How to restore the legendary acoustics of Notre Dame

For centuries, the inside of Notre Dame never saw much sunlight. But when Brian Katz stepped inside the cathedral last July, the place was drenched light, its famous arched ceiling hospitable the sky. Nearly three months before, on April 15, 2019, a fireplace had ripped through the Paris cathedral. Now, charred wood lay heaped on the ground , mingled with toxic lead dust. The acrid scent of fireside lingered. But Katz and his colleague Mylène Pardoen had one main concern: the sound.

Something fundamental to Notre Dame’s voice was missing: its reverberance, that echolike quality that the grandest cathedrals are known for. “You didn’t hear the building anymore,” Katz says.

Before the hearth , the faucet of a heel or a cough would persevere the air for several seconds, a feature that imbued visitors with a bent to step softly and keep voices low. Notre Dame de Paris — which translates as “Our Lady of Paris” — had how of imposing silence upon her guests. To Katz and Pardoen, the cathedral’s personality had been erased.

But there was reason for hope. Much of the cathedral remained relatively untouched by the fire; wooden chairs still stood neatly in rows, and paintings and sculptures — though covered in dust — remained intact.

Preliminary repairs had already begun. Damaged pillars and flying buttresses were reinforced, and nets hung high within the arches to catch falling debris. Robotic devices swept up rubble in places too dangerous for humans to line foot.
As architects, builders and historians begin the method of rebuilding Notre Dame, Katz — an acoustics researcher at CNRS, the Centre National de la Recherche Scientifique, and Sorbonne University in Paris — is on a mission to assist restore the building’s sonic signature.

Similar work has been happening at other historical places, too. The disaster at Notre Dame has put a field referred to as heritage acoustics within the spotlight. Science has made it possible to document the acoustics and re-create the symphonic grandeur of destroyed or altered structures. Researchers are wielding their knowledge of physics to unveil a hidden history of sound in historical buildings.
“The past wasn’t a silent place,” says acoustician Damian Murphy of the University of York in England. “Sound may be a fundamental a part of our human experience.”

At Notre Dame, Katz and colleagues had a fortuitous start . employing a simulation and acoustic measurements the group made within the intact cathedral in 2013, the researchers had already digitally reproduced the building’s reverberance. Katz is using that employment to predict how choices made during the reconstruction might alter Notre Dame’s effect on the ears.

He also can resurrect the acoustics of Notre Dame of old, showing the impact of renovations from previous eras within the medieval cathedral’s past, that specialize in how the building would have skilled the sounds within. Meanwhile Pardoen, of CNRS and therefore the Maisons des Sciences de l’Homme in Lyon, aims to re-create those long-ago sounds.
Sound may be a transient, ethereal phenomenon, and it tends to be neglected in historical records. While photographs and drawings can preserve the visual impact of a building or scene, documenting the sonic impact of an area is more complicated. except for many of us , sound provides an intimate a part of the feeling , the je ne sais quoi of being during a particular place. Eyes closed, you’ll still tell immediately whether an area is small or soaring and grand.
Aural history

Cathedrals are a classic study subject for heritage acoustics. But sonic scrutiny has been applied to other spaces, including other religious buildings, theaters and even prehistoric caves (SN Online: 7/6/17; 6/26/17). Murphy, as an example , has studied the acoustics of a beloved chocolate factory and an underground reactor cavern.

For cathedrals especially , “the sound and therefore the feeling you get once you are inside … is vital for the character” of the buildings, says acoustic engineer Lidia Álvarez-Morales of the University of York. She and colleagues recently measured the acoustics of 4 English cathedrals, including York Minster. That Gothic structure is larger than Notre Dame and suffered a catastrophic fire in 1984. The cathedral was later restored.

The acoustics within an area are all about how the sound reflects off the surfaces inside. once you clap your hands, for instance , vibrations of air molecules travel during a wave, causing variations in pressure. a number of those waves travel on to your ear, which registers an instantaneous sound. But others travel altogether directions until they reach a surface like a wall, floor or object within the space . Sound waves can bounce off that surface and reach your ear at a later time (SN: 7/13/13, p. 10).

In a place with one reflecting surface, like the distant wall of a canyon, the reflected waves produce an echo, a delayed repetition of the first sound. But during a cathedral, reverberation is that the rule. “Reverberation happens once we have, say, thousand reflections that are all returning to us so fast that we can’t resolve a person one among them with our sensory system ,” says acoustician Braxton Boren of yank University in Washington, D.C. As a result, the sound is drawn out, slowly trailing off to silence over several seconds.

Materials that tend to reflect sound waves and enhance reverberation, like marble and limestone, are common in cathedrals. In contrast, a more typical room has surfaces — carpets, drapes and even the people within the space — that mostly absorb sound waves (SN: 11/15/03, p. 308). Larger rooms also boost sound’s endurance , because the waves take longer to travel between surfaces. Before the hearth , with its arched limestone ceiling reaching 33 meters high and a 4,800-square-meter marble floor, Notre Dame was sort of a giant, mirrored fun house for sound, bouncing the waves around and around.

The reverberation time of an area is that the number of seconds it takes for an initial, loud sound to become so quiet that it can not be heard. Specifically, it’s an estimate of how long it takes a sound to fade by 60 decibels. While a typical front room may need a reverberation time of half a second, and a hall might reverberate for 2 seconds, cathedrals can have reverberation times in more than five seconds.
With long reverberation times, fast-moving music or speech are often muddied, with notes and words stepping on top of 1 another. Gothic cathedrals were designed to be grand spaces — their long reverberation may are a by-product. But music evolved to suit the space: For organ music or religious chanting, “the acoustic conditions are specialized , because this type of music has been designed for those buildings,” Álvarez-Morales says.

In fact, Notre Dame’s special sound may have inspired the birth of polyphony during which different voices sing separate notes, rather than an equivalent pitch — within the 12th and 13th centuries. The Gregorian chants sung within the cathedral in medieval times were monophonic, featuring just one note at a time. But the drawn-out acoustics meant that consecutive notes attended overlap.

Some acousticians believe this effect may have provided an opportunity to experiment with which notes sounded good together, eventually developing into voices singing consonant . This practice is now so common it seems obvious. But at the time, it had been revolutionary. As a result, the roots of recent Western music may are shaped by the acoustics of Notre Dame. “It’s incredibly historically significant,” Boren says.
Sound of silence

On the day of the hearth , Parisians gathered to observe the dramatic blaze. When Katz first heard the news, he didn’t quite believe it. Like numerous others, he decided he had to ascertain for himself.

Despite the throngs, Paris was mostly silent, Katz says. “No one was really talking above a whisper. to possess that a lot of people staring in awe was really strange.” Katz opens his eyes wide while remembering the scene. “No one knew what to mention or what to try to to , but we were all standing there.”

The next day, Katz realized there was something he could do. The 2013 data his group had taken were the sole detailed measurements of the acoustics of Notre Dame. He also had his simulation of the cathedral. Such acoustic models include the locations of the varied surfaces within an area along side estimates of how well each material would absorb sound. And despite the destruction of the cathedral’s roof and medieval timbers, talk about restoring the wounded edifice had already started.

Inside the cathedral, Katz had measured a property referred to as “room impulse response,” which captures how the sound levels within an area vary over time after a quick initial noise. From that impulse response, researchers can derive the reverberation time and subtle characteristics which will affect how a listener perceives an area . One such property is that the length of the delay between when the primary sound waves reach a listener and therefore the arrival of the second, reflected set of sound waves.
Using these measurements of the cathedral, Katz had calibrated his computer model, which allowed him to accurately reproduce Notre Dame’s lost acoustics. And now he could tell architects what they needed to try to to to make sure the building would maintain its acoustic splendor.

Katz exudes an almost constant air of bemusement, as if he can’t fully grasp the cosmic circumstances that led him to become the foremost expert on Notre Dame’s acoustics. With a graying beard and long wavy hair tied back during a loose knot, his look is halfway between musician and physicist. But neither category quite fits: He doesn’t play any instruments, and he’s not a standard physicist.

As a child, Katz’s attempts at learning musical instruments fizzled: He abandoned both the cello and therefore the saxophone. While studying physics at Brandeis University in Waltham, Mass., Katz diverged from his college classmates, who were fascinated with astrophysics or subatomic particles. “That wasn’t really my thing,” Katz says: He stuck to the human scale.

Eventually, Katz stumbled into acoustics because of his experience fixing sound systems for events at Brandeis. With a Ph.D. from Penn State, he eventually became an acoustics researcher in Paris. But he’s no audio-gear geek either. He declares that his home sound system is “crap.”
Music from ruins

The acoustic properties of damaged or demolished buildings are resurrected before. Murphy and colleagues re-created the 16th century sound of a ruined church called St. Mary’s Abbey, founded in 1088 in York. Today, only remnants of the abbey’s walls endure — arched windows frame sky and trees within a city park. But Murphy and colleagues pieced together the architecture of the lost church as best they might , consulting with archaeologists and studying historical references. By putting that information into a simulation , the group got a way of how the space would reverberate.

In 2015, singers performed a concert within the ruins, with the first reverberation of the abbey applied to their voices in real time. Audience members seated within the church’s footprint heard what the music would have seemed like within the intact space.

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