Image copyright Nick Rochowski Image caption The organ has been reinstalled in a new position to create extra stage space

It's a vast beast of an instrument. Taking up the entire backstage wall of the Royal Festival Hall, the Southbank Centre's newly restored organ is a true behemoth, as much a feat of engineering as it is a musical force.

And at full tilt, it makes the very air shake, leaving you feeling as if your molecular structure has been re-ordered. The sheer size of the sound it can produce is astonishing.

"It needs to be louder than an entire orchestra", says William McVicker, the Southbank's Organ Curator, demonstrating the instrument's massive, full-throated tones with a passage from Jean Langlais's Suite Breve.

But this huge machine, which dates back to 1954 and comprises almost 8,000 pipes, is also built to play quiet, tender tones, all of which will be on display for Tuesday's gala, which publicly re-launches the organ.

"The organ hasn't been with us for nine years so it's amazing to think we're going to have a welcome back concert with hundreds of people taking part", says Gillian Moore, Southbank's head of classical music, who as a teenager herself earned extra cash as a wedding organist.

The organ has been out of use since 2005, when the Royal Festival Hall itself was closed for a refit.

Image caption "It needs to be louder than an entire orchestra", says William McVicker, the Southbank's organ curator

The instrument was removed and refurbished by organ-makers Harrison & Harrison, who originally built it, before the company reinstalled the entire instrument in a new position that created extra stage space.

Standing next to the console as McVicker works across the organ's four keyboards - playing the bass notes with foot pedals - it is easy to feel intimidated by this colossal array of pipes and stops. And to understand Gillian Moore's excitement.

"We're going to hear the organ in all its glory and then we're going to have a festival lasting two weeks to really formally and properly and joyfully welcome it back to the Royal Festival Hall", she enthuses.

World premieres

The gala concert will feature the world premiere of Sir Peter Maxwell-Davies's new work, A Wall of Music for organ, brass and choir - based on a poem by Forward Poetry prize-winner Jo Shapcott, scored for a children's choir.

The piece features two interludes for organ which are "contemplative rather than loud" Sir Peter explains.

"I thought there would be plenty of triumphant organ sound in the programme, and decided to explore its more intimate possibilities. The ending, however, uniting choir, organ and brass, is suitably affirmative."

Image copyright Shona Branigan Image caption Organ-makers Harrison & Harrison refurbished the organ at their workshops before reinstalling it

The new work will be brought to life by Eton College organist, David Goode, who also played the organ before its restoration.

"It's an enormously versatile instrument and very characterful. It was revolutionary in its day and rather brash - but I think it's mellowed", he says.

"The sound is a little warmer but more direct in a curious way than it was, and the acoustics of the hall have been warmed up - which is good for the organ. There's more warmth generally in the bass."

Goode says he feels honoured and excited to be back playing the Royal Festival Hall organ, and to be working with Maxwell-Davies, whose score has allowed the organist some freedom in how the instrument is set up.

"He (Maxwell-Davies) said he wanted there to be the chance for some softer colours", says Goode, "so I've been looking for that when I tried different stop combinations - this colour here or that effect there - I could have sat for hours trying different things."

'Poignant but joyful'

The gala will also feature another premiere by Sir John Tavener, who died in November last year.

His work, Monument to Beethoven, was commissioned by Southbank two years ago. Moore says hearing the piece will be poignant but joyful.

"We thought he'd be the perfect composer to write a new piece for the organ", says Moore.

"I'm so pleased we'll have his music."

Moore says it's been a huge project to bring the organ back in to use, that has depended on thousands of people who "love and care for this organ".

A £950,000 Heritage Lottery Fund grant left the restoration budget with a shortfall of more than £1.3m. A sponsored bike ride raised another £100,000, with the rest donated by members of the public.

Image copyright Tim Cochrane Image caption Individual pipes have been sponsored by individuals in order to reach the fundraising target to repair the organ

"Individual people have helped to make it happen by sponsoring an individual pipe" says Moore, "anything from a large amount of money for one of the great big 30-foot (9.14 metres) pipes, to a relatively small amount of money for one of the very small pipes. And I think that's been a great thing that's put the organ into people's hearts."

Moore concedes the cost of restoring the organ has been enormous, but she never doubted it was the right thing to do.

"This is an extraordinary, unique instrument, a historic instrument, and it's part of the hall," she insists.

"When we reopened the Royal Festival Hall in 2007 after two years of refurbishment, here was this wonderful 1950s jewel of a room, in all its mid-century modern glory, but there was something missing at the heart of it, and that was this organ - and we knew that it was so important to do everything we possibly could to get it back."

The gala concert launches a two-week organ frenzy at Southbank, which will see the newly restored instrument put through its paces with performances of huge works for organ and orchestra, such as Janacek's Glagolitic Mass and Organ Symphony by Saint-Saens, as well as new works from Finnish composer Kaija Saariaho, and Turner Prize winner Martin Creed.

Image caption Organist David Goode believes the organ has a warmer sound now

'Bad boy of the organ world'

The festival will also include a performance by the flamboyant American organ virtuoso, Cameron Carpenter, who has been described as the 'bad boy of the organ world'. He'll be improvising a live soundtrack to the 1920s German silent horror film, The Cabinet of Dr Caligari.

Image copyright Royal Festival Hall Image caption Cameron Carpenter says the organ is "a psychologically gripping instrument"

The film is "thought by many to be the ultimate summation of German expressionism in film and is one of the first films dealing with mental illness," says Cameron. "It's an extraordinarily rich tapestry on which to build a score."

It's the first time Cameron will play the Royal Festival Hall organ, and he says he doesn't know for certain how his improvised soundtrack will turn out on the night. But whatever happens, he says he will draw on the instrument's unique properties.

"The organ is a psychologically gripping instrument because it's the only instrument which sets the player in a different relation to time by its ability to sustain notes indefinitely without drawing on the player's own energy. So naturally its ability as a psychological machine is really useful in a setting like this."

It is one of a series of performances that Moore hopes will enable the organ to set new directions for music at Southbank for years to come.

"Every day I just smile at the thought of all the possibilities, at all the artists calling us up saying they want to do something with the organ," she says.

"It's a thrill to think that that massive sound will once more be available to the audiences who come here."

The Organ Gala Launch concert takes place at the Royal Festival Hall in London at 7.30pm on Tuesday 18 March and is being broadcast live on BBC Radio 3.