Audio for film can also be referred to as sound effects. In the context of motion pictures and television, sound effects refers to an entire hierarchy of sound elements, whose production encompass many different disciplines, including: Hard sound effects are common sounds that appear on screen, such as door slams, weapons firing, and cars driving by. Background (or BG) sound effects are sounds that do not explicitly synchronize with the picture, but indicate setting to the audience, such as forest sounds, the buzzing of fluorescent lights, and car interiors. The sound of people talking in the background is also considered a “BG,” but only if the speaker is unintelligible and the language is unrecognizable (this is known as walla). These background noises are also called ambience or atmos (“atmosphere”).
Foley sound effects are sounds that synchronize on screen, and require the expertise of a Foley artist to record properly. Footsteps, the movement of hand props (e.g., a tea cup and saucer), and the rustling of cloth are common foley units. Design sound effects are sounds that do not normally occur in nature, or are impossible to record in nature and all elements of audio for film These sounds are used to suggest futuristic technology in a science fiction film, or are used in a musical fashion to create an emotional mood. Each of these sound effect categories are specialized, with sound editors known as specialists in an area of sound effects (e.g. a “Car cutter” or “Guns cutter”). The process can be separated into two steps: the recording of the effects, and the processing. Large libraries of commercial sound effects are available to content producers (such as the famous Wilhelm scream), but on large projects sound effects may be custom-recorded for the purpose. Although effects libraries may contain every effect a producer requires, they are seldom in correct sequence and never in the required time frame.
In the early days of film and radio, library effects were held on analogue discs and an expert technician could play six effects, on six turntables, in five seconds. Today, with effects held in digital format, it is easy to create any required sequence to be played in any desired timeline. Also, if the soundtrack is processed through a foley, it can make the smallest sound look perfect on screen and the audience can never guess how much work went into the making of that specific sound. Audio for film is a sweet science.