A siren is a loud noise making device. Most modern ones are civil defense

Civil defense siren

A civil defense siren is a mechanical or electronic device for generating sound to...

 or air raid sirens, tornado sirens, or the sirens on emergency service

Emergency service

Emergency services are organizations which ensure public safety and health by addressing different emergencies. Some agencies exist solely for addressing certain types of emergencies whilst others deal with ad hoc emergencies as part of their normal responsibilities...

 vehicles such as ambulance

Ambulance

An ambulance is a vehicle for transportation of sick or injured people to, from or between places of treatment for an illness or injury, and in some instances will also provide out of hospital medical care to the patient...

s, police car

Police car

A police car is a ground vehicle used by police, to assist with their duties in patrolling and responding to incidents. Typical uses of a police car include transportation for officers to reach the scene of an incident quickly, to transport criminal suspects, or to patrol an area, while providing a...

s and fire trucks

Fire apparatus

A fire apparatus, fire engine, fire truck, or fire appliance is a vehicle designed to assist in fighting fires by transporting firefighters to the scene and providing them with access to the fire, along with water or other equipment...

. There are two general types: pneumatic and electronic.

Many fire sirens serve double duty as tornado

Tornado

A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

 or civil defense

Civil defense

Civil defense, civil defence or civil protection is an effort to protect the citizens of a state from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery...

 sirens, alerting an entire community of impending danger

Risk

Risk is the potential that a chosen action or activity will lead to a loss . The notion implies that a choice having an influence on the outcome exists . Potential losses themselves may also be called "risks"...

. 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

Government

Government refers to the legislators, administrators, and arbitrators in the administrative bureaucracy who control a state at a given time, and to the system of government by which they are organized...

 buildings, on top of tall structures such as water tower

Water tower

A water tower or elevated water tower is a large elevated drinking water storage container constructed to hold a water supply at a height sufficient to pressurize a water distribution system....

s, 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

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 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 to 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).

History

Some time before 1799, the first siren was invented by the Scottish natural philosopher (physicist) John Robison

John Robison (physicist)

John Robison FRSE was a Scottish physicist and mathematician. He was a professor of philosophy at the University of Edinburgh....

. 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

Charles Cagniard de la Tour

Charles Cagniard de la Tour was a French engineer and physicist.-Biography:Charles Cagniard was born in Paris, and after attending the École Polytechnique became one of the ingénieurs géographiques. He was made a baron in 1818...

. 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 from the fixed disk, producing a tone. De la Tour's siren could produce sound under water, suggesting a link with the siren

Siren

In Greek mythology, the Sirens were three dangerous mermaid like creatures, portrayed as seductresses who lured nearby sailors with their enchanting music and voices to shipwreck on the rocky coast of their island. Roman poets placed them on an island called Sirenum scopuli...

s 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 siren was made shortly before 1887 by a “Mr. Slight” for 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

Centrifugal fan

A centrifugal fan is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing...

, 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

Bell (instrument)

A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

, either mounted atop the fire station, or in the belfry of a local church. As electricity

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 became available, the first fire sirens were manufactured. Two early manufacturers of 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.

Pneumatic

The pneumatic siren, which is a free aerophone

Aerophone

An aerophone is any musical instrument which produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes, and without the vibration of the instrument itself adding considerably to the sound...

, consists of a rotating disk with holes in it (called a chopper, siren disk

Siren disk

A siren disk is a disk with holes variously spaced, such that when spun in front of an air jet, the holes modulate the jet in a primarily binary fashion, to directly produce sound.-Typical usages of siren disks:...

 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

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

. Such sirens can consume large amounts of energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

. 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

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 English language

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 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

Q2B

The Q2B siren is an electromechanical siren that has one of the most noticeable sounds in the world. The Q2B or "Q-siren" is famous for its sound, which is trademarked, and the look of the siren...

 siren sold by Federal Signal Corporation

Federal Signal Corporation

Federal Signal Corporation is a global corporation with about 2,800 employees and annual revenue of about US$727 million. Oak Brook, Illinois-based Federal Signal designs, develops and deploys solutions intended to protect people, property and the environment under brands such as Federal Signal,...

. Because of its high current draw (280 amps

Ampere

The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

 when power is applied) its application is normally limited to fire apparatus, though it has seen increasing use on type IV ambulance

Ambulance

An ambulance is a vehicle for transportation of sick or injured people to, from or between places of treatment for an illness or injury, and in some instances will also provide out of hospital medical care to the patient...

s 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

Electronic sirens incorporate circuits such as oscillators, modulator

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

s, and amplifier

Amplifier

Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

s 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 whistle

Steam whistle

A steam whistle is a device used to produce sound with the aid of live steam, which acts as a vibrating system .- Operation :...

s 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 instrument

Musical instrument

A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

s, such as in Edgard Varèse

Edgard Varèse

Edgard Victor Achille Charles Varèse, , whose name was also spelled Edgar Varèse , was an innovative French-born composer who spent the greater part of his career in the United States....

's compositions Amériques

Amériques

Amériques is a musical composition by the French-born composer Edgard Varèse.Written between 1918 and 1921 and revised in 1927, it is scored for a very large, romantic orchestra with additional percussion including sirens...

(1918–21, rev. 1927), Hyperprism (1924), and Ionisation

Ionisation (Varèse)

Ionisation is a musical composition by Edgard Varèse written for thirteen percussionists, the first concert hall composition for percussion ensemble alone. The premiere was at Steinway Hall, on March 6, 1933, conducted by Nicolas Slonimsky, to whom the piece was later dedicated...

(1931); in George Antheil

George Antheil

George Antheil was an American avant-garde composer, pianist, author and inventor. A self-described "Bad Boy of Music", his modernist compositions amazed and appalled listeners in Europe and the US during the 1920s with their cacophonous celebration of mechanical devices.Returning permanently to...

's Ballet Mécanique

Ballet mécanique

Ballet Mécanique was a project by the American composer George Antheil and the filmmaker/artists Fernand Léger and Dudley Murphy. Although the film was intended to use Antheil's score as a soundtrack, the two parts were not brought together until the 1990s. As a composition, Ballet Mécanique is...

(1926); in Henry Fillmore

Henry Fillmore

Henry Fillmore was an American musician, composer, publisher, and bandleader, best-known for his many marches and screamers.-Biography:James Henry Fillmore Jr. was born in Cincinnati, Ohio as the eldest of five children...

's "The Klaxon: March of the Automobiles" (1929); in The Chemical Brothers

The Chemical Brothers

The Chemical Brothers are a British electronic music duo comprising Tom Rowlands and Ed Simons. Originating in Manchester in 1991, along with The Prodigy, Fatboy Slim, The Crystal Method, and fellow acts, they were pioneers at bringing the big beat genre to the forefront of pop culture.- Background...

's "Song to the Siren"; and, in a CBS News 60 Minutes segment, by experimental percussionist Evelyn Glennie

Evelyn Glennie

Dame Evelyn Elizabeth Ann Glennie, DBE is a Scottish virtuoso percussionist. She was the first full-time solo percussionist in 20th-century western society.-Early life:Glennie was born and raised in Aberdeenshire...

. A variation of a siren, played on a keyboard, are the opening notes of the REO Speedwagon

REO Speedwagon

REO Speedwagon is an American rock band. Formed in 1967, the band grew in popularity during the 1970s and peaked in the early 1980s. Hi Infidelity is the group's most commercially successful album, selling over ten million copies and charting four Top 40 hits in the US...

 song "Ridin' the Storm Out".

Vehicle-mounted

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

California Highway Patrol

The California Highway Patrol is a law enforcement agency of the U.S. state of California. The CHP has patrol jurisdiction over all California highways and also acts as the state police....

 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.

Electric-motor-driven mechanical sirens may draw 50 to 200 amperes at 12 volts (DC

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

) 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. One must realize, that these phase differences also account for increases, based upon the frequency and the speaker spacing. You must remember though, that 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.

External links

• Various examples
• Whistle Museum - Pictures of Siren whistles
• The Siren Archive - pictures of fire sirens