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Concept
Civil defense siren
Summary
A civil defense siren, also known as an air-raid siren or tornado siren, is a siren used to provide an emergency population warning to the general population of approaching danger. It is sometimes sounded again to indicate the danger has passed. Some sirens, especially within small municipalities, are also used to alert the fire department when needed. Initially designed to warn city dwellers of air raids during World War II, they were later used to warn of nuclear attack and natural disasters, such as tornadoes. The generalized nature of sirens led to many of them being replaced with more specific warnings, such as the broadcast-based Emergency Alert System and the Cell Broadcast-based Wireless Emergency Alerts and EU-Alert mobile technologies.
A mechanical siren generates sound by spinning a slotted chopper wheel to interrupt a stream of air at a regular rate. Modern sirens can develop a sound level of up to 135 decibels at . The Chrysler air raid siren, driven by a Chrysler Hemi gasoline engine, generates 138 dB at .
By use of varying tones or binary patterns of sound, different alert conditions can be called. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.
Sirens are sometimes integrated into a warning system that links sirens with other warning media, such as the radio and TV Emergency Alert System, NOAA Weather Radio, telephone alerting systems, Reverse 911, Cable Override, and wireless alerting systems in the United States and the National Public Alerting System, Alert Ready, in Canada. This fluid approach enhances the credibility of warnings and reduces the risk of assumed false alarms by corroborating warning messages through multiple forms of media. The Common Alerting Protocol is a technical standard for this sort of multi-system integration.
Siren installations have many ways of being activated.
Official source
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