and prestigious concert halls in Britain and the world.

It's 150 years old,

and basically every world famous musician and band has performed here at some point.

The trouble is, the science of acoustics

wasn't really well developed back in the 19th century.

A tall, oval-shaped auditorium with a domed glass roof,

well, it looked impressive and regal,

but it created an accidental echo chamber,

the exact opposite of the focused sound that you'd want for a concert.

Beautiful location, terrible sound.

To see how they fixed it, we need to go up,

to the parts that the public aren't allowed into.

- It was constructed without any thoughts

of acoustics, really, but it was also glazed,

it was daylit, naturally lit.

The Prince of Wales who became King Edward VII

made a speech there at the public opening.

Everyone heard his voice twice.

So immediately they had to do something to solve the acoustics,

and they installed what's called a calico velarium,

it's like a sail cloth hung from the roof.

It was partially successful,

the inner-glazed roof suffered from leakage,

broken glass panes as a result of the Second World War,

and also daylight really wasn't suitable for a modern hall.

The inner glazing of the dome was removed and replaced in 1949

by an aluminium velarium, which is what you see there now.

In the late 1960s, the now-iconic "mushrooms" were added to the hall.

They are suspended, convex diffusers,

and they help cut down the echo tremendously.

In 2019, the sound system was replaced completely

within the hall, and the subjective response,

the reaction from audiences, has been absolutely tremendous.

- This is right at the top of the hall.

That is the glass roof, it is 40 meters down to the floor,

and just through there is an air vent called the "corona",

and running across it is a tension steel mesh

to let the technicians access it.

I am assured by the team here that it is fail-safe

and I can walk on it.

Guy, hello!

- Hello, welcome to the corona.

- Oh, it's a lot cooler in here.

[laughing] Must be... oh, I looked down!

[laughing] I should not have looked down.

I mean, you're health and safety officer here, right?

- That's right, yeah.

- You're just casually standing on the mesh?

- Mm-hm. Yep. Feel free to step out with me.

- That's a full fear response from me, that.

I was going to say, is there anything I can grab onto?

I was going to- - That's very greasy.

So most people hold my hand.

Including the fire brigade! - [laughing] I was going to ask,

like, who has to come out here?

Oh my god, it bounces. It bounces.

Who has to come out here?

- Technicians, people who need to put shows on and rigging.

- Haah, ha-haah. Putting all my weight on that foot

is so, so difficult, bloody hell.

Oh my god, don't bounce!

- The weight of the Royal Albert Hall roof,

the dead weight of the wrought-iron structure is about 340 tonnes.

The roof itself in total can take about a thousand tonnes,

inclusive of all permanent and imposed loads, such as wind and snow loading.

There is approximately a hundred motorized chain hoists

which hang from the original wrought iron structure of the roof,

which vary in their load rating,

but some can carry up to 1-2 tonnes of show infrastructure.

We have lighting rails, which support lighting,

sound speakers, as well as the mushrooms

which provide the acoustic dampening of the auditorium.

The mushrooms, despite their quite substantial size,

only weight around 40kg.

All of the mushrooms, show infrastructure, speakers,

is all hung from the original wrought iron roof structure.

- I am not sure I can move right now.

I'm genuinely...

- Just look forward, just look forward and walk.

Just walk, there's no way you can fall.

There's no way you can fall.

Just walk, look, just look at the pole,

and that's it, you've done it, well done.

- Thing is-

- Congratulations(!) - Thank you.

I know it's all my head

because the pole is connected exactly the same

as this grid is.

- So this is the corona, the crown,

and that is the oculus above us.

- It's creaking.

- That's the heat expanding,

so temperatures are regularly in the thirties and forties.

- Typically, whenever there's a fall-from-height risk,

the factor of safety on the material properties

that would go into the design of the tension wire grid

is 5:1, which means there's a

fivefold weight allowance of the material property which it can support.

- But the most surprising thing of all about this roof

is not how they fixed the acoustics,

it's not this terrifying technicians' trampoline,

it's just out there,

and the best way to see that is from the outside.

I'm going to need a hand, I just-

- Feel free.

[Tom gasping]

- The roof was constructed in full in Manchester

and then it was dismantled

and reassembled in London through internal scaffold shoring

from the inside of the hall.

Every beam and element of the wrought iron trusses

was then winched up to the top of the roof

and then assembled in situ.

The shoring was then removed once the roof

had been completed in full

and dropped by about 5/16ths of an inch

or about 8mm, which would've been a heart stopping moment

I think for all those involved.

- All of which leads to the most starting thing for me,

the reason I made this video, this roof,

this iron and glass dome with all that equipment hanging from it,

it's not actually connected to the rest of the building.

It's touching, sure, it's resting just here,

you can see where the two structures meet,

but there's no cement, no bolts, no attachments at all.

This dome is just so incredibly heavy

that it's not going to move, and it hasn't for 150 years.

600 tons of iron and glass, and a load of Victorian engineering.

It doesn't need anything more

because anything powerful enough to shift this

wouldn't be stopped by a couple of bolts.

The roof of the Royal Albert Hall isn't actually attached.