A sound source that is based on the regular interruption of a stream of fluid (usually air) by a perforated rotating disk or cylinder. The components of a siren are a source of air, a rotor containing a number of ports which interrupt the airflow at the desired frequency, and ports in a stator through which the air escapes. The air is supplied by a compressor, and a motor drives the rotor. The frequency of the sound wave produced by the siren is the product of the speed of rotation and the number of ports in the rotor. The shape of the rotor and stator ports determines the wave shape at the entrance of the stator port. The stator ports feed into a horn in order to improve radiation.

Siren performance parameters are sound power output, acoustic pressure, and efficiency, that is, the ratio of acoustic power output to compressor power.