Active Noise Control in a Three-Dimensional Space

In this research, a new approach is presented for optimizing the performance of active noise control in three-dimensional space. The work focuses on the critical role of determining the locations for secondary sources in active noise suppression. A systematic technique is proposed for estimating optimal locations of secondary sources on the boundary of a three-dimensional domain for both local and global active control of harmonic sound fields. The theoretical foundation of this work is Huygen's principle. The indirect boundary element method (IBEM) which is a numerical implementation of Huygen's principle is employed as a basis for the numerical analysis. The fictitious source distribution used to represent the boundary in the indirect boundary element method is the focus of this investigation. Using numerical predictions of the fictitious source strength distribution on the boundary, it is found that the optimal locations for secondary sources are the boundary node locations with high fictitious source strengths. The strengths of the secondary sources, normal velocities in this research, can then be optimized using a minimum squared pressure criterion at selected observation points. Examples are presented to verify both the validity of the computer implementation and the conception of this approach. These examples include active cancellation of an interior sound field with rigid wall boundary conditions and normal acoustic impedance boundary conditions, as well as reduction of sound radiation. Selected numerical predictions are compared with published experimental results and numerical predictions obtained by other researchers using alternative numerical techniques. Numerical predictions using the proposed method are found to be impressive and promising. The results demonstrate that significant attenuation can be achieved by locating secondary sources using the method presented in this work. It can be concluded from these results that the fictitious source strength on the boundary of the domain provides a powerful method for estimating optimal locations for secondary sources in active cancellation of three-dimensional sound fields. The results also suggest promising potential applications of the approach in active control design for vehicle and aircraft passenger compartment. A particularly useful feature of the approach in the design of active noise control systems is that the fictitious source distribution for a given situation is determined as an intermediate step in characterizing the primary sound field. The proposed process for source location is thus computationally efficient because it builds directly upon other essential intermediate results.