1MC is the term for the shipboard intercom on U.S. Navy vessels. This system broadcasts throughout the entire ship, and is loud enough that all embarked personnel are able to (normally) hear it. In movies, calls to General Quarters are made over the 1MC.
1. General Quarters - "This is not a drill, this is not a drill, General Quarters, General Quarters! All hands man your battlestations.....
2. Sweepers - "Sweepers, Sweepers, man your brooms. Sweep down all decks fore and aft. Sweep all passageways, ladderwells, and ......Take all trash to the fantail. Now sweepers."