Architectural manoeuvres on the Tyne

Tyneside is soon to gain its fourth landmark. The River Tyne is already garlanded by two spectacular bridges, the Tyne Bridge and the "blinking eye" Millennium Bridge, and its south bank is home to the former flour mill that last year became the Baltic Centre for Contemporary Art. This trio is about to be joined by another stunning attraction, the Foster and Partners-designed Gateshead Music Centre – or Sage Gateshead, as it is now known, in honour of its sponsor.

This latest icon is situated roughly midway between the great-arched Tyne Bridge and the tilting Millennium Bridge. The £70m Sage comprises three performance spaces strung out along the Gateshead bank of the river: a main 1650-seat hall – the new home of the Northern Sinfonia – to the east; a theatre in the round for jazz and folk music to the west; and, slotted between them, the orchestra's rehearsal hall. The three halls are linked by a spectacular public concourse with views over the river, beneath which will be a regional music school. All three auditoriums are united beneath a curvaceous stainless steel-clad roof, shaped like a giant wave breaking over them.

"The nicety of the roof is one thing, but for this building to function it's all down to the acoustics," explains Wilf Hannon, main contractor Laing's construction director. The proximity of the two performance halls – one accommodating the Northern Sinfonia's classical repertoire, the other Folkworks' programme of jazz, blues and folk – has dictated much of the building's form and the way it has been constructed. It is essential that when the Sinfonia is sashaying through a quiet movement, the audience is unaware that the jazz club is in full swing next door.

"The hall has been designed to be incredibly silent," explains Jason Flanagan, Foster and Partners' project architect. The somewhat unsophisticated method of achieving this has been to place the hall in a very large, rectangular concrete box, or "shoebox", as Flanagan describes it. The box was formed around a heavy concrete frame, constructed by concrete specialist Hayrod Construction. The spaces between the concrete elements were then filled with concrete blocks to give the walls sufficient mass to prevent sound breaking in or out. A massive 300 mm thick concrete lid seals the box, 22 m above the audience's heads.

For the contractor, getting the blockwork right was essential if the acoustic performance was to be achieved. Any air gaps in the block's mortar bedding would allow sound to percolate either in or out of the hall. The ideal solution would have been to use very large blocks to cut down on mortar joints. But the Construction (Design and Management) Regulations precluded their use on the basis that they would have been too heavy for one man to lift. Instead, dense concrete blocks 440 mm long by 215 mm high by 100 mm deep and weighing 20 kg were placed on their side so that the 215 mm dimension formed the width of the wall. "All joints were very carefully inspected," says Hannon.

"We designed the building from the inside out with acoustic excellence as our goal," says Flanagan. Working with acoustic consultant Arup, the practice focused its attention initially on getting the design of the main concert auditorium, Hall 1, spot-on. To find the holy grail of perfect acoustics, the designers opted to make the hall long, narrow and tall. "It follows the form of the great performance spaces of the past," Flanagan explains.

The room's 1650-seat capacity is the optimum for concerts, says Flanagan. If the room had been any bigger, it would have begun to lose its acoustic perfection as reverberation times increased; any smaller and its reduced audience capacity would make performances uneconomical.

At the moment, the inside of the hall is lined with scaffolding, through which the two rows of balconies that wrap around the walls are just visible. "The balconies are an essential part of the acoustics," says Flanagan. The interior of the hall is about to be clad in timber – hence the high-level scaffold platform. "The wall's acoustic mass drives the solution, while the timber cladding gives timbre," says Flanagan, quoting an acoustician's idea of a joke. The hall's acoustics will then be fine-tuned depending on individual performances and the size of the audience through movable ceiling-mounted acoustic baffles and fabric banners on the walls.

Hall 2 is much more intimate – a theatre in the round. Again, the space is enclosed within a concrete box, but this time the room's circular plan is formed by a series of 10 faceted walls alternating between blockwork infill panel and strips of concrete shear wall to provide stability. Flanagan describes the hall as "a courtyard", and with its two rows of balconies ringing the space, it is not difficult to see what he means. Each faceted wall bows outward slightly to bounce the sound around the space. Again, provision has been made to alter the room's acoustic quality by moving fabric banners around the perimeter.

Sandwiched between the two main auditoriums is the Sinfonia's rehearsal room. The space has been sized to allow a small orchestra to practice, and its acoustics are designed to be much less forgiving than those of the main hall, to allow the players to hone their performance to perfection. The finishes in this space will be much less ostentatious; work is about to commence on installing the room's triple-skin of plasterboard drylining.

Like the acoustic designers, the construction team has had to treat each auditorium as a separate entity with its own dedicated construction manager. A 50 mm acoustic gap separates one auditorium from the next, to eliminate the transmission of sound between venues.

However, the key factor was not just isolation from adjacent auditoriums; plant noise also had to be eliminated. Consequently, each auditorium has its own dedicated plant room, tucked out of the way at the rear of the building. These, too, are isolated from the auditoriums by a 50 mm gap in the structure.

The challenge of achieving acoustic excellence has extended to the regional music school, housed beneath the public concourse at the front of the building. In addition to its administration and teaching spaces, the school has practice rooms, which Flanagan describes as "a box within a box". Again, the first line of defence against noise in the practice rooms is the outer frame, with its dense, concrete blockwork walls and carefully applied mortar. The box is capped top and bottom by concrete floor and ceiling slabs.

The floor of the inner box is also made of concrete but is raised 37 mm above the ground-floor slab to prevent sound being transmitted through the structure into the room. This raised floor was cast in situ on a sheet of polyethylene. However, before the concrete was poured, mini screw jacks mounted on neoprene pads were strategically positioned around the floor.

After the concrete had been poured and cured, an operator using a special key wound in the screw jacks, now cast into the slab, to raise the floor. Flanagan calls it a "jack-up" floor. The inner walls and ceiling of the box are then constructed from three layers of plasterboard dry lining mounted on studs fixed to the raised floor.

Viewed from the Tyne bridge, the building's three concrete-encased auditoriums appear draped by a rippling exoskeleton of steelwork as work on the centre's roof structure nears completion. Above the main auditorium, to the east, the final few sections of the roof structure are being bolted into place. On the west elevation, work is under way on the huge glazed wall that will eventually nestle beneath the blanket of the overhanging roof. On the river front, a forest of scaffolding is steadily growing, from which the steelwork contractor will soon begin to install the roof's stainless-steel rainscreen cladding.

The roof hovers over the auditoriums, providing a sense of cohesion and making a bold architectural statement at the same time. The roof pulled down at sides of the building to shade its glazed end elevations, while at the front, it plunges down to meet the sloping riverbank. Foster and Partners originally considered blanketing the halls under a membrane roof. However, this was rejected on the grounds that it gave "no sense of permanence", according to Flanagan. Instead, working with consultant Buro Happold, the architect eventually opted for a propped shell structure that was draped over the halls and shaped to hint at the three auditoriums beneath. "We've shrink-wrapped the halls," says Flanagan.

It is on the building's river frontage that the genius of the roofing solution is most apparent. "The bonus with this roof is that it gives us the foyer space," explains Flanagan. The practice has exploited this exciting space by wrapping each hall in horizontal layers of cafe-bars and breakout spaces, all orientated to overhang an enclosed riverside concourse. This is intended to be a space where musicians and the public will mingle. Three glazed bulges in the roof lattice allow concert-goers in the bars to soak up the spectacular view of Newcastle and the river below.

Building a one-off roof, equivalent in size to two football pitches, is tricky. So tricky, in fact, that Laing has handed over the responsibility, and most of the risk, to Austrian steelwork specialist Waagner Biro. The building is being constructed under the JCT98 form of contract amended for two-stage tender. "We pre-tendered the work with the highest degree of risk," explains Hannon.

In addition to the roof package, the concrete frame contract and the mechanical and electrical works were also pre-tendered.

"We then value-engineered the packages, drawing on the subcontractor's expertise," he adds. The contractor was helped in this by the project's 18-month lead-in time, a by-product of Gateshead council's difficulty with the remediation of the site.

The roof works were originally tendered as three separate packages: steel, glass and cladding. However, Waagner Biro bid competitively for all three, which were then bundled into one £8m package. From the main contractor's perspective, this made perfect sense, as it created a single point of contact for all the specialists involved in the roof construction, eliminating many interface issues.

Buro Happold's design concept used four primary steel arches to support the roof. These rise up steeply from the riverbank before flattening out to curve gently over the building and then down to meet the ground behind. Curved sections of secondary steelwork span between these arches to create the roof's undulating form. Each arch is supported on a series of four tubular steel props. Again, the building's acoustic performance has influenced the design of the roof structure. The roof structure is completely separate from the structure of the auditoriums. Even the props supporting the main arches pass through oval cut-outs in the concrete floor slabs between the auditoriums rather than providing a bridge for sound to cross between halls.

Detail design of the roof structure was left to Waagner Biro. As Martin Wallbank, Waagner Biro's project manager says, the structure is even more complicated than the roof of the British Museum's Great Court – another of the firm's projects. "There are five different phases of behaviour of the roof structure during construction," says Wallbank. This meant the steelwork initially had to be constructed out of position, and it was only as the structure was loaded that it began to assume its final form.

Steelwork erection was one thing that did not go as planned. Waagner Biro had intended to preassemble sections of secondary and tertiary steelwork flat on the ground before craning them into place. But it proved so difficult to align the preassembled units in the air that the contractor had to crane it in piece by piece.

With only a small section of the structure above Hall 1 left to complete, work started on the cladding last month. Despite the roof's curving form, it is a faceted construction. First, a skin of trapezoidal steel decking will be fixed to the roof structure. This will then be covered by a vapour-control membrane, 75 mm of insulation and a single-ply waterproofing membrane. Unusually, there is a 700 mm gap between the waterproofing membrane and the roof's stainless steel rainscreen cladding panels and a secondary layer of insulation. A 35 mm gap between each flat, stainless-steel panel will allow rainwater to drain onto the waterproofing layer below.

With the roof's stainless-steel cladding now taking shape, the building's iconic status is likely to be consolidated long before its opening date of summer 2004. But for the contractor, the acid test will be when the acousticians arrive with their testing equipment some months before the official opening. Only then will Hannon be sure that every mortar joint has been successfully completed.