But there's nothing soft about the way Davies sings. It's an unmistakably male sound, pure and strong—albeit an octave higher than you might expect. Newcomers are rarely ambivalent: Either it will leave you with feelings of aversion, or it will shake your world.

"When I think about full-body gooseflesh, there are a couple of moments that only the countertenor voice has been able to reproduce for me," says Nico Muhly, the 30-year-old hotshot composer who, when he's not collaborating with Grizzly Bear and Björk, writes operas and film scores and is generally reimagining classical music for the 21st century. "There's a Purcell duet for countertenors that drives me insane with happiness, and there's a David Daniels recording of Berlioz's Les Nuits d'Été—little things in the last 20 years of music that have changed my shit up."

Muhly's response is backed up by science. When we hear high-pitched notes, our brains bombard us with the feel-good hormones epinephrine and norepinephrine, our hearts beat faster, and we become aroused. For Muhly, who arranged four folk songs that Davies performed at Carnegie Hall, the feeling is akin to that induced by a cinematic cliffhanger. "The higher you get, the more it's like walking on a tightrope," he says. "That's part of the thrill: The countertenor voice is like watching someone levitate. Something could go wrong because it appears so curious and fragile."

Davies is perplexed when I float the idea that our current fixation on falsetto-driven pop and rock (Bon Iver, Fleet Foxes, Ariel Pink, Glee's Chris Colfer) dovetails with the rise of the countertenor. "People often say [Coldplay's] Chris Martin is a falsetto," which Davies grants, "but it's totally different—that's just karaoke gone high." But pop stars' affection for opera is undeniable. Rufus Wainwright wrote the opera Prima Donna, which premiered in 2009; Davies' teen hero, Damon Albarn of Gorillaz, composed the opera Dr Dee, which will open in London in June; and even Lady Gaga recently tweeted that her 2012 tour would be an "ELECTRO POP OPERA!" And young, affluent audiences are looking back to Baroque music, too. "There's definitely renewed interest in countertenors," says John Gilhooly, the director of Wigmore Hall in London, one of the world's most respected recital venues. "Iestyn is an instant sellout here, and he's particularly popular with the under-forties."

As his upcoming engagements in America attest, Davies is poised for a singular breakthrough. His two main rivals, Daniels and Scholl, are both in their mid-forties, and, inevitably, their recent performances fail to match the quality of their early recordings. "It's a young man's game at the moment," Davies says—so he's working as much as his voice will allow: In June he releases Arias for Guadagni, a collection of pieces composed for and/or sung by the 18th-century castrato Gaetano Guadagni. He's lined up another collaboration with Muhly, who has "many, many schemes" in mind for Davies, all of which are sure to raise his profile among his biggest fans—particularly, gay men and older women. Many admirers are shocked when they learn that Davies is happily shacked up with his girlfriend, a high-school French teacher. "I get a lot of people saying, 'Wow, you're not gay!' But my voice has no connection with my sexuality. There are quite a lot of straight countertenors, actually, and plenty of gay baritones with very butch voices."

In fact, Davies points out, the castrati were the playboys of their time. "They were a kind of walking contraception," he says, smiling mischievously. "They could have sex with women without impregnating them." Davies gets it: He knows an ethereal voice is enough to pique people's interest—but he also knows that entertaining them is what will make them into fans.

Also on Details.com:
The Confessions of Rufus Wainwright
Q&A: Noel Gallagher
Benjamin Millepied: Lord of the Dance