What separates opera singers from other vocalists?
12 June 2014 Last updated at 11:01 BST
Opera "shows humanity at its greatest", American mezzo-soprano Joyce DiDonato has said.
She explained to BBC Radio 4's Today programme that the art of an opera singer is to perform without amplification, often for hours at a time.
Ms DiDonato added that it was this skill, above all, that separates opera singers from other classical vocalists.
First broadcast on BBC Radio 4's Today programme on Thursday 12 June.