A siren is a noise-making warning device. There are two general types: mechanical sirens and electronic sirens. 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.
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 or most railroad crossings in Indonesia) 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.See: 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 404–405 . 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 7–10 . 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.Charles Cagniard de la Tour (1819) "Sur la Sirène, nouvelle machine d'acoustique destinée à mésures les vibrations de l'air qui contient la son" (On the siren, new acoustic machine to be used for measuring the vibrations of sound in air) Annales de chimie et de physique, vol. 12, pages 167–171. 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.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 30 . De la Tour's siren is illustrated on page 31. De la Tour's siren is also illustrated on page 12 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 Heritage .) De la Tour's siren could produce sound under water,See; 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 37 . suggesting a link with the sirens of Greek mythology; hence the name he gave to the instrument.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 171 : 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.
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.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. The earliest such sirens were developed during 1877–1880 by James Douglass and George Slight (1859–1934)Alan Renton, Lost Sounds: The Story of Coast Fog Signals (Latheronwheel, Scotland: Whittles Publishing, 2001), page 51 . For a brief biography of George Slight, see the Spanish Wikipedia article "George Slight" (in Spanish). of Trinity House; the final version was first installed in 1887 at the Ailsa Craig lighthouse in Scotland's Firth of Clyde.See: David A. Stevenson (1887) “Ailsa Craig lighthouse and fog signals,” Minutes of the Proceedings of the Institution of Civil Engineers, vol. 89, pages 297-303. Regarding the siren, see pages 300 and 301. Frederick A. Talbot, Lighthouses and Lightships (Philadelphia, Pennsylvania: J. B. Lippincott & Co., 1913), page 62 . 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 Oregon . "Lighthouse," Encyclopædia Britannica (1911), 11th ed., vol. 16, page 647 . 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 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.
Types Pneumatic
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) 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 125 dB, 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.