Abstract: The acoustic character controlled by the active noise attenuation system in ducts has been analyzed in detail on premise of semi-infinite ducts. The studying results have proved that under the ideal noise cancellation condition the reflection coefficient module of standing wave field between the noise source and the secondary source is equal to 1, the reflection coefficient at the secondary source is equal to 1, the acoustic impedance at the position is equal to zero. And the acoustic radiation impedance of noise source is close to a pure imaginary number, namely, it cannot radiate acoustic energy again.