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.

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 interrupt a flow 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 U.S. English usage, vehicular sirens are sometimes referred to as mechanical or coaster sirens, to differentiate them from devices which make noise electronically.

Electronic

Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, yelp, pierce/priority, hi-lo, scan, airhorn. and a few more) which is played through external speakers.

Electronic sirens seem to be better for clearing traffic in front of vehicles, while pneumatic sirens seem better at protecting vehicles passing or turning through an intersection. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens.

As art

Sirens are also used as musical instruments, such as in Edgard Varese's Hyperprism (1924), The Klaxon: March of the Automobiles (1929 by Henry Fillmore, Ionisation (1931), recorded, in his Poeme Electronique (1958) and, (in a CBS News 60 Minutes segment) by experimental percussionist Evelyn Glennie.

Vehicle-mounted

Approvals or certifications

Most governments 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. In practical use, the approval process is not important because some officer will write you a ticket if your ambulance has an unapproved siren. Instead, 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 slighly 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 while the siren is operating. It also puts the noise where it will serve a useful purpose. Studies in some agencies operating emergency vehicles show sound pressure levels over 120dba in the passenger compartment. In one study, a specific vehicle's engine sounds and the siren produced sound levels over 123dba in the passenger compatment.

Electric-motor-driven mechanical sirens may draw 50- to 200-amperes at 12VDC when spinning up. Appropriate wiring and transient protection for modern engine control computers is a necessary part of an installation. Wiring should look appropriate for the engine's 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 the emergency vehicle arrives at scene or is cancelled en route, the vehicle operator can press a button to rapidly spin-down 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 in speaker wiring. 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.

External links

Firefighting | Rotating aerophones | Scottish inventions | Sound production

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