Film sound is carefully designed and mixed in post-production to emphasize important sounds and set the atmosphere. There are two main types of sound: diegetic sounds that come from within the story world like characters' voices, and non-diegetic sounds from outside the world like music soundtracks. Sound can also be synchronous to match visuals, asynchronous to originate outside the story, or contrapuntal to create a different meaning by clashing with visuals. Key elements of sound design include selective and ambient sounds, sound bridges, effects, voiceovers, theme music, and musical scores.