An efficient numerical method based on a rigorous integral formulation is used to calculate precisely the acoustic eigenvalues of complex shaped objects and their associated eigenvectors. These eigenvalues are obtained and later used in acoustic nondestructive evaluation. This study uses the eigenvalues to implement a simple acoustic shape differentiation algorithm that is the key in our direct nondestructive analysis. Stability and convergence of the Galerkin boundary element method used herein are discussed. Finally, some numerical examples are shown.