The sound defies mere singing, pulsing though the church sanctuary with sufficient force to put hell on alert.
For three days this month singers from the United States, Canada and Britain gathered to celebrate an Elizabethan-era form of sacred choral music that took root and is preserved in the American south.
Nearly 700 registered for the 32nd annual National Sacred Harp Convention, which maintains the tradition of the a cappella singing style in four-part harmony.
In the rural U.S. South Sacred Harp singing is a weekly occurrence in a string of family churches.
"We went to a small wooden church with people who had been singing together their whole lives and the sound was just wonderful," said Briton Judy Whiting, of her visit to Gum Pond, Alabama.
Whiting, who had traveled from her home in West Yorkshire, England for the event, was one of many international visitors.
Sacred Harp began in the singing schools in England in Elizabethan times. Participants sit in a square with a hollow center, facing each other, in groups separated by voice range.
In the 1800s, a written form of the choral style emerged in a "shape note" style of musical signature. The notes are shaped according to sound to help people who cannot read music to be able to quickly learn how to sing in key.
John Merritt, a sixth generation singer who claims to have been involved in the singing for 70 of his 69 years said William Shakespeare made the first reference to it when he mentioned church music in four notes.
Merritt led his first song at age five in 1947, and despite his oxygen tank, he took part in this year's convention.
INCLUSIVE AND EGALITARIAN
Anyone can step to the center and call a song number. The first tenor will chime in with the first note, followed by the bass, alto and treble, which is a mix of men and women hitting the higher notes, without accompaniment.
The songs are slower and more somber than the rousing gospel so often heard in modern Southern churches. Sin, redemption, death and resurrection dominate the lyrics with an oddly joyful delivery.
"The first time I heard it, it grabbed me. I knew right then one day I would be singing in the South," said Rebecca Over from Yorkshire, England.
The singing style, which migrated from England, flourished in the American South until the eve of the Civil War, with President Abraham Lincoln participating, according to historian Steven Sabot.
In the grim years after the war, followers of the music faded until only about 1,000 people were still singing it in the 1970s, when the folksong movement emerged.
"The folkies found this strange stuff with weird shapes and fortunately respected the tradition," said Ginny Ely of Colorado.
Word spread and today Sacred Harp singing groups gather around the world, with the newest group in Poland. A stronghold formed in Chicago, where a group found this strange music and attempted to figure out how to voice the old songs.
"We didn't know what we were doing and we were told 'ya'll need to come south,'" said Kathryn Bearer, of Chicago. "That's how we found out how to get from Chicago to nowhere," she adds.
In recent years, interest in the music has been renewed, partly in movies such as "Cold Mountain," and "Oh, Brother Where Art Thou?"
The first night of the convention was dedicated to new songs by composers such as Randy Webber of Kentucky.
"We preserve it by singing the old songs and by creating new songs," Webber said.
The seats fill again. Books open, voices pour out.
"It gives me goose bumps just hearing that sound," said Ely, who headed to the center to lead the group in her favorite song.