A rotary shutter valve 10 positioned in accordance with the speed of the engine by linkage 15, 16 throttles the main exhaust duct 5 and continuously changes the cross- section of secondary exhaust ducts 6 and 7 from fully closed at low speed to fully-open at high speed. At low speed the valve 10 also connects the main exhaust 5 to a resonant cavity 14 but as the speed rises this connection is progressively closed as the flow cross-section of the main exhaust duct is increased.
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