Arts: She whispered, she roared, she wept, she soared ...

From the moment of La Norman's grand entrance in shimmering blue chiffon, her strong, creamy voice and sense of drama proved more than equal to Berlioz's testing song collection `Les Nuits d'ete'. Adrian Jack marvels at the diva's Barbican concert.

You'd expect a singer's solo spot to be in the middle of an orchestral concert, not at the end. But if Brahms's G minor Piano Quartet orchestrated by Schoenberg had come after the interval on Thursday evening, La Norman's fans might have left a lot of empty seats.

It's a long way from the artists' entrance to the Barbican platform, and Norman did not want to be seen struggling the distance. After all, she is a diva, and should simply appear, by as close to supernatural means as the Barbican's carpenters could devise. So screens were specially constructed each side to conceal the goddess until she was safely on the stage. The sight of her scarcely needed enhancing. She had tugged her hair upwards in a dark blue chiffon, so that it climaxed in a sort of exclamation mark. Her dress was simple black crepe, draped with a diaphanous blue over-garment. The style evoked the salons and boudoirs of the early 19th century, the colours suggested the erotic nocturnal melancholy of Berlioz's six settings of Theophile Gautier, Les Nuits d'ete.

Originally, Berlioz wrote the songs for different types of voice, though they are nearly always sung by a mezzo-soprano. The strain on one singer can tell, and the effect of a collection, which wasn't conceived in the first place as a cycle, can become monotonous. Although Berlioz kept his orchestration light, the music is grand, and Norman's strong, creamy voice was certainly built for grandeur. What surpassed expectation were the many ways in which she varied it. In the delicate, tripping "Villanelle", she reduced her potentially enormous sound playfully, without any sign of awkwardness. "Le spectre de la rose", rapturous and swaying, called for a more ardent, sustained line, a bigger voice, twisting upwards perilously at the end of the first verse, later hushed to a whisper against softly bubbling harp, to the words "Ce leger parfum est mon ame / Et j'arrive du paradis" ("This delicate perfume is my soul and I come from Paradise").

At the end of the song, Norman looked upwards, smiled, opened her arms, then closed her eyes. She may scorn acting for intellectually inclined opera producers, but she can certainly do it on the concert platform. Besides, there was enough acting in her voice to make her expansive bodily gestures an optional extra. In "Sur les lagunes", she lamented her dead lover with a sepulchral resonance on the word "toujours", and softly squeezed "femme" and "elle" as if she were kissing them. The refrain of "Absence" - "Reviens, reviens, ma bien aimee" ("Come back, my beloved"), pitched cruelly high and stretched in time by Berlioz - receded in the back of her head like a hopeless appeal. It was a brilliant stroke of expressive colouring. These details all contributed to a sustained view of each song as a whole. The only thing that left a little to be desired was the way Norman sometimes distorted the sounds of words for the sake of musical smoothness; so "la blanche tombe" in "Au cimetiere" scarcely had any consonants, and in "L'ile inconnue", the repeated word "aile" sometimes began with an "ah", so much nicer to sing than "ai". Still, it's a very rare singer indeed who never resorts to that sort of self-help.

The London Symphony Orchestra under Michael Tilson Thomas supported Norman wonderfully. Only here did Tilson Thomas follow a score. He conducted Berlioz's Overture Le Carnaval Romain and the Brahms/ Schoenberg Quartet from memory. Schoenberg's arrangement was commissioned by Otto Klemperer and the Los Angeles Philharmonic. It stopped just short of frivolity with touches of triangle and xylophone in the scherzo-like second movement, and even a tambourine and glockenspiel in the final gipsy rondo.

Brahms would have been surprised, too, at the bass clarinet and trombones, the latter sometimes muted. But then Schoenberg's love for Brahms's music didn't bind him to academic reverence. The LSO's playing was certainly sumptuous, though Michael Tilson Thomas coarsened the third movement by indulging what Schoenberg had unduly amplified - that march-like middle section didn't merit such bombast. Bernard Shaw, who called Brahms a sensualist with intellectual pretensions, might have said "I told you so."