Civil defense siren
From Freepedia
A civil defense siren, air raid siren, outdoor warning siren, or tornado siren is usually considered to be an 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 Alert 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 up to, but not commonly, 135 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. These "electronic sirens" are similar to electromechanical sirens except for the fact that they rely on a series of large sound drivers to produce sound.
Some companies that manufacture outdoor warning/civil defense sirens are:
- American Signal Corp (http://www.americansignal.com/)
- Federal Warning Systems (http://www.federalwarningsystems.com/)
- Whelen Engineering (http://www.whelen.com/)
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Historical Warning Tones
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. After World War 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. These were not used in the United States.
Current Warning Tones
Today, the most common tones sirens in the United States produce are "Alert," "Attack," and "Hi-Lo." While the tones used vary in some locations, the most common tone, the "Alert" is widely used by municipalities to warn citizens of impending extremely severe weather, particularly tornadoes. The sound of the "Alert" is the same as the "White Warning" described above, whereas the siren produces a steady continuous note. The "Attack" tone may also be used by some communities to warn of severe weather, but it is widely believed that this tone is to be used only in case of an impending attack on the United States. This tone is best described as having a rising and falling sound ("wailing"), similar to the "Red Warning" used during WW II. Finally, the "Hi-Lo" tone can sometimes be used by local volunteer fire departments to alert firefighters of an emergency. The "Hi-Lo" tone is also used in some cities to warn of the potential for flash flooding. The "Hi-Lo" tone is just that, a continous alternating higher pitched and lower pitched sound, similar to police sirens in Europe. To emphasize a point again, some cities and towns may use siren tones for different purposes, such as to warn of a hazardous materials situation, other than what is described here. It all depends on what is a more common occurrence from area to area.
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.
Sirens in Integrated Public Warning Systems
Sirens are sometimes used as part of an integrated warning system that 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.
Links
- Air Raid Sirens dot com - The only remaining active and useful Outdoor Warning Siren site on the net. Active community
- Civil Defense Museum - Overview of sirens since their inception
- Los Angeles air raid sirens - Pictures of unused nuclear-era civil defense sirens still extant in Los Angeles, California
