“We wanted the best scientific testing possible to understand what it meant when the Byzantines said, ‘It sounds like there are angels in the buildings,’” Gerstel said.

So Gerstel and a team that included an audio and acoustics professor from USC, a recording engineer, an archaeologist, an architectural historian and a musicologist who specializes in transcriptions of 14th-century Byzantine music spent two weeks in northern Greece last summer trying to capture the ethereal.

The 14th century was significant to the team’s work for several reasons: It marked the end of a five-centuries-long period when Byzantine churches were shrinking in size. In that time period, the paintings and mosaics in these churches depicted the singers as well as the lyrics of the chants. And the music in Byzantium changed into a form called kalophonic (beautiful voice), putting the emphasis on soloists who would riff on the music and create beautiful melodies.