*The Jennifer Lopez era of “American Idol” has started off with a ratings plummet, compared to last year’s season opener.

Numbers for Wednesday night’s tenth season premiere fell 13 percent among total viewers and 18 percent in the ad-coveted demographic of 18-49-year olds, according to preliminary data from Nielsen.

The much-hyped two-hour opener, featuring new judges Lopez and Steven Tyler with returning judge Randy Jackson, was watched by 26.1 million viewers with a 9.7 in the demo.

Last year’s premiere – which kicked off the final year of Simon Cowell, considered by many observers to be the show’s linchpin – was watched by 29.9 million viewers and averaged an 11.8 demo rating. That was down slightly compared to the 2009 premiere, which averaged 30.4 million viewers and an 11.7 rating in the demo.

American Idol has been declining since the 2007 season when the premiere installment attracted a high of 37.4 million viewers with a 15.8 in the demo.

Below, a clip from Wednesday’s premiere…