Musical Variations: This music school opens an issue on leisure because it is not a teaching machine but an institution which promises to enrich the life of the city and all its citizens.

I never tire of saying that Sydney is one of the most magnificent cities in the world. It's not that many of its buildings are particularly outstanding, but the whole composition of the centre is magnificent, with the skyscrapers of the CBD jostling each other on the tight Georgian grid that runs up the hillside from Circular Quay, the place from which all the ferries take you up and down the harbour. The dense picturesque composition is offset by the splendid parks that flow down to the sea and the white sails of the Utzon Opera House.

One of these parks is the Botanical Gardens, some of the very best in the world. In them was set the Govenor's stables, built in 1816 by Francis Greenway, a pupil of Nash transported to New South Wales for forgery. He quickly became NSW Government Architect, and his several styles echoed the work of the master. The stable block resembles Nash's Gothick work, and is basically a rectangular Classical building, with economical brick battlements on top. By 1913, it had become redundant, and Seymour Wells, the Government Architect, converted it to the Conservatorium of Music (the Con), his main move being to roof over the inner court and create a concert hail in it, named after Henri Verbrugghen, the first director.

By the '90s, the Con was clearly inadequate in almost every way: far too small, decrepit and with many acoustic problems. Several places were examined for relocation. In the end, it was decided to let it stay on its original site. But there were many problems: masses of new accommodation had to be added, conservationists were determined to preserve the gardens and the old building, from which blurring '60s additions had to be pulled off. Chris Johnson, the New South Wales Government Architect, produced a basic parti, in which Greenway's building would be cleaned up, and the new 30 000 square metres of space were to be placed underground. So, on top, a landscaped terrace overlooks the gardens and is part of them. Underneath this grassy plateau is an extraordinarily complex maze, toplit as far as possible, with numerous small studios and practice rooms.

Daryl Jackson Robin Dyke Pty, became the project architects, working with acoustic consultants Kirkegaard & Associates of Chicago. The main obvious new move is the long foyer that runs out from Greenway's building towards Macquarie Street (the city's equivalent of Park Lane or Fifth Avenue). What you see from outside is mainly a lightweight metal and glass pavilion and of course, the Greenway building, now rendered white. But the pavilion is the entrance space for the whole organization and it leads down to the underground levels as well as giving access to the old building. Its long volume is full of light from clerestoreys and rooflights. Cut down into the soft rock of the ridge, the space is dramatic, with the buttery sandstone strata exposed under the old building. Remains of a lost convict-built road are exposed, as are many other relics of the past, displayed in museum-like cases. The whole place is full of students and visitors, happy and chattering against a muted background of music from the practic e and recital rooms.

One of the major problems of a music school is of course noise, both from outside and in. The site is nothing if not noisy: Macquarie Street is busy with traffic, and the roaring Cahilll expressway emerges from its under-park route almost opposite Greenway's west front. Underneath the new terrace are two busy railway tunnels, one of which actually goes through the building. To counteract all this vibration, the main performance spaces are supported on movement-reducing bearings -- steel springs and rubber pads, so that the main performance volumes are separated from the structure in general, and from the surrounding ground.

Walls between practice rooms and studios are made of heavy concrete blocks with independent plasterboard inner skins to prevent lateral sound transmission. Their stiff floors, made of composite layers of plywood and particle board are supported independent of the heavy walls by resilient pads, so the inner shells are virtually independent rooms, carefully sealed, and with two layers of laminated glass where they have windows.

The Verbrugghen Hall has been restored as a vibration isolated structure. Its 1960s proscenium was removed and its original gallery has been demolished because it caused sound shading below it. Its roof is a new precast concrete shell, giving mass to counteract airborne road noise. The heavy terrace with its layer of soil, between 600 and 750mm deep, clearly helps to exclude external noise from the studios and offices below. Light courts and skylights provide luminance from the sky. In its north-eastern corner, where the Conservatorium's High School is housed, the terrace plan has been eroded to conserve important trees in the Botanic Gardens, some of which are over 150 years old.

To me, the only place which does not work properly is the forecourt. Almost opposite Renzo Piano's spectacular tower is a sea of asphalt leaking out northwards to Government House Drive. But once you are past the car stuff, the Con offers generous welcome and clearly makes an important contribution to civic life. As Elizabeth Farrelly said in the Sydney Morning Herald, 'Whether it was right to keep the Con downtown depends on your opinion of cities, music, gardens, archaeology and heritage ... [The] way it puts students in the streets, music in the air, light in the earth, energy in the Greenway and archaeology in that enchanted public space is worth every penny. It could so easily have been just another Sydney con game'.