The 30.7 million people who watched Sparks win on Wednesday was a sharp drop from the 36.4 million people who saw Taylor Hicks in last year's finale, according to Nielsen Media Research.
Industry observers said chances are that the 2006 season will stand as the peak for audience interest.
A lackluster pool of contestants, the general malaise among viewers this spring and the simple laws of TV gravity are all factors.
"People are getting tired of it," said Marc Berman, an analyst for Media Week Online. "I know I am."
Mind you, most TV shows would kill for this kind of "bad news." The Fox phenomenon was still TV's most popular program this season. Barring a major surprise, it likely will be again next year.
Fox executives cautioned against counting "Idol" out. For the season as a whole, "American Idol" ratings will be slightly down or even identical to last year, when DVR viewing is taken into account. To some degree interest is driving by the personalities involved, and a whole new cast of characters join next spring, said Fox's Scott Grogin.
This year's finale compares to the 30.3 million who watched in 2005, and 28.8 million in 2004. The showdown between Clay Aiken and Ruben Studdard in 2003 attracted 38 million viewers, Nielsen said.
For the first half of this season, ratings outstripped last year. When water-cooler favorite Sanjaya Malakar was voted off in April, much of the interest left with him.
"The competition down at the end didn't have the excitement that it had in years past," said Tim Brooks, author of "The Complete Directory to Prime-Time Network and Cable Television." "It's kind of a rehash to viewers."
Producers have tweaked the show each year to keep it fresh, this season adding a songwriters' contest and charity drive for those starving in Africa. In past years, famous musicians were brought in to work with the singers.
The long hiatus each year has also kept interest keen. "American Idol" won't be back for new episodes until next January.
However, producers have agreed to make a spinoff for Fox, a band contest instead of one for individual singers, that will air Fridays in the fall.
While Berman said it's likely to fail in the same way that "American Juniors" did, the spinoff still represents a risk in diluting the brand.
"I would steer clear of it," he said.