Active sound control by directional secondary sources in the free field

A general formula for calculating the optimum complex strengths of secondary sources was derived based on the criterion of minimizing the total acoustic power output while both the primary and secondary sources are with directivity. Theoretical analysis shows that the formula is of universal significance. Besides, the specific properties of four types of typical secondary sources commonly used in the active sound control were discussed, and such a discussion will provide the necessary theoretical guide for the selection of secondary sources and for their optimum arrangement.