In a electromechanical sirens, sound is generated by a motor driving a shaft with aspecial turbine (known as a rotor or chopper)on one or both ends.
It will have only one fan if it is a single-toned siren, while if it is a dual-toned siren, it will have either one fan on each end or two fans in a stack on one end, with one fan having a few more blades than the other.
Around each faní¬or chopper or rotorí¬is a housing with a number of rectangular holes to match the number of fan blades. This housing is known as a stator. The end of each blade has a plate whose shape is matched with the rectangular holes and circular curve of the stator. The blades draw air in at the end and force it out through the slots in the housing in rapid pulses, as the plates on the end of the blades interrupts that flow, which is what produces the sound