the inlet header 5 and also a collar 25 which is coaxial with an outlet collar 27 formed in the outlet header except for the collars the partition 17 is preferably imperforate an inlet bushing 29 is mounted in and spot welded to the collar 23 and has a reduced diameter inner portion 31 which receives and is spotwelded to the outer end of an inlet tube member 33 the other end of the tube being supported in and spot welded to the partition collar 21 an outlet tube member 35 comprises a tubular outlet shell 36 which has a bushing portion 37 mounted in and spotwelded to the collar 27 in header 7 the outer shell 36 has reduced diameter portions 39 which provide annular inner surfaces that engage and support a gas flow tube 41 the tube 41 has an inner end that fits within the collar 25 and is spotwelded to it and an outer end which opens into the bushing 37 and is spotwelded to the outermost of the reduced diameter portions 39 in the embodiment illustrated the outlet tube 41 has a pair of louver patches or sets of openings 43 and 45 which open respectively into closed annular chambers 47 and 49 that are formed between the tube 41 and the outer shell 36 and between the reduced diameter portions 39 the louver patches 43 and 45 with the chambers form high frequency attenuating spit chambers as is well known in the embodiment shown the inlet tube 33 also has a patch of louvers 51 extending over a major part of.