Historically the bosun whistle (boatswain's call) was used to pass commands to the crew when the voice could not be heard over the sounds of the sea. Because of its high pitch, it could be heard over the activities of the crew and bad weather.
This bosun whistle measures 5 x .5 inches and can actually be used! The pipe of the whistle consists of a narrow tube (the gun) which directs air over a metal sphere (the buoy) with a hole in the top. Simply open and close your hand over the hole to change the pitch.