the invention relates to a muffler which includes a first expansion chamber , a second expansion chamber , a resonator chamber , and an inlet pipe having a first open end and a second open end . the first open end of the inlet pipe is inserted into the first expansion chamber so as to communicate with the latter . a resonator pipe runs substantially in a coaxial relationship with the inlet pipe so as to establish communication between the first expansion chamber and the resonant chamber . a return pipe is disposed between the resonator chamber and the second expansion chamber and passes through the first expansion chamber for establishing communication between the first expansion chamber and the second expansion chamber . an outlet pipe communicates with the second expansion chamber . the improvement comprises that said return pipe has an open end and is inserted with the open end into said resonator chamber , while the inserted open end thereof is sealed with a plug . a portion of the return pipe , passing through the first expansion chamber and the resonator chamber , is provided with at least a throughhole .