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Dual mode mufflers are extensively used in Japan, the construction involves a valve to control the exhaust gas flow. The exhaust gas passes through a small pipe at low engine speed, and a larger pipe at higher engine speed. This has the effect of a large noise reduction at low engine speed, and lower back pressure at high engine speed. This paper clarifies the resonator effect of the dual mode muffler that depends on the valve opening angle and rate of exhaust gas flow, and detailed relationship between the valve opening angle and noise reduction, how the exhaust gas flow is the major influence for the noise reduction of the dual mode muffler.