Siren (noisemaker)

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Siren - a noise-maker. The original version would yield sounds under water, suggesting a link with the sirens of Greek mythology.

Image:Pneumatic siren.jpg Sirens are devices making sound to alarm others, such as the air raid siren, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire engines. There are two general types, pneumatic and electronic.

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.

Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, sawtooth, bellring, or beebaw) 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.

Sirens are also used as musical instruments, such as in Edgard Varese's Hyperprism (1924), Ionisation (1931), and, recorded, in his Poeme Electronique (1958).

The device was invented by the Scottish natural-philosopher John Robison . It was improved and given its name by Charles Cagniard de la Tour.

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