Acoustics of Heritage in the Restored Notre Dame

The Architectural Symphony of Notre-Dame

Notre-Dame de Paris has long stood as a monument to history, yet its role in shaping music remains less celebrated. Initially, medieval clergy filled its halls with Gregorian chants, their voices rising in unison through the nave. Over time, however, composers began experimenting with polyphony, transforming sacred music forever. By the late 12th century, the cathedral’s soaring vaults and vast stone surfaces inspired musicians to layer melodies, creating harmonies that danced across its arches. This shift marked the birth of polyphonic music, a radical departure from the monophonic plainsong dominating European worship.

Notre-Dame's Architecture: Shaping Eight Centuries of Sound and Harmony

Architecturally, Notre-Dame’s design played a direct role in this evolution. Its ribbed ceilings, towering columns, and expansive nave created a unique acoustic environment where sound could linger for seconds. For instance, reverberation times—the duration a note persists in the air—reached up to eight seconds in certain areas, allowing complex harmonies to blend seamlessly. Consequently, composers like Léonin and Pérotin, key figures in the Notre-Dame School of polyphony, crafted intricate motets that exploited these acoustic properties. Their innovations laid the groundwork for Western musical traditions, influencing genres from classical symphonies to modern pop.

Interestingly, the cathedral’s physical structure continues to shape performances today. Brian Katz, an acoustics expert at Sorbonne University, notes that medieval builders unknowingly designed an “instrument” of stone. “They prioritised light and height,” he explains, “but in doing so, they created a space where sound behaves unpredictably.” Katz’s research, which began in 2015, involved mapping the cathedral’s acoustics using 16 microphones and a spherical loudspeaker. The data revealed how sound waves ricocheted off surfaces, creating pockets of clarity and resonance. These findings not only shed light on historical performances but also guided modern restorations after the 2019 fire.

Echoes Through Time: A Musical Revolution

While Notre-Dame’s architectural grandeur is undeniable, its sonic legacy often goes unnoticed. During the medieval period, the cathedral became a laboratory for musical experimentation. Léonin’s Magnus Liber Organi, a collection of polyphonic compositions, exemplified this shift. By weaving multiple vocal lines together, these works transformed static chants into dynamic dialogues between voices. Meanwhile, Pérotin expanded the complexity, introducing four-part harmonies that shimmered through the nave.

Centuries later, these innovations resonate globally. Jazz improvisation, choral arrangements, and even electronic music owe a debt to Notre-Dame’s early pioneers. Gustavo Dudamel, conductor of the Orchestre Philharmonique de Radio France, emphasises this continuity. “The cathedral’s acoustics force musicians to listen differently,” he says. “Notes blend in ways that feel almost alive.” During rehearsals for the 2024 reopening concert, Dudamel adjusted tempos to accommodate the space’s reverberation, a practice medieval musicians likely employed.

To preserve this heritage, Katz’s team developed a “digital twin” of Notre-Dame. Using 3D scans and acoustic data, they recreated the cathedral’s soundscape across different eras. One simulation, for example, transports listeners to 1804, when Napoleon’s coronation filled the space with thick draperies, dampening reverberation. Another explores the post-fire acoustics, where exposed stone and cleaned surfaces amplified brightness. This project, titled Vaulted Harmonies, premiered as an immersive documentary in January 2025, offering audiences a chance to hear history unfold.

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The Fire’s Aftermath: Silence and Reconstruction

On 15 April 2019, flames engulfed Notre-Dame’s roof, collapsing its spire and charring centuries-old timber. While the world watched in horror, scientists like Katz faced an urgent question: had the fire altered the cathedral’s voice? Initial assessments revealed a 8% reduction in reverberation time, a consequence of soot-coated walls and gaping roof holes. “The space felt muted,” Katz recalls. “Sound decayed faster, as if the soul had been drained.”

Rebuilding efforts, funded by €700m in donations, prioritised historical accuracy. President Emmanuel Macron’s pledge to restore “every stone” included subtle acoustic considerations. For instance, architects debated relocating the choir organ to the triforium, a gallery above the nave. Katz’s simulations, however, showed this would trap sound in the upper vaults, reducing clarity for congregants below. Ultimately, the organ remained in its original position, preserving balance between architectural and auditory heritage.

Polishing History: Notre-Dame's Acoustic Transformation Through Restoration

Material choices also played a role. Workers removed a 1980s-era carpet, exposing the original marble floor. Similarly, 1,500 solid oak pews replaced older, fabric-upholstered seats. These changes, while visually minor, brightened the acoustics by reducing sound absorption. Meanwhile, cleaning methods—such as spraying latex to extract lead dust—left stone surfaces smoother, enhancing reflectivity. “It’s akin to polishing an instrument,” says Katz. “The notes ring truer now.”

As the cathedral reopened in December 2024, performers noted subtle shifts. Henri Chalet, director of the Maîtrise Notre-Dame choir, observed that voices carried further, yet required precise articulation to avoid muddiness. “The space demands discipline,” he explains. “But when you harmonise with its echoes, the result is transcendent.” For visitors, the interplay of light and sound adds another layer. New LED installations illuminate once-darkened chapels, creating a visual rhythm that mirrors the acoustics.

Heritage Acoustics: Resurrecting Lost Soundscapes

The science of heritage acoustics has gained prominence since Notre-Dame’s fire, offering tools to preserve and recreate historical soundscapes. Brian Katz’s work exemplifies this field, blending physics with history to decode how architectural choices shape auditory experiences. In 2013, his team conducted a pivotal acoustic survey, capturing the cathedral’s “room impulse response” — a measure of how sound decays within a space. Using a slide-whistle-like test tone, they recorded reverberation times averaging eight seconds for middle C, a benchmark that underscored Notre-Dame’s sonic grandeur.

Comparatively, Damian Murphy, an acoustician at the University of York, highlights similar efforts at sites like York Minster, where medieval acoustics inform modern restorations. “Soundscapes are as vital as visual aesthetics,” Murphy argues. At Notre-Dame, this philosophy guided post-fire decisions. For example, architects initially proposed relocating the choir organ to the triforium. Katz’s simulations, however, revealed that elevated placement would trap 70% of sound energy in upper galleries, leaving congregants below with muted tones. Consequently, the organ stayed grounded, preserving its historical resonance.

Meanwhile, the removal of a 1980s-era carpet — initially installed to dampen tourist footsteps — revealed another layer of acoustic history. Pre-fire measurements showed the carpet reduced reverberation by 15%, a figure confirmed by comparing 1987 and 2015 data. Its absence post-restoration has restored the marble floor’s reflective properties, allowing sound waves to bounce freely. “It’s like lifting a veil,” Katz notes. “The space now echoes with a clarity absent for decades.”

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The Digital Twin: Bridging Past and Present

Central to Notre-Dame’s acoustic restoration is Katz’s “digital twin,” a 3D model integrating architectural scans with sound propagation data. Developed from 16 microphone arrays and spherical speakers, this virtual replica allows researchers to simulate how music, sermons, and even ambient noise would have travelled through the cathedral across eras. During reconstruction, the model became indispensable. Builders tested material choices — such as replacing fire-damaged oak pews with solid wood — by inputting their acoustic properties into the simulation.

One striking application emerged in recreating Napoleon’s 1804 coronation. Historical records describe draperies lining the walls, absorbing sound and shortening reverberation. By adjusting the digital twin’s parameters, Katz’s team found post-fire acoustics eerily mirrored this muted environment. “The fire’s soot acted like fabric,” he explains, “dulling the space just as it was during Napoleon’s era.” This insight validated efforts to clean stone surfaces thoroughly, using liquid latex sprays to extract lead dust without damaging medieval masonry.

The twin also underpins Vaulted Harmonies, an immersive documentary premiering at UNESCO’s 2025 Week of Sound. Viewers experience virtual concerts spanning eight centuries, from Pérotin’s polyphony to Dudamel’s post-restoration performances. Audiences report chills as the simulation shifts from the 12th-century nave — its rougher stone surfaces scattering sound unevenly — to the polished 2024 interior, where notes ring with crystalline precision. “It’s time travel for the ears,” remarks Henri Chalet, whose choir features in the film.

Light, Sound, and Perception: A Multisensory Experience

Since reopening, visitors often describe Notre-Dame’s acoustics as “brighter” — a shift attributed not just to physical changes but sensory interplay. The installation of 21st-century LED lighting, for instance, illuminates previously shadowed vaults, altering spatial perception. Gustavo Dudamel observes, “When light reveals new details, the mind interprets sound differently. It’s as if the music gains dimension.”

Research supports this phenomenon. A 2022 study by the University of York found that brighter environments enhance auditory clarity, as visual stimuli prime the brain to detect finer sonic details. At Notre-Dame, cleaned stained-glass windows — restored using €30m in donations — diffuse coloured light that syncs with liturgical music, creating synesthetic experiences. During Advent services, cobalt hues from the Virgin Mary windows accompany choir harmonies, evoking what Chalet calls “a dialogue between light and sound.”

Yet challenges persist. The new sound system, designed to enhance sermon intelligibility, required meticulous calibration. Engineers focused directional speakers onto pews, minimising reverberation interference. Katz praises the setup: “It’s like whispering into someone’s ear in a crowded room.” However, purists argue it detracts from the cathedral’s raw acoustic character. Organist Olivier Latry, who has played Notre-Dame since 1985, admits, “The space feels both familiar and new. Adapting requires respecting its history while embracing its rebirth.”

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Public Engagement: Soundscapes for the Digital Age

For performers, this duality inspires creativity. The Maîtrise choir now rehearses at dusk, leveraging the interplay of fading light and emerging artificial illumination. “As shadows lengthen, the acoustics soften,” Chalet explains. “We adjust dynamics instinctively, almost conversing with the building.” Similarly, Dudamel’s orchestra varies repertoire based on time of day — opting for Baroque pieces in morning light and Romantic works under evening chandeliers.

Notre-Dame’s acoustic revival extends beyond physical restoration, embracing digital innovation to share its sonic legacy globally. The Vaulted Harmonies documentary, premiering at UNESCO’s Week of Sound in January 2025, allows audiences to experience eight centuries of the cathedral’s acoustics through virtual reality. Viewers navigate a 3D model, hearing Pérotin’s 12th-century motets resonate in a bare stone nave or Dudamel’s 2024 performance echo through cleaned vaults. “It’s not just about preservation,” Katz emphasises. “It’s about making history audible.”

Echoes of History: Notre-Dame's Acoustic Revival Through Innovation

Complementing this, the Notre-Dame Whispers app offers an augmented reality tour. Visitors exploring the cathedral’s exterior trigger location-based audio clips — Gregorian chants near the choir entrance, revolutionary crowds murmuring by the west facade. Developed with historian Mylène Pardoen, the app layers historical soundscapes over modern Paris. For instance, tapping a smartphone near the Seine plays recordings of medieval fishmongers, reconstructed using period tools. “Sound connects us to the past viscerally,” Pardoen notes. “You don’t just see history; you feel it in your bones.”

These projects reflect broader trends in heritage acoustics. Institutions like York Minster and Rome’s Pantheon now use similar models to recreate ancient soundscapes. Meanwhile, Notre-Dame’s restored organ — its 8,000 pipes cleaned of fire debris — features in live-streamed concerts, reaching global audiences. Henri Chalet’s choir, for example, performed a 2025 Easter service broadcast in Dolby Atmos, capturing the nave’s eight-second reverberation. “Technology lets us share the cathedral’s voice,” Chalet says, “even with those who’ll never climb its towers.”

The Future of Heritage Acoustics

Notre-Dame’s restoration has cemented acoustics as a priority in heritage conservation. The Association of Scientists in the Service of Notre Dame, formed days after the fire, now advises on projects from Chartres Cathedral to Cambodia’s Angkor Wat. Their guidelines, published in March 2025, stress interdisciplinary collaboration — architects consulting acousticians, historians informing engineers. “We’ve moved beyond just saving stones,” says Damian Murphy. “We’re preserving sensory experiences.”

Critical to this shift is Katz’s digital twin, now open-source. Researchers worldwide adapt it to simulate threats like climate change — predicting how humidity might warp wooden choir stalls or heatwaves crack stone, altering sound propagation. At Notre-Dame, the model informs ongoing tweaks; engineers recently adjusted ventilation grilles after simulations showed they dampened high frequencies. “Restoration isn’t a one-time fix,” Katz explains. “It’s a conversation between past and present.”

Public appetite for such work grows. Over 500,000 downloaded the Notre-Dame Whispers app in its first month, while UNESCO’s Vaulted Harmonies stream garnered two million views. Educational programmes leverage these tools too — Parisian schoolchildren use simplified models to compose “Gothic hip-hop,” layering beats over medieval reverberation profiles. “Acoustics isn’t niche anymore,” says Dudamel. “It’s how we make heritage resonate with new generations.”

Conclusion: A Cathedral Reborn

Notre-Dame’s story, post-fire, transcends architectural triumph. It reveals how spaces shape sound — and sound, in turn, shapes culture. The medieval composers who pioneered polyphony responded to their environment as instinctively as Katz’s robots mapping soot-stained vaults. Today, the cathedral stands not as a relic but a living laboratory, where 12th-century stone meets 21st-century science.

The restoration’s success lies in balancing fidelity with innovation. By retaining the organ’s original position, yet installing discreet speakers, the team honoured history while enhancing accessibility. Similarly, removing modern carpets revived centuries-old acoustics, yet LED lighting reimagined visual storytelling. “It’s a dialogue,” Chalet reflects. “We respect what was, but aren’t afraid to evolve.”

For visitors, the experience remains transformative. As daylight filters through restored stained glass, casting ruby hues over Dudamel’s orchestra, sound and sight fuse into something transcendent. Notes linger, light shifts, and history feels palpably alive. In this interplay, Notre-Dame achieves its greatest miracle — bridging eight centuries without freezing in time. Katz puts it simply: “Cathedrals breathe. This one just needed help remembering how.”