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Sirens are devices making sound to alarm others, such as the air raid siren 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 disk with holes in it (called a rotor), rotated by a high speed electrical motor so that the holes interrupt the air coming out of fixed holes on the outside of the unit (which is called a stator). Such sirens can consume large amounts of power, and may be three phase. 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 from in front of the vehicle, while pneumatic sirens seem to protect a vehicle 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. Overall, sirens provide warning to a group of people.
Sirens are also used as musical instruments, such as in Edgard Varese's Hyperprism (1924), Ionisation (1931), and, recorded, in his Poeme Electronique (1958).
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