Fire Apparatus USA 29
Fire Apparatus USA 29 : Design and Construction (3) Audible Warnings: In addition to visual warnings, most appliances are also fitted with audible warnings, sometimes known as sirens, which can alert people to the presence of an emergency vehicle before they can be seen. The first audible warnings were mechanical bells, mounted on the front or roof of the truck. Most vehicles are now fitted with electronic sirens, which can produce a range of different sounds. Fire service driving training often includes the use of different sounds depending on traffic conditions and maneuver being performed. For instance, on a clear road, approaching a junction, the "wail" setting may be used, which gives a long up and down variation, with an unbroken tone, whereas, in heavy slow traffic, a "yelp" setting may be preferred, which is like a wail, but faster. The speakers for modern sirens can be located in several places on the vehicle, including being integral to the lightbar, or hidden in the grille. Some vehicles may also be fitted with airhorn audible warnings. A number of North American fire departments have returned to the "acoustic" or "air" traditional siren, as its overtones help the public "locate" and avoid the fire truckthe newer electronic signals disperse almost pure electronic sine wave tones, which are hard to locate, especially in city "canyons" of buildings. In Chile, many vehicles are fitted with three types of audible warnings, which are sounded all at once: the classic "air" siren, the electronic "yelp", and the European two-tone air horns (sometimes newer vehicles are fitted with the electronic version of the latter, commonly called "Hi-Lo"). A development is the use of the RDS system of car radios, whereby the vehicle can be fitted with a short range FM transmitter, set to RDS code 31, which interrupts the radio of all cars within range, in the manner of a traffic broadcast, but in such a way that the user of the receiving radio is unable to opt out of the message (as with traffic broadcasts). This feature is built in to all RDS radios for use in national emergency broadcast systems, but short range units on emergency vehicles can prove an effective means of alerting traffic to their presence, although is not able to alert pedestrians and non-RDS radio users
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