Active sound control by diectional secondary sources in the free field

In this paper, a general formula for caculating the complex strengths of secondary sources is derived according to the criterion of minimizing the total acoustic power output while both of primary and secondary sources are with difectivity. Analytic results show that the formula is of universal significance. In the last, the specific properties are presented for some typical secondary sources commonly used in active sound control, and these results are shown in figures.