Film sound is carefully designed and mixed in post-production to emphasize important sounds and set the atmosphere. There are different types of sounds: diegetic sounds exist within the film's world while non-diegetic sounds come from outside. Synchronous sounds match the visuals while asynchronous sounds come from outside the diegetic reality, like musical scores. Other elements include voiceovers, theme music to introduce characters, and silence which is sometimes used provoke a reaction.