An expansion-chamber muffler consists of one or more chambers of larger cross-sectional area than the main pipe, which are in series with the pipe.
The expansion-chamber muffler provides attenuation by taking advantage of the reflections from the junctures at which the cross sectional area changes.
The theory shows that below a certain frequency, which is called the cut-off frequency, the muffler is relatively ineffective [1].

[1] Davis,D., et al. National Advisory Committee for Aeronautics,Technical Note 2893, Theoretical and Measured Attenuation of Mufflers at Room Temperature without Flow,  with Comments on Engine-Exhaust Muffler Design, Langley Aeronautical Laboratory 1953, pp. 49