A cappella singing means singing without accompaniment. But how do you define accompaniment? Doesn’t amplification accompany? What about extended vocal techniques like Korean p’ansori singing or Tuvan throat singing? If you’re a soprano and you’re yodeling into a mike while three other women are spreading a pad of mellifluous vocal sound underneath your hard-edged tones, aren’t you being accompanied?

It probably doesn’t matter. The salient point is that it’s unusual (which is a given) and worth hearing (which is not). The yodeling piece is called “Cesca’s View,” by the composer Rinde Eckert, and you can hear it on the self-titled debut album of the vocal octet Roomful of Teeth — that is, if you missed the group’s concert at the Atlas on Monday night.

Between its own new-music series and Library of Congress presentations, the Atlas has become Washington’s go-to venue for those interested in the burgeoning “alt-classical” scene.

Like a number of other groups on the series, Roomful of Teeth records with New Amsterdam Records, and all three of that label’s composer-founders were represented on the program — including Judd Greenstein, whose “AEIOU” offered the vowels in question, produced with a raucous edge that carried them away from the traditional sound world of beautiful vocal production, until the singers finally arrived at a round “U” sound, suspended on notes that formed an intriguing, sculptural chord, twisting gently in midair.

To the group’s credit, only five of the nine works on Monday’s hour-long program were on the recording, so there was plenty that was new and interesting for those who had already heard it. These included “Otherwise” by Brad Wells, the group’s founder and conductor, who juxtaposed an insistent, propulsive, nasal droning taken from Sicilian folk music with the rich soaring baritone of the singer Dashon Burton, the latter taking on extra warmth and beauty in contrast to the graininess of the other voices below it.

Roomful of Teeth played at the Atlas on Monday night. (Nicholas Whitman)

Another of the eight singers, Virginia Warnken, is an early music specialist, according to her bio, but she dug into pop-style belting as if to the manner born in, for example, “Amid the Minotaurs” by William Brittelle, another New Amsterdam founder.

The concert ended with “Quizassa,” an exuberant and raucous offering by Merrill Garbus, whose own group, tUnE-yArDs, is a genre-buster of its own. The enthusiastic audience would have been happy to have an encore, but the group, Wells said, hadn’t prepared one.