In order to understand the role of sound in film, one must examine the relationship between sounds and images. Since film is considered a predominantly visual medium, for many filmmakers and viewers, sound exists in movies to enhance the impact of an image. However, sounds can interact with images in infinite ways, and strategies used to combine the two fundamentally affect our understanding of film. Sound is an important aspect in guiding our perceptions of cinematic realism.

Synchronous sound has a visible onscreen source, while asynchronous sound does not (these can also be referred to as onscreen sound and off screen sound, respectively). We can further differentiate between parallelism in the use of sound, which occurs when the soundtrack and image “say the same thing,” and counterpoint (or contrapuntal sound), which occurs when two different meanings are implied by these elements.

Diegetic sound, such as dialogue, has its source in the narrative world of film, while nondiegetic sound, such as background music or certain kinds of narration, does not belong to the characters’ world.

During production, sound recording takes place simultaneously with the filming of a scene. Microphones for recording synchronous sound may be placed on the actor or positioned overhead with the use of a device resembling a long pole called a boom. The snap of a clapboard is recorded at the beginning of each take to synchronize sound and image. When a cut of the film is prepared, the crucial and increasingly complex phase of postproduction sound work begins. Sound editing interacts with the image track to create rhythmic relationships, establish connections between sound and onscreen sources, and smooth or mark transitions. Sound effects may be gathered, produced by sound-effects editors on computers, retrieved from a sound library, or generated by foley artists. Postsynchronous sound is recorded after the fact and then synchronized with onscreen sources. During automated dialogue replacement (ADR), actors watch the film footage and re-record their lines to be dubbed into the soundtrack (a process also known as looping because actors watch a continuous loop of their scenes). During the sound mixing stage, the three elements of a soundtrack – voice, music, and sound effects – are combined. Although the three sound elements (voice, music, and sound effects) can all be present and combined in relation to any given image, conventions have evolved governing these relationships.

Human voice is often central to narrative film’s intelligibility, primarily in the form of dialogue. Speech is used to expose a character’s motivation and goals and convey plot information, and it is therefore typically mixed to be the most audible sound heard by the audience. Used famously by filmmaker Robert Altman, overlapping dialogue is a technique that makes individual lines less distinct and is often used to approximate the everyday experience of hearing multiple competing speakers and sounds at the same time. A voice-off is a voice that originates from a speaker who can be inferred to be present in the scene but who is not currently visible onscreen, while a voiceover describes a voice whose source is not visible in the frame yet acts as the organizing principle behind the film’s images, such as the narration in a documentary film.

Music is a crucial element in the film experience, providing rhythm and deepening emotional responses. Soundtrack music can work to guide an audience’s attention, provide character information, and cue emotional responses. Background music, or underscoring, literally underscores what is happening dramatically. Narrative cueing is how music tells us what is happening in the plot. The most noticeable examples are called stingers, sounds that force us to notice the significance of something onscreen.

Popular songs have long had a place in the movies, promoting audience participation and identification by appealing to tastes shared by age or ethnic groups. Sheet music and recordings were profitable tie-ins even before sound cinema. Since the 1980s, pop songs began to dominate many film soundtracks, and now rock and pop soundtrack tie-ins have become increasingly prevalent.

Much of the impression of reality in cinema comes from the use of sound effects, although, like other aspects of the soundtrack, they may not be consciously noticed by viewers. When reproduced in the three-dimensional space of the theater, sound effects are also one of the most effective techniques used to add depth to the two-dimensional image of a film.