The primary objective of the production sound mixer and the boom operator is to capture clean dialogue on set.
Movie soundtracks are made up of 5 basic types of audio tracks - dialogue which includes all spoken words in the movie, Foley - which is the sounds of the movement of the actors including footsteps, clothing movement, props and interaction with the environment. Often time Foley is systematically recorded later in a sound recording studio. The third track is the ambience of the location, the fourth is sound effects - explosions, planes flying overhead, essentially anything outside the actor's interactions with the environment, and lastly, the music track. Because most of these tracks are created in the controlled environment of the studio, the only element that needs to be pristine is the actor's dialogue. And getting good dialogue is the primary responsibility of the production sound mixer.
Recording on set dialogue can be difficult, but through the proper choice of microphone, microphone placement, recording techniques, and most importantly the location itself, the production sound mixer can isolate the diallogue for clean recording.
Much like a camera can bring the audience closer to or farther from a subject through the use of the focal length of the lens, so too can the boom operator control the perspective of the sound by changing the proximity of the microphone to the subject.
Sound perspective is an important consideration when recording sound, as the boom operator must always be aware ofthe frame size and position the microphone accordingly. A long shot where a lavalier is within 8" of the actor, will sound to an audience as though the actor was 8" away from them, when the shot speaks to the contrary.
In some instances, actors may speak at different volume levels, requiring the boom operator to compensate by moving the microphone closer to the quieter actor and farther from the louder actor. This live, real-time "mixing" uses perspective as a way of balancing the audio levels betwen actors.
Ambient sound is the background noise of a location. For example, waves, seagulls, and wind are the ambient sounds of a beach, and traffic, horns, and people talking are the ambient sounds of a city street. Although ambient sounds are important in helping establish the location, they must be reduced as much as possible so that on set, the only sounds recorded are the actors’ dialog and their movements. Ambient sounds are then added to the entire scene in postproduction, helping add consistency to a scene. There are several techniques to help minimize the ambience. Most importantly, listen on set during the location scout to determine what sounds are present. Most often, the ambient sounds will be air conditioning units and refrigerators. Talk to the location owner about turning these off on the day on the shoot.
Interior locations are easier to control because the walls absorb much of the sound from the outside. Shooting outdoors makes controlling ambient sounds more difficult. If you’re in a noisy location, bring several packing blankets (available from your local mover) and hang them on C-stands to create movable “sound walls” that can be positioned between the actors and the source of the sound. Place the sound blankets behind lights and the camera after all the equipment has been set up. If you’re recording in a church or a large room with a lot of echo, try placing the packing blankets on the floor under the actors to absorb the sound. It is important to minimize the ambience as much as possible to save time and money in postproduction. If there is too much ambient noise, you may need to ADR the actors’ dialog and then recreate the Foley and ambience. This is a time-consuming and expensive process that can be avoided by using these techniques on set.
Every environment has an ambient sound, regardless of how quiet it may seem. This ambience becomes apparent in the editing room when the dialogue begins to piece together lines of dilaogue to create a continuous track. Any gaps in the dilaogue need to be filled with the ambience so the audio doesn't "drop out." For this reason, the production sound mixer should always record at least 30 seconds of room tone.
At the end of every scene, ask the crew to stand still, with everything the same as when the cameras were rolling, and, with the microphone positioned in the same place it was during the scene, record 30 seconds of room tone. This will be an invaluable sound source for the dialgue mixer in post.
Recording No Sound
Every scene has sound- even though there may be no dialogue. It could be the handling of props, footsteps, the rustling of clothes, even the sound of the actors breathing. With the low cost of digital media, the director has no excuse for always rolling sound. Remember, it's easier to have it and not need it than to need it and not have it.
Working with Extras
There may be scenes that involve background actors or extras, such as a restaurant, bus station, auditorium, or any other public place. Although in real life you can hear the ambience of people talking, laughing, chatting on cell phones, arguing, and otherwise interacting with their environment, when you film that moment in a movie, filmmakers instruct the extras to ACT as if they are talking but to SAY NOTHING. The only sound that should be heard on set when the cameras roll is the sound of the principal actors delivering their lines and their interaction with the environment. The rest of the ambience is recorded later and mixed in during the editing process, giving the filmmakers control over the balance between background sounds and dialog.
The production sound mixer should maintain complete and comprehensive sound logs, which list the audio take, the scene and setup, the timecode of each take, and any notes regarding the quality of each take. These sound logs will help the editor determine which take is best to use and aid in finding a particular take.