Abstract
In the experimental study of active noise control for ducted fan, the sound sources are installed on the outer periphery of the duct and are generating acoustic modes with circumferential distribution by vibrating in the same frequency but with different phase. To achieve the substantial reduction of the fan noise, the acoustic output of the sound sources has to be comparable with that of fan noise, therefore a large driver unit is usually adopted as a sound source. The driver unit is connected to the duct wall via a cone-shaped horn. When increasing the number of sound sources attached to the periphery of the duct, it is necessary to extend the length of the horn to avoid the interference with adjacent driver units. Because conical horn has its frequency response, it is important to predict the effect of modifying the geometry of the horn on the acoustic field inside the duct. In this study, by forming the combination of a linear acoustic model for the three-dimensional cylindrical duct and an analytical model for conical horn, the impact of the geometry of a horn on the active noise control is predicted and the experimental verification is also presented.
