Loud noise-making alarm used to notify for emergency or warning
This article is about the alarm device. For other uses, see Siren.
Electronic Sirens Pavian by TelegrafiaA Whelen WPS electronic siren imported to Saudi Arabia by HSS Engineering for use as a Civil Defense siren.There are 8,200 alarm sirens for civil protection throughout Switzerland. They are tested once a year, on the first Wednesday in February. Sound sampleAn 1860s-era siren.
A siren is a loud noise-making device. Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks. Sirens are used on emergency service vehicles such as ambulances, police cars, and fire engines. There are two general types: mechanical and electronic.
Many fire sirens (used for summoning volunteer firefighters) serve double duty as tornado or civil defense sirens, alerting an entire community of impending danger. Most fire sirens are either mounted on the roof of a fire station or on a pole next to the fire station. Fire sirens can also be mounted on or near government buildings, on tall structures such as water towers, as well as in systems where several sirens are distributed around a town for better sound coverage. Most fire sirens are single tone and mechanically driven by electric motors with a rotor attached to the shaft. Some newer sirens are electronically driven speakers.
Fire sirens are often called fire whistles, fire alarms, or fire horns. Although there is no standard signaling of fire sirens, some utilize codes to inform firefighters of the location of the fire. Civil defense sirens also used as fire sirens often can produce an alternating "hi-lo" signal (similar to emergency vehicles in many European countries) as the fire signal, or attack (slow wail), typically 3x, as to not confuse the public with the standard civil defense signals of alert (steady tone) and fast wail (fast wavering tone). Fire sirens are often tested once a day at noon and are also called noon sirens or noon whistles.
The first emergency vehicles relied on a bell. In the 1970s, they switched to a duotone airhorn, which was itself overtaken in the 1980s by an electronic wail.
History
Some time before 1799, the siren was invented by the Scottish natural philosopher John Robison.[3] Robison's sirens were used as musical instruments; specifically, they powered some of the pipes in an organ. Robison's siren consisted of a stopcock that opened and closed a pneumatic tube. The stopcock was apparently driven by the rotation of a wheel.
In 1819, an improved siren was developed and named by Baron Charles Cagniard de la Tour.[4] De la Tour's siren consisted of two perforated disks that were mounted coaxially at the outlet of a pneumatic tube. One disk was stationary, while the other disk rotated. The rotating disk periodically interrupted the flow of air through the fixed disk, producing a tone.[5][6] De la Tour's siren could produce sound under water,[7] suggesting a link with the sirens of Greek mythology; hence the name he gave to the instrument.[8]
Instead of disks, most modern mechanical sirens use two concentric cylinders, which have slots parallel to their length. The inner cylinder rotates while the outer one remains stationary. As air under pressure flows out of the slots of the inner cylinder and then escapes through the slots of the outer cylinder, the flow is periodically interrupted, creating a tone.[9] The earliest such sirens were developed during 1877–1880 by James Douglass and George Slight (1859–1934)[10] of Trinity House; the final version was first installed in 1887 at the Ailsa Craig lighthouse in Scotland's Firth of Clyde.[11] When commercial electric power became available, sirens were no longer driven by external sources of compressed air, but by electric motors, which generated the necessary flow of air via a simple centrifugal fan, which was incorporated into the siren's inner cylinder.
To direct a siren's sound and to maximize its power output, a siren is often fitted with a horn, which transforms the high-pressure sound waves in the siren to lower-pressure sound waves in the open air.
The electrically powered cylinder siren used to alert the town of Lowestoft during World War Two.
The earliest way of summoning volunteer firemen to a fire was by ringing of a bell, either mounted atop the fire station, or in the belfry of a local church. As electricity became available, the first fire sirens were manufactured. In 1886 French electrical engineer Gustave Trouvé developed a siren to announce the silent arrival of his electric boats. Two early fire siren manufacturers were William A. Box Iron Works, who made the "Denver" sirens as early as 1905, and the Inter-State Machine Company (later the Sterling Siren Fire Alarm Company) who made the ubiquitous Model "M" electric siren, which was the first dual tone siren. The popularity of fire sirens took off by the 1920s, with many manufacturers including the Federal Electric Company and Decot Machine Works creating their own sirens. Since the 1970s, many communities have since deactivated their fire sirens as pagers became available for fire department use. Some sirens still remain as a backup to pager systems.
During the Second World War, the British civil defence used a network of sirens to alert the general population to the imminence of an air raid. A single tone denoted an "all clear". A series of tones denoted an air raid.
The pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a chopper, siren disk or rotor), such that the material between the holes interrupts a flow of air from fixed holes on the outside of the unit (called a stator). As the holes in the rotating disk alternately prevent and allow air to flow it results in alternating compressed and rarefied air pressure, i.e. sound. Such sirens can consume large amounts of energy. To reduce the energy consumption without losing sound volume, some designs of pneumatic sirens are boosted by forcing compressed air from a tank that can be refilled by a low powered compressor through the siren disk.
In United States English language usage, vehicular pneumatic sirens are sometimes referred to as mechanical or coaster sirens, to differentiate them from electronic devices. Mechanical sirens driven by an electric motor are often called "electromechanical". One example is the Q2B siren sold by Federal Signal Corporation. Because of its high current draw (100 amps when power is applied)[citation needed] its application is normally limited to fire apparatus, though it has seen increasing use on type IV ambulances and rescue-squad vehicles. Its distinct tone of urgency, high sound pressure level (123 dB at 10 feet) and square sound waves account for its effectiveness.
In Germany and some other European countries, the pneumatic two-tone (hi-lo) siren consists of two sets of air horns, one high pitched and the other low pitched. An air compressor blows the air into one set of horns, and then it automatically switches to the other set. As this back and forth switching occurs, the sound changes tones. Its sound power varies, but could get as high as approximately 125dB, depending on the compressor and the horns. Comparing with the mechanical sirens, it uses much less electricity but needs more maintenance.
In a pneumatic siren, the stator is the part which cuts off and reopens air as rotating blades of a chopper move past the port holes of the stator, generating sound. The pitch of the siren's sound is a function of the speed of the rotor and the number of holes in the stator. A siren with only one row of ports is called a single tone siren. A siren with two rows of ports is known as a dual tone siren. By placing a second stator over the main stator and attaching a solenoid to it, one can repeatedly close and open all of the stator ports thus creating a tone called a pulse. If this is done while the siren is wailing (rather than sounding a steady tone) then it is called a pulse wail. By doing this separately over each row of ports on a dual tone siren, one can alternately sound each of the two tones back and forth, creating a tone known as Hi/Lo. If this is done while the siren is wailing, it is called a Hi/Lo wail. This equipment can also do pulse or pulse wail. The ports can be opened and closed to send Morse code. A siren which can do both pulse and Morse code is known as a code siren.
Electronic
HLS electronic siren
Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, yelp, pierce/priority/phaser, hi-lo, scan, airhorn, manual, and a few more) which is played through external speakers. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens. Often, police sirens also use the interval of a tritone to help draw attention. The first electronic siren that mimicked the sound of a mechanical siren was invented in 1965 by Motorola employees Ronald H. Chapman and Charles W. Stephens.[12]
Steam whistles were also used as a warning device if a supply of steam was present, such as a sawmill or factory. These were common before fire sirens became widely available, particularly in the former Soviet Union. Fire horns, large compressed air horns, also were and still are used as an alternative to a fire siren. Many fire horn systems were wired to fire pull boxes that were located around a town, and this would "blast out" a code in respect to that box's location. For example, pull box number 233, when pulled, would trigger the fire horn to sound two blasts, followed by a pause, followed by three blasts, followed by a pause, followed by three more blasts. In the days before telephones, this was the only way firefighters would know the location of a fire. The coded blasts were usually repeated several times. This technology was also applied to many steam whistles as well. Some fire sirens are fitted with brakes and dampers, enabling them to sound out codes as well. These units tended to be unreliable, and are now uncommon.[citation needed]
Physics of the sound
Mechanical sirens blow air through a slotted disk or rotor. The cyclic waves of air pressure are the physical form of sound. In many sirens, a centrifugal blower and rotor are integrated into a single piece of material, spun by an electric motor.
Electronic sirens are high efficiency loudspeakers, with specialized amplifiers and tone generation. They usually imitate the sounds of mechanical sirens in order to be recognizable as sirens.
To improve the efficiency of the siren, it uses a relatively low frequency, usually several hundred hertz. Lower frequency sound waves go around corners and through holes better.
Sirens often use horns to aim the pressure waves. This uses the siren's energy more efficiently by aiming it. Exponential horns achieve similar efficiencies with less material.
The frequency, i.e. the cycles per second of the sound of a mechanical siren is controlled by the speed of its rotor, and the number of openings. The wailing of a mechanical siren occurs as the rotor speeds and slows. Wailing usually identifies an attack or urgent emergency.
The characteristic timbre or musical quality of a mechanical siren is caused because it is a triangle wave, when graphed as pressure over time. As the openings widen, the emitted pressure increases. As they close, it decreases. So, the characteristic frequency distribution of the sound has harmonics at odd (1, 3, 5...) multiples of the fundamental. The power of the harmonics roll off in an inverse square to their frequency. Distant sirens sound more "mellow" or "warmer" because their harsh high frequencies are absorbed by nearby objects.
Two tone sirens are often designed to emit a minor third, musically considered a "sad" sound. To do this, they have two rotors with different numbers of openings. The upper tone is produced by a rotor with a count of openings divisible by six. The lower tone's rotor has a count of openings divisible by five. Unlike an organ, a mechanical siren's minor third is almost always physical, not tempered. To achieve tempered ratios in a mechanical siren, the rotors must either be geared, run by different motors, or have very large numbers of openings. Electronic sirens can easily produce a tempered minor third.
A mechanical siren that can alternate between its tones uses solenoids to move rotary shutters that cut off the air supply to one rotor, then the other. This is often used to identify a fire warning.
When testing, a frightening sound is not desirable. So, electronic sirens then usually emit musical tones: Westminster chimes is common. Mechanical sirens sometimes self-test by "growling", i.e. operating at low speeds.
Governments may have standards for vehicle-mounted sirens. For example, in California, sirens are designated Class A or Class B. A Class A siren is loud enough that it can be mounted nearly anywhere on a vehicle. Class B sirens are not as loud and must be mounted on a plane parallel to the level roadway and parallel to the direction the vehicle travels when driving in a straight line.[15]
Sirens must also be approved by local agencies, in some cases. For example, the California Highway Patrol approves specific models for use on emergency vehicles in the state.[16] The approval is important because it ensures the devices perform adequately. Moreover, using unapproved devices could be a factor in determining fault if a collision occurs.[citation needed]
The SAE International Emergency Warning Lights and Devices committee oversees the SAE emergency vehicle lighting practices and the siren practice, J1849.[lower-alpha 1] This practice was updated through cooperation between the SAE and the National Institute of Standards and Technology. Though this version remains quite similar to the California Title 13 standard for sound output at various angles, this updated practice enables an acoustic laboratory to test a dual speaker siren system for compliant sound output.[17]
Siren speakers, or mechanical sirens, should always be mounted ahead of the passenger compartment. This reduces the noise for occupants and makes two-way radio and mobile telephone audio more intelligible during siren use. It also puts the sound where it will be useful. A 2007 study found passenger compartment sound levels could exceed 90dB(A).[18]
Research has shown that sirens mounted behind the engine grille or under the wheel arches produces less unwanted noise inside the passenger cabin and to the side and rear of the vehicle while maintaining noise levels to give adequate warnings.[19] The inclusion of broadband sound to sirens has the ability to increase localisation of sirens, as in a directional siren, as a spread of frequencies makes use of the three ways the brain detects a direction of a sound: Interaural level difference, interaural time difference and head-related transfer function.[20]
The worst installations are those where the siren sound is emitted above and slightly behind the vehicle occupants such as cases where a light-bar mounted speaker is used on a sedan or pickup. Vehicles with concealed sirens also tend to have high noise levels inside. In some cases, concealed or poor installations produce noise levels which can permanently damage vehicle occupants' hearing.[21]
Electric-motor-driven mechanical sirens may draw 50 to 200 amperes at 12 volts (DC) when spinning up to operating speed. Appropriate wiring and transient protection for engine control computers is a necessary part of an installation. Wiring should be similar in size to the wiring to the vehicle engine starter motor. Mechanical vehicle mounted devices usually have an electric brake, a solenoid that presses a friction pad against the siren rotor. When an emergency vehicle arrives on-scene or is cancelled en route, the operator can rapidly stop the siren.
Multi-speaker electronic sirens often are alleged to have dead spots at certain angles to the vehicle's direction of travel. These are caused by phase differences. The sound coming from the speaker array can phase cancel in some situations. This phase cancellation occurs at single frequencies, based upon the spacing of the speakers. These phase differences also account for increases, based upon the frequency and the speaker spacing. However, sirens are designed to sweep the frequency of their sound output, typically, no less than one octave. This sweeping minimizes the effects of phase cancellation. The result is that the average sound output from a dual speaker siren system is 3dB greater than a single speaker system.
↑ The dials atop the siren are connected, via reduction gears, to the perforated disks (in the cylinder beneath the dials) which produce the siren's sound. The dials allow the siren's frequency to be determined. During the 19th century, sirens were among the few sources of sound having a known frequency. Hence they were used in research on hearing and sound.
John Robison, Encyclopædia Britannica, 3rd ed., 1799.
"Temperament of the scale of music" in: John Robison with David Brewster and James Watt, ed.s, A System of Mechanical Philosophy (Edinburgh, Scotland: 1822), vol. 4, pages 404-405Archived 2022-11-13 at the Wayback Machine .
Ernst Robel, Die Sirenen: Ein Beitrag zur Entwickelungsgeschichte der Akustik [Sirens: A contribution to the history of the development of acoustics] (Berlin, Germany: R. Gaertners, 1891), part 1, pages 7–10Archived 2022-11-13 at the Wayback Machine .
↑ For descriptions of Robison's and de la Tour's sirens, see:
Robert T. Beyer, Sounds of Our Times: Two Hundred Years of Acoustics (New York: Springer Verlag, 1998), page 30Archived 2022-11-13 at the Wayback Machine . De la Tour's siren is illustrated on page 31.
De la Tour's siren is also illustrated on page 12Archived 2022-11-13 at the Wayback Machine of: Hermann von Helmholtz, On the Sensations of Tone as a Physiological Basis for the Theory of Music, 3rd ed. (London, England: Longmans, Green, and Co., 1875). (Reprinted in 1954 by Dover Publishing Inc. of N.Y., N.Y.)
↑ The rotating disk of such a siren is driven by air pressure alone: The holes in each disk are not drilled perpendicularly to the disk. Instead, the holes slope in a clockwise direction in one disk and counterclockwise in the other. To escape, the flowing air must therefore change direction sharply, driving the rotating disk like a turbine. See:
Robert T. Beyer, Sounds of Our Times: Two Hundred Years of Acoustics (New York: Springer Verlag, 1998), page 30.
See also: Michael Lamm, "Feel the noise: The art and science of making sound alarming," Invention and Technology Magazine, vol. 18, no. 3, pages 22–27 (Winter 2003). (Lamm's article is available on-line at: American HeritageArchived 2008-02-18 at the Wayback Machine .)
Adolphe Ganot (translator: Edmund Atkinson), Elementary Treatise on Physics: Experimental and Applied, 8th ed. (N.Y., N.Y.: Wm. Wood and Co., 1877), pages 188-189.
J.H. Poynting and J.J. Thomson, Sound (London: Charles Griffin and Co., 1899), page 37Archived 2022-11-13 at the Wayback Machine .
↑ de la Tour (1819) "Sur la Sirène, nouvelle machine d'acoustique destinée à mésures les vibrations de l'air qui contient la son," Annales de chimie et de physique, vol. 12, page 171Archived 2015-10-15 at the Wayback Machine :
Original: "Si l'on fait passer de l'eau dans la sirène, au lieu d'air, elle produit également le son, lors même qu'elle est entièrement immergée dans ce fluide, et les mêmes nombres de chocs produisent les mêmes nombre de notes par l'air. C'est à cause de cette propriété d'être sonore dans l'eau, que j'ai cru pouvoir lui donner le nom sous lequel elle est désignée."
Translation: If one runs water through the siren instead of air, it still produces sound even though it is fully immersed in this fluid, and the same number of shocks produce the same number of audible vibrations as in air. It is because of this property of making sound in water that I thought I could give it the name by which it is designated.
↑ Some sirens have two pairs of slotted cylinders, allowing such a siren to produce two tones having a musical interval of a minor or a major third.
↑ Alan Renton, Lost Sounds: The Story of Coast Fog Signals (Latheronwheel, Scotland: Whittles Publishing, 2001), page 51Archived 2022-11-13 at the Wayback Machine . For a brief biography of George Slight, see the Spanish Wikipedia article "George Slight" (in Spanish).
Frederick A. Talbot, Lighthouses and Lightships (Philadelphia, Pennsylvania: J. B. Lippincott & Co., 1913), page 62Archived 2022-11-13 at the Wayback Machine .
Wayne Wheeler, "The history of fog signals – part 1," The Keeper's Log, vol. 6, no. 4, pages 20–23 (Summer 1990) and "The history of fog signals – part 2," The Keeper's Log, vol. 7, no.1, pages 8–17 (Fall 1990). See especially page 11 of part 2.
Brian Clearman, Transportation-markings: A Historical Survey, 1750–2000 (St. Benedict, Oregon: Mount Angel Abbey, 2002), pages 170-171. Available on-line at: University of OregonArchived 2016-03-03 at the Wayback Machine .
A civil defense siren, also known as an air-raid siren or tornado siren, is a siren used to provide an emergency population warning to the general population of approaching danger. It is sometimes sounded again to indicate the danger has passed. Some sirens, especially within small municipalities, are also used to alert the fire department when needed. 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.
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors, which is typically contrasted with rotor. Energy flows through a stator to or from the rotating component of the system. In an electric motor, the stator provides a magnetic field that drives the rotating armature; in a generator, the stator converts the rotating magnetic field to electric current. In fluid powered devices, the stator guides the flow of fluid to or from the rotating part of the system.
An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar, an electric bass, electric organ, electric piano, synthesizers and drum machine to convert the signal from the pickup or other sound source into an electronic signal that has enough power, due to being routed through a power amplifier, capable of driving one or more loudspeaker that can be heard by the performers and audience.
A foghorn or fog signal is a device that uses sound to warn vehicles of navigational hazards such as rocky coastlines, or boats of the presence of other vessels, in foggy conditions. The term is most often used in relation to marine transport. When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to shipping.
A police car is a ground vehicle used by police and law enforcement for transportation during patrols and responses to calls for service. A type of emergency vehicle, police cars are used by police officers to patrol a beat, quickly reach incident scenes, and transport and temporarily detain suspects, all while establishing a police presence and providing visible crime deterrence.
An air horn is a pneumatic device designed to create an extremely loud noise for signaling purposes. It usually consists of a source which produces compressed air, which passes into a horn through a reed or diaphragm. The stream of air causes the reed or diaphragm to vibrate, creating sound waves, then the horn amplifies the sound making it louder. Air horns are widely employed as vehicle horns, installed on large buses, semi-trailer trucks, fire trucks, trains, and some ambulances as a warning device, and on ships as a signaling device.
Emergency vehicle equipment is any equipment fitted to, or carried by, an emergency vehicle, other than the equipment that a standard non-emergency vehicle is fitted with.
A siren disk is used in pneumatic sirens and has holes which are variously spaced apart. When the disk is spun in front of a jet of air, the holes modulate the air-jet which produces a sound. The pitch of a siren is produced by "the frequency of the impulses of compressed air passing through the openings in a rotating disk." The pitch is therefore determined by the speed at which the disk rotates, the number of holes which air passes through, the size of the holes and their spacing apart.
A swashplate, also known as slant disk, is a mechanical engineering device used to translate the motion of a rotating shaft into reciprocating motion, or vice versa. The working principle is similar to crankshaft, Scotch yoke, or wobble/nutator/Z-crank drives, in engine designs. It was originally invented to replace a crankshaft, and is one of the most popular concepts used in crankless engines. It was invented by Anthony Michell in 1917.
The Fender Vibratone was a Leslie speaker designed for use with electric guitars, manufactured by Fender from 1967-1972. Named after the first Leslie speaker made for the Hammond Organ in 1941, the Vibratone was associated with the electric guitar, although it was used in vocals on many famous songs. The Vibratone was essentially an equivalent of the Leslie 16. A prime example of the Vibratone's sound is on the song "Cold Shot" by Stevie Ray Vaughan.
The Q2B siren ("Q-siren") is an electromechanical siren that is produced by the Federal Signal Corporation and is most recognizable for its sound, which is trademarked, and the look of the siren.
Federal Signal Modulators are electronic warning devices produced by Federal Signal Corporation that are used to alert the public about tornadoes, severe weather, earthquakes, fires, lahars, tsunamis, or any other disaster. They are identified mostly by their distinctive stacked "flying saucer" design. The Modulator II is sold based on the more compact chassis of the siren compared to the original Modulators.
SiraTone was a brand of electronic outdoor warning sirens produced by Federal Signal Corporation which began production in the early 1980s. These sirens were designed to broadcast high-intensity warning signals over a large area. SiraTone products are/were used for natural disaster notification, HAZMAT incident notification, fire call systems, and more. The SiraTone product line is no longer in production. The product line was succeeded by the Modulator and DSA electronic outdoor warning sirens.
The ATI Soundblaster HPSS is an electronic outdoor warning siren manufactured by Acoustic Technology, Inc that is used to alert citizens of tornadoes, hurricanes, severe thunderstorms, fires and so on. The recent HPSS systems use high-powered electronic speakers, which can be independently arranged to provide directional or omnidirectional coverage, depending on installation. The HPSS16 have 4 speakers and the HPSS32 have 8 speakers. They can also be used as public announcement (PA) systems, and can play a number of programmed tones. Options include solar-powered operation pre-recorded voice messages. ATI also sold rotational units of the HPSS16, and HPSS32, called the "HPSS16R" and "HPSS32R", that are now discontinued.
A vehicle horn is a sound-making device installed on motor vehicles, trains, boats, and other types of vehicles. The horn is activated to warn others of the vehicle's presence or approach, or to call attention to some hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Trams, trollies, streetcars, and even bicycles are also legally required to have an audible warning device in many areas.
The Federal Signal 3T22 / 2T22 was a dual tone, mechanical outdoor warning siren made by Federal Signal Corporation from 1952 through the early 1990s. It has a very recognizable design, having a ten-port rotor (chopper) on the bottom with ten cones (horns) and a 12-port one on top with twelve cones.
A firefighting apparatus or firefighting appliance describes any vehicle that has been customized for use during firefighting operations. These vehicles are highly customized depending on their needs and the duty they will be performing. These duties can include firefighting and emergency medical services.
Canadian Line Materials LTD. (CLM) was a Canadian siren manufacturing company which built civil defense sirens. These sirens were built and installed all over Canada to warn residents and military about incoming nuclear attacks from the USSR during the Cold War.
In engineering, a solenoid is a device that converts electrical energy to mechanical energy, using an electromagnet formed from a coil of wire. The device creates a magnetic field from electric current, and uses the magnetic field to create linear motion. In electromagnetic technology, a solenoid is an actuator assembly with a sliding ferromagnetic plunger inside the coil. Without power, the plunger extends for part of its length outside the coil; applying power pulls the plunger into the coil. Electromagnets with fixed cores are not considered solenoids. In simple terms, a solenoid converts electrical energy into mechanical work. Typically, it has a multiturn coil of magnet wire surrounded by a frame, which is also a magnetic flux carrier to enhance its efficiency. In engineering, the term may also refer to a variety of transducer devices that convert energy into linear motion, more sophisticated than simple two–position actuators. The term "solenoid" also often refers to a solenoid valve, an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid or linear solenoid. Solenoid bolts, a type of electromechanical locking mechanism, also exist.
The Thunderbolt is a discontinued mechanical outdoor warning siren made by Federal Signal Corporation. It has a very recognizable design, being a unidirectional siren with a large yellow exponential horn.
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