Turquoise carpets covered the marble floor, with its geometric designs. White drapes concealed the mosaic of the Virgin and Christ. Scaffolding obscured crosses and other Christian symbols.
Footage broadcast around the world last week captured some of these striking changes to Hagia Sophia, the Byzantine cathedral in Istanbul, which served as a mosque under Ottoman rule before becoming a museum in 1934. On the orders of Turkey’s president, Recep Tayyip Erdogan, it is now once again used as a mosque.
But for a group of scholars, scientists and musicians, Hagia Sophia’s rededication as a Muslim place of worship threatens to cloak a less tangible treasure: its sound. Bissera Pentcheva, an art historian at Stanford University and an expert in the burgeoning field of acoustic archaeology, has spent the past decade studying the building’s extravagantly reverberant acoustics to reconstruct the sonic world of Byzantine cathedral music. Ms. Pentcheva argues that Hagia Sophia’s mystical brilliance reveals itself fully only if it is viewed as a vessel for animated light — and sound.
“The void is a stage,” she said in a recent interview over Zoom.
Conducting research inside this contested monument has required a mixture of diplomacy, ingenuity and technology. Turkish authorities forbade singing inside Hagia Sophia, even when it was operated as a museum. Now that the building falls under the jurisdiction of religious authorities, that ban will harden, and further research may be even more difficult.
But Ms. Pentcheva’s existing work culminated last fall in the release of “The Lost Voices of Hagia Sophia,” an album that brings to life the stately mystery of Byzantine cathedral liturgy, bathed in the glittering acoustics of the space for which it was written — even though it was recorded in a studio in California.
For about 20 years, it has been possible to superimpose the acoustics of a particular space onto recorded music during postproduction. A pioneer was Altiverb, a plug-in software that draws on a large library of virtual spaces so that a recorded track can be retrofitted to seem like it was done in, for example, the Berlin Philharmonie or the King’s Chamber inside the Great Pyramid of Giza.
But in what has become known as live virtual acoustics, processors and speakers provide the acoustic feedback of a particular space in real time, so that musicians can adjust their performance as if they were really in another building.
Jonathan Abel, a consulting professor at the Center for Computer Research in Music and Acoustics at Stanford, devised a plan with Ms. Bissera that allowed her to capture vital information about the acoustic properties of Hagia Sophia with the help of a balloon, discreet recording equipment and a cooperative security guard.
In the winter of 2010, Ms. Pentcheva obtained permission to enter what was then a museum at dawn, when Istanbul was quiet. She persuaded a guard to stand in a spot that would have been occupied by singers during the Byzantine era and to pop a balloon. In the meantime, she stationed herself where a privileged member of the public might have experienced mass. Microphones captured the explosion of sound and the ensuing wash of reverberations.
Ms. Pentcheva was allowed to capture only four such pops over two visits. But those bursts of sound yielded a wealth of data.
“That little balloon pop brings back all the information about the material and the size of the space,” Mr. Abel said. “You can think of a human voice as being made up of a whole bunch of balloon pops. Each voice drags behind it a bunch of impulse responses, like streamers behind a wedding car.”
The balloon noises, along with maps of the interior, allowed Mr. Abel to identify what he called the acoustic fingerprint of the building, including the multidirectional refraction of sound as it bounces off the dome and marble colonnades. His computer simulation was then integrated into a set of microphones and speakers.
Thus the members of Cappella Romana, a vocal ensemble based in Portland, Ore., specializing in Byzantine chant, recorded “The Lost Voices” in a space that persuasively mimicked the acoustics of Hagia Sophia — with its luscious reverberation, cross echoes and amplification of particular frequencies.
Alexander Lingas, a musicologist and the music director of Cappella Romana, said that the live virtual acoustics were transformative to his understanding of the group’s repertory. The long reverberation time dictated slower tempos. Basses singing drones made subtle pitch adjustments to match frequencies of maximum resonance.
Mr. Lingas said that some pieces only “made sense” inside the simulated acoustics. One example featured on the album is a cherubic hymn that likens the singers to angels.
“The music is designed to convey that,” Mr. Lingas said. “But I remember editing this piece and thinking, ‘My, this is really strange.’” Yet, he added, as the group rehearsed it with the virtual acoustics, a pattern of repeated undulating motifs built up rippling momentum until, as he described it, “the sound essentially achieved liftoff.”
Ms. Pentcheva believed that in Byzantine cathedral chant, reverberation was key to invoking the divine presence. She pointed to the exuberant amount of melisma in the repertory, where a single syllable is stretched over multiple notes. In the liquid acoustics of Hagia Sophia, words sung in this way blur, the way a line drawn in ink bleeds on wet paper.
“Rather than containing this smearing of semantics, the music itself actually intensifies it,” Ms. Pentcheva said. “So there is this process of alienation and estrangement from the register of human language that happens in Hagia Sophia, and is a desired goal.”
In Greek Orthodox rites, Ms. Pentcheva argued, acoustics and chant interact in a way that “is not about sound carrying information, but sound precipitating experience. It is a fully corporeal investment.”
The recording provides a glimpse of that experience. Phrases chanted in unison leave a ghostly imprint. Rhythmic shudders and grace notes set off blurry squiggles of overlapping echoes. Chords unfurl in reverberant bloom.
The acoustic drama of Hagia Sophia would have unfolded alongside the changing light and curling smoke of burning incense, enveloping the senses. The effect is described in a 6th-century description of the building by Paul the Silentiary, an aristocrat and poet at the court of Justinian.
“He speaks about a human action that brings into presence the divine reaction, the divine voice,” Ms. Pentcheva said. “In a sense that is the reverberation of the space: After the human voice stops singing, the building continues.”