An electromechanical siren is a siren that uses an electric motor to drive a spinning rotor, or chopper, inside of a stationary stator. A (usually) equal number of holes, or ports, are inside the rotor and stator, with vanes in the rotor directing air through the ports. As air moves through the ports as the rotor spins, it is chopped at a certain frequency, creating sound. The more ports on the siren and the faster the rotor spins, the higher the peak pitch of the siren.