The usage of directional secondary sources to actively control a finite line source

Active noise control in free-space always is challenging, especially for an overall attenuation. Conventionally, the secondary sources for introducing destructive sound wave are loudspeakers theoretically regarded as acoustic monopoles, which can result, according to previous literatures, in relatively remarkable performance. This paper studies on the active control system for a primary simple line source, a common type of noise sources usually encountered in the traffic noise control. A variety of parameters, e.g. the location, strength and number of the secondary sources, concerning the ANC system are determined by minimizing specified objective function defined on an observation plane covering a desirable range for quietness. The numerical simulation and optimization suggest that the usage of directional secondary sources is necessary. Through the comparison among several kinds of control sources with different directivity patterns, a novel type of secondary sources consisting of an inner-piston and an outer-concentric-annulus together, is introduced. The multi-part novel source can achieve a significant control result within a realistic physical size, which makes the radical proposition practical.