- I'm inside the Notre Dame Cathedral.
This place is renowned
for its gothic architecture and iconic acoustics.
Except the real cathedral was devastated by a fire in 2019
and I'm actually in a sound studio
at Sorbonne University about a kilometer away.
The audio is being fed through a computer program
to accurately represent what it would actually sound like
if I were standing in the pre-fire cathedral.
In today's episode,
we'll be finding out how this program works,
how it's being used to renovate the real cathedral,
and we'll learn a bit more about our musical heritage.
If you're trying to simulate the real world
inside a computer,
then the best place to begin is the real world.
This is the actual Notre Dame Cathedral,
undergoing some pretty serious repairs of the moment,
but still very much at the heart of Paris.
Despite having roughly the same musical talent
as a sack of potatoes,
even I can appreciate how much better things sound
when you bring them from being performed out in the open
to inside Notre Dame.
In 2020, a year and a half after the fire,
a small group of performers were allowed inside
for a televised Christmas concert.
They were devastated to find out
that the building's acoustics had changed,
somehow becoming deadened.
With a quintessentially French reverence for history,
musicians demanded that the repaired Notre Dame
was to sound identical to the one before the fire.
For that, we need to understand
why cathedrals sound the way that they do.
This is the Basilica of Saint-Denis.
The original Dennis was a third century Roman bishop
who was sent to Paris
to help convert the local tribes to Christianity.
According to legend, he was decapitated at Montmartre,
where, after apparently dying,
his decapitated corpse picked up the head
and walked at some 20 kilometers
before deciding to die properly at this location.
The cathedral was eventually constructed here in his honor.
When we talk about acoustics,
what we really mean is reverberation or reverb.
By filling up this balloon with air and then popping it,
we should release an impulsive sound.
By listening to it,
we can quantitatively determine
how much reverb the space has.
If we pop it outside,
(balloon pops)
no reverb.
Let's compare that to in a bathroom,
(ballon pops)
a metro station,
(ballon pops)
and of course, here in the cathedral.
(ballon pops with extra ~r~e~v~e~r~b~)
Sound is just a series of pressure waves.
When these hit an object,
some of the wave energy bounces back and some gets absorbed.
The returning energy can then bounce off objects
again and again and again
until eventually all of the energy is lost.
We hear these progressively fainter returns
as reverberation.
In a bigger space where reflective services are far apart,
this means that it takes longer
for reverberation to fade away.
Breaking up our recordings by frequency,
you can see how lower frequencies persist longer
than higher ones.
This is what gives reverberation its iconic rolloff sound.
With the help of a robot,
this same balloon burst test
was performed in the post-fire cathedral.
Compared with historic data,
the reverberation time was significantly shorter,
especially for the low frequencies.
We could fix Notre Dame by just making it bigger.
But given the truly ridiculous land prices in Paris
and the fact that different frequencies
were affected by different amounts,
this wouldn't solve the problem very effectively.
Instead, we need to look for other approaches.
When we drop a ball onto a hard material such as stone,
then it returns to pretty much the point we dropped it from.
That's because very little of the energy
is lost in the bounce.
On the other hand,
if we drop it onto a soft material like a jacket
or a person,
(James laughs)
then much less of this energy is returned.
So dumb.
In much the same way,
sound waves bouncing off of hard materials
have most energy come back to them,
allowing for many, many reverberations to occur.
Adding in a whole lot of stone
increases reverberation time for Notre Dame,
but to fine-tune it for different frequencies,
we'll also need to add some extra approaches.
This is Sainte-Chapelle,
originally constructed to house relics
from the Holy Cross, the Crown of Thorns, sacred sponge,
and, of course, the Holy Hand Grenade of Antioch.
These massive 15-meter-tall stained glass windows
show stories from the Bible
as these relics were created and eventually moved to Paris.
As these relics were moved away,
people noticed that the sounds inside the space
started to change.
Windows have no effect on high frequencies,
which bounce off of them as if they were made of stone,
but lower frequencies cause the panels
to bow like a membrane,
absorbing a good chunk of their energy.
Likewise, rougher surfaces like unpolished stone,
ancient relics, and really cool detailing
don't affect low frequencies
but will scatter higher ones
due to lots of very small crevasses.
Depending on the ratio of relics to glass,
we should be able to fine-tune
many of the building's high and low reverberant frequencies.
Our final factor to consider is structure
and it's what brings us to the top of Montmartre.
Coincidentally, this is where Saint Denise was beheaded
before walking all that way
to the balloon cathedral from before.
Now, not wanting to miss a perfectly good opportunity
to give Paris yet another cathedral,
eventually people built this,
the Sacre-Coeur Basilica.
Its prime location makes it clearly visible
from the rest of Paris
where its domes,
more similar to those of a Russian Orthodox cathedral
than anything else here,
are very clearly visible.
These have an interesting acoustic effect
where they're able to capture a lot of the reverberation
before only later reflecting it back down into the audience.
This gives the impression
as if the angels in heaven are singing.
It's a cool effect
and would definitely add to reverberation time
but probably isn't quite
what we're going for with Notre Dame.
Columns stop the roof from caving in
but also affect how sound operates.
High frequencies are able to bounce off the columns
while lower frequencies effectively pass straight through.
Depending on where you're seated,
this means that different frequencies
will reach you at different times.
Interconnected chapels, crypts, and corridors
will also change how sound flows through the structure.
Understanding how all these factors interconnect
is actually pretty complicated.
Just moving one thing will change the entire acoustics.
Therefore, in order to properly understand
how Notre Dame is going to sound,
we can't just make the renovations,
we've got to stimulate them first.
And to do that,
we'll be heading back down the hill to Sorbonne University.
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And to now back to the video.
It turns out that you can simulate sound
in much the same way
as you might do a high-end light simulation.
We model sound as a series of particles,
which emanate from the source.
Like with actual sound waves,
when these pascals hit an object,
they bounce off,
losing some energy
as defined by what material they're hitting.
By replicating the building in 3D space,
populating it with materials,
and then placing in some virtual microphones
before running the simulation,
we can replicate what being at those microphone locations
would actually sound like.
At the moment, we have the fire-destroyed cathedral
and a very accurate calibrated simulation
of that same fire destroyed structure.
Engineers use this model
to find out which additions or substitutions are required
to match the recordings made in the pre-fire Notre Dame.
In the early days
when there were some pretty outlandish rebuild proposals,
this simulation would've been used
to suggest design alterations
to ensure that the new acoustics
match those of the original,
even if appearances did not.
Want a lot of glass?
Fine, but since that will dampen the low frequencies,
we'll have to have some extra ancient relics
for rounding out the high ones.
That is extra of the ancient relics,
not an extra old handkerchief from Saint Denis.
Today, the architectural mission
is to bring things looking like they were before,
which makes the acoustics loads easier.
Even so, the materials and decorative items
that are physically available,
as well as the new ways
which people are planned to walk through the space,
will fundamentally change the acoustics.
To counteract it, we can add in more stone,
put in some carpet, perform a blood sacrifice,
knock out a wall to replace with glass,
and just move around some seats,
of course, making these alterations
in the digital simulation first
before doing it for real.
While it will take at least another two years
to open Notre Dame to real performances,
for the moment, the tools being used
to renovate the cathedral
can be jury rigged
to give a virtual reality performance
acoustically identical to the real thing.
When you bring all of that together,
you get this, the ghost orchestra of Notre Dame.
It was made by recording individual groups of musicians
and then simulating them inside a computer.
This is incredible.
When I watched it for the first time,
it felt as if someone had put a drone
inside the real Notre Dame
and then just flown it through, holding a microphone.
Had to keep reminding myself
that all the reverb was simulated.
Graphics though, yep, I can tell they're fake.
As we continue to restore this mighty cathedral,
it is the simulations like those made at Sorbonne University
which will guide our acoustical restoration
of this incredible monument.
The ghost orchestra of Notre Dame will be ghosts no more.
This has been James Dingley from the Atomic Frontier.
Keep looking up.