PRINCETON - It all feels so easy, looks so casual. But it sounds unlike anything.

Roomful of Teeth, the vocal octet whose somewhat regular presence here Philadelphians should envy, seemed to be just dropping in at the McCarter Theatre Center for an 80-minute-or-so Sunday program that may have radically changed forever the way some listeners hear voices in concert.

Far more inviting than alienating (as innovation can be on first encounter), this group founded by Brad Wells uses amplification - each singer has a microphone - to create intimacy with the audience, sometimes as though the voices are whispering in your ear. In a darkened auditorium with singers in everyday street clothes, there's almost a campfire effect.

Besides having the visceral appeal of beautifully produced vocalism, the repertoire often shows antecedents in traditional hymns, folkish songs, and the Beach Boys while spiraling into uncharted harmonic galaxies and timbres that could be conceived only in our higher-tech age.

If the group has a manifesto, it's Partita for 8 Voices, the Pulitzer Prize-winning piece by group member Caroline Shaw that puts all sorts of extremes in close proximity and that occupied the program's first half. Spoken words are employed both for their rhythm and content - and contrasting with longer lyrical lines going on elsewhere. Saturated harmonies give way to powerful unison singing. Rhythm - often the weak point of many voice-only groups - is downright propulsive.

Many hear lots of Meredith Monk in the way Roomful of Teeth taps into the power of wordless vocalism - and go beyond that in, say, the neo-yodeling of Rinde Eckert's Cesca's View. But especially in Shaw, I hear reverberation from Luciano Berio's 1969 Sinfonia, which is entering Roomful of Teeth's repertoire and which is one of the great 20th-century works, with its speech and singing amid symphonic writing. Berio uses concerted voices for a Tower of Babel effect; Shaw has similar moments conveying the information overload of our age.

Later in the concert, Michael Harrison's Just Constellations explored the opposite of reckless heterogeneity. Influenced by seminal minimalist La Monte Young, the music isn't repetitive so much as it settles onto a single chord and evolves so subtly it's like watching a landscape in changing light. Just when one fears Roomful of Teeth's ability to do most anything could prompt mere novelty, a concentrated piece like this with tight, difficult-to-tune chords shows how extraordinary these singers are.