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Federal Signal Thunderbolt 1003 Civil Defense siren.
A civil defense siren (also referred to as an air raid siren, tornado siren, tsunami siren, or other outdoor warning siren and also rarely referred to as the doomsday siren) is a mechanical or electronic device (modern-day sirens are electrically-powered) for generating sound to provide warning of approaching danger and sometimes to indicate when the danger has passed. In some areas in the United States, civil defense sirens may sound in the late morning on a regular basis.

Initially designed to warn of air raids in World War II, 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.

In a mechanical siren, sound is generated by a motor driving a shaft with a special fan (known as a rotor or chopper) on one or both ends. It will have only one fan if it's a single-toned siren, while if it's a dual-toned siren, it will have either one fan on each end or two fans in a stack on one end, with one fan having a few more blades than the other. Around each fan—or chopper or rotor—is a housing with a number of rectangular holes to match the number of fan blades. This housing is known as a stator. The end of each blade has a plate whose shape is matched with the rectangular holes and circular curve of the stator. The blades draw air in at the end and force it out through the slots in the housing in rapid pulses, as the plates on the end of the blades interrupts that flow, which is what produces the sound. Some mechanical sirens, such as the Federal Signal Thunderbolt series, also employ compressed air that is blown at the rotor to "supercharge" the sound from the siren, which causes the sound to be sharper and much louder than it would be with the chopper and stator alone.

Modern sirens can reach up to, but not commonly, 135 decibels when measured away from the siren; the loudest confirmed siren ever produced was the Chrysler Air Raid Siren, producing 138 dB at 100 feet. The Chrysler Air Raid Siren used a 180 hp V-8 Hemi engine to drive the siren and weighed 5543 lb. Most sirens are mounted on poles that are usually around 40 to 50 feet off the ground, but some older sirens can be found mounted on buildings.

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 (an untempered minor third).

Some newer sirens have the ability to broadcast voice messages over large areas, depending on winds and noise. These electronic sirens differ from electromechanical sirens in that they rely on a series of large loudspeakers 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 – not only does the sound echo off some surfaces, the sound could have multiple arrival times from widely-spaced siren sites.

Historical warning tones

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.

Current warning tones

Usage in the United States

Federal Signal SD-10.
, the most common tones produces by sirens in the United States are "Alert (Steady)" "Attack (Wail)" and "High-Low (Hi-Lo)." Only a few siren models are capable of producing the hi-lo signal. Other tones include Chimes, Whoop, Pulse, Air Horn, Hazard, & Scream.

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. In towns by the ocean, the "Alert" may also be used to warn of a tsunami.

The "Attack" tone is the iconic siren of a air raid or nuclear attack and may also be used to warn of severe weather or tsunamis, 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 wail.

Finally, the "Hi-Lo" is a fire siren; this 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 or a tornado warning or a Hazardous Materials spill. The "Hi-Lo" tone is a continuous alternating higher pitched and lower pitched sound, similar to police sirens in Europe.

Many fire departments in rural areas use sirens to alert volunteers of fire calls. In some instances, such as in the dry areas of the western states, residents may be required to shut off outdoor water systems to ensure adequate pressure at fire hydrants upon hearing the signal. There is no standardized fire signal in the United States, and it can vary from one community to another. Three long blasts on a siren is one common signal, which is also used in Germany and other countries as a standard fire signal. Other locales use the hi-lo signal described above, and some communities (particularly in New England and northern California) make use of coded blasts from alarm boxes on the street over a diaphone or air horn for fire signals, reserving the use of sirens for more serious situations. Some communities make use of an "all clear" signal, or sound separate signals for fire calls and ambulance runs.

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. They may also be used for Nuclear Power Plant Emergencies.

Also in Pierce County, Washington there is a system of sirens installed along the Puyallup and Carbon River Valleys to warn residents of volcanic eruptions and lahars (giant mudslides) from Mt. Rainier.

In Hawaiimarker, sirens are used to give warning to an approaching hurricane or tsunami.

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, Reverse 911, Cable Override and wireless alerting systems in the United Statesmarker and the Emergency Public Warning System in the Canadian province of Albertamarker. 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.

Siren installations themselves have many ways of being activated. Commonly used are DTMF broadcasts over phone lines (direct connection or standard PSTN) or over radio broadcast. This does leave room for exploitation, but there are protections from false alarms. These sirens can also be tied into other networks such as a fire departments volunteer notification/paging system. The basics of this type of installation would be a device (possibly the same pager the firefighters have) connected to the controller/timer system of the siren. When a page is received, the siren is activated.

Types of sirens

Mechanical sirens (direct driven)

Watch an ACA Allertor 125 in "Alert" and "Attack" modes
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 Federal Signal 2T22, 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 isthe same in every direction at all times, it is described asomnidirectional. The Federal 2T22 was also marketed in a 3-signal configuration known as the Federal 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. 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 series. Introduced in the late 80s, it is capable of battery backup. There are two separate motors in the 2001. One powers the siren rotor, while another powers the rotator mechanism.

One rare type of mechanical siren does not rotate or produce equal sound output in all directions. The Federal Signal RSH-10 "Thunderbeam" siren employs a slowly rotating angled 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

Watch a Federal Signal Thunderbolt 1003 in alert mode
The Federal Signal Thunderbolt series is arguably 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" .

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 primarily used as a fire signal.

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

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.

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 horn loudspeaker typically incorporating one or a multiple of compression drivers.

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 certain directions.

An example of a rotating electronic siren is the Whelen Engineering "Vortex", American Signal Alertronic RE1600 and Federal Signal Siratone 408, 612, & 812, 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, Whelen Engineering WPS2700, WPS2800, and WPS2900, 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 HPSS series, and Federal Signal DSA series.

Defunct manufacturers

  • CLM Industries – Maker of Toronto 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, although in real life Chicago used Federal Thunderbolt sirens. Toronto has removed the majority of these sirens.

  • SoundMaster Signals Inc. – was a siren company that manufactured sirens similar to Alerting Communications of America (ACA), which then became American Signal. SoundMaster started producing sirens based on plans designed by ACA and sold them to municipalities across the midwest. The company fell apart when owner/founder Jim Morgan died in the early 1970s. The addition of the motors these sirens used not being built anymore, due to icing defects, also added on to the company's demise. Very few SoundMaster sirens still exist. They made 5 different kinds of outdoor sirens. SoundMasters were once numerous in Wisconsin, and throughout the midwest at one point in time. Iron Ridge, WI and Browntown, WI have operating rigs.

  • Fosters Engineering- A Newark, New Jerseymarker Company, Made sirens in the 1940s. Every city that ordered from the company, would get 19 sirens for a system.

  • Clayton - A Daytona Beachmarker, Florida, siren manufacturer that made the Clayton Whirlwind.

  • Harrington - Makers of a 2V8 siren used to fill in "gaps" in siren systems in the Northeastern United States.

  • CTI Siren - Oklahomamarker based siren manufacturer. Three models are known to exist and can be found in Oklahoma.

  • Toshiba - The company is still in business, but, they do not manufacture their unusual omni directional siren anymore.

  • Decot - A Wisconsin-based manufacturer. They mainly produced fire sirens as early as 1910's. They also produced CD Sirens in their later days. Defunct around 1940's to 1950's, they invented one of the first dual tone siren, around the 1910s.

  • Erick - A Minnesota-based manufacturer started in the 1920s and liquidated around the 1950s. Although the sirens were intended for fire dept. use, they could've easily been used to warn of air raids and nuclear attacks. These types can be found in the Midwest.

  • Hendie And Bolthoff mfg and Supply Company - A Denver, Coloradomarker based company that produced mining supplies and then entered the business of making municipal goods. They produced the "Denver Siren", an unusually massive siren with up to 4 stator/rotors on a shaft protruding from a 3-phase, 220/440 volt, 5-h.p. Westinghouse Electric type CSP Induction motor. Dual stator/rotor sirens where known as Denver "Duplex" Sirens, mfg name M-210 (2 rotors 10 ports each) and quad stator/rotors where known as Denver "Quadplex" Sirens but there mfg. name was the M-410 (4 rotors, 10 ports each). Most were horizontal but few were vertical. Some also had two stator/rotors on opposite sides of the motor much like a Castle Castings siren. They produced sirens from the early 1910s to the mid 1940-50's. Most were mainly used for flood, fire and tornado warnings, yet some were used through the Cold War. These are very rarely seen though Michigan seems to have quite a few still in use. Cities still using these sirens include Capac, MI; Grand Ledge, MI; Kalamazoo, MI; Bellevue, MI (moved to new location in 90's when water tower was replaced); Holdon, MO; Lebanon, TN; Ponchatoula, LA, Lockhart, Texas;. Many more sirens are still out there, waiting to be found. Several other variations where produced, such as the extremly rare M-610, M-810, and a prototype M-1210, and M1410 which was powerd by a 10 hp motor, and weighed anywhere from 2-3 tons.

  • W.S. Darley and co. - Made sirens using Federal internals, and their own custom based casings. This went on until around the 80s, when they started selling spare ACA sirens. They now sell Sentry Siren sirens. The Darley Champion was one of the best selling for its time period, and resembled a Sterling Little Giant

  • Sparton - A somewhat unknown manufacturer of 2-HP 4-port sirens. Their sirens were very compact in size, and were mainly seen around fire departments.

Civil Defense sirens around the world

One of the many public address systems setup in Sydney, Australia as a result of APEC Australia 2007.
Manufactured by Bosch.


A series of 98 electronic sirens making up a large scale public address system (the "Sydney CBD Emergency Warning System", also including 13 variable-message signs) are installed in the Sydney central business district. While installed in the months preceding the 2007 APEC conference, they are designed as a permanent fixture and are tested on a monthly basis.

Aside from this, there are also usually small scale sirens, like the Model 5 or Model A which are found at fire stations for call-outs. These are also used at Sydney's beaches for shark alarms. When the alarm goes off, everyone must leave the water with no delay. Alarms are also used around prisons for break-outs.


In Canada a nation-wide network of sirens was established in the 1950s to warn urban populations of a possible Soviet nuclear attack. The system was maintained until the 1970s, when advancements in military technology reduced the Soviet nuclear missile strike time from 3–5 hours to less than 15 minutes. Sirens can still be found in many Canadian cities, all in various state of repair. In Toronto, for instance, the network has been abandoned to the point that no level of government will take responsibility for its ownership.

Sirens have recently been built within 3 kilometers of the Darlington and Pickering nuclear power plants in the province of Ontario. (Both plants are within 30 kilometers of each other.) These sirens will sound in the event of a nuclear emergency that could result in a release of radioactivity. There are also many sirens in Sarnia, Ontario because of local chemical plants that are tested weekly.


In France the emergency population warning network is called the "Réseau national d'alerte" (RNA). The system is inherited from the air-raid siren network (défense passive) developed before the World War II. It consists of about 4,500 electronic or electromechanical sirens placed all over France.

The system is tested every month, the first Wednesday at 12 noon.

In small municipalities hosting a centre for fire-fighters first intervention, the siren is also sometimes calling volunteer fire-fighters when their intervention is required, but the signal is different, and the system is increasingly being replaced by pagers.


Mumbaimarker has around 450 sirens, located in all parts of the city. Some 200-250 are still functional. The government is thinking of repairing sirens, and installing new ones.

In Mumbai civil defence, sirens were used during the Indo-Pakistani War of 1971, warning civilans about air raids by the Pakistan Air Force. At night, sirens were also used to indicate blackout, when all lights in Mumbai were switched off. The sirens are tested every day at 9 am and at 5 pm. They are controlled by the Indian Defence Services. Sirens are also use to denote a minute's silence at special occasions.


Air raid sirens used are typically sounded to warn of air raids or missile attacks on civilian population. The "all-clear" signal is used three times yearly to denote a moment of silence (of one or two minutes), once on Israel's Holocaust Remembrance Day and twice on Day of Remembrance, and, in a few religious neighborhoods, to announce the arrival of candle-lighting times for Shabbat or religious holidays. Most of the sirens in urban areas are German-made HLS (supercharged) sirens, model F71. The Air Raid Sirens are called אזעקה ("Az'aka", litterally "alarm"), and consist of a continuous, ascending and descending tone. The "all-clear" signal (called צפירת ארגעה, "Tzfirat Arga'ah") is a continuous single-pitch sound. However, in recent conflicts use of the "all-clear" signal has been discontinued, as it was seen as causing needless confusion and alarm.

The Netherlands

The Netherlandsmarker tests its air raid sirens once a month, every first Monday at 12:00, to keep the public aware of the system. There are about 4,200 sirens placed all across the country.


Switzerlandmarker currently has 7750 mobile and stationary civil defence sirens, which can alert 99% of the population. There are also 750 sirens located near dams. Every year, on the first Wednesday of February Switzerland's sirens are tested. During this test, general alert sirens as well as the sirens near dams are tested to see if they are in working order. The Population is informed of the test in the days before it by radio, television, teletext and newspapers. The siren tests do not require the population to take any special measures.

Swiss Alarm Signals

The tones of the different sirens is provided on the last page of all phone books as well as on the Internet.

  • The general alert siren goes off when there is a possible threat to the population. Sirens for this alert have a regularly ascending and descending tone lasting a minute and repeated after a two-minute interval. The population is instructed to inform those around them to proceed inside. Once inside, people are instructed to listen to emergency broadcasts made by the DRS, RSR, RTSI and/or RTR.

  • Flood alerts consist of twelve low continuous tones each lasting 20 seconds. The flood alert is activated once the general siren is sounding. If heard by the population in danger zones (near dams, therefore) they must leave the danger area immediately or find shelter.

United Kingdom

Every village, town and city in the United Kingdom used to have a network of Carter sirens to warn of incoming air-raids during World War II, and latterly as sirens for nuclear attack during the cold war. With the end of the cold war towards the end of the 20th century, the siren network was decommissioned in 1993 and very few remain.

Where they do remain, they are mostly sounded on receipt of a Severe Flood Warning for an area from the Environment Agency. Sirens are also used for public warning near gas or Nuclear power plants, Nuclear submarine bases, Oil refineries and Chemical plants. They consist of about 1200 sirens, a mix of older motor driven sirens usually from World War II, such as the Carter siren manufactured by Gents of Leicester, and the Cold War and newer electronic sirens. They are located in almost every village, town or city that is near the sea or has a river running through it. They are tested once yearly between the months of August and September.

United States

In the United States, sirens are usually used to warn of impending natural disasters, as well as threats of military attacks, which in the United States are rare. Throughout the Great Plains, Midwest, and South, they are used to notify the public of a tornado warning. They are usually also present in areas within a ten-mile radius of nuclear power plants. Such power plants that include sirens are North Anna (Domain), Three Mile Island (TMI), and Saint Lucie (FPL). Coastal communities, especially in northern Californiamarker, Oregonmarker, Washingtonmarker, Alaskamarker, and Hawaiimarker use siren systems to warn of incoming tsunamis. Also in Pierce County, Washington there is a system of sirens set up along the Puyallup and Carbon River valleys to warn residents of volcanic eruptions and lahars from Mt.marker Rainiermarker.

Many cities in the US periodically sound their sirens as a test, either weekly, monthly or yearly, at a day and hour set by each individual city.

Some volunteer fire departments within the United States in particularly rural areas use sirens to call all volunteers to assembly at the fire house. Some areas utilize their sirens as a last resort, relying more on cellular and paging technology, however some rural brigades are outside of the range of modern wireless communications and still must rely on sirens to activate the local volunteer brigade.


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