An Eminem song comes on your radio. You’re expecting beats that bounce like a head-bobbing doll, rhymes delivered at a madcap pace and scabrous humor that’s bound to offend at least somebody. But instead, on his new single “Superman,” you get the blond provocateur doing his best shower-stall serenade.

That’s right, he’s singing. And it’s not the first time. 

Eminem stretched his pipes on the chorus of last year’s single “Cleanin Out My Closet.” And on “Hailie’s Song” — which isn’t an official single but is getting some radio airplay — he boldly chirps throughout the whole tune.

This is old news to any of the 6 million folks who own a copy of “The Eminem Show,” the album that includes all of the above songs. But what’s interesting is that Eminem is one of an increasing number of rappers who are choosing to sing as well as rap.

The usually hyper Busta Rhymes offers up some smooth vocals on his new single, “I Know What You Want.” Ja Rule bursts into a raspy croon on his hit pairings with Jennifer Lopez (“I’m Real’), Mary J. Blige (“Rainy Dayz”) and Ashanti (“Always on Time” and the new “Mesmerize”). And Nelly trades melodic lines with Kelly Rowland of Destiny’s Child on his recent No. 1 cut “Dilemma.”

But it’s not just the mainstream chart-toppers who are breaking into song. Bohemian rhymesmith Common pipes an ode to rocker Jimi Hendrix on his new album, “Electric Circus.” And Cee-lo, formerly of the rap group Goodie Mob, showcases his gospel-like voice on several tracks on his debut solo album, “Cee-lo Green and his Perfect Imperfections.”

What makes this trend so surprising is that, in many ways, rap was born as the antithesis of singing. Like punk rock, it was conceived as a raw form of expression, stripped of the artifice and sentimentality often associated with traditional crooning.

Throughout the 1980s and early ’90s, rappers provided a stark contrast to rhythm-and-blues balladeers such as Anita Baker, Luther Vandross, Whitney Houston and Boyz II Men. But starting in the mid-’90s, the line between hip-hop and R&B became increasingly blurred. Important producers such as Dr. Dre and Sean “P. Diddy” Combs discovered that rap numbers received more radio airplay — and hence became bigger hits — if they featured singing on the chorus.

The most popular tracks by legendary rappers Tupac Shakur and the Notorious B.I.G. include sung choruses. And successful R&B vocalists like Blige, Faith Evans and Kelly Price got their start by singing on rap records.

The current trend of rappers singing seems to be a natural evolution. Instead of relying solely on other vocalists, rappers are now singing for themselves.

In some cases, they seem to do it because of the personal nature of the material. Eminem’s “Hailie’s Song,” for example, is an uncharacteristically earnest number about how his daughter has given meaning to his life. “It all makes sense when I look in her eyes,” he sings on the song.

But sometimes rappers chose to croon simply because they can. “I’m like every normal person who wants to sing but can’t sing,” Ja Rule told Jet magazine. “I harmonize and do my own way of singing.”

And that’s a good point. None of the current rapper-singers has the vocal chops to make it past even the first cut on “American Idol.” But, to some extent, that’s what keeps their singing in line with hip-hop’s rugged aesthetic.

“They’re not trying to be the next R&B superstar,” says Tosha Love, music director of Atlanta urban radio station V-103. “It just enhances what they already do.”

Just as hip-hop experienced a politically conscious era in the 1980s and a “bling bling” period of materialism and flash in the ’90s, this current moment will probably go down as the age of the platinum throat. In any event, it’s yet another interesting development in a genre that continues to surprise.