passage 20 connected to an outer tube 21 by a pair of baffles 22 23 to which the tubes are welded or otherwise suitably secured the inside of tube 20 comprises a gas flow passage which offers relatively low resistance to fluid flow and hence presents only slight back pressure to an engine for example whose exhaust gas contains a component of acoustical energy to be removed the space between tubes 20 and 21 and baffles 22 and 23 is designed as a resonating chamber 24 for the attenuation of acoustical energy in a predetermined band and may be one of several such chambers having different design frequency bands and located along tube 20 the wall of the passage formed by tube 20 is provided with a field of sharp edged perforations 25 which provide acoustical connection between the passage and the chamber in a typical device of this kind the inner tube was 4 02 inches in inside diameter of 16 gauge material the holes were round 1 4 inch in diameter arranged in 40 staggered circumferential rows of 17 holes per row so that the field extended for 12 inches along the tube and occupied 17 of the tube area chamber 24 was 12 inches long and 6 inches in internal diameter to deal with a band of frequencies in the audible range extending between 20 and 20 000 hertz the structure just described is known to generate severe whistles of which the number intensity and frequency of principal components vary when the flow through the tube is varied as is shown in fig1 and 2 the wall of tube 20 is formed with a plurality of inward beads 26 which extend completely around the tube and are spaced axially therealong in one embodiment of.