Development of NDIF Method to Solve the Non-extraction Problem of Low-frequency Eigenvalues for Arbitrarily-shaped Acoustic Cavities with a Mixed Boundary

The NDIF method is a numerical analysis technique that was developed for high-precision eigenvalue analysis of arbitrarily shaped acoustic cavities. It is characterized by a limitation where the precision of higher-order eigenvalues increases, but low-order eigenvalues are not extracted as the number of nodes increases. In a recent study, a practical method was developed to overcome this limitation in acoustic cavities with rigid-wall boundaries. In this study, the practical method was applied and extended to acoustic cavities with mixed boundaries, comprising rigid-wall and open boundaries. To verify the validity and accuracy of the proposed method, example studies were conducted on both a rectangular acoustic cavity with a known exact solution and an arbitrarily shaped acoustic cavity. The verification demonstrated that lower-order eigenvalues can be successfully extracted regardless of the number of nodes. In future studies, this method will be applied to acoustic cavities with more diverse shapes and boundary conditions.