Abstract: An Adaptive Active Noise Control (AANC) system is of practical use if andonly if its stability is guaranteed and a substantial broadband noise reduction is achieved. Inthis paper,steady state performances of the broadband AANC system are discussed and theeffect of system parameters which include the noise bandwidth, the acoustic transmissionpath and the parameters of the adaptive filter, etc. on Attenuation Level (AL) is analyzedand a physical mechanism of AANC is explained clearly taking AANC in the free farfieldsituation. The purpose of this paper is to provide a theoretical basis for improvement ofperformances of the broadband AANC system.