A civil defense siren (also referred to as an air raid siren, tornado siren, tsunami siren, or other outdoor warning siren and also rarely referred to as the doomsday siren) is a mechanical or electronic device (modern-day sirens are electrically-powered) for generating sound to provide warning of approaching danger and sometimes to indicate when the danger has passed. In some areas in the United States, civil defense sirens may sound in the late morning on a regular basis. Initially designed to warn of air raids in World War II, they were adapted to warn of nuclear attack and of natural phenomena such as tornadoes. The generalized nature of the siren led to many of them being replaced with more specialized warnings, such as the Emergency Alert System. In a mechanical siren, sound is generated by a motor driving a shaft with a special fan (known as a rotor or chopper) on one or both ends. It will have only one fan if it's a single-toned siren, while if it's a dual-toned siren, itwill have either one fan on each end or two fans in a stack on one end, with one fan having a few more blades than the other.