Describing the sound of Sacred Harp singing can be difficult. Arcane, otherworldly, and overwhelming, it’s a unique a cappella form of shape note Christian hymn singing that was born in England in the 18th century, but took root in the American south. It’s not something you listen to so much something as experience. This week on Transmissions, co-director Matt Hinton discusses his documentary on the subject, Awake, My Soul.