Although Mercury's speaking voice naturally fell in the baritone range, he delivered most songs in the tenor range. His known vocal range extended from bass low F (F) to soprano high F (F). He could belt up to tenor high F (F). Biographer David Bret described his voice as "escalating within a few bars from a deep, throaty rock-growl to tender, vibrant tenor, then on to a high-pitched, perfect coloratura, pure and crystalline in the upper reaches." Spanish soprano Montserrat Caballé, with whom Mercury recorded an album, expressed her opinion that "the difference between Freddie and almost all the other rock stars was that he was selling the voice". She adds: