The vibrations of vocal cords fill your ears well before you step inside Evergreen Cemetery’s historic chapel.

What magnificent choir is this, you wonder, singing so powerfully it operates like smelling salts, opening up long-forgotten neural pathways in your brain, through which emotions hurtle like blood.

For this All Souls’ Day Cemetery Sing in November, about five-dozen people, bundled up in thick sweaters, scarves and woolen hats, sit side-by-side on plastic chairs inside the unheated 1910 chapel’s mint-green walls, drawn together by a mutual obsession: sacred harp singing, also called shape note singing.

This is no regular church or community choir. There is no performing for an audience, and there are no harps. There is only singing loudly to each other because you love to sing, and you love the sound of four-part harmonies.

“The sacred harp is the human voice,” said Kathryn Eastburn, author of the 2013 book “A Sacred Feast: Reflections on Sacred Harp Singing and Dinner on the Ground. “It’s the musical instrument we’re born with.”

Today’s annual event is an all-day affair hosted by the Pikes Peak Sacred Harp Singers – a morning sing, a potluck lunch on the cemetery grounds and then an afternoon sing. It’s a sacred harp tradition that goes back more than a century. The birth of the music dates back centuries, originating in England before moving to colonial New England and finally to the South.

People have come from all over Colorado to spend the day singing at the cemetery, including a man from West Texas, who drove 10 hours. His grandmother initiated him into the tradition in the 1960s.

“It’s pretty powerful, isn’t it?” he said.

Pikes Peak Sacred Harp Singers also meets on the first and third Mondays of the month at Switchback Coffee. Everyone is welcome, just like today.

“Singing is natural to humans,” said Mia Alvarado, the chairperson of the All Souls’ Day event. “It’s only recently in human history that we’ve lost that. Human history is singing history, and we are singing people.”

There are no rules to sacred harp singing. People can come and go. Most of them can carry a tune. Some can’t, but they’re still welcome to attend and sing, or sit and listen.

“We all learn together. It’s an anachronistic tradition. It’s folk art,” Alvarado said. “Some are born into this tradition, but it’s had a resurgence in popularity. Most people learn by showing up at sings and trying.”

Tenor, treble, bass and alto voices are seated in separate sections that make up a hollow square. As they all face each other, different leaders rotate in and out of the middle.

Before each song, Eastburn, seated to the side of the square, calls out the name and home city of the song leader and the number of the song they’ve selected in B.F. White’s 1844 “The Sacred Harp” songbook.

As Crock-Pots and foil-wrapped casserole dishes get piled onto a side table for lunch, and a baby is overcome by the spell of sleep in their father’s arms, the ephemeral songs come one right after the other. There is no time to dilly-dally on superfluous conversation when you could be belting old hymns, spirituals and anthems, some of which date back to the 1600s.

“A lot of people love it for the lyrics,” Eastburn said. “They’re a lot about death and dying and eternal life. A lot of the songs are written in a minor key, so they have a haunting sound.”

A man sings the pitch, and the quiet of the chapel goes from zero to roar in the tick of a metronome. The first time through the song, singers don’t use the lyrics. Instead, they sing the tune using the words fa-sol-la-mi. The next time through, and for however many remaining verses, they use the lyrics. This is the curious world of a cappella shape note singing.

The early American tradition started as a way to teach singing to rural congregations that didn’t have choirs or people who knew how to read music. Itinerant song leaders went into communities and churches and taught a method of reading note shapes (triangle, circle, square, diamond), so singers could learn to sight-read hymns and spirituals.

“People sing in their full natural voices. There’s not the aesthetic of blending, like in choral music,” Alvarado said. “There are no dynamics written into the songs, like piano forte. It’s maybe a rougher sound than people would expect. Most of us love that it’s a tradition that comes from before people would use microphones and speakers.”

DETAILS

Pikes Peak Sacred Harp Singers, 6-8 p.m. first and third Mondays monthly, Switchback Coffee, 330 N. Institute St., free; coloradosacredharp.org