How researchers hope to restore the unique sound of Notre Dame

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Marco Werman: One year ago today, tragedy in Paris: A fire broke out on the roof of Notre Dame cathedral. France quickly pledged to rebuild the church. The restoration will certainly take years because Notre Dame isn't just any building. It's a place with a unique sound.

Emma Jacobs reports on what it will take for that to be restored.

Emma Jacobs: Vincent Dubois is one of the three organists of Notre Dame. He was outside Paris when he learned that the cathedral had caught fire.

Dubois: It's like one of the closest members of your family, your husband, your partner or your brother or sister, was burning beside you and there was nothing you could do.

Jacobs: It wasn't until 6 a.m. the morning after the fire that he got a text message. It said the organ had survived.

Dubois: We got this news. And that was reassuring, even miraculous.

Jacobs: Dubois describes the acoustics of Notre Dame as extraordinary, unlike any other cathedral he's heard.

Dubois: The resonance is unlike anywhere else. It's both precise, while at the same time lasting. And the way the sound reverberates.

Jacobs: Brian Katz studies sound. He's an acoustician by trade and he could hear the difference when he was able to enter Notre Dame last July. He described the way it suddenly streamed through the now open roof.

Brian Katz: What was the most stark at that time that I remember, visually, was the number of things that weren't burned and weren't damaged, kind of side by side — having a big piece of burnt roof sitting next to a perfectly fine altar or statuette that was undamaged.

Jacobs: Though the cathedral sounded, he says, completely different than before the fire.

Katz: The fact that there are these holes in the roof, the reverberation time has dropped significantly. So you don't feel like you're walking into a cathedral, acoustically, anymore.

Jacobs: But Katz has an acoustic map of Notre Dame, created by his research lab at the Sorbonne University in Paris. He later learned that they're the only set of measurements of the cathedral's acoustics before the fire.

Katz: The measurements that we did in 2013 were after the concert that we recorded. So it was, I think, between 10 p.m. and midnight or maybe one o'clock in the morning. There were about ten of us moving microphones around.

Jacobs: Playing sounds in one spot and recording when they reach other spots around the cathedral.

Katz: The sound that I tend to typically use is called a chirp.

Jacobs: A chirp is a tone that swoops from very low frequencies to very high.

Katz: We make these measurements in a lot of different combinations of where the sound could be coming from and where the sound can be listened to, which then we use to calibrate the acoustic model that we create afterwards.

Jacobs: These recordings, this acoustic model of the cathedral, were an experiment. Katz's team used them to make a video simulation that lets you fly around Notre Dame on a magic carpet and hear the music of a concert transformed.

Katz: It was not planned to be as important as it has turned out to be.

Jacobs: They realized after the fire last April that these measurements could help in Notre Dame's reconstruction.

Katz: Acoustics is an effect of the choices of all the other disciplines. So, the structural engineers and the stonemasons and the architectural finishes, all those choices of details are what creates the acoustics of a space.

All cathedrals have lots of big, flat, reflective surfaces, and even small modifications can change the resonance of the space. In some places, changing the painting or the cleaning, the stone — that’s been known to have happened.

Jacobs: And you can imagine rebuilding Notre Dame will involve many, many changes. But with the measurements and the type of software used to design concert halls, Katz can advise on how to make all these choices add up to something that sounds like the Notre Dame of before the fire.

Katz: The earlier the acoustics is thought of and considered then the more integrated it can be in the design, and important decisions can be made at the right time, as opposed to later in the project when it's more difficult to change things.

Jacobs: Katz's team planned to return to Notre Dame this spring to take new measurements of what it sounds like now. That trip got postponed when Paris shut down because of the coronavirus. But the French government has pledged to rebuild the cathedral.

Organist Vincent Dubois says his instrument can be restored and retuned. Still, even after the best restoration effort, he thinks it will take a few years for Notre Dame to sound like it once did.

Dubois: It's also the dust of the space, simply put. It's a place in which there is so much foot traffic. There are many candles to be burned, so lots of soot, which ends up all over everything.

Jacobs: The only thing that can restore that, Dubois thinks, is time — a short time, really, in the life of the centuries-old cathedral.