When you are recording sound on location, someone on your crew should be responsible for audio. It might be the camera person or it might be a separate audio person who also controls the boom microphone. This person should be listening to the audio on headphones and should also be responsible for making sure that the set is quiet before the tape rolls. Audio technicians or directors frequently call out "Quiet on the set! Tape's rolling!" before the director calls "Action," to let the crew know that the scene has started and they need to immediately cease making noise. The audio technician will also be sure to record 30 seconds or so of "room tone," which is the ambient sound of the room. People and equipment affect the acoustics of a room as sound bounces off them or is absorbed, so it's important that everybody who is in the room while you record the dialog is also in the room (but completely silent) when you record the room tone. You can use the room tone to overdub or "erase" extemporaneous noises recorded during the take. The prop masters who worked on the 1960s Star Trek television series wanted the actors' tools (phasers and communicators) to mount magically onto their clothes without visible belts - so they used Velcro. Every time Captain Kirk pulled out his communicator to call back to the Enterprise, there was a zip! noise as the Velcro detached. Overdubbing with room tone made the sound vanish. You may want to overdub the sound of a passing truck or a falling lamp of…