Civil defense siren

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Thunderbolt 1000/1000T Civil Defense siren. 127dBc @ 100 ft or 30 m
Thunderbolt 1000/1000T Civil Defense siren. 127dBc @ 100 ft or 30 m

A civil defense siren (also referred to as an air raid siren, tornado siren, or outdoor warning siren) is 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 in World War 2, they were adapted to warn of nuclear attack and of natural phenomena such as tornadoes. The generalized nature of the siren led to many of them being replaced with more specialized warnings, such as the Emergency Alert System.

Sound is generated by a motor driving a shaft with fans on each end, 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 draw air in at the end and force it out through the slots in the housing in rapid pulses, which produces the sound. Modern sirens can reach up to, but not commonly, 135 decibels when measured 100 feet away from the siren; the loudest confirmed siren ever produced was the Chrysler Air Raid Siren, producing 138 dB at 100 feet.

Many warning 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 (a minor third).

Some newer sirens have the ability to broadcast voice messages over large areas, depending on winds and noise. These electronic sirens are similar to electromechanical sirens; however, they rely on a series of large loudspeakers to produce sound. However, there is some question about the ability of a system of electronic sirens to broadcast a voice message with sufficient intelligibility over long distances - not only does the sound echo off some surfaces, the sound could have multiple arrival times from widely-spaced siren sites.

However, the relative simplicity, efficiency, and easily-serviceable 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.

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[edit] Historical warning tones

An air-raid siren in Nice, France is still operational.
An air-raid siren in Nice, France is still operational.

During World War II for a "Red Warning" of approaching danger, the siren produced a tone that rose and fell regularly between one high and one low tone.

A "White Warning" (All Clear) was a single continuous tone. Another alternating tone signified a "take cover" warning for immediate danger.

After World War II, two further warnings were introduced for nuclear attack - a "Grey Warning" indicated approaching nuclear fallout with a 2½ minute warning of long steady tones divided by equal periods of silence, the silence being created with a manual shutter or electric solenoid.

A "Black Warning", also for manual sirens, was either a Morse code 'D' (–··) or three quick tones, indicating imminent danger of fallout.

These warnings were not used in the United States.

[edit] Current warning tones

ACA P-50 Civil Defense Siren. 135dBc @ 100 ft or 30 m
ACA P-50 Civil Defense Siren. 135dBc @ 100 ft or 30 m

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 a steady continuous note.

The "Attack" tone may also be used to warn of severe weather, but it is widely believed that this tone is to be reserved for an impending attack on the United States. This tone is best described as having a rising and falling sound ("wailing").

Finally, the "Hi-Lo" Fire siren tone can sometimes be used by local volunteer fire departments to alert firefighters of an emergency. This tone is also used in some cities to warn of the potential for flash flooding. The "Hi-Lo" tone is a continuous alternating higher pitched and lower pitched sound, similar to police sirens in Europe.

In some locations, sirens are also used to alert the public to releases of hazardous materials, such as chemicals from a derailed train car or a manufacturing plant.

[edit] 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 in the United States and the Emergency Public Warning System in the Canadian province of Alberta. 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.

[edit] Civil defense sirens today

In the United States, sirens are usually used to warn of tornadoes and flash floods, as threats of military attacks on the United States are rare. Many cities in the US and other countries around the world periodically sound their sirens as a test, either weekly or monthly or yearly at a day and hour set by each individual city.

[edit] Major U.S. cities using outdoor PAS

Note: Chicago is home to Federal Signal and Milwaukee is home to American Signal.

[edit] Major international cities using outdoor PAS

[edit] Types of sirens

[edit] Mechanical sirens (direct driven)

Federal Signal Thunderbolt 1003
Federal Signal Thunderbolt 1003

A mechanical siren uses a rotor and stator to "chop" an air stream, which is forced through the siren by radial vanes in the spinning rotor. An example of this type of siren is the The Federal Signal "2T22" [1], which was originally developed during the Cold War and produced from the early 1950s to the late 1980s. This particular design employs dual rotors and stators to sound each pitch. Because the sound power output of this type of siren is the same in every direction at all times, it is described as omnidirectional. The 2T22 was also marketed in a 3-signal configuration known as the 3T22, which had capabilities for a "hi-lo" signal.

While some mechanical sirens produce sound in all directions simultaneously, other designs produce sound in only one direction, while employing a rotator mechanism to turn the siren head throughout 360 degrees. One such siren is the American Signal "Allertor" [2]. 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.

An example of such a siren being produced today is the Federal Signal Model 2001 [3].

One rare type of mechanical siren does not rotate or produce equal sound output in all directions. The Federal Signal "Thunderbeam" siren [4] 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.

[edit] "Supercharged" electromechanical sirens

Federal Signal SD-10
Federal Signal SD-10

The Federal Signal Thunderbolt series is the most recognizable of all warning sirens due to its unique shape and design. It creates a very distinct tone, made specifically to get the attention of people. "Thunderbolt" sirens use a separate blower to force air through the rotor and produce greater air movement with each pulse, thus they are described as supercharged[citation needed].

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, and the Thunderbolt 1000T is a dual tone siren. The Thunderbolt 1003 is essentially the same as the Thunderbolt 1000T, except that it employs solenoid-actuated slide valves to create a "hi-lo" signal.

Other "supercharged" sirens (having a separate blower for supplying air) include the American Signal (ASC) "Hurricane" [5].

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. Soon afterward, Hörmann improved on the design to create the HLS 273, which did away with the massive siren head of the original in favor of a more compact 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. The later HLS 273 located the large (6000 liter) air tank underground beside the machinery vault, instead of in the mast itself as in the earlier HLS units. [6]

[edit] Electronic sirens

American Signal AL-8000 in O'Fallon, Illinois
American Signal AL-8000 in O'Fallon, Illinois

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 horn speaker 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 omni-directional and rotating categories, though Whelen Engineering 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. Also, these sirens can be set to rotate any amount from 0 to 360 degrees, allowing sirens to broadcast only in (a) certain direction(s).

An example of a rotating electronic siren is the Whelen Engineering "Vortex" [7], American Signal Alertronic 6000R[8] and Federal Signal Siratone 408, 612[9], & 816, whose design incorporates 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.

Examples of omni-directional electronic sirens are the Federal Signal "Modulator" series [10], Whelen Engineering WPS2700, WPS2800, and WPS2900 [11], and American Signal I~Force, 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 omni-directional sirens simply array directional re-entrant horn modules in all directions to produce a continuous coverage pattern, such as the ATI Sound Blaster series [12], and Federal Signal DSA series[13].

[edit] 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, such as the 2001 and Modulator sirens.

[edit] Sirens around the world

Australia currently has no need for any major outdoor warning system, and the only sirens that are in place are usually small scale sirens, like the Model 5 or Model A which are found at fire stations for call-outs.

[edit] Other uses

Sirens have been used in a number of pieces of music for special effect including Ionization by Edgard Varèse, Fireman by Lil Wayne, In Memoriam by Robert Steadman, Danjer by O.G.C., Pulse Of The Maggots by Slipknot, and "Christmas at Ground Zero" by Weird Al Yankovic.

[edit] External links

[edit] Manufacturers

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