: lonely soul-searching transformed into a moment of cathartic, communal, euphoria.