Siren (noisemaker)

A siren is a loud noise maker. The original version would yield sounds under water, suggesting a link with the sirens of Greek mythology. Most modern ones are civil defense or "air raid" sirens, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types, pneumatic and electronic. The device was invented by the Scottish natural philosopher (physicist) John Robison. It was improved and given its name by Charles Cagniard de la Tour.

History
The earliest way of summoning volunteers 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 manufacturers of fire sirens where Federal Signal and Sterling Siren. Both started manufacturing fire sirens around 1900 to 1905.

Many communities have since deactivated the fire sirens due to noise complaints.

Pneumatic
The pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a 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.

In United States English language usage, vehicular pneumatic sirens are sometimes referred to as mechanical or coaster sirens, to differentiate them from devices which make noise electronically. One example is the Q2B electromechanical 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, sound power (123 dB at 10 feet) and square sound waves help account for its effectiveness.

Electronic
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 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.

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. Also, some fire sirens are fitted with brakes and dampers to enable them to sound out codes as well. These units tended to break down a lot, and thus, aren't common or produced anymore. Other bizarre methods of alerting firemen have been incorporated as well.

As art
Sirens are also used as musical instruments, such as in Edgard Varèse's Hyperprism (1924), Ionisation (1931), recorded, in his Poeme Electronique (1958), George Antheil's "Ballet Mécanique" (1926), The Klaxon: March of the Automobiles (1929 by Henry Fillmore, The Chemical Brothers's Song to the Siren and, (in a CBS News 60 Minutes segment) by experimental percussionist Evelyn Glennie.

Approvals or certifications
Governments may have standards for vehicle-mounted sirens. For example, in California, sirens are designated Class A or Class B. In non-technical terms, a Class A siren is so loud 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 every emergency vehicle 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.

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 serve a useful purpose. 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) when spinning up to operating speed. Appropriate wiring and transient protection for modern 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 pad of friction material against the siren rotor. When an emergency vehicle arrives on-scene or is cancelled en route, the vehicle 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. A crude, static test for dead spots is to apply white noise from an unsquelched F.M. two-way radio to the siren amplifier's auxiliary input then walk around the vehicle making sure the sound doesn't have any unexpected nulls.