A siren comprising: a base having a central axis (A-A) and a peripheral wall circumscribed about the central axis (A-A) to define an internal chamber, the peripheral wall including a plurality of apertures in communication with the internal chamber, the base further including an inlet having an inlet end for receiving a compressed gas and a discharge end for discharging the gas, the discharge end being axially spaced from the inlet end; a rotor disposed within the chamber and centrally aligned with the central axis (A-A), the rotor having a plurality of fins radially spaced and disposed about the central axis (A-A), each of the plurality of fins extending radially inward along a fin axis (C-C) that intersects the central axis (A-A), each fin extending from a peripheral edge of the rotor to a tip portion, each fin having a pair of lateral edges that extend symmetrically about a plane of symmetry containing both the fin axis (C-C) and the central axis (A-A) from the peripheral edge to the tip portion; an external housing that cooperates with the base to enclose the rotor within the internal chamber, the external housing including a first plurality of ports and a second plurality of ports, the first and second plurality of ports (16, 18) being in fluid communication with the internal chamber of the base; a driver disposed externally of the internal chamber adjacent and coupled to the rotor to rotate the rotor about the central axis (A-A) such that air is drawn into the siren housing through the first plurality of ports and is expelled from the siren housing through the second plurality of ports for generation of a sound; the driver having a surface in line with the discharge end of the inlet to be impacted with the discharged gas; and a nozzle insert disposed within the inlet between the inlet end and the discharge end, the nozzle insert defining a converging-diverging internal passageway for the compressed gas in the direction from the inlet end to discharge end in order to condition the compressed gas before discharge from the discharge end of the inlet. a base having a central axis (A-A) and a peripheral wall circumscribed about the central axis (A-A) to define an internal chamber, the peripheral wall including a plurality of apertures in communication with the internal chamber, the base further including an inlet having an inlet end for receiving a compressed gas and a discharge end for discharging the gas, the discharge end being axially spaced from the inlet end; a rotor disposed within the chamber and centrally aligned with the central axis (A-A), the rotor having a plurality of fins radially spaced and disposed about the central axis (A-A), each of the plurality of fins extending radially inward along a fin axis (C-C) that intersects the central axis (A-A), each fin extending from a peripheral edge of the rotor to a tip portion, each fin having a pair of lateral edges that extend symmetrically about a plane of symmetry containing both the fin axis (C-C) and the central axis (A-A) from the peripheral edge to the tip portion; an external housing that cooperates with the base to enclose the rotor within the internal chamber, the external housing including a first plurality of ports and a second plurality of ports, the first and second plurality of ports (16, 18) being in fluid communication with the internal chamber of the base; a driver disposed externally of the internal chamber adjacent and coupled to the rotor to rotate the rotor about the central axis (A-A) such that air is drawn into the siren housing through the first plurality of ports and is expelled from the siren housing through the second plurality of ports for generation of a sound; the driver having a surface in line with the discharge end of the inlet to be impacted with the discharged gas; and a nozzle insert disposed within the inlet between the inlet end and the discharge end, the nozzle insert defining a converging-diverging internal passageway for the compressed gas in the direction from the inlet end to discharge end in order to condition the compressed gas before discharge from the discharge end of the inlet.