"The Voice" season 7 all set with Gwen Stefani and Pharell Williams on board. Coach Adam Levine also acknowledged the reality show's lack of capacity to launch singers with breakout success.

Gwen Stefani and Pharell Williams are set to make their respective debuts on the spinning chair when "The Voice" season 7 returns on September 22, Sunday night on NBC and according to E! Online, "it's safe to say their chemistry could rival that of Blake and Adam's epic friendship."

Both the "Don't Speak" singer and the "Happy" singer have collaborated once before for Gwen Stefani's monster hit "Hollaback Girl" back in 2005. The entertainment gossip site revealed that their first promo for "The Voice" season 7 will feature the same song.

Gwen Sefani also posted on her Twitter a photo of her and Pharrel Williams, while Blake Shelton wore the rapper's signature hat and posting in on his own social media account, along with the caption: "You know what they say about big hats...right @Pharrell?"

Meanwhile, "The Voice" coach Adam Levine acknowledged that the reality show failed so far to produce a breakout singer like Kelly Clarkson or Carrie Underwood, who both won "American Idol."

"The Voice" coach Adam Levine told Rolling Stone magazine: "'The Voice' is very successful, but it's got a ways to go if it wants to have as profound an impact as Idol. I'm not going to lie to you: I think there are problems... Millions of people watch the show every week and fall in love with a singer, then you don't hear much from them."

According to Yahoo TV's Lyndsey Parker, "The Voice" coach Adam Levine's admission was warranted considering that season 1 champ Javier Colon's album only sold 46,000 copies and "was subsequently dropped from Universal." Season 2's champion Jermaine Paul did not release an album at all.

"The Voice" season 7 will return on September 22 on NBC.