Air Raid Siren

Air raid sirens are essential for civil defense. The devices are electrically powered to create loud dual tone sounds. The noises emitted from the sirens can both alert the public, as well as inform them as to when danger has passed. A motor driving a shaft with unique fans located at both ends of the device help run the machine. The blades of the fans draw air in at the end of the contraption and force it out through slots in rapid pulses; as the plates on the end of the blades interrupt the flow, the sound is produced. Originally used to warn the public of air raids in World War II, sirens are used today to alert the public of tornadoes, tsunamis, and other extreme weather conditions.