A siren is a loud noise making device. Most modern ones are civil defense or air raid sirens, tornado sirens, nuclear test sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumatic and electronic.
Many fire sirens 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 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 by speakers, though these are not as common.
Fire sirens are often called "fire whistles", "fire alarms", "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 pulling double duty as a fire siren often can produce an alternating "hi-lo" signal (similar to a British police car) as the fire signal, or a slow wail (typically 3x) as to not confuse the public with the standard civil defense signals of alert (steady tone) and attack (fast wavering tone).
Some time before 1799, the first siren was invented by the Scottish natural philosopher (physicist) John Robison. 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 invented and named by Baron Charles Cagniard de la Tour. 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. De la Tour's siren could produce sound under water, suggesting a link with the sirens of Greek mythology; hence the name of the instrument.
Instead of disks, most modern 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. The earliest such sirens were developed during 1877-1880 by James Douglass and George Slight (1859 - 1934) of Trinity House; the final version was first installed in 1887 at the Ailsa Craig lighthouse in Scotland’s Firth of Clyde. 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. Two early fire sirens were the Decot siren and Sterling Siren. Both started manufacturing fire sirens around 1900 to 1905. Many communities have since deactivated their fire sirens as pagers became available for fire department use.
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 (280 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.
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.
Other types 
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. 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 thus would "blast out" a code in respect to that boxes location. For example, pull box number 233, when pulled, would trigger the fire horn to sound a 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 aren't common anymore.
In music 
Sirens are also used as musical instruments, such as in Edgard Varèse's compositions Amériques (1918–21, rev. 1927), Hyperprism (1924), and Ionisation (1931); in George Antheil's Ballet Mécanique (1926); in Henry Fillmore's "The Klaxon: March of the Automobiles" (1929); in The Chemical Brothers's "Song to the Siren"; and, in a CBS News 60 Minutes segment, by experimental percussionist Evelyn Glennie. A variation of a siren, played on a keyboard, are the opening notes of the REO Speedwagon song "Ridin' the Storm Out".
Approvals or certifications 
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.
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. 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.
The Society of Automotive Engineers, (SAE), Emergency Warning Lights and Devices committee oversees the SAE emergency vehicle lighting practices and the siren practice, J1849. This practice was updated through cooperation between the SAE and NIST, the National Institute of Standards. 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.
Best practices 
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 cause permanent hearing damage to vehicle occupants.
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. Studies in some agencies operating emergency vehicles show sound levels over 120 dB(A) in the passenger compartment. In one study, a specific vehicle's engine sounds and the siren produced sound levels over 123 dB(A) in the passenger compartment.
Research has shown that sirens mounted behind the engine grill 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. The inclusion of broadband sound to sirens has the ability to increase localisation of sirens, 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.
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 end result is that the average sound output from a dual speaker siren system is 3 dB greater than a single speaker system.
See also 
- 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.
- "Temperment 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.
- 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.
- For descriptions of Robison’s and de la Tour’s sirens, see:
- Robert T. Beyer, Sounds of Our Times: Two Hundred Years of Acoustics (N.Y., N.Y.: 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 (N.Y., N.Y.: 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: http://www.americanheritage.com/articles/magazine/it/2003/3/2003_3_22.shtml .)
- 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.
- 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.
- 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 51. For a brief biography of George Slight, see the Spanish Wikipedia article "George Slight" (in Spanish).
- 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.P. Lippincott, 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: https://scholarsbank.uoregon.edu/dspace/bitstream/1794/4213/1/TM_survey.pdf .
- "Lighthouse," Encyclopædia Britannica (1911), 11th ed., vol. 16, page 647.
- Catchpole, Ken; Denis Mckeown (08). Ergonomics 50 (8): 1287–1301. doi:10.1080/0014013070138780.
- "Alarms and Sirens". Physics.org. Retrieved 25 October 2012.
- Various examples
- PDF (206 KiB)
- Whistle Museum - Pictures of Siren whistles
- The Siren Archive - pictures of fire sirens