Sibelius Festival, Sibelius Hall, Lahti, Finland

They're shy people, the Finns, but sometimes they take it too far. What kind of a festival has two Sibelius world premieres to boast about – but doesn't announce them in the programme or publicity and springs them on an unsuspecting public only when the conductor announces an encore? The Finnish tourist authorities are trying to make a cultural package out of Lahti and its environs – and it's true that the blend of unruffled forest-and-lake nature by day and first-rate music-making in the evening makes for an unusually satisfying break. But the discovery of two long-forgotten orchestral movements by Sibelius surely deserved at least little noise aforehand.

The Sibelius Festival is the third since the opening of Lahti's Sibelius Hall on the shores of Lake Vesijärvi (more Finnish understatement: it means "water-lake"). Each year focuses on a particular aspect of Sibelius' output: for the first festival, and again now, it was the symphonies; last year saw the symphonic poems under scrutiny and next year, more adventurously, presents some of the stage music.

Bringing the symphonies around again might court routine with a lesser conductor, but the interpretations of Osmo Vänskä – chief conductor in Lahti, formerly with the BBC Scottish in Glasgow and now off to Minnesota – are so intense, demand such concentration from the listener, that you emerge from each performance with nothing in your mind but the music itself.

His reading of the Fourth Symphony in particular, the bleakest and austerest of the lot, took on an almost maniacal intensity. Even you disagree with what he is doing, Vänskä's sheer passion – and the Lahti players' virtuoso response – insist on your attention. He took the Second Symphony, for example, at such breakneck speed that the strings couldn't articulate the dagada-dagada opening of the third movement, and the finale lost much of its monumental glory. But you do notice, as if for the first time, how hard, how dissonant, how modern this new Sibelius is.

The two world premieres were movements from a suite written in 1914, the first of which Sibelius appears to have recycled in the tone-poem The Oceanides later in the same year; that may explain his inscription, at the top of the score, "Predecessor of The Oceanides". The first, just over four minutes in length, begins with a perky tune in the flutes, answered by the strings in antiphonal exchange; harmonically, it looks both backwards to the Sibelius of the early 1890s and forwards to the late masterpieces of the decade to come. The other movement, three minutes long, is a transparent, elliptical exercise in wringing the maximum effect out of the sparsest material, Sibelius as master of suggestion. If you can make it to Lahti on 24 October, you'll catch the official premiere.