Siren

(engineering acoustics) An apparatus for generating sound by the mechanical interruption of the flow of fluid (usually air) by a perforated disk or cylinder.

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. See also Sound pressure.

Applications of sirens include acoustic levitation (the use of radiation pressure to levitate small objects), broadband underwater sound projectors, and sonic fatigue (fatigue life and failure of structures subjected to fluctuating pressures generated by acoustic waves). See also Acoustic levitation.

Electromechanical sirens use an electric motor instead of a compressed air supply to generate the acoustic signal. A second motor spins the rotor. The stator and horn increase the sound power output and efficiency. Electromechanical sirens are widely used as warning devices. See also Sound.