When creating the music for the Dark Knight, Christopher Nolan and Hans Zimmer wanted the main theme of the soundtrack to be inspired by the joker. They wanted it to describe his character and motives, they wanted it that so when you hear the music you’d get that the jokers presence was near.
To do this Hans Zimmer decided to go as far out into the deep as possible with the sounds he was creating. He described the sounds he was making to be unsettling and unnerving which was exactly what he wanted to capture.
One of the main pieces used for the soundtrack was produced by continually playing two notes on a cello, this in result gave a really sinister feeling of a string getting tighter and tighter from tension.
Christopher Nolan and Hans Zimmer thought that this described the Joker perfectly and in the movie was very effective at making the audience feel on edge and griped by the jokers precense.