Early in Kaufmann's career, [singing the Wagnerian role of] Siegmund wouldn't even have been a possibility. He started out in the 1990's sounding "like a typical young German tenor with a very light, white voice."
As for his lower register, he said, "I had none at all. I think F or F sharp (below middle C) was the lowest note I had. Under that it was only hot air."
He also found that when he tried to sing out to express emotion, "I lost control. My voice just wasn't reliable."
That led him in 1995 to a teacher, American baritone Michael Rhodes, who had moved to Germany after World War II.
Rhodes said in an email reply to questions that he instructed Kaufmann in the use of a technique he calls "sbadigliare," from the Italian for yawning, which he himself had learned from the great baritone Giuseppe De Luca.
"Sbadigliare requires one to relax the whole body and really yawn – an involuntary reaction," Rhodes said. "In this state of relaxation, the voice is completely free, as Jonas's is."
Kaufmann said that after he had studied with Rhodes, many colleagues who knew him from before predicted he would quickly come to ruin. "They said, `Maybe another two to three years, maybe five, but then it's over.' Because everyone expected me to overdo, to artificially darken the sound. It was impossible to believe that this would be my own voice."