LOS ANGELES - Pop music's celebrity universe has widened in recent years to include producers and songwriters; they're as crucial to what you hear on the radio as the stars, and increasingly known to the public. But there are deeper levels of highly specialized talent that often go unrecognized. Kuk Harrell is among them.

The job of Mr. Harrell, 47, whose clients include Justin Bieber, Rihanna and Jennifer Lopez, is to make sure a star's vocal is as powerful and flawless as it can be.

In the studio, rarely, if ever, does a star sing a song the whole way through. A singer working with Mr. Harrell covers a few bars over and over, with different emphases and inflections, until Mr. Harrell hears what he wants. The process repeats for each section. Only after the singer is gone are the best pieces stitched together into the song you hear.

Mr. Harrell's work begins long before his clients show up in the studio, as he picks what kinds of sound equipment will be best.

As the artist sings, Mr. Harrell's eyes remain fixed on the computer screen, where each new take is represented in Pro Tools, the production software, by a jagged line. The data pile up. "I'll take it in chunks," he explained. "If they sang it amazing, I'll get the first chunk and go, 'Oh that was beautiful.' Boom. I'll drag that up."

The process then repeats. "All the way down, until I get to the end of the record."

After an hour or two of takes the singer's work is done. Mr. Harrell then "tunes" the compiled vocal, making sure of the pitch, and "grooves" it, matching it to the rhythm of the backing track.

He says most of his work is getting stars to trust him. "It's never, 'Man, you screwed up,'" he said. "I can tell Jennifer she's not singing it the right way without telling her that she's not singing it the right way: 'Give it a sexy vibe like you're singing in the shower.'"

Ms. Lopez said Mr. Harrell "can find your strengths."

"One of his favorite lines," she says, is: "'That's a superstar performance right there! That's it!' And it just makes you feel so great."

With Rihanna he'll push her with "There she is! She just showed up!"

The final product then, blends the songwriter's original lyric and melody, the singer's particular tone and approach, and Mr. Harrell's instincts.

"He knows where my voice can sit and what notes I can hit," said Mr. Bieber, for whom Mr. Harrell has "become kind of an uncle."

But Mr. Harrell noted, "If I'm not careful, I can let that turn into, 'We're buddies, we hang out,' and I can't press him."

When superstars work with Mr. Harrell, they aren't running to the machines and away from their own voices. Rather, they're trying to ensure that they sound as engaged and alive as possible.

"We want to enhance the artist's authenticity," said Chris Hicks, who was executive vice president at Island Def Jam, home to Mr. Bieber, Ms. Lopez and Rihanna. "You buy a Bieber or Rihanna because you believe in them, and this is part of that."

It falls to Mr. Harrell to highlight what's distinctive about each voice: Ms. Lopez's blend of husk and flirt, Rihanna's petulant purr, Mr. Bieber's sweet coo.

"Rihanna, you hear two bars - Oh, my God, that's Rihanna," Mr. Harrell said.

Having the certainty of Mr. Harrell's ear comes with a price: several thousand dollars per song and a cut of the royalties.

"Believe," Mr. Bieber's second full-length album, which was released last month, had the biggest first week of any album this year, selling 374,000 copies, according to Nielsen SoundScan. Its first single, "Boyfriend," has already sold more than 2.5 million copies.

Mr. Harrell said that he can make $1 million a year.

One February night in the studio, Mr. Bieber, a day away from his 18th birthday, was working on "Believe," the new album's title track. He took the song's soaring hook: "I don't know how I got here/I knew it wouldn't be easy/But your faith in me was so clear/It didn't matter how many times I got knocked on the floor."

Mr. Harrell shouted, "I love it."

Mr. Bieber called back, "Do it again?"

"Absolutely," Mr. Harrell said. "You killin' it. I just need to understand you just a little bit more."

Mr. Bieber tried again.

"Wooooooo!" Mr. Harrell exulted. "That's incredible."

Mr. Bieber came out of the booth. "I'm so excited for my birthday, I can hardly even concentrate," he said.

"But you're killin' it, though," Mr. Harrell replied. And with that, Mr. Bieber went back to work.

The New York Times