"Sacred Harp" refers to the human voice.
In East Texas, I plumb growed up being dragged to Sacred Harp and Gospel Singin's, which my dad loved. I have a couple of his Sacred Harp books--donated his tapes to a friend in Kilgore, who's saving them until some museum is organized in Gilmer.
Biggest Sacred Harp Convention I know of is in Henderson, Texas, each August, or whenever temperatures and humidity reach 120 degrees. County Judge--must be dead by now--always saw to it that courthouse was available. The singers face each other, square. The women's tones are especially nasal and grating. Supposedly, this singing dates back to Shakespearian England. Rhythms are set and led by one distinguished person at a time. I think requirement is that the person be 101 years old.
I know. It's worth preserving. We took (my husband's) niece and nephew (both Ph.D's in music,Indiana) to one Henderson singin', and they both got all excited about the tonal distinctions.
I just remember being five years old, and desperately wanting to get out of whatever littlecountry church I was trapped in, while my dad was quite absorbed. Maybe my reward was growing up in Kilgore, listening in Baptist church and at school to Van Cliburn, who's a year older, and plays the music of my heart.