Justin Bieber's voice 'blossoming' with age

Justin Bieber's voice is improving with age, according to Island Def Jam boss Steve Bartels.

The music group president said that the 17-year-old's new Christmas album Under the Mistletoe shows off how much his voice has changed over the past year.

"The album really shows his growth as a young man and his voice has just blossomed," Bartels told Billboard. "It will pleasantly surprise many people. It's not your typical holiday album."

"Vocally, his balls have dropped," added Bieber's manager Scooter Braun.

Braun revealed that a duet with Mariah Carey on her hit 'All I Want For Christmas Is You' posed a particular problem during recording, owing to her insistence that Bieber sing in her original key.

"That was the hardest song to do," Braun admitted. "A year ago that would have been no problem."

Bieber said: "Yes that was high, but I grinded it out and we got it down."

Bieber released the album's first single 'Mistletoe' earlier this month. The song reached number 11 on the US Hot 100 and 21 on the UK Single Chart.