The seven doors backstage at the Royal Opera House wouldn't be out of place in Wagner's Valhalla – one weighs 64 tonnes and is 17 m high. Why did they have to be so big? And how do you open one?
Stepping backstage at London's Royal Opera House is a startling experience. A vast, cavernous space, more akin to a factory than a theatre, contains a host of behind-the-scenes activities. This space is partitioned by seven huge doors, splitting it into rooms and isolating each of the activities.

Doors do not come much bigger than this. All custom-built, the largest is more than 18 m wide by 17 m high and weighs 64 tonnes.

The auditorium itself takes up little more than 10% of the newly extended opera house's 1 ha floor area. Apart from the rebuilt public areas and offices, the remainder is given over to backstage activities. Two opera rehearsal rooms, a scenery assembly space and a set-builder's workshop compete for space with the actual backstage areas.

"The doors are large enough to allow immense sections of the set to be moved between the different backstage areas," says Jeffrey Phillips, assistant technical director of the Royal Opera House's in-house design team. "It means we'll be able to stage two different shows a day and rehearse for a third."

Keeping out backstage noise

Increasing the number of shows may be good for revenue, but it results in a lot of backstage activity during performances. This noise could interrupt the performers or the audience.

So, the doors' acoustic performance will be critical. And it is not only the doors leading to the stage that need to kill sound – the backstage layout is such that one of the opera rehearsal rooms is situated behind the rear stage area, and the other next to the scenery assembly area.

For the acoustic consultant, Arup Acoustics, the doors presented a real challenge. Before their construction could be specified, the practice had to determine the type of activity to be carried out in each area to assess how much sound would be produced. This exercise showed that most of the doors had to reduce sound transmission by a massive 40 to 50 dB between spaces.

The most demanding acoustic requirement was for the door between the scenery assembly area and opera rehearsal room two, which had to reduce transmitted sound by 65 dB so "angle-grinders could be used by set-builders while rehearsals are taking place on the other side of the door", says Phillips.

The doors are constructed on a top-hung bolted steel I-section frame. A 5 mm steel sheet forms the outer skin, adding mass while providing a durable finish. It is attached to an 18 mm thick sheet of plywood, mounted on plasterboard and attached to the frame.

"The doors' different acoustic requirements are met by adding sheets of plasterboard to increase their mass," says Phillips. A neoprene layer between the frame and the skin stops sound being transmitted directly from the outer skin through the steel frame to the inner steel lining. Rockwool is used to fill the voids in the frame. Rob Harris, director and principal of Arup Acoustics, admits that this is not ideal acoustic insulation, but says "it does give the door some extra mass".

To meet rehearsal room two's 65 dB sound reduction criteria, two sliding doors were installed to ensure rehearsing singers are not disturbed by the whine of angle grinders.

Pumped up to keep the volume down

The doors were made in Germany by specialist door fabricator Bayerische Buhnenbau. Because of their size and weight, they were delivered to site in kit form and assembled in situ, top down, by operatives in cradles.

Arup Acoustics specified an inflatable seal around the edges of the doors. This resembles a bicycle inner-tube and is recessed into the doors' lower edge and the walls at both sides of the doors. When a door opens, the seal deflates, but when closed an air compressor inflates the tube to fill the gap and stop sound passing through.

The 40 m high ceiling of the theatre's new fly-tower allows the doors separating backstage from stage to open vertically. Large electric motors open and close the doors, counterweights reduce the effort required and a hydraulic brake ensures they do not come crashing down. Elsewhere, the top-hung doors slide open horizontally, using pulleys and gears powered by electric motors.

As the last few sections are put in place, the team is steeling itself for the moment of truth: will they do the job? Arup won't have long to wait to find out. "Acoustic testing is scheduled for June," says Phillips.