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...Hz rasp (when correctly wired). It also has been used as the base for the "Mark 2" gearbox sirens. It is not known if the NP3S and NP2[S] share the same ro ...it one of CEN's heaviest sirens just behind the Mark 1 gearbox sirens and Mark 2 gearbox (NGx-series) sirens) of the NP3S is rumored to exist, however, no ...