Civil defense siren
The civil defense siren is a form of siren used to warn civilians of approaching danger - a form of Emergency population warning. Initially designed to warn city dwellers of air raids during World War II, they were later used to warn of nuclear attack and natural disasters, such as tornadoes. The generalized nature of sirens led to many of them being replaced with more specific warnings, such as the broadcast-based Emergency Alert System and the Cell Broadcast-based Wireless Emergency Alerts and EU-Alert mobile technologies.
By use of varying tones or binary patterns of sound, different alert conditions can be called. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.
Integrated public warning systems
Sirens are sometimes integrated into a 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 fluid approach enhances the credibility of warnings and reduces the risk of assumed false alarms by corroborating warning messages through multiple forms of media. The Common Alerting Protocol is a technical standard for this sort of multi-system integration.Siren installations have many ways of being activated. Commonly used methods are dual-tone multi-frequency signaling or public switched telephone network using telephone lines, but activation can also be done via radio broadcast. This method opens up vulnerability for exploitation, but there are protections from false alarms. These sirens can also be tied into other networks such as a fire department's volunteer notification/paging system. The basics of this type of installation would consist of a device connected to the controller/timer system of the siren. When a page is received, the siren is activated.
Types
Mechanical
Basic design
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 Signal 2T22 was also marketed in a three-signal configuration known as the Federal Signal 3T22, with the capability for a high–low 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 through 360 degrees of rotation. One rare type of mechanical siren, the Federal Signal RSH-10, does not rotate or produce equal sound output in all directions. It instead uses a slowly rotating angled disc below the siren which directs the siren's output throughout 360 degrees.
Supercharged
A variation of the electromechanical siren is a supercharged siren. A supercharged siren uses a separate source, usually a supercharger-like blower, which forces air into the rotor assembly of the siren. This increases the air pressure in the rotor assembly, causing the sound output of the siren to increase heavily, which in return increases the sound range by a large amount. The blower is generally driven by an electric motor, but in rare cases it is driven by an engine. Federal Signal took advantage of this design and created their Thunderbolt Siren Series, utilizing Sutorbilt Roots Blowers of different varieties and outputs.A very early model was the Thunderbolt 2000. The difference between the Thunderbolt 2000 and later editions is that its blower is driven by an Onan two cylinder gas engine. Another example of a siren that has a separate blower is the Alerting Communicators of America Hurricane. One more example of a siren with a blower is the SoCal Edison Model 120, utilizing a Centrifugal Style Blower, built specifically for the San Onofre Nuclear Generating Station. The SoCal Edison Model 120 is no longer standing out in public, as only one exists, and is owned privately.
Pneumatic
Another variation on the electromechanical siren is the pneumatic siren. Similar to supercharged sirens, pneumatic sirens also force air into the rotor assembly of the siren. However, these sirens use a pressurized air reservoir instead of a motor-driven blower. hochleistungssirene, first 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 placed the large air tank underground beside the machinery vault, instead of in the mast itself as in the earlier HLS units.Electronic
Electronic sirens consist of an electronic tone generator, a high-power amplifier, and a horn loudspeaker. Typically, the loudspeaker unit incorporates horn loading, causing 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 uses two doughnut-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. Most newer electronic sirens have the ability to store digital audio files. These audio files could be custom sounds, or emergency messages. Depending on the situation, the stored sound file can broadcast through the siren. These sirens could also come with a Microphone Jack to broadcast live messages via microphone.As with electromechanical sirens, there are both omnidirectional, directional, and rotating categories. 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. These sirens can also be set to rotate any amount from 0 to 360 degrees, allowing sirens to broadcast only in certain directions.
Electronic sirens may be mechanically rotated to cover a wide area, or may have transducers facing in all directions to make an omnidirectional pattern. A directional siren may be applied where notification is only required for a defined area in one direction.
Around the world
Middle East
Iran, Kuwait, and Iraq
Kuwait has an entire siren system of Federal Signal Modulators. They once had an entire system of Thunderbolt system 7000's, until they all got replaced. However, one System 7000 remains on display.Iran & Iraq, since the 80's, they got a whole system of the Pneumatic sirens made by Hörmann, specifically, the Hörmann HLS 273, and Iraq replaced them for Comtel manufactured sirens
Israel
Israel has more than 3,100 warning sirens. Most of the sirens in urban areas are German-made HLS sirens, models F71, 273 and ECN3000. All the other sirens are HPSS32 models made by Acoustic Technologies and the Federal Signal Modulator 6048. During the early 2010s, mechanical sirens were gradually phased out and replaced by electronic ones, although the mechanical ones were generally left standing. The air-raid sirens are called אזעקה az'aka, 'alarm', and consist of a continuous ascending and descending tone. The all clear signal, called צפירת הרגעה tzfirat harga'a, is a constant single-pitch sound. In recent conflicts, use of the all clear signal has been discontinued, as it was seen as causing unnecessary 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 used.There is an earthquake warning system in Israel, which uses the sirens, called Trua.
The all clear signal is used three times per year to denote a moment of silence : once on Israel's Holocaust Remembrance Day and twice on the Day of Remembrance.