Civil defense siren
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A civil defense siren (also known as an air-raid siren or tornado siren) is a siren used to provide emergency population warning of approaching danger and sometimes to indicate when the danger has passed. Some (that are mostly located in small towns) are also used to call the volunteer fire department to go fight a fire. Initially designed to warn of air raids in World War II, they were adapted to warn of nuclear attack and of natural destructive weather patterns 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.
A mechanical siren generates sound by spinning a slotted chopper wheel to interrupt a stream of air at a regular rate. Modern sirens can develop a sound level of up to 135 decibels at 100 feet (30 m). The Chrysler air raid siren, driven by a 331-cubic-inch Chrysler Hemi gasoline engine, generates 138 dB at 100 feet away.
By use of varying tones or on/off patterns of sound, different alert conditions can be signaled. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.
- 1 Historical warning tones
- 2 Sirens in integrated public warning systems
- 3 Types of sirens
- 4 Civil defense sirens around the world
- 4.1 Asia
- 4.2 Europe
- 4.3 North America
- 4.4 Oceania
- 5 Defunct manufacturers
- 6 See also
- 7 References
- 8 External links
Historical warning tones
Many warning sirens have a sound that is made distinguishable from that used by emergency vehicles by use of two simultaneous tones, with pitches usually in a 5:6 frequency ratio (an untempered minor third).
In the United States, several sets of warning tones have been used which varied over time, by government structure, and by manufacturer. The initial alerts used during World War II were the Alert Signal (a 3–5 minute steady continuous siren tone), and the Attack Signal (a 3–5 minute wail siren tone, or series of short tone bursts on devices incapable of wavering, such as whistles). The Victory Siren manual stated that when manual generation of the warbling tone was required, it could be achieved by holding the Signal switch on for 8 seconds and off for 4 seconds. In 1950, the Federal Civil Defense Administration revised the signals, naming the alert signal "red alert" and adding an "all-clear" signal, defined as three one-minute steady blasts, with two minutes of silence between blasts.
Beginning in 1952, the Bell and Lights Air Raid Warning System, developed by AT&T, was made available to provide automated transmission of an expanded set of alert signals:
- Red Alert (attack imminent)
- Yellow Alert (attack likely)
- White Alert (all-clear)
- Blue Alert (undefined)
The Yellow Alert and Red Alert signals correspond to the earlier Alert Signal and Attack Signal, respectively, and the early Federal Signal AR timer siren control units featured the Take Cover button labeled with a red background, and the Alert button labeled with a yellow background. Later AF timers changed the color-coding, coloring the Alert button blue, the Take Cover button yellow, and the Fire button red (used to call out volunteer fire fighters), thus confusing the color-coding of the alerts. In 1955, the Federal Civil Defense Administration again revised the warning signals, altering them to adapt to deal with concern over nuclear fallout. The new set of signals were the Alert Signal (unchanged) and the Take-Cover Signal (previously the Attack Signal). The All-Clear signal was deleted because leaving a shelter while fallout was present would be hazardous.
During World War II Britain had two warning tones:
- Red Warning (Attack In Progress Or Imminent)
- All Clear (Attack Over)
These tones would be initialized by the Royal Observer Corps spotting Luftwaffe aircraft coming towards Britain, helped by coastal radar stations. The Attack Warning would be sounded when the Royal Observer Corps spotted enemy aircraft in the immediate area. The sirens were tested periodically. This was done by emitting the tones in reverse order, with the All Clear tone followed by the Red Warning tone. This ensured the public would not confuse the test with a real warning.
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 States and the National Public Alerting System, Alert Ready in Canada. 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
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 is the same in every direction at all times, it is described as omnidirectional. The Federal 2T22 was also marketed in a 3-signal configuration known as the Federal Signal 3T22, which had capabilities for a "hi-lo" signal. Some sirens, like the Federal Signal Thunderbolt series, had a blower so that more air could be pumped into the siren.
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 rare type of mechanical siren, the Federal Signal RSH-10 (a.k.a. "Thunderbeam") does not rotate or produce equal sound output in all directions but uses a slowly rotating angled disc below the siren, which directs the siren's output throughout 360 degrees. The chopper of this siren is taken from another one of Federal Signal's sirens, the STH-10.
The Federal Signal Thunderbolt series creates a distinct tone using a separate blower to force air through the rotor. Horns having an exponential profile amplify the sound. 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. Another example of a siren that has a separate blower is the Alerting Communicators of America (ACA) Hurricane.
A variation on the 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 a 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 (6,000 liter) air tank underground beside the machinery vault, instead of in the mast itself as in the earlier HLS units.
Electronic sirens consist of an electronic tone generator, a high-power amplifier, and a horn loudspeaker. Typically the loudspeaker unit incorporates horn loading, which causes them to be similar in appearance to some electromechanical sirens. Many of these loudspeakers incorporate a vertical array of horns to achieve pattern control in the vertical plane. Each cell of the loudspeaker horn is driven by one or more compression drivers. One type of compression driver for this type of loudspeaker handles 400 watts of electrical power and uses two donut-shaped permanent magnet slugs to provide magnetic 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 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 omnidirectional electronic sirens are the Federal Signal EOWS1212, Federal Signal Modulator series, Whelen Engineering WPS2700, WPS2800, and WPS2900,> ATI HPSS 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 omnidirectional sirens simply array directional re-entrant horn modules in all directions to produce a continuous coverage pattern, such as the ASC AL-8000 series, and Federal Signal DSA series.
Civil defense sirens around the world
The People's Republic of China
China has sirens located in most cities and towns, particularly those located in or near disputed territories. If the state declares a state of emergency due to attacks, invasion, or when there is a very high risk of military conflict, sirens will warn the public of possible attacks or invasion. The sirens are controlled by the People's Liberation Army.
In addition, Nanjing in China will blare air raid sirens at 10 a.m. annually on December 13, followed by a moment of silence, to commemorate the "Rape of Nanking" (see Nanking Massacre.) Same as other sirens, the sirens that only serve memorial purpose are controlled by the People's Liberation Army.
The air raid warning comes in 3 types:
- Pre-raid warning: signifies a likely air raid about to take place. Consists of a 36-sec high-tone followed by a 24-sec low-tone. 3 cycles as a period.
- Raid warning: signifies that the air raid is imminent. A 6-sec high-tone followed by a 6-sec low tone. 15 cycles as a period.
- Post-raid warning: signifies an end of a raid or a cool-down of the wartime situation. A single 3-minute high-tone as a period.
The Republic of China (Taiwan)
Taiwan has sirens that cover at least some metropolitan areas. The government holds annual air-raid drills. During the air raid siren, the populace is not supposed to walk on the streets, and cars must pull over for at least 30 minutes.
Mumbai has around 450 sirens, located in all parts of the city. Some 200–250 are still functional. The government is planning to change the system by incorporating the modern wireless/digital technology, in place of the present landline switching system.
In Mumbai civil defense, sirens were used during the Indo-Pakistani War of 1965, and 1971, warning civilians 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 were tested every day at 9 am. More recently, after the request from civil society, these sirens are tested once every month, and monitored by civil defense personnel deputed for the purpose . They are controlled by the Regional Civil Defense Control Center, Mumbai with inputs from Indian Defense Services. Sirens are also used to denote a minute's worth of 'silence' at special occasions.
Israel has more than 3,100 warning sirens and most of the sirens in urban areas are German-made HLS sirens, model F71 and ECN3000. All the other sirens are HPSS32 made by ATI Systems (Acoustic Technologies, Inc). During the early 2010s the mechanical sirens were gradually put out of use and replaced by electronic ones, although the mechanical ones were generally left standing. The air-raid sirens are called אזעקה ("az'aka", literally "alarm"), and consist of a continuous ascending and descending tone. The "all-clear" signal (called צפירת הרגעה, "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. In certain regions in the south of Israel, which regularly undergo rocket attacks from Gaza, a specialized system called Red Color is installed.
Singapore currently has a network of over 2,000 stationary sirens named the Public Warning System which warns the entire country of war air raids, man-made and natural disasters (except earth tremors). On the first day of every month Singapore's sirens are tested. During the test, the sirens sounds a light but cheerful chime instead of any of its three signals, which led to the population's speculations of the sirens reminding them of the start of each month. The sirens look very similar to the ECN3000 Israel version pictured on the upper right.
Nearly all towns and cities are equipped with civil defense sirens in case of either natural disasters or in case of missile attacks from North Korea. South Korea holds civil defense drills every month to prepare for such scenarios.
Austria is fully covered with an operational air-raid siren system consisting of 8,203 devices (10/2012). They are tested weekly at noon on Saturday with the "sirenenprobe" signal, a 15-second continuous tone. Every year on the first Saturday of October, the whole range of alarm signals (with exception of the fire alert) is sounded as a system test ("Zivilschutz-Probealarm") and to familiarize the population with the signals.
- Warning: a three-minute continuous tone. People are warned of an incoming danger and advised to tune in the appropriate Ö2 regional radio station or ORF 2 for further instruction.
- General alarm: a one-minute ascending and descending tone. Danger is imminent, people should seek shelter immediately and listen to radio or TV.
- End of danger: a one-minute continuous tone. The danger is over, shelters can be left. Any hazards potentially encountered during normal life are announced in the media.
- Fire alert: three 15-second continuous tones separated by seven-second intervals. All firemen of volunteer fire brigades should report to their fire station immediately. This signal is used less and less, as many fire brigades alert their members by use of pagers.
Belgium tests its air-raid sirens every first Thursday of the trimester. When the air-raid sirens are tested, the message "proefsignaal" or "signal d'essai" is pronounced every time the sirens work. There are 540 sirens placed all across the country.
1,078 electronic warning sirens are installed in Denmark. The sirens are placed on the top of buildings or on masts. This warning system makes it possible to warn the population in all urban areas with more than 1,000 inhabitants. This means that about 80% of Denmark's population can be warned by means of the stationary sirens. The remaining 20% will be warned by mobile sirens mounted on police cars. The function of the sirens is tested every night, without producing any sound. Once every year, on the first Wednesday of May at 12:00, the sirens will be tested with sound.
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 World War II. It consists of about 4,500 electronic or electromechanical sirens placed all over France. The system is tested each month at noon on the first Wednesday.
In Germany, the "Warnämter" (warning authorities) were closed in the 1990s after the Cold War threat no longer existed and the ability to alert the public was considered unnecessary. As the civil defense sirens were also frequently used to alert volunteer fire fighters, many sirens were sold to the municipalities for a symbolic price. Others were dismantled. In the 2000s it was realized that the ability to warn the public is not only necessary in cases of war, but also in events like natural disasters, chemical or nuclear accidents or terrorist attacks. As a result, some cities like Düsseldorf or Dresden began to rebuild their warning sirens. In Hamburg, the sirens are still operational. They should warn the public during storm surges. The majority of operational sirens in Germany are either electric-mechanical type E57 or electronic sirens.
During World War II, Berlins air raid sirens became known as "Meiers trumpets" or "Meiers hunting horns" by the city's residents due to Luftwaffe chief Hermann Goering's boast that "If a single bomb ever falls on Berlin, you can call me Meier!".
The Italian War Ministry began installing air raid sirens and issuing air defence regulations in 1938. Production was entrusted to company La Sonora, founded in 1911 and still active today.
During World War II every town had one, and several were present in each large city. Even after the danger of bombings had ended, they were kept in order to provide warning in case of any danger (e.g. high water in Venice).
In addition, the Protezione Civile (Civil Protection) operates sirens to warn the public in case of a threat to the citizen population. Protezione Civile also provides transport needs and military defense for the Government of Italy. These defence systems were put in place in the 1990s and are occasionally still used today.
The Netherlands tests its air-raid sirens once a month, every first Monday at noon, to keep the public aware of the system. There are about 4,200 sirens placed all across the country. In March 2015 it was announced that due to high maintenance costs the sirens will be taken out of service by the end of 2017. The government is implementing a cell-broadcast system, NL-Alert, to replace the sirens. During initial tests of the cell-broadcast alert: 12% received the alert, 22% configured their phone to receive it, but did not, 26% did not configure their cellphone and 40% did not have a phone or mobile-operator supporting the system.
Norway has about 1,250 operational sirens (mostly Kockums air horn units rather than motorized sirens), primarily located in cities. Three different signals are used.
The "air raid, take cover" signal is an intermittent signal for about a minute, the "all-clear" message is a continuous signal for about 30 seconds, and the "critical message, listen to radio" is 3 periods with three signals, separated by one minute between the periods. The critical-message signal is followed by a radio broadcast. The sirens are tested twice each year. At noon the second Wednesday of January and June. As of 2014 only the critical- message signal is used, but prior to this the signal tested in June would be "air raid" and "all clear." The latter two will no longer be used in peace time.
The critical-message signal is used in peace time to warn the population about major accidents, big fires and gas leaks.
In Romania, civil defense sirens have been used since the early 1930s. Originally, each street had a small siren on top of highrise buildings. Each siren could be powered mechanically. During World War II, the sirens had a single continuous tone, that could be heard to warn of an air strike.
Throughout the Cold War, bigger sirens manufactured locally have been installed on various public buildings and residences. The sirens were able to transmit a comprehensive variety of tones, each with a different meaning, such as a chemical disaster, an earthquake, a flood, an upcoming air/nuclear strike; each of these tones required the population to either move to high ground or an ABC shelter. An 'all clear' signal was played after the area has been deemed safe for the general public or decontaminated.
Since the 1990s, civil defense sirens have been replaced by electronic sirens and the procedure has been simplified. As of 2013, there are four playable tones; a natural disaster warning, an upcoming air/nuclear strike, an imminent air/nuclear strike, and an 'all clear' signal. Taking shelter is no longer a legal requirement, though ABC shelters are operational.
Three siren tones are used in the whole country:
- Warning: 2 minutes long steady tone. Used to warn of impending danger of a fire, environmental and other disaster or high water level.
- Immediate danger: 1 minute wailing tone. Warns of the danger of a major fire, flood, radiological and chemical danger, air raid.
- All clear: 30 second steady tone that marks testing of sirens every first Saturday each month at noon and the end of an emergency for which the immediate danger signal was sounded.
Municipalities of Brežice, Krško, Sevnica, Hrastnik and Trbovlje use a special signal for the immediate danger of an accident involving chlorine when there is a danger of chlorine leaks in the environment. The signal is 100 seconds long – it consists of 30 second wailing tone followed by 40 second steady tone and 30 second wailing tone. And municipalities of Muta, Vuzenica, Podvelka, Ribnica na Pohorju and Radlje ob Dravi use a 100-second signal – 4 second wailing followed by 4 seconds of silence for the immediate danger of flash floods used in the event of overflow or collapse of a hydroelectric dam.
At emergencies that impact two or more regions at the same time or the whole country, people are advised to listen to the first channel of Radio Slovenia, Val 202 or watch TV Slovenia 1/TV Slovenia 2. Emergencies of smaller extent are announced via regional radio and TV stations.
Few alarms for civil defense against bombing during the Spanish Civil War are preserved. The Guernica siren has a high symbolical value because the impact of the Bombing of Guernica. Barcelona City History Museum preserves one related to the Bombing of Barcelona. Another siren from civil war years is also preserved in Valencia
The Swedish alarm system uses outdoor sirens as well as information transmitted through radio and television. Special radio receivers for the purpose are handed out to residents living near nuclear power plants.
The outdoor signals used are as follows:
- Readiness alarm: 30 second tones with 15 second gaps. Used when imminent danger of war exists.
- Air raid alarm: Short tonebursts for a duration of one minute. The country will be or is being attacked from the sky.
- Important message to the public, general alarm: Seven second long tones with 14 second long gaps. Go inside, close windows and doors, close ventilation and listen to radio channel P4.
- All clear: Used for all above signals when danger is over. 30 second long single tone.
The outdoor sirens are tested 4 times per year, the first non-holiday Monday of March, June, September and December, at 15:00 local time. The test consists of the general alarm for two minutes, followed by a 90-second gap before the All clear is sounded.
Switzerland currently has 8,500 mobile and stationary civil defense sirens, which can alert 99% of the population. There are also 700 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.
The tones of the different sirens are 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 broadcasting networks SRF, RTS, RSI and RSR.
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 (such as near dams) they must leave the danger area immediately or find shelter.
Every village, town and city in the United Kingdom used to have a network of dual-tone sirens to warn of incoming air raids during World War II, the operation of the sirens was coordinated by a wire broadcast system via police stations, in towns and cities with a population of 3000+ power sirens were used, in rural areas hand operated sirens were used, which were later put to use as warnings for nuclear attack during the Cold War. With the end of the Cold War, the siren network was decommissioned in 1993 and very few remain. These sirens, mostly built by Carter, Gents, Castle Castings, and Klaxon Signal Co., have 10 and 12 ports to create a minor third interval (B♭ and D♭ notes) and are probably the world's most recognized World War II air-raid siren sound. In fact, recordings of British sirens are often dubbed into movies set in countries which never used this type of siren.
Where they do remain, they are mostly sounded on receipt of a severe flood warning for an area from the Environment Agency. Broadmoor Hospital has use of 13 sirens which are tested weekly. 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 Carter's of Nelson or the "syren" manufactured by Gent's of Leicester, and the Cold War and newer electronic sirens. They are tested once yearly between the months of August and September. The metal band Black Sabbath incorporated a siren into their song "War Pigs", though it was absent from the song when it was first known as "Walpurgis".
In Canada, a nationwide network of sirens was established in the 1950s to warn urban populations of a possible Soviet nuclear attack. This system was tested nationwide twice in 1961, under codenames Exercise Tocsin and "Tocsin B". 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 states 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. A handful of sirens still remain in Toronto in older established neighbourhoods:
- Princess and Bayview Avenue – Bayview Village Park
- Dundas Street West and Shaw Street
- York Quay, Harbourfront
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. Sirens have also been placed (and are tested weekly) in Sarnia, Ontario due to the large number of chemical plants in the vicinity.
Many warning sirens in provinces such as Ontario, Alberta and Saskatchewan are now used as tornado warning instruments. Smithers, British Columbia uses an old air raid siren as a noon-day whistle. One of the warning sirens was even used as a goal horn for the Quebec Nordiques between the mid 1980s and 1991.
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Sirens began to replace bells for municipal warning in the early 1900s, but became commonplace following America's entry into World War II. Most siren models of this time were single-tone models which often sounded almost an octave higher in pitch than their European counterparts. Dual-tone sirens became more common in the 1950s, but have been used in some areas since about 1915. During the Cold War, standard signals were used throughout the country for civil defense purposes, referred to as "alert" and "attack". Volunteer fire departments generally used a different siren signal. Many towns, especially in California and New England, used coded air horns or diaphones for fire calls and reserved sirens for civil defense use.
Today, signals are determined by state and local authorities and can vary from one region to another. The most common tones produced by sirens in the United States are "alert" (Steady) and "attack" (wail). Other tones include Westminster Chimes (commonly used for the testing of electronic sirens), Hi-Lo, Whoop, Pulse, Air Horn, and fast wail.
The U.S. federal standard regarding emergency warning signals is defined in FEMA's Outdoor Warning Systems Guide, CPG 1-17, published on 01-March-1980, which describes the Civil Defense Warning System (CDWS) and its warning signals. The language was slightly revised by FEMA's National Warning System Operations Manual, Manual 1550.2 published 2001-03-30:
- Attack Warning – A 3 to 5 minute wavering tone on sirens or a series of short blasts on horns or other devices. The Attack Warning signal means detection of an actual attack or accidental missile launch. Take protective action immediately. The Attack Warning will be repeated as often as deemed necessary by local government authorities to obtain the required response by the population, including taking protective action related to the arrival of fallout. This signal will have no other meaning and will be used for no other purpose. (However, sometimes the attack modes are used for tornado warnings.)
- Attention or Alert Warning – A 3 to 5 minute steady signal from sirens, horns, or other devices. Local government officials may authorize use of this signal to alert the public of peacetime emergencies. Besides any other meaning or requirement for action as determined by local government officials, the Attention or Alert signal will indicate to all persons in the United States, "Turn on your radio or television and listen for essential emergency information."
- A third distinctive signal may be used for other purposes, such as a local fire signal.
- No all-clear signal is defined by either document.
The most common tone, "alert", is widely used by municipalities to warn citizens of impending severe weather, particularly tornadoes. This practice is nearly universal in the Midwest and parts of the deep South, where intense and fast moving thunderstorms that can produce tornadoes occur frequently. The sound of "alert" is a steady continuous note. In seaside towns, "alert" may also be used to warn of a tsunami. Sirens that rotate will have a rising-and-falling tone as the direction of the horn changes.
The "attack" tone is the rising-falling sound of an air raid or nuclear attack, frequently heard in war movies. It was once reserved for imminent enemy attack, but is today sometimes used to warn of severe weather, tsunamis, or even fire calls, depending on local ordinance.
There is no standard "fire" signal in the United States, and while the use of sirens by volunteer fire departments is still common, it is diminishing. 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. The fire signal can vary from one community to another. Three long blasts on a siren is one common signal, similar to the signal used by volunteer brigades in Germany and other countries. Other locales use the hi-lo signal described above, and some communities (particularly in New England and northern California) make use of coded blasts over a diaphone or air horn for fire signals, reserving the use of sirens for more serious situations. Some cities use the Attack tone as their fire call. Some communities make use of an "all-clear" signal, or sound separate signals for fire calls and ambulance runs. Some fire signals in the U.S. are often blasted at least once a day mostly at noon, to test the system (and are often referred to as "noon sirens" or "noon whistles").
CPG 1-17 recommends that a monthly test be conducted, consisting of the steady "attention" signal for no more than one minute, one minute of silence, followed by the "attack" signal for no more than one minute. A "growl test" signal is described by CPG 1-17, when a siren must be tested more than once a month. This is typically a 1-second burst of sound to verify the proper operation of the siren without causing a significant number of people to interpret the test as an actual alert. 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.
In the United States, there is no national level alert system. Normally, sirens are controlled on county or local level, or, in Hawaii, on state level. Sirens are usually used to warn of impending natural disasters. They are also used to warn of threats of military attacks, which in the United States are rare. Throughout the Great Plains, Midwest, and South, they are used to warn the public to take cover when a tornado warning is issued. They are generally required in areas within a ten-mile radius of nuclear power plants. In the South and East Coast (except from Texas, Maine, Florida and New Hampshire), they use sirens to inform people about approaching hurricanes. 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 (giant mudslides) from Mt. Rainier.
Coastal communities, especially in northern California, Oregon, Washington, Alaska, and Hawaii use siren systems to warn of incoming tsunamis. In 2011, the city of Honolulu created an "Adopt-A-Siren" website for its tsunami sirens. The site is modeled after Code for America's "Adopt-a-Hydrant", which helps volunteers in Boston sign up to shovel out fire hydrants after storms.
Some US volunteer fire departments, particularly in rural areas, use sirens to call volunteers to assemble at the fire house, but to a decreasing degree than in years past due to technological advancements. Some areas utilize their sirens as a last resort, relying more on cellular and paging technology; however, a decreasing number of rural brigades are outside the range of wireless communications and rely on sirens to activate the local volunteer brigade.
Many college campuses in the US, especially in the wake of the Virginia Tech shooting, have begun installing sirens to warn students in the event of dangerous incidents.
A series of 98 electronic sirens, making up a large-scale public-address system (the "Sydney CBD Emergency Warning System") and 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.
Smaller-scale sirens are also deployed, like the Model 5 or Model A, used at fire stations for call-outs and at Sydney's beaches for shark alarms. Alarms are also used around prisons for break-outs and at many factories and schools to announce start and finish times.
A siren is located at the Kwinana BP plant south of Perth, which is tested every Monday. It is used to evacuate the plant in case of an emergency and can be heard in Kwinana and certain parts of Rockingham. It can also be used to warn of severe weather and potential dangerous emergencies on the Kwinana Industrial Strip.
In South Australia a number of Country Fire Service stations have a mechanical siren on, or near the station. These are only activated when the brigade are responded to bushfire or grassfire events and for testing, they are not activated for every call out as they are used as public alert to bushfires.
In Victoria, many Country Fire Authority stations have a siren installed that are used to summon volunteers to an emergency callout, as well as consequently alerting the local community of brigade activity. Due to a variety of siren types in use across the state, there are two signals that are used in the state, which are differentiated by length:
- Emergency callout (siren sounds for no longer than 90 seconds); and,
- Community alert (siren sounds for no shorter than five minutes).
In Queensland Whelen Vortex 4 sirens have been installed as part of the Somerset Regional Council Flood Warning System. At nearby Grantham, a Whelen WPS2906 which features both warning tones and pre-recorded messages provides early warning in the event of flooding.
Lower Hutt, Napier, Wanganui, and the former Waitakere City area of Auckland each has a network of civil defense sirens. The networks in Lower Hutt and Napier are bolsted by fire sirens pulling double duty as civil defense sirens. Lower Hutt's network is further bolstered by selected industrial sirens pulling double duty as civil defence sirens. In the Western Bay of Plenty several fire sirens pull double duty as civil defense sirens and there is a dedicated civil defense siren at the Bay Park Raceway in Mount Maunganui; in the South Waikato the Tokoroa, Putaruru and Tirau fire sirens pull double duty as civil defense sirens and Tokoroa also has a dedicated civil defense siren; and Whangamata has two dedicated Civil defense sirens and the fire siren pulls double duty as a civil defense siren. In the years following the tsunamis of the Indian Ocean earthquake in 2004, Meerkat electronic sirens were installed in all populated areas of the west coast lower than 10 metres.
Warning sounds vary from area to area, including rising and falling notes and Morse code sirens. Communities with volunteer fire brigades use a continuous note on all sirens for civil defense, and a warbling siren on the fire station siren only for fire callouts. Civil defense uses a distinctive "sting" siren that is used by all radio stations nationwide, but is currently only used for civil defense sirens in Wanganui.
- CLM Industries – A Canadian company which produced warning sirens during the Cold War. They have a distinctive appearance, with one large horn and a smaller, rectangular horn above it. The smaller horn was connected to a rear rotor, added on to comply with Canadian regulations requiring a dual-tone siren for civil defense. 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. There was also a vertical version available, with only 2 known to still exist.
- 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 discontinuation of the particular motors used by SoundMaster, mainly due to icing defects, added to the company's demise. Very few SoundMaster sirens still exist. They made five different kinds of outdoor sirens, including a version in which a directional horn rotated around a stationary siren. SoundMasters were once numerous in Wisconsin, and throughout the midwest at one point in time. Iron Ridge and Browntown, Wisconsin, have operating units.
- Fosters Engineering – A Newark, New Jersey company, made steam sirens in the 1940s, which required a supply of steam or compressed air to operate. Every city that ordered from the company would get 19 sirens for a system.
- Clayton – A Daytona Beach, Florida, siren manufacturer that made the Clayton Whirlwind.
- Harrington – Makers of a 2V8 siren used to fill in gaps in siren systems in the north-eastern United States.
- CTI Siren – Oklahoma-based siren manufacturer. Three models are known to exist and can be found in Oklahoma.
- Toshiba – The company is still in business, but no longer manufactures its unusual omnidirectional, compressor-charged siren.
- Decot – A Wisconsin-based manufacturer of fire sirens as early as the 1910s. They also produced CD sirens in their later days. Defunct since the 1950s, they invented one of the first dual-tone sirens around the 1910s.
- Erick – A Minnesota-based manufacturer, started in the 1920s and liquidated around the 1950s. Although the sirens were intended for fire-department use, they were also suitable for warning of air raids and nuclear attacks. These types can be found in the Midwest.
- Hendrie and Bolthoff Manufacturing and Supply Co – A Denver, Colorado-based company that was famous for selling mining equipment through the early 20th century. They began supplying the local mines with tools and equipment, and later entered the business of producing municipal Supplies. They built the "Denver Siren", Pat. by William A. Box, a large siren that was sold by the Hendie & Bolthoff Mfg. and Supply Co. as early as 1915 until the early 1920s. These sirens used a centrifugal clutch connected to a 5 HP motor, usually General Electric. These distinctive sirens had one or two large, open-ended, double sided, 20-port rotors connected by a long shaft to the motor and mounted to a common horizontal frame. single head models were known as the "Duplex" and double head models were known as "Quadruplex". Two smaller models were also available, connected directly to a single phase motor, the small double 12 port Type A or the slightly larger double 20 port Type B. Sometime in the early 1920s, Sterling Fire Alarm of Rochester NY. Sold a variant of the Denver siren under the name "Sterling Siren Fire Alarm", and produced them for s short while. These models have a window-frame grill bolted to the stator, most likely to keep birds from nesting in the rotor. Due to their weight, (the largest being nearly 6 feet long and 2 feet wide, made completely of iron with bronze or aluminum rotors on the Sterling models grossing near 2000 lbs) they were usually mounted on top of high buildings rather than poles. An article from "Electrical Review and Western Electrician" once stated when tested, local official departments received calls from several miles away by people wondering what the cause of the noise was. Few of the sirens still exist and only a handful of these are still in use.
- W.S. Darley & Co. – Made sirens labeled "Darley Champion Siren". In the 1980s, Darley changed over to selling surplus ACA sirens. They currently sell Sentry sirens.
- Sparton Corporation – A Brookesville, Florida manufacturer  (formerly Sparks-Withington Co of Jackson, Michigan, whose engineers produced the first all-electric horn for cars in 1911.) Made 2-HP 4-port sirens. Their sirens were very compact in size, resembling a louvered vertical cylinder, and were mainly seen around fire departments.
- H.O.R. – A New York company that started around 1930. They made vertical sirens 3, 5, and 7.5 HP. They also produced the Super Sirex, a large, double-ended siren usually available in single tone, dual tone, and rotating versions with a 50 Hp. motor. Their dual-tone sirens had 7 and 10 ports, producing the infamous "Devil's interval" of a diminished fifth, or tritone. They went out of business by the 1950s.
- B&M Siren Mfg. Co. – A Los Angeles company that started out in 1913 making sirens for Police and Fire vehicle use. Later produced the B&M Firehouse Siren, a Siren that resembled a horizontal British siren with an unusual 12-14 port ratio. B&M Siren Mfg Co. is still in business today producing two models of vehicle siren: the Super-Chief and the Siro-Drift.
- Federal Signal - A global corporation located in Oak Brook, Illinois. Federal Signal is best known for its variety of emergency lighting, sirens, industrial equipment, and public safety solutions. Federal Signal was founded in Chicago, Illinois, in 1901 as Federal Electric Co. and by the 1950s, the company was manufacturing outdoor warning sirens, most notably the Thunderbolt series, primarily intended for warning of air raids or fallout during the Cold War.
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|Wikimedia Commons has media related to Civil defense siren.|
- Air Raid Sirens – outdoor warning siren website
- 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
- The Siren Archive – Over 1,000 siren photographs coupled with a few recordings from around the world
- The World's loudest and largest sirens ever
- "Tocsin B", Canada's dry run of its Nuclear Warning System from 1961 (CBC Radio News Special, Nov. 13, 1961)
- Toronto Star story about air raid sirens in Toronto.
- Michigan Civil Defense Museum
- Old German World War II Air Raid Siren YouTube
- British World War II Air Raid Siren with all-clear YouTube
- Federal Thunderbolt 1000T air raid siren in alert signal YouTube