UPDATE 3-In upset, rocker David Cook wins 'American Idol'

(Adds details, Cook and Archuleta quotes from backstage)

By Nichola Groom

LOS ANGELES, May 21 (Reuters) - Rocker David Cook won the coveted title of "American Idol" on Wednesday, dealing an unexpected but decisive defeat to a silken-voiced teenager, also named David, on the most popular U.S. television show.

Cook, 25, who was tending bar and playing in a band before he auditioned for the singing competition, stood back to applaud rival David Archuleta, 17, and then bent over crying after host Ryan Seacrest said he had won by 12 million votes.

"This is amazing. Thank you," said the Blue Springs, Missouri, native, who in addition to being crowned this year's "Idol" will receive a recording contract.

In its seventh season, "American Idol" has grown from a cheesy summer talent show into a pop culture phenomenon that draws millions of viewers to News Corp's (NWSa.N: Quote, Profile, Research, Stock Buzz) Fox network at least twice a week.

It has produced mega-stars from both winners and losers, including Kelly Clarkson, Carrie Underwood and Chris Daughtry.

This season, however, average audiences on the show have slipped from just over 30 million viewers last year to roughly 28 million this year, the second straight year of declines.

Nevertheless, the dueling Davids garnered a record 97.5 million votes between them, smashing the previous record by 23 million. The contest was decided by a telephone and text-messaging vote in which Cook received 56 percent to Archuleta's 44 percent.