This study proposes an optimization approach to maximize the transmission loss (TL) of a muffler by optimizing the thickness of the embedded porous layer. The objective function, i.e. the TL value, is computed by the four-pole parameters based on the boundary element analysis, and the thicknesses assigned to the discretized elements are naturally chosen as the design variables, resulting in a continuous optimization problem. The sound-absorbing property of the porous layer is simulated using the local admittance boundary condition in the boundary element method (BEM) to link the objective function with the design variable. An efficient sensitivity analysis is developed on the basis of the adjoint variable method and the BEM. Finally, the method of moving asymptotes is applied to solve the optimization problem with the assistance of sensitivity information. As the design variable only affects the (local) admittance value, resulting in a diagonal admittance matrix, and the coefficient matrices of the boundary element analysis will not be influenced by the design variables, we can regard the overall optimization as highly efficient. Moreover, the proposed and previously developed (density-based) optimization approaches are compared. Results show high similarities in the optimization results, thus validating the presented optimization approach. The numerical results also indicate the frequency-dependency of the optimized design. The frequency-averaged TL in the frequency band of interest is selected as the objective function to achieve a multi-frequency optimization.