Civil defense siren

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 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). 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 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, typically electrodynamic and horn-loaded loudspeaker drivers to produce sound. There is some question about the ability of a system of electronic sirens to broadcast a voice message with sufficient intelligibility over long distances due to reverberant surfaces near the sound source and detrimental effects of multiple arrival times from widely-spaced siren sites. However, the relative simplicity, efficiency, and easily-servicable design of modern electronic warning systems, in addition to the ability to broadcast any type of warning signal desired, makes them an attractive alternative for many municipalities.

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 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. Residents of municipalities having outdoor siren warning systems should familiarize themselves with the tones used for various emergencies in their particular municipality.

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.

Civil Defense Sirens Today

Civil Defense Sirens are still used in Switzerland, where all of the country's Air Raid, Nuclear Attack/Fallout and Dam Burst/Flooding Sirens are sounded on the first Wedensday of February every year. Almost every village, town and city in Switzerland has at least 1 siren. As the risk of air raids and nuclear blast/fallout in Switzerland is extremely low, today the sirens are most likely to be sounded in the event of a terrorist attack on Switzerland or if there is a risk of a dam breaking, although by the time this would be detected, there is a high chance that it would be too late.

In various cities of France, public warning sirens are sounded once a month. In Grenoble, the siren warns of flooding. In Nice, the siren is sounded on the first Wednesday of each month ^*.

In the Channel Island of Guernsey, the Civil Defence Siren is sounded to mark certain special occasions. The 'Alert' and 'All-clear' mark the beginning and end of the two minutes silence on Remembrance Day, as well as being sounded at 11am on 9 May to mark Liberation Day. It would also be used if there were a meltdown at the nearby French nuclear power stations at Flamanville and Cap de la Hague.

In Seoul, South Korea, Civil Defense sirens sound as part of a test and drill every Thursday at 10am. The system was incoprorated since the early 1940's and has been used (and updated) throughout the decades to protect the public against the threat of attack from North Korea which is mainly Nuclear.

In Taiwan, sirens sound every Monday morning at 11am as part of a test of the tsunami early warning system, which typically consists of a Carter siren that sounds across the city.

In Crowthorne, Berkshire in the United Kingdom, A Klaxon CS8 Siren is sounded every Monday morning at 10am as part of the Broadmoor Hospital security system. The siren is followed by an all clear and has a distinctive Hi-Low Sound.

Major U.S. Cities With Outdoor Public Alert Systems

Major International Cities with Outdoor Public Alert Systems

Types of sirens

Electromechanical Sirens(Direct driven)

The Federal Signal "2T22" ^* is an example of an electromechanical siren, which uses a rotator and stator to "chop" an air stream, which is forced through the siren by radial vanes in the spinning rotor. The Federal Signal Model 2T22 dual tone civil defense siren was originally developed during the Cold War, but thanks to the longevity of its mechanical design, many can be seen in use today. Like many dual-tone electromechanical sirens, this particular design employs dual rotors and stators to sound each pitch. The upper rotor/stator combination has 12 ports, while the lower rotor/stator combination has 10, generating a distinctive musical interval. Because the sound power output of this type of electromechanical siren is the same in every direction at all times, this type of siren is described as omnidirectional. 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" signal. The solenoids also allowed for an optional "pulsed" signal. The 2T22 was produced from the early 1950s to the late 1980s. Its successor is the Federal Signal "Eclipse" omnidirectional siren, which offers a battery backup feature.

While some electromechanical sirens like the Model 2T22 described above produce sound in all directions simultaneously, other designs produce sound in only one direction, with their siren heads employing a rotator mechanism to rotate the directional siren head throughout 360 degrees. One such siren is the Alerting Communicators of America "Allertor" siren which features a prominent hand-laid fiberglass horn. The lower horn simply serves an aerodynamic purpose, minimizing the restriction to airflow at the intake of the siren, allowing the vanes in the rotor to more efficiently do their job. This siren also produces two pitches simultaneously in a musical interval, but in this case the rotor and stator incorporate separate sections for producing each pitch. Another type of rotating electromechanical siren which is produced today is the Federal Signal Model 2001 siren [http://www.jmarcoz.com/sirens/porthuron1_mi.jpg, which is particularly common. The motor of the 2001 siren is located in the large cylinder at the center of the siren's megaphone-like projector, with the rotor and stator being located at the rear of the projector. Air intake is through the large rectangular shroud at the rear of the projector.

One rare type of electromechanical siren does not rotate or produce equal sound output in all directions. The Federal Signal "Thunderbeam" siren ^* employs a slowly rotating disc below the siren, which directs the siren's output throughout 360 degrees. This same method, applied differently, produces the distinctive sound character of the Leslie loudspeaker, but the rotation of the disc in the Thunderbeam is far too slow to produce the "warbling" sound associated with the Leslie loudspeaker.

"Supercharged" Electromechanical Sirens

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 is characterized as having a "buzz-like" quality when compared to other sirens because of the unique way that the sounding mechanism ("chopper") receives air. All Federal Signal "Thunderbolt" sirens use a separate blower to force air through the "chopper" rotor and produce greater air movement with each pulse, thus they are described as supercharged. Specially-designed horns having an exponential profile amplify the sound, causing the air at the end of the horn to be displaced the same distance as air in the throat of the horn with the passage of each wavefront. This lends a unique "distorted" character to the sound of these sirens as the throat of the horn is overloaded. Within the Thunderbolt product line, three different configurations were offered. The Thunderbolt 1000 is a single tone siren, having one row of ports on the rotor (4, 5, or 6 ports in the 1000's case). Acoustically, the Model 1000 is rated for 131 Decibels at 100 feet. The Thunderbolt 1000T 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, creating a dual-toned musical interval, originally intended to differentiate its tone from the sound of the sirens of responding emergency vehicles. Some used a 4/6 or 4/5 chopper ratio for distinguishing the sound of other sirens. The 1000T is rated for 129 Decibels at 100 feet due to the inherent reduced efficiency of a dual-tone system. The Thunderbolt 1003 is essentially the same as the Thunderbolt 1000T, except that it employs dampers and solenoids to close and open the upper or lower port of the chopper's exit in sequence creating a "hi-lo" signal. The 1003 is also rated for 129 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.

Other "supercharged" sirens (having a separate blower for supplying air) include the Alerting Communicators of America (ACA) "Hurricane" which features a distinctive fiberglass horn with a central divider, and a characteristic fast "wind-up" (initial ramping-up of the pitch). Also, the sirens surrounding the San Onofre Nuclear Plant in San Clemente, CA [http://www.jmarcoz.com/sirens/sanclemente_ca.jpg do not employ blowers, though the original patent details the use of a blower, and the presence of the exponential horns suggests that the original design incorporated a blower. This siren employs eight horns, yet the rotor only opens four of the horns at a time, separated by ninety degrees. It has been suggested that this was intended to produce the maximum sound pressure level without requiring a blower of exceptional size.

A variation on the 'Supercharged' electromechanical siren is the pneumatic Hochleistungssirene (HLS), produced by the German firm Pintsch-Bamag, and later by the German firm Hörmann which improved on the design to create the HLS 273 which did away with the massive siren head in favor of a more compact siren head and cast aluminum exponential-profile horns. These sirens stored an enormous reservoir of compressed air, recharged periodically by a diesel engine-driven compressor in a vault in the base of the massive siren unit, typically underground. The later HLS 273 located the large (6000 litre) air tank underground beside the machinery vault, instead of in the mast itself as in the earlier HLS units. Another examples of gas powered sirens includes the ACA Allertor G, and the Federal Signal Thunderbolt 1002. ^*

Electronic Sirens

Electronic sirens produce their sound in a fashion that is fundamentally different from electromechanical sirens. Instead of a motor-driven rotor spinning inside a stator, electronic sirens consist of an electronic tone generator, a high-power amplifier, and a loudspeaker typically incorporating one or a multiple of electrodynamic transducers. Typically the loudspeaker unit incorporates horn loading, which causes them to be similar in appearance to some supercharged electromechanical sirens. Many of these loudspeakers incorporate a vertical array of horns, in order to achieve a practical yet effective high-power audio source with pattern control in the vertical plane. Each cell of the loudspeaker horn is driven by one or a multiple of compression drivers, which are typically purpose-built for siren applications. One type of purpose-built compression driver for this type of loudspeaker handles 400 watts of electrical power and features an enormous magnetic structure with two donut-shaped magnet slugs stacked on top of each other to provide enormous flux. For siren applications, high-fidelity sound is a secondary concern to high output, and siren drivers typically produce large amounts of distortion which would not be tolerable in an audio system where fidelity is important. As with electromechanical sirens, there are both omnidirectional and rotating categories, though one manufacturer produces sirens which oscillate through 360 degrees, rotating in one direction and then in the other to allow a hard-wired connection between the amplifiers and the siren drivers. An example of a rotating electronic siren is the Whelen Engineering "Vortex" whose design incoprorates four vertically-arrayed loudspeaker cells exiting into a common manifold. This horn design accomplishes pattern control in the vertical plane and focuses the output into a high-penetration beam. An example of an omnidirectional electronic siren is the Federal Signal "Modulator" series [http://www.jmarcoz.com/sirens/hanalei2_hi.jpg, in which compression drivers located in each cell exit into the center of the cell. The contour of each cell forms the horn profile in this case, though other omnidirectional sirens simply array directional re-entrant horn modules in all directions to produce a continuous coverage pattern.

Defunct Manufacturers

CLM Industries - Maker of Toronto and Chicago warning sirens during the Cold War. One was most famously featured in the 1980 Chicago based film, The Blues Brothers, used as a vehicle mounted PA system. Toronto and Chicago have removed the majority of these sirens and, in the case of Chicago, they have been replaced by newer Federal Signal models.

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

Manufacturers

Warning systems

Varslingssirene | Sirene (Gerät) | Luchtalarm

Gourt Home::Gourt :: Civil defense siren