How did Heath Ledger change his voice for the Dark Knight?

He's so good at doing that! In every movie he had to change his voice,he did it perfectly! How did he get so good?How did Heath Ledger change his voice for the Dark Knight?
easily, I read that he stayed in his hotel or apartment ? whichever for a long time developing th perfect way to play the joker.How did Heath Ledger change his voice for the Dark Knight?
As an actor, you learn to put different range in your voice and speak with different dialects

fever

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