“The harmonium is the ocean and the wind.” That’s how Shanti Curran describes the role the hand-pumped organ plays in a song by her band Arborea about a woman who loses her lover to the sea. And it’s a fitting intro — the natural world suffuses their music. Curran is the band’s singer, guitarist and — assuming this is a word — Ban-Jammerist. (A Ban-Jammer is a hybrid instrument, part banjo and part dulcimer, built by Tennessee luthier Mike Clemmer.) Her husband Buck adds spectral moans with an E-Bowed electric guitar, providing an amorphous shroud around his wife’s incantatory plucks and strums. Shanti’s singing can be piercingly spectral — high and plaintive. At other times, it’s earthy and almost painfully hushed — more breath than voice, a way of singing that emphasizes the physical effort of pushing these notes out into the world. The results are transfixing. Sharing the bill are dream-folk singer Mariee Sioux — “so sweet you almost killed me,” she sings on her otherworldly ballad “Homeopathic” — and Nashville’s own avant-acoustic guitar savant William Tyler.