Justin Bieber (REUTERS/Gil Montano)

Adulthood has robbed Justin Bieber of his high notes.

According to the pop star’s manager Scooter Braun, Bieber struggled to record his Christmas duet with Mariah Carey, revealing the teen's voice broke just before he hit the studio with Carey to re-record her festive hit All I Want For Christmas.

And Braun admits his young charge's tone is not as high as it used to be.

The manager tells Billboard.com, "Vocally, his b---- have dropped... A year ago that (singing with Carey) would have been no problem."

And Island Def Jam president Steve Bartels is convinced fans will notice the more adult sound to Bieber's voice on his new holiday album, Underneath The Mistletoe, that is set to be released in the U.S. on Tuesday.

Bartels adds, "The album really shows his growth as a young man, and his voice has just blossomed."