Major improvements are being made to the acoustics of the Concert Hall in the Sydney Opera House as part of a $200 million upgrade.

Temporary sound reflectors have been installed after decades of criticism about the quality of the acoustics.

After a successful trial with a lowered ceiling, a permanent upgrade has been approved which will include upgrades to the entry and foyer.

The renovation is the most significant since its opening and aims to improve accessibility, upgrade stage and backstage areas and replace ageing theatre systems.

The managing director of the Sydney Symphony Orchestra, Rory Jeffes, said it would be like taking cotton wool out of the ears of the audience.

"I always say that the Sydney Opera House is like a body and the body needs a heartbeat and that heartbeat that truly keeps the body truly living is the Sydney Symphony Orchestra,"

"Like any body after time it needs a bit of care and attention and a bit of love, and that's what's happening here.

"Finally, I think the ambition that we are trying to achieve is to give this wonderful hall an acoustic that will make it truly world class so that this building will then be an absolute world class icon both inside and out."

The reflectors will allow concert-goers to hear more detail at performances. ( ABC News: Jayne Margetts )

Concert Hall to close during renovations

The Concert Hall will be closed from mid 2019 until the start of 2021 during the renovations, but Mr Jeffes said it would be worth the wait.

"In the middle of 2019, we will be closing the hall with a big festival we will be announcing this year and then we will be reopening the hall at the beginning of our 2021 season," he said.

An artist's impression of how the Concert Hall will look after its upgrade. ( Supplied: Sydney Opera House )

"This is just a temporary structure. The permanent structure will then get put in place and will enable the stage both to have this amphitheatre shape and also then to be flat, and as you can imagine, that is a pretty significant project.

"I've had audience members in the 12 years I've been with the orchestra who have said that they've been coming to the orchestra for 40 years and they can't believe that finally this is happening."

Sydney Symphony Orchestra concert master, Andrew Haveron, said concert-goers currently hear only 60 per cent of what is going on.

"My seat is the best seat in the house and always has been, I hear all the detail and all the passion and all the skill of my fellow musicians here," he said.

"But I have attended many concerts in this hall, sitting in different places and I know it's like having cotton wool in your ears, which is a shame.

"These reflectors will actually be sending all the detail, all the gritty stuff that goes on, the way we shape a phrase and possibly whisper and change the sound subtly, which doesn't always get to the back or even the front in this form weirdly either."

The funding from the New South Wales Government also allows for a new creative learning centre and a function centre.