Film sound is carefully designed and added in post-production to emphasize important sounds and create the proper atmosphere for scenes. Sound can be either diegetic, coming from visible or implied sources within the story, or non-diegetic, coming from outside the story. Transitions between scenes are aided by sound bridges, while effects and music are used to add realism, psychological impact, or inform the audience. Careful use of sound ensures the viewer has the intended emotional experience of the film.