© 2016 Institute of Noise Control Engineering. Mufflers are widely used in engine exhaust systems for providing sound attenuation in a specific frequency range. In this case study, an engineering problem on reducing the noise radiation from a truck is investigated. The original muffler used by the truck consists of four reactive sub-chambers with similar geometries. Both experiments and numerical simulations revealed the narrow band transmission loss (TL) characteristics of the muffler which limit its attenuation performance, resulting in noise leakage at some particular frequencies. With an aim to broadening the attenuation bandwidth, a recently proposed sub-chamber design method is implemented to retrofit the muffler. The inner structure of the muffler is modified under the guidance of numerical simulations, in order to create extra acoustic stop-bands for an overall broadband performance. The retrofitted muffler is experimentally verified with a better TL response in the design frequency of interest. Analyses demonstrate the effectiveness of using the proposed simulation-based design method, which also show potentials to a wide range of muffler design applications.