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.
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 this Thunderbolt, 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.
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.
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.
These warnings were not used in the United States.
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 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 ("wavering").
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 or a tornado warning. 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 insure 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 on 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.
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.
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 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 "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.
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 .
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 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", American Signal Alertronic 4000 and Federal Signal Siratone 408, 612, & 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, 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 Sound Blaster series, and Federal Signal DSA series.
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.
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.
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.
The Netherlands tests its air raid sirens once a month, every first Monday at 12:00 pm. There are about 4,200 sirens placed all across the country.
Switzerland 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.
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.
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. They are usually also present in areas within a ten-mile radius of nuclear power plants. 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. There are towns that have their sirens go off daily at 12:00, and sometimes at other times as a local time signal. Coastal communities, especially in northern California, Oregon, Washington, Alaska, and Hawaii use siren systems to warn of incoming tsunamis.
Note: Chicago is home to Federal Signal, Milwaukee is home to American Signal and Cańon City, Colorado is home to Sentry Siren.