An　active　adaptive　noise　control　system　in　ducts　is　proposed,　which　adopts　a　motional　feedback　loudspeaker　as　an　noise　controlling　actuator　and　a　detecting　sensor.　By　analysing　each　acoustical　path,　identification　based　adaptive　linear　neural　networks　is　made.　This　identifying　method　can　be　achieved　expediently,　and　it　wouldn＇t　bring　disturbance　to　working　environment.　The　estimated　result　of　each　acoustical　path　agrees　well　with　its　real　acoustical　character.