Flow-generated tones inside and outside mufflers with expansion chambers

Detailed experiments are conducted to investigate flow-generated tones inside and outside mufflers with expansion chambers. Three types of tones are captured inside and outside the mufflers. The first type of tones is captured both inside and outside the mufflers with three length-to-diameter ratios (L/D). They are generated by tailpipe resonances and can be predicted accurately by the natural acoustic mode frequencies of tailpipe. The second type of tones is captured both inside and outside the muffler at medium values of L/D but only inside at large values of L/D. These types of tones are induced by the chamber resonance of the muffler and agree with the natural acoustic mode frequencies of the chamber inside the muffler. These tones are captured only inside the muffler at small and medium values of L/D and are closely related to a complex flow-acoustic feedback loop inside the muffler. Moreover, the tailpipe works like an acoustic filter which influences the sound pressure levels (SPL) outside the muffler.