When seeking to enhance the acoustic attenuation performance of a reactive muffler, it is necessary to ensure that the flow resistance does not increase significantly. To date, there have been very few attempts to simultaneously optimize the transmission loss and pressure drop of a muffler. In this study, a multiobjective topology optimization problem is formulated to maximize the transmission loss at a target frequency and minimize the pressure drop simultaneously. The objective function in the formulation is given as the sum of weighted transmission loss and weighted pressure drop. The effect of the weighting factors on the optimal topologies of a muffler is investigated. Furthermore, the physical interpretation of partition layouts of optimized mufflers is discussed. The proposed muffler design method involving multiobjective topology optimization is compared with the previous muffler design method that involves single-objective topology optimization to maximize only the transmission loss. The most important advantage of this study is shown by considering numerical results; the proposed muffler design method is applicable to nonconcentric expansion chamber mufflers, unlike the previous muffler design method. Copyright © 2012 John Wiley & Sons, Ltd.