Christopher Nolan's vision for "The Dark Knight" was more gritty, urban crime drama than slick superhero movie. At the same time, there were fantastic gadgets, vehicles and weapons that required cool and exciting sound effects. These kinds of sounds are not captured on the set, of course, but are created in postproduction.
The goal of sound design is to use sound to create feeling, to give inanimate objects an emotional component. We worked hard to give each element a distinct personality or audio signature.
The Batmobile had to sound huge, like a military vehicle crossed with a dragster. For this, we recorded the sound of big race boat engines, adding large animal roars and growls for emphasis.
The Batpod is slicker and more refined. I used the concept of the Shepard tone to make the sound appear to continually rise in pitch. The basic idea is to slightly overlap a sound with a distinct pitch (a large A/C electric motor, in this case) in different octaves. When played on a keyboard, it gives the illusion of greater and greater speed; the pod appears unstoppable.