Answer:
Air
Step-by-step explanation:
A siren is a device that produces sound waves due to the flow of air through a series of holes. It consists of a rotating disk which has series of holes drilled at required distances to one another, and a nozzle through which air flows.
The nozzle is arranged in such a way that it forces air through the holes of the rotating disk, thus producing sound with appropriate frequency.
Sirens are used for security purpose, in ambulances etc.