Civil defense siren
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A civil defense siren, air raid siren, or tornado siren is a electrically-powered mechanical device for generating sound to provide warning of approaching danger and to indicate when the danger has passed. Initially designed to warn of air raids they were adapted to warn of nuclear attack and of natural phenomena such as tornadoes. The generalized nature of the siren led to them being largely replaced with more considered warnings, such as the U.S. Emergency Broadcast System.
Sound is generated by having a motor drive a shaft at either end of which are mounted fans, one fan having a few more blades than the other. Around each fan is a housing with a number of cut slots to match the number of fan blades. The blades are designed to draw air in at the end and force it out through the slots in the housing. Due to the design, the air output is cut on and off alternately thus producing the sound. Modern sirens can reach 140 dB at 30 metres (100 feet).
Newer "sirens" are far more sophisticated, with the ability to broadcast voice messages over large areas, depending on winds and noise.
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Attack warnings
A number of different sound forms could be created. During World War II for a "red warning" of approaching danger the siren would be run normally producing a tone that rose and fell regularly between one high and one low tone, corresponding to the number of blades on each fan and the speed at which they turned. A "white warning" (All Clear) was a single continuous tone. Sometimes there was a "take cover" warning for immediate danger, the power to the motor was cut for a moment at intervals to change the tone produced. Post WW II two further warnings were introduced for nuclear attack - "grey warning" indicated approaching fall out with a 2½ minute warning of long steady tones divided by equal periods of silence, the silence being created with a manual shutter. A "black warning", also for manual sirens, was either a Morse code 'D' (–··) or three quick tones, indicating imminent danger of fall out.
Natural disasters
Public sirens are often sounded by local municipalities when a tornado warning has been issued for an area by a government agency, such as the National Weather Service in the U.S., or the Meteorological Service of Canada. Sirens may also be sounded even before a warning, if a tornado, waterspout, or other funnel cloud is spotted by police, firefighters, or other personnel trained as a spotter. In some places, such as Cobb County, Georgia, sirens are sounded if a severe thunderstorm warning is issued while there is a tornado watch out for the region.
Some areas, such as Mexico City, have warning systems for major earthquakes. Because the seismic detection system can give several seconds notice of earthquakes (which generally occur over 100 km away on the Pacific coast), lives can be saved when people can scramble to greater safety, or at least less danger. This is not as effective where major earthquakes occur very near or even right under cities, such as Los Angeles or San Francisco.
Other emergencies
Sirens are sometimes used to alert the public to releases of hazardous materials, such as chemicals from a derailed train car or a manufacturing plant. nl:Luchtalarm
Sirens in integrated public warning systems
Sirens are sometimes used as part of an integrated warning system that automatically links sirens with other warning media such as the radio and TV Emergency Alert System, NOAA Weather radio, telephone alerting systems and wireless alerting systems. This integrated approach enhances the credibility of warnings and reduces the risk of their being dismissed as false alarms by corroborating the warning messages through multiple media. The Common Alerting Protocol is a technical standard for this sort of multi-system integration.