Unless you’re an audio engineer, you’ll have little reason to know what the term “convolution reverb” means. But it’s a fascinating concept nonetheless. Technicians bring high-end microphones, speakers, and recording equipment to a particularly resonant space—a grain silo, for example, or famous concert hall. They capture what are called “impulse responses,” signals that contain the acoustic characteristics of the location. The technique produces a three dimensional audio imprint—enabling us to recreate what it would sound like to sing, play the piano or guitar, or stage an entire concert in that space. As Adrienne LaFrance writes in The Atlantic, “you can apply [impulse responses] to a recording captured in another space and make it sound as though that recording had taken place in the original building.”
This kind of mapping, writes Allison Meier at Hyperallergic, allows researchers to “build an archive of a building’s sound, with all its nuances, echoes, and ricochets, that could survive even if the building fell.” And that is precisely what researchers have been doing since 2014 in ancient Greek Byzantine churches. The project began when Sharon Gerstel, Professor of Byzantine Art History and Archeology at UCLA, and Chris Kyriakakis, director of the Immersive Audio Laboratory at the University of Southern California, met to discuss their mutual interest in capturing the sound of these spaces.
(Hear them both explain the genesis of the project in the CBC interview above.) The two researchers traveled to Thessaloniki, coincidentally, Kyriakakis’ hometown, and began, as Gerstel puts it, to “measure the churches.” LaFrance’s Atlantic article gives us a detailed description of the measurement process, which involves playing and recording a tone that sweeps through the audible frequency spectrum. You’ll hear it in the video at the top of the post as a “chirp”---bouncing off the various architectural surfaces as the voices of singers would have hundreds of years ago.
In that video and in the audio recording above, chanters in a studio had the audio characteristics of these churches applied to their voices, recreating the sounds that filled the spaces in the early Christian centuries. As another member of the team, James Donahue---Professor of Music Production and Engineering at Berklee College of Music---discovered, the churches had been acoustically designed to produce specific sound effects. “It wasn’t just about the architecture,” says Donahue, “they had these big jugs that were put up there to sip certain frequencies out of the air… They built diffusion, a way to break up the sound waves… They were actively trying to tune the space.” In addition, the builders “discovered something that we call slap echo. [In the ancient world], they described it as the sound of angels’ wings.”
The project not only allows art historians to enter the past, but it also preserves that past far into the future, creating what LaFrance calls a “museum of lost sound.” After all, the churches themselves will eventually recede into history. “Some of these buildings may not exist later,” says Kyriakakis, “Some of these historic buildings are being destroyed.” With immersive video and audio technology, we will still be able to experience much of their grandeur long after they’re gone.