soundtrack - Why are most sounds dubbed in afterwards for movies, instead of being recorded live?

The typical big budget movie has 50–150 people standing just outside but close to the shot:

• actors not in the shot


• stand ins


• stunt performers


• extras


• cinematographer, camera crew, dolly crew


• script supervision


• lighting crew, grips, electricians, light controller


• hair and makeup


• wardrobe and assistants


• props


• greenery


• set supervisor


• continuity


• capturing sound (!)


• stunt supervision


• assistant directors: 1st, 2nd, and 3rd


• production assistants


• behind-the-scenes photographers


• still photographer


• runners


• producers and studio execs—usually by the director at the video tap monitors


• etc., etc.

Further away are tradesman who do

• set construction: carpenters, finish, set construction supervision


• set painting


• riggers


• equipment movers


• more electricians and a generator operator


• plumbers


• drivers


• and a huge set of electrical generators

Someway into calling the roll, there will be a call for quiet—possibly including briefly ringing bells and/or flashing red lights coming on around the sound stage—at which time all hammering, drilling, pounding, sawing, etc., air compressors, and air conditioning are silenced.

The transition is profound—from total bedlam to near silence—but there are still dozens and dozens of people breathing, drinking, chewing, or moving—much more slowly than normal—and further away talking quietly (if they dare). All that activity adds up to background noise that even the most directional microphone can't help but capture.

Then there is the inevitable unavoidable sounds: actors (or the camera) on turntables for revolving shots: the turntable always makes some creaking and grinding noises. Or fake elevator doors have the wrong sound. Or the subfloor on a sound stage is always two layers of 3/4 inch plywood over a frame 4 feet above the floor. Even if the flooring is granite, an actor walking over that just doesn't sound right.

The general acoustics of a sound stage also aren't conducive. A conference room won't really have a ceiling: it is open or missing for lighting and perhaps so the boom folks can reach in to get a good microphone angle on an actor. The windows aren't really there—to avoid troublesome reflections, and so the acoustic ambiance of almost any onset room will sound slightly wrong.

For comedy, most directors and actors prefer capturing the sound live for its more spontaneous and natural effect. But for drama, it is easier and more compelling to ADR and/or foley in a soundproof room with only one or two people present to eliminate all the subliminal distractions.

Also, films which are dubbed into other languages need to have the set's apparent acoustic ambiance separate so that its sound can be layered with the dubbed audio.