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A civil defense siren, '''air raid siren''', '''tornado siren''', or other kind of '''outdoor warning 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 Design ed to warn of Air Raid s, they were adapted to warn of Nuclear Attack and of natural phenomena such as Tornado es. 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 Fan s, 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 Blade s. 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 Metre s (100 Feet ). Many air raid sirens have a sound that is distinct from that used by emergency vehicles due to use of two simultaneous tones, usually in a 5:6 frequency ratio. Newer sirens are far more sophisticated, with the ability to Broadcast Voice Message s over large areas, depending on Wind s 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. 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 Tone s, 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 continuous 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 System s for major Earthquake s. Because the Seismic Detection system can give several Second s 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 Material s, such as Chemical s from a Derail ed 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. MAJOR CITIES WITH OUTDOOR PUBLIC ALERT SYSTEMS TYPES OF SIRENS Thunderbolt 1000, 1000T, 1003 ]] ]] The Federal Signal Thunderbolt is the most common and well known civil defense siren ever made. It creates a very distinct tone, made specifically to get the attention of people. The tone has more of a "buzz-like" quality than other sirens because of the unique way that the sounding mechanism receives air. All Thunderbolts use a separate blower to force air through the rotor (most normal mechanical sirens just draw outside air into the rotor) and amplify the sound. Within the Thunderbolt category, there are three different configurations. The Thunderbolt 1000 is a single tone siren, meaning it has one row of ports on the rotor (5 ports in the 1000's case). Acoustically, the 1000 is rated for 130 Decibels at 100 feet. The Thunderbolt 1000T (T standing for Dual Tone) is a dual tone siren. It has two rows of ports on its rotor: 6 on the upper row and 5 on the lower row, thus creating two different tones at once. The 1000T is rated for 125 Decibels at 100 feet. The Thunderbolt 1003 is the same as the Thunderbolt 1000T, except that it has dampers and solenoids to close and open the upper or lower port in sequence creating a "hi-lo" signal. The 1003 is also rated for 125 decibels at 100 feet. Today, most Thunderbolts are either used in active warning systems or have been disconnected since the end of the cold war and are sitting at the tops of their poles rusting away. Alertronic High Powered Voice Siren American Signal Corporation produces AL-model sirens series 1000, 2000, 4000, 6000, and 8000 designed for community and industrial use. These models produce both tone and high quality voice output. The 8000 model produces 126dB rated at 100 feet in all directions. {Link without Title} 2T22 The 2T22 is a dual tone civil defense siren that was originally intended for use in the Cold War Era. The Federal Signal corporation produced this 10/12 port siren and was the only company to make such a device. Rather than warning a circular area by mechanically rotating a single horn on a pole like the Thunderbolt, the 2T22 featured smaller horns around its circumference which sounded the warning to all directions at once. The 2T22 was also marketed in a 3-signal configuration known as the 3T22, which had solenoids that opened and closed dampers in the air intake tubes to create what is known as the "fire signal" or the "hi-lo." The solenoids also allowed for an optional "pulsed" fire signal. Both the 2T22 and 3T22 sirens are rated 115 dB @ 100 ft. Total sound coverage was about 2600 feet in all directions. It also came in other colors besides Civil Defense Yellow.[http://www.jmarcoz.com/sirens/moore0060_ok.jpg The 2T22 was made from the early 1950's all the way up to the late 1980's. Today, the 2T22 is still widely used in many communities as part of an early warning system. The model number 2T22 is from the siren specifications of two tones and 22 horns. The 3T22 model number reflects the third hi-lo signal. DEFUNCT MANUFACTURERS
OTHER USES Sirens have been used in a number of pieces of music for special effect including Ionisation by Edgar Varese , Fireman by Lil Wayne , In Memoriam by Robert Steadman , and in Pulse Of The Maggots by Slipknot . EXTERNAL LINKS
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