The inner cylinder rotates while the outer one remains stationary. Instead of disks, most modern mechanical sirens use two concentric cylinders, which have slots parallel to their length. De la Tour's siren could produce sound under water, suggesting a link with the sirens of Greek mythology hence the name he gave to the instrument. The rotating disk periodically interrupted the flow of air through the fixed disk, producing a tone. One disk was stationary, while the other disk rotated. De la Tour's siren consisted of two perforated disks that were mounted coaxially at the outlet of a pneumatic tube. In 1819, an improved siren was developed and named by Baron Charles Cagniard de la Tour. The stopcock was apparently driven by the rotation of a wheel. Robison's siren consisted of a stopcock that opened and closed a pneumatic tube. Robison's sirens were used as musical instruments specifically, they powered some of the pipes in an organ. Some time before 1799, the siren was invented by the Scottish natural philosopher John Robison. Then in the 70s, they switched to a duotone airhorn, which was itself overtaken in the 80s by an electronic wail. The first emergency vehicles relied on a bell. Fire sirens are often tested once a day at noon and are also called "noon sirens" or "noon whistles". Civil defense sirens also used as fire sirens often can produce an alternating "hi-lo" signal (similar to emergency vehicles in many European countries) as the fire signal, or attack (slow wail), typically 3x, as to not confuse the public with the standard civil defense signals of alert (steady tone) and fast wail (fast wavering tone). Although there is no standard signaling of fire sirens, some utilize codes to inform firefighters of the location of the fire. Some newer sirens are electronically driven speakers.įire sirens are often called "fire whistles", "fire alarms", or "fire horns". Most fire sirens are single tone and mechanically driven by electric motors with a rotor attached to the shaft. Fire sirens can also be mounted on or 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 either mounted on the roof of a fire station or on a pole next to the fire station. Many fire sirens (used for calling the volunteer fire fighters) serve double duty as tornado or civil defense sirens, alerting an entire community of impending danger. There are two general types: mechanical and electronic. Sirens are used on emergency service vehicles such as ambulances, police cars, and fire trucks. Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks.
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