How to make sure a restored Notre-Dame Cathedral gets the acoustics right

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While engineers and architects from around the world debate the aesthetics of the Notre-Dame Cathedral reconstruction — it's acoustics that Brian Katz is thinking about. 

Katz is a senior acoustics researcher at Sorbonne University in Paris, who is working to help ensure that changes made to the cathedral sound right. 

The 2019 fire that destroyed much of the building's roof and iconic 19th-century spire also drastically changed the acoustics within the building. 

As It Happens host Carol Off spoke to Katz about the unique sound of the cathedral and his efforts to restore it to what it was before the fire. 

Here is part of their conversation.

First of all, can you describe the way Notre-Dame sounded before the fire? What was special about its acoustics?

Notre-Dame, like many cathedrals, is a very large, hard space. So it has a very long reverberation. A very full and rich sound.

The reverberation time of Notre-Dame was about six to eight seconds ... which really gave a full, really immersive, kind of rich sound to the space. 

You've been there since the fire, so what does it sound like now? 

Basically there [are] a number of things that have changed.

One is there's no seating because it was all moved because of the fire. There's a fair amount of dust ... and there's three big holes in the ceiling, which mean that the reverberation time is less.

It doesn't feel at all the same — like the same cathedral that it was before. 

What can you do now? What are the possibilities for restoring the cathedral's acoustics to what they were?

The site is still going through the process of figuring out what's safe and what's not and what parts of the building can remain and what parts of the building would have to be torn down.

What we're trying to do is kind of to highlight ... that the acoustics of the space, like the architecture, has been evolving almost since the beginning of the construction. 

What we want to do is kind of highlight when people say, "We want to make it sound as as it did before." Well, when is before?

Over 800 years, the acoustics have changed a lot. 

For you, 'what it was' is something you actually recorded in 2013. Is that right? 

Yes. So we made ... the only detailed acoustic measurements in the Notre-Dame Cathedral in 2013 as part of a project. And then the same team that I'm with now — in the late '80s — had done some minor measurements. They were doing a study on installing a new organ. 

And we were even able to see that between the late '80s and now, or a few years ago, the acoustics had changed ... and it was just the installation of a carpet in the circulation area. 

So if we look at how other things have changed over time, for example, the less religious activity and the more tourist activity has had an impact on the acoustics.

So that's kind of why it's a strange question. I mean we can make it as it was but as it was in 2013 is not actually what made the acoustics famous and interesting through history.

When the work is done and you've done your work, will they be able to calibrate it? Will they make any adjustments based on your recommendations if you feel you can improve on the sound inside the cathedral?

With the model that we created after the measurements in 2013, we have a calibrated acoustic model of the space.

And what we're hoping to do is ... enter into discussions over the next couple of years with both politicians and especially with the architectural team ... to provide them with tools so that they can test ideas and look at the acoustical impacts of changes that they are proposing. 

If I take the extreme, if they want to make a glass ceiling that's flat, we can ... show how that will make the acoustics different or not. So our goal is really to provide them with a tool to make educated guesses. 

Written by Chloe Shantz-Hilkes and John McGill. Interview produced by Chloe Shantz-Hilkes. Q&A has been edited for length and clarity.