An Overview of Sound for Film
The soundtrack of a film can be broken into three major categories: sound effects, dialogue, and music. From there, each of the categories is broken into subcategories. See below for a brief explanation of each. For a more visual overview of sound for film, see this explanation from two of the lead sound practitioners for the film Ford v Ferrari (0:30-1:53):
Sound Effects
Sound effects (SFX) encompass all sounds that are not dialogue or music. The branches of SFX include hard effects, backgrounds, foley, and designed sounds.
Hard Effects
Hard effects are sounds that match the action on-screen. Examples include a door slamming, an alarm clock, or a car starting. These sounds can either be production sounds or library sounds. Production sounds are those that are recorded on set. While a production sound mixer’s focus is on recording clean dialogue, they can and will record other narratively important sounds. While these are often recorded simultaneously with the image, they can also be recorded “wild,” or without video. Library sounds are effects that are pulled from large data-bases of pre-recorded sounds. These data-bases can be personal, purchased, or subscribed to. The most common sounds found in libraries are those that are difficult or expensive to capture (Schrader). Examples of an explosion, a lion's roar, and glass shattering can be heard here:
Backgrounds
Atmospheric sounds (a.k.a backgrounds or atmos) are the ambient sounds that fill out a sonic environment and help immerse the audience in a scene. These sounds can either be recorded on set or can be pulled from a production sound library. Examples include the sound of traffic in a city, birds in a jungle, and people talking at a party.
Foley
Foley is the art of performing sounds to picture in post-production. Foley artists use both conventional and unconventional techniques to replicate the sound of an action or object seen on screen. Foley sounds are often further broken into categories such as footsteps, cloth tracks, props, and weather. The following video gives an overview of the typical foley process.
Designed Sounds
Designed sounds are brand new sounds that are created for objects that do not already have a sound. This is commonly required for science-fiction films that feature fictional technology or creatures. In the following video, legendary sound designing Ben Burtt explains the process of creating the voice of Chewbacca for Star Wars.
Dialogue
Dialogue consists of any speech produced by a character. Dialogue can be broken into two categories: production dialogue and ADR.
Production Dialogue
Production dialogue is the dialogue that is recorded on set. The primary goal of a production sound recordist is to record clean dialogue that can later be edited and used in post-production.
ADR
Automated dialogue replacement, referred to as ADR, is the process of re-recording dialogue in post-production. ADR is most commonly used to replace dialogue that could not be captured cleanly on set. However, ADR can also be used for more creative purposes such as changing inflection, changing words, and even overbudding an actor’s voice with another. For an excellent explanation of the history and process of ADR, see the following video from Filmmaker IQ:
Music
Music in a film is either non-diegetic or source.
Non-Diegetic
Non-diegetic music cannot be heard by the characters and serves a multitude of functions that are discussed in great depth here. This is the most common form of music in film. An example is John Williams' score in this scene from E.T. the Extra-Terrestrial.
Source
Source music is diegetic, or heard by the characters in the film. An example of source music is Bob Seger’s “Old Time Rock in Roll” in Risky Business.