Americans turned off the finale of American Idol in their droves on Wednesday night, according to early Nielsen ratings. The once mega juggernaut’s 11th finale had its smallest every audience of 21.5 million. That compares with more than 30 million viewers in 2006 and especially 2007, when it peaked at a huge 37.3 million viewers. It’s a fall of around 43% over the last five years. The program has suffered the loss of key judge Simon Cowell, who left in 2010 to concentrate on the X Factor.

The Voice has made a dent in Idol‘s US ratings and audience figures and has taken viewers in the 18 to 49 group, especially female and teenage viewers. The Voice‘s finale earlier this month on NBC averaged 11.6 million viewers, up 5% from the previous year. But US analysts point out that that was actually a poor outcome because the 2011 finale aired out of ratings in June when TV use is lower (by a third on some nights, as we find in Australia in January). Because more sets are in use in early may, the audience should have been much higher.

More worrying than the drop in the total number of viewers for Idol was the very nasty 32% fall in 18 to 49 year group viewers, the most important in the US for advertisers. Last year Idol averaged 25 million viewers (give or take a few thousand); this season it fell to around 19 million an ep. It will be back, but executives of Rupert Murdoch’s Fox TV network said earlier this month that the audience drop-off this season had been greater than anticipated. Changes are planned for 2013.

Idol‘s ratings successes may have been reduced this year in terms of audience size, but it still helped Fox to its eighth straight season victory among 18-49 year-olds. It beat CBS, NBC and ABC in that order. Overall, CBS claimed it was the most-watched network among US audiences, with an average 11.74 million viewers a night, followed by Fox with 8.87 million, ABC at 8.36 million and NBC in 4th place at 7.38 million.