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 rotating disk with holes in it (called a 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 power.

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.

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