LOS ANGELES (Reuters) - The microphone that singers on Fox television's "American Idol" will use for a live broadcast on Wednesday has traveled to Hollywood after an Olympic torch-style journey through 13 cities.

Producers hope the "Idol Across America" relay helps build enthusiasm for the rest of the 12th season of the long-time TV ratings champ, which has seen viewership fall over the past six years. This season's finale is scheduled for May.

Last year, the show that launched the careers of Kelly Clarkson and Jennifer Hudson lost its eight-year-old title as the most-watched series on U.S. television when it fell behind NBC's "Sunday Night Football."

Ratings slipped again when the 12th season began in January, with season-to-date viewership down roughly 20 percent overall and in the 18 to 49 age group that advertisers most want, despite a revamped judges panel and new interactive features.

Among 18- to 49-year-olds, "Idol" on Wednesday ranks third this season behind "Sunday Night Football" and CBS comedy "The Big Bang Theory." The Thursday broadcast comes in sixth, with ABC comedy "Modern Family" in fourth place and Monday night's "The Voice" at No. 5, according to ratings data provided by Fox, a unit of News Corp.

"You look at the declines and you get a little bit concerned when you see double-digit drops," said Jason Maltby, who purchases commercial time for advertisers as the lead TV ad buyer at Mindshare.

"You wonder how much of it is due to personality changes and how much is due to how long it's been on the air. Has it started to run its course?" he said.

To freshen the broadcast, "Idol" for the first time replaced three judges in one move, leaving only Randy Jackson, who has assessed the talent since the show's 2002 debut. Producers brought in judges from different musical styles - pop diva Mariah Carey, hip-hop star Nicki Minaj and country singer Keith Urban.

The show also created an online "super vote," where fans can cast 50 votes at once. Viewers were encouraged to tweet #idolagree or #idoldisagree in response to the judges' decisions in early rounds, with results appearing on-screen.

During its heyday in 2006 and 2007, "Idol" could pull in more than 30 million viewers for an episode. This season's Wednesday night shows average 16.5 million, according to Nielsen.

Even with eroding viewership, "American Idol" ranks in the top five shows each week, which so far has allayed any fears among its major advertisers, none of which has dropped their sponsorship. "It's still pulling in numbers that other networks can't get on any night," said Lauren Israel, vice president at media agency J3.

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Even so, as ratings have slipped, the cost for 30 seconds on "Idol" has declined, according to an annual survey released by Ad Age in October. The average commercial price on Wednesday nights dropped 32 percent this season to $340,800, the publication said. Coke, AT&T and Ford remain primary sponsors.

Minaj has earned rave reviews for candid and entertaining comments. Rolling Stone writer Rob Sheffield called her "the best judge 'Idol' has ever had."

Fox executives still can pitch "Idol's" relatively youthful audience to advertisers who seek out viewers below age 50. The median age of "Idol" viewers dropped by nearly four years to 45, a rare feat for a show on the air for more than a decade.

"It brings young and old, and it brings in females," said Darcy Bowe, director at media agency Starcom USA. "It still has a high value to advertisers."