which causes an additional phase change in the sonic wave during its propagation thus providing additional attenuation another change is that the diameter of the inlet tube 20 including the perforate section 26 is smaller in the embodiment of fig5 then in fig1 the significance of this modification is discussed below the choice between the embodiments of fig1 and 5 can be made on the basis of test results with individual engines each of which have differing exhaust noise characteristics a third embodiment of the invention is shown in fig6 again the muffler comprises an outer shell 10 and inlet and outlet tubes 20 and 24 respectively the inlet tube 20 is perforated as at 26 and is separated from a venturi 22 by a gap 28 the venturi 22 again comprises a bell reducer 30 a straight section 32 and a gradual diffuser 34 and is immediately followed by a section of perforated tube 42 however in the embodiment of fig6 the relative dimensions of the straight section 32 and the diffuser 34 are comparatively short and the section of tube 22 having the perforations 42 is comparatively long and contains a block 44 forcing the exhaust gases to pass out of a first section of the perforate tube 42 flow around the block 44 and back into a second section of the perforate tube 42 so as to exit the muffler again the block 44 together with the diffusion of gas flow through the perforations provides an additional impedance and phase change to gas flow finally a fourth embodiment of the invention is shown in fig7 this is essentially a combination of the embodiments of fig5 and 6 incorporating as it does a block 46 in the inlet.