'The space is like an instrument': How Notre-Dame found its voice after fire muffled it

Performers and visitors to the famous gothic cathedral in the midst of the River Seine may find some subtle differences to the way sound bounces around its walls.

The Cathedral of Notre-Dame de Paris has borne witness to many turning points in history. The building's striking gothic stonework has stood sentry on an island in the midst of the River Seine since the late 12th Century as coronations, wars and revolutions have unfolded in its shadow. What you might not realise, however, is that the cathedral has played a key part in shaping the music you hear when you turn on the radio or stream a playlist.

Notre-Dame was, for a brief time, the beating heart of a musical revolution – one that changed music forever and laid the foundations for many of the songs we listen to today.

Up until the middle of the 12th Century in Medieval Europe, it was common to hear the haunting melodic chants of the clergy echoing through churches and cathedrals. Gregorian chant, or plainsong, was the musical style of the day, where sacred texts were sung either by a single voice or a choir in unison – something known as monophony.

But faced with the Notre-Dame's soaring nave, ribbed vaults and towering columns, a group of composers began to try something different to take advantage of the way sound rattled around inside the building. They introduced multiple lines of melody simultaneously to produce elaborate polyphonic arrangements, or motets – the early beginnings of a musical texture that is a common feature in modern music, from jazz to pop and hip-hop.

"They had to figure out a way to sing in there that worked with the architecture," says Brian Katz, an expert on acoustics and research director at the Institut D'Alembert at Sorbonne University in France.

Katz has spent more than a decade studying the acoustics of Notre-Dame, painstakingly hanging microphones and taking measurements to reconstruct what it would have sounded like to play music, sing and preach inside the cathedral at different points in its evolution. Now, Katz hopes to return with his microphones and equipment as Notre-Dame enters a new phase. The cathedral recently reopened for the first time after the devastating fire that destroyed the roof, spire and much of the interior of the cathedral in April 2019. The reconstruction – which reportedly cost €700m (£582m, $758) – saw the building restored stone by stone to how it had been before the blaze.

But Katz believes something may have changed – the way it sounds.

"It's hard to tell exactly how it has changed but we expect it to be more reverberant," he says. Katz has visited Notre-Dame several times to attend masses and performances since it officially reopened on 7 December 2024. The experience has been "incredibly moving", he says.

But as he sat in different parts of the cathedral, he also couldn't shake the sensation that it sounded different. A new sound system and the extensive cleaning of the organ's 8,000 pipes during the restoration work have certainly contributed to that, but he believes that the way sound interplays with the fabric of the building itself is also subtly different.

"The organist has already stated that it will take a bit of time to adapt to the subtle changes in the restored acoustics and organ," he says. "This is very typical of say a new concert hall, where the resident orchestra – and audience – need a few months to adjust to new conditions. The space is like an instrument, and one needs to become familiar with it to get the best out of it."

Katz and his research group at Sorbonne University have worked alongside the architects and builders charged with restoring the cathedral since the fire. They were eager not to lose the unique acoustics of Notre-Dame as the physical building was pieced back together.

"The next day [after the fire], I started getting calls," says Katz. "Colleagues were saying, 'you have those acoustic measurements at Notre-Dame and nobody else has any'."

Katz and his colleagues had gathered these during a series of extensive acoustic surveys of the building in 2015, where they hung microphones in 16 different positions at a time and played sounds from an omnidirectional loudspeaker – a sphere with 12 separate speakers pointing in different directions. They repeated this multiple times in different locations to build up a full acoustic model of the cathedral's interior.

For those who perform inside Notre-Dame cathedral, it is an enlivening experience. The complex stonework and vaulted ceilings make it a challenging but interesting place to sing.

"The greatest privilege is to be able to sing in the evening after closing time, to prepare a concert, and to enjoy the empty cathedral and make its stones vibrate with music," says Henri Chalet, director of the Maîtrise Notre-Dame de Paris music school, which has had an association with the cathedral since it was founded in the 12th Century. "There is an extra soul in this cathedral, which everyone can define in their own way. It is clearly alive."

Gustavo Dundamel, the Venezuelan conductor who led the Orchestre Philharmonique de Radio France at the reopening of Notre-Dame on 7 December 2024, and who led performances there before the fire, agrees there is something special about the building.

"Notre Dame is one of the most magical performance spaces in the world," he says. "Of course there are challenges to coordinating any kind of live performance in such a reverberant acoustic, but the sheer transcendental nature of the cathedral turns that work into a gift."

In the aftermath of the fire in 2019, Katz's team visited the cathedral again, sending their microphones bobbing around debris and scaffolding on a robot normally used to inspect sewer pipes. When they did the measurements, it became apparent just how much of Notre-Dame's special sound had been lost.

"It's dark, you can still smell the carbon and smoke and there's basically 3-4 big holes in the roof," recalls Katz. Those holes combined with the blanket of soot had a dulling effect on the sound. The reverberation time – how long it takes for an acoustic signal to decay within a space – decreased by around 8% compared with before the fire.

When they plugged the data into their "digital twin" of Notre-Dame, they found the acoustics were similar to another key moment in the Cathedral's past.

"It corresponds to the most [sound] absorbing condition we have predicted in Notre-Dame," says Katz. "If you look at Napoleon's coronation – the records of that and the paintings of that show every surface is covered in fabric. When we try to replicate that, we end up with something quite similar to the post fire condition."

In other-words, the voice of Notre-Dame had been muffled by the blaze.

Over the months that followed the fire, there were no shortages of proposals for how the grand cathedral could be reconstructed. Architects suggested radical changes such as a glass roof or even a garden on top of the building before the French President Emmanuel Macron announced the cathedral would be restored to its condition before the fire. 

Even so, Katz and his colleagues were called upon to help with critical decisions that could influence the overall acoustics inside the cathedral. "One of the things that we were tasked with in the reconstruction was to evaluate the proposals for the new choir organ," says Katz. "They were thinking about moving it and making it larger." One of the proposals was to put the organ up in the triforium – the gallery tucked high above the nave, the main open space of the cathedral – rather than beneath it, where it had been before the fire.

"Putting it up higher would mean it had a better view," says Katz. "But the prediction we got from our digital twin was that most of the energy from an organ up there would stay up in the upper space. It would just go from triforium to triforium and didn't really get down to the audience in the way they wanted. So, in the end, they kept it where it was before."

It is a clear illustration of just what a complex space Notre-Dame is from an acoustic perspective. The triforium in Notre-Dame runs on either side of the nave, but then stops at the transept.

"It creates this barrier between the choir side and the nave side, which affects how sound propagates from one to the other," says Katz. He experienced this for himself before the fire when he watched a performance of choir soloists.

"During the performance they would move back and forth, taking four or five steps," he says. "When they came closer to us, all of a sudden we were in a small room with them, and they would take a couple of steps back, and their voices excited the whole space, and they felt much farther away. You really get the feel of that barrier."

While Notre-Dame was reconstructed to replicate how it had been before the fire, there are a few crucial changes that Katz believes have altered the sound inside the cathedral. Among those is the removal of a long carpet that ran around much of the outer perimeter of the building's interior. It had been installed in the late eighties to absorb footfall noise from tourists on the hard marble floor.

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During their research, Katz and his colleagues had discovered recordings from an acoustic survey conducted in 1987 when there were proposals to install a new organ in the cathedral. When they compared the data with the survey Katz had conducted in 2015, it revealed a strange difference.

"There was a noticeable shift in the reverberation time, in a way that should be perceptible," Katz says. With the carpet now gone and the bare marble floor once more exposed, Katz says it is likely the reverberation will have increased again to how it was nearly half a century ago.

Many of the other soft furnishings such as wall hangings have also gone after being damaged in the fire while 1,500 solid wood chairs now sit where there were once rows of chairs with woven seats.

Another change that may affect the acoustics lies with the stonework itself. During the reconstruction process the building was given what effectively amounts to a giant facemask.

"They had to remove all the lead dust so what they effectively did was spray liquid latex onto the walls and peel it off like a skin," says Katz. "It gets into all the crevices and pulls out all the dust in there. The paint work and plasterwork is new too. So in that respect we expect it to be much more reverberant than it was before the fire."

For those performing in the cathedral, it has taken only a little time to get used to singing with the unique acoustics inside Notre-Dame again after a five-year absence.

"The Maîtrise used to sing there all the time," says Chalet. "Then for more than five years, we discovered many different places in Paris, in France and even abroad. This allowed us to learn to adapt very quickly to a new acoustic. Now we take advantage of each service and each concert to tame this demanding but benevolent acoustic."

Both he and Dudamel say they haven't noticed any significant difference in how the space sounds during performances. Dudamel says due to the difference in the set up between the performances he led in the cathedral before and after the fire, it is particularly difficult to compare.

"No doubt acoustic studies will show differences, but honestly, the cathedral is very large and has been rebuilt exactly as it was," adds Chalet. "It is just that the stone has been cleaned which can make the sound reflect a little differently. But my memories of it before the fire are broadly the same."

But both men have another theory for why the auditory experience inside the cathedral may feel different to visitors today.

"Due to the lighting – which is a lot brighter – and the cleanliness of the windows, which puts a huge amount of added brightness in the space, I think there is a palpable difference in how the musicians and audience feed that information into what they are hearing, perhaps creating the impression that the sound is brighter and clearer now," says Dudamel.

Chalet agrees. "The sight of a cathedral, the discovery of the restored chapels that you could not see before because they were so dark, gives the impression that the cathedral has become wider. The sight clearly has an impact on listening."

Certainly, there is some research that suggests visual information can affect our experience of what we hear. 

Of course, to determine exactly how much it has changed will require more tests by Katz and his colleagues. But in many ways, he feels, the cathedral has been given a new life acoustically, that is potentially allowing visitors to experience sound in Notre-Dame in a way that probably hasn't been possible for hundreds of years.

"In some ways it is like a brand-new cathedral," says Katz. "One that is 800 years old."

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