Previously, for me, the term “Chamber Orchestra” has meant an ordinary orchestra, only smaller: apart from the sound being somewhat thinned out and consequently cleaner, I don't expect a fundamentally different experience. Or, didn't, that is, until last night at Cadogan Hall, where I saw the Australian Chamber Orchestra for the first time. It was an experience far more akin to listening to a giant string quartet: from the point at which Richard Tognetti and ten other string players launched into Tognetti's Caprice on Caprices, it was remarkable to see so many musicians interacting in the way quartet players do - watching each other intently, listening to breathing, each reacting to the others immediately and directly. The effect was hardly dimmed when the number of musicians on stage increased to 17 later in the concert.

Caprice on Caprices was a delicious appetiser, light and fluffy: the tunes from two of Paganini's Caprices stirred together and sprinkled with every violin effect you care to name: harmonics, sautillé, jeté, con sordino, various different pizzicati. Quite apart from displaying the fact that we had a stage full of top league string players, this was a true flight of fancy, exactly as the name described.

We then moved to an altogether more serious item: the late Australian composer Richard Meale's Cantilena Pacifica, an elegy for a friend dead from cancer which wandered achingly through a gentle landscape of mourning, pensive themes emerging from a contrapuntal wash. Next was the main item of the first half: Tognetti's arrangement of the Ravel F major String Quartet. Ravel's original is a fascinating piece, not least for its use of harmony. While all of the Impressionists made considerable use of the four-note chords that were to become a staple of jazz, they often used them mainly to add lushness of texture: Ravel has the same harmonic density but employs it in rhythms that are continually in motion - his landscape may be lush but there's no time to sit back and wallow: this is a brisk journey, not a ramble. Tognetti's arrangement preserves the sense of motion and the variety of shifting moods, while adding a depth of texture that can only be achieved by quadrupling the number of musicians. And while you might expect the larger number of players to cause some of the crispness of the original quartet to be lost, the ACO play so tightly together that this was not so. Most spectacular was the second movement scherzo, a piece of pizzicato bravura into which they injected enormous pace and vibrancy.

As the second half began, we were joined by soprano Dawn Upshaw in a set of three lieder, one each from Schumann, Schubert and Schoenberg. Upshaw's many fans may have been disappointed at the quantity of her performance (she was only on stage for around fifteen minutes), but they won't have been disappointed by the quality: this was a marvellously flexible voice which produced beautiful timbre and a full range of expression. My favourite was the first of the three: Schumann's Mondnacht, an unashamedly romantic depiction of the soaring of one's soul after meeting one's beloved. Next was the Litanei from Schoenberg's String Quartet no. 2, based on a somewhat enigmatic poem and written when the composer discovered that his wife was having an affair. This showed Upshaw at her most versatile, with a wide range of expressions from anger to despair to resignation,

The most substantial work in this concert was the last: Schoenberg's own arrangement for string orchestra of his 1899 sextet Verklärte Nacht (“Transfigured Night”). It's a work that's difficult to describe because, in the course of half an hour of music, it goes through such an unbelievable variety of forms, textures and emotions. The gentlest of openings gives way to an immense outburst of passion, to be followed by wave upon wave of music of intensity that increases until it verges on the manic. The waves are interspersed with reflective passages, which serve merely to prepare you for the next emotional wrench. In the heavier passages, the orchestra is split into many independent parts in writing of extreme complexity. There's no formula involved - different groups of instruments get totally different roles as the work progresses - and for the first time, I found myself understanding that the process of composing such a piece would be sufficiently mind bending to make a composer desperate to create some unifying theory to help make sense of it all.

Verklärte Nacht provided the perfect showcase for the ACO. They gave the firm impression that every phrase, every note had been pored over, debated and perfected, with every member of the ensemble knowing exactly how it was going to be played. The musicians were acutely aware of each other and, most notably, seemed to be really enjoying themselves: I've never seen so many broad grins whenever a phrase turned out perfectly. The increase in expressive power that results from players who are perfectly mentally aligned was palpable. This was a remarkable process of music making, one that sets a high bar for other chamber ensembles, and it was a privilege to watch it at close quarters.