How restorers are making the Notre Dame Cathedral sound the same after restoration

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The Notre Dame Cathedral in Paris is on target to reopen at the end of 2024. The medieval landmark was ravaged by fire four years ago, and now hundreds of artisans are working to restore Notre Dame. There are stone carvers and iron workers, and there are other workers focused on less visible aspects. NPR's Eleanor Beardsley spent some time with the people restoring the cathedral's acoustics.

BRIAN KATZ: So my name is Brian Katz. I'm an acoustician, and I work in room acoustics and virtual reality.

ELEANOR BEARDSLEY, BYLINE: Katz, an American, is with the French National Scientific Research Center. We meet on the bridge behind Notre Dame, where he watched the cathedral burn on the night of April 15, 2019.

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KATZ: And we're just surrounded by masses and masses of people all watching the fire in almost complete silence. It was a very eerie feeling.

BEARDSLEY: There were gasps when the spire fell. With a gaping hole now in its roof, Katz thought of the acoustics world that had been erased. Then came the plans to rebuild the spire.

KATZ: There were some really wild ideas, such as, like, a glass ceiling or a park or a pool, at which point we really started wondering how all of that could affect the acoustics. And that's when we became more and more involved in the reconstruction efforts.

BEARDSLEY: The decision was made to rebuild the original spire, and Katz was ready. Luckily, he had recorded a concert inside the cathedral in 2013 celebrating its 850th anniversary. So he was able to map out the cathedral's acoustics, calculating how sound reverberates against each interior feature of the building.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED CHOIR #1: (Singing, inaudible).

BEARDSLEY: You can listen to that concert on YouTube. It's called the "Ghost Orchestra Project." Katz says it's like a magic carpet ride through the cathedral.

KATZ: We put the recordings of all the close mic instruments into the computer model, and from that you can fly over the orchestra while it's playing and see - well, hear how the acoustics varies.

(SOUNDBITE OF STONES BEING CHISELED)

BEARDSLEY: Katz is not working alone.

MYLENE PARDOEN: (Speaking French).

BEARDSLEY: Mylene Pardoen is a soundscape archaeologist. She says she and Katz are painting audio frescoes. She's recording stone carvers at a site in Burgundy, where a medieval castle is being built using methods and tools from the 12th century. It's part of their effort to recapture the original sounds of Notre Dame's construction.

PARDOEN: (Through interpreter) Here, we have the real gestures and sounds of how these crafts were practiced 800 years ago, not like in movies where it's all simulated. Here, they are carving the stone. And all these sounds will be used to reconstitute the historic soundscapes around Notre Dame.

BEARDSLEY: Katz says Notre Dame is a living building, and as it's evolved over the centuries, so have its acoustics.

KATZ: The Middle Ages, before there was all this seating, the floor would have been probably covered with straw or hay to absorb, you know, water and mud from people.

BEARDSLEY: Then there was the transformation from a religious to a mass tourist site. Carpeting was added in the 1990s to reduce footfall.

KATZ: So the acoustics has evolved quite a lot, and that's what we're kind of interested in.

BEARDSLEY: Katz says an important part of the cathedral's future is recapturing the acoustics of its past. I ask him which era of Notre Dame was best suited for singing.

(SOUNDBITE OF MUSIC)

UNIDENTIFIED CHOIR #2: (Vocalizing).

KATZ: Probably at the time when that singing was written.

BEARDSLEY: He's talking about polyphonic music from the 12th and 13th centuries.

KATZ: We're assuming that that was probably the best acoustics suited to that music because it was written for that building.

BEARDSLEY: The cathedral itself won't reopen until the end of 2024. But you can already plunge into its history and restoration in one of Katz and Pardoen's tours.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED ACTOR: (As Notre Dame Cathedral, speaking French).

BEARDSLEY: "Whispers Of Notre Dame" is narrated by an actor playing the cathedral herself. An English language version will soon be available.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED ACTOR: (As Notre Dame Cathedral, speaking French).

(SOUNDBITE OF STONES BEING CHISELED)

BEARDSLEY: Notre Dame describes her stones being chiseled...

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED ACTOR: (As Notre Dame Cathedral, speaking French).

BEARDSLEY: ...And how it takes a full eight seconds for sounds inside her to completely fade away.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED ACTOR: (As Notre Dame Cathedral, speaking French).

BEARDSLEY: "My acoustics are what make me exceptional," she says. "I help songs and prayers rise to the heavens." Eleanor Beardsley, NPR News, Paris.

(SOUNDBITE OF MUSIC)

UNIDENTIFIED CHOIR #2: (Vocalizing). Transcript provided by NPR, Copyright NPR.