"American Idol" producers definitely appear to be sending a strong message to hopefuls who are vying to make it to season nine's competition: Your guest judges know exactly what they're talking about.

Following the shocking departure of Paula Abdul last month, the rotating list of celebrity judges warming up the fourth seat on the panel keeps growing. Fox announced Wednesday morning (September 2) that pop-punk princess Avril Lavigne will switch off with previously announced fourth-seater Katy Perry when prospective "Idol" contestants line up in Los Angeles for the final days of auditions this week. The pair will sit in separately to offer their multi-platinum advice to the singers vying for a golden ticket to the show's Hollywood round.

They join a list of confirmed judges who have or will offer hopefuls some advice that includes Joe Jonas, Victoria Beckham, Mary J.Blige, Neil Patrick Harris, Shania Twain and singer/actress Kristin Chenoweth.

The eclectic mix of singers — and Neil Patrick Harris — should make for an interesting dynamic with the permanent judging panel of resident baddie Simon Cowell, good-cop/bad-cop Randy Jackson and returning sophomore judge Kara DioGuardi. Producers have not yet announced when they will permanently fill the fourth seat, but Abdul has made it seem unlikely that she will stage a surprise return to the program.

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