This study is to describe the effects of the duct termination conditions upon the active noise attenuation system.
The adaptive filtering algorithm using FIR filter is implemented for duct noise attenuation. To avoid the instability caused by the acoustic feedback, two methods are considered. One is to use a compensating filter which cancels out the acoustic feedback plant using FIR filter. The other is to constitute an uni-directional detecting microphone and uni-directional control speaker
Experiment results show that the reflections of sound from duct terminations may reduce the performance of an ANC system. The directionality of detecting microphone and control speaker is a major factor to decide ANC performance. When there is some reflections from both duct terminations and two omnidirectional coupler are used, the noise attenuation using finite FIR filter is not enough to model the duct plant. Especially, the reflection from the upstream termination reduces the noise attenuation in the frequencies related to the distance between control speaker and upstream termination.
Finally, the performance of the noise attenuation is largely enhanced by using uni-directional coupler; both detecting microphone and control speaker, even if the duct system has an arbitrary termination conditions.
