The more Gerstel thought about it, the more this bothered her. The music of the Byzantine era, she decided, was a key to understanding her area of expertise—and not just the music itself, but understanding the experience of hearing it, and what it would have been like 700 years ago. “As an art historian, I could look at the pictures and say, ‘this is a nice painting of the hymn,’ but I couldn’t say anything about how the audience perceived that painting within a ritual setting.”

Which means she also couldn’t fully appreciate why churches began to change shape and size in the 13th century. Perhaps, she realized, it was to optimize the sound of chanting. “It seemed to me that the only way to think about these paintings and their meaning was to think about the music,” Gerstel said.

So she teamed up with Kyriakakis and James Donahue, an associate professor of music production and engineering at Berklee College of Music, and together they devised a plan to map the acoustic fingerprint of several churches, starting in Thessaloniki, Greece. Even before their technical analysis began, it was clear that these ancient spaces were designed to shift a person’s sensory experience.

“You cross the threshold and your eyes immediately have to adjust,” Gerstel said. “It seems pitch black inside. The first thing you notice is images of saints, who are your size, staring at you. Gold halos against dark background, and they seem to loom. It smells of incense. You’re in this world of myrrh. The temperature is different as well. Inside, you’re in a much cooler space. Your entire body adjusts ... and then to have music at the same time? That hits every sense.”

“What was truly surprising for me,” Donahue said, “was going into a space that was ancient, and to crawl around the ceiling and look at the walls and realize that they were looking at things acoustically. It wasn’t just about the architecture. 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 by putting striations in the walls. They were actively trying to tune the space.”

“They also discovered something that we call slap echo,” Donahue added, “when you have walls fairly close to one another and the frequencies go back and forth. It goes ta-ta-ta, ta-ta-ta, ta-ta-ta, ta-ta-ta, ta-ta-ta. [In the ancient world,] they described it as the sound of angels’ wings.”

When Donahue and his colleagues were in Hagia Sophia—once the cathedral of Thessaloniki——they used a test tone at different frequencies to see how the space responded to sound.* The tone ranged from about 50 hertz, which sounds like a low buzz, to 20 kilohertz, a high-pitched whine. “I heard the standard sweep tone until it hit 6 kilohertz, and then it just spread out everywhere,” Donahue said. “I could hear the fluttering. I said, ‘Wow, those are the angels.’”