such as for example u s pat no 4 325 460 turning to fig1 exhaust gases enter intake tube 18 and pass generally through the tube until they reach the end of the tube these gases then enter the inlet end of tube 30 because there is not alternative flow path the constricted throat of tube 30 produces sound reduction in the same manner as is known in the prior art for sonic chokes or venturis a portion of the gas flow escapes passage through the constriction and instead flows through the bypass trough 52 the effect is to increase silencing and reduce backpressure through the tube this is apparently caused by the interaction of the two flow patterns one coming through the constricted portion and the other passing unimpeded through the trough the sound attenuating operation of the embodiments shown in the remaining figures is the same in fig1 there is shown an alternative placement for the integral fluted tubes 330a and 330b either before or after a standard muffler in line with an exhaust tube the tube may have perforations and packing or may be without those features and may be formed separately from or integrally with exhaust pipes 380 381 test studies on prototypes of the integral formed tubes operated optimally show that a lower pressure drop and higher maximum velocity can be achieved over a similarly sized radially symmetrical choke the preferred number of troughs appears to be three with four troughs being substantially less advantageous.