Patent GB-417935-A (GB)
Title: Improvements in and relating to sound wave attenuating apparatus
IPC: F01N1/02
Filed: 1933-04-10
Abstract: 417,935. Exhaust and like silencers. GENERAL MOTORS RESEARCH CORPORATION, Detroit, Michigan, U.S.A. April 10, 1933, No. 10668. Convention date, Sept. 15, 1932. [Class 7 (ii).] Apparatus for attenuating sound waves travelling through a passage comprises one or more chambers proportioned to respond by resonance to, or communicating with the passage at or near a nodal point of, the fundamental or lower harmonics of the passage. The form shown in Fig. 2 comprises a central perforated double-walled passage 23, 52 connected at its ends 19 and 21 to the exhaust pipe, and surrounded by an imperforate casing 16 closed at its ends by walls 17, 18, the annular space thus formed being subdivided by partitions 39, 47, 48 into several resonance chambers which communicate with the central passage through the perforations therein. The perforations 24 in the inner tube 23 are larger and less closely spaced than the perforations 53 in the outer tube 52. The first three annular chambers 36, 37, 38 comprising the section A of the silencer constitute a set of simple resonance units which are designed to attenuate the sound waves of higher frequencies. The remaining section B of the silencer is designed to attenuate sound waves of lower frequency, and comprises chambers 40 ... 46 which are fitted with tubular members 25, 32, 34 surrounding the central passage. The tube 25 is divided into sections 26...30 by rings 31, and a ring 35 divides the tube 34. The chambers 28 and 29 are respectively open to the chambers 41 and 42, and tubes 51 connect chambers 43, 44 and 45, 46 as shown. The chamber 30 forms a single resonator, and the chambers 26-40, 28-11, 29-42, 27-44-43 and 33-45-46 form series-compound resonance units. The perforations in the central passage connecting it to the resonance chambers are small and closely spaced, preferably 1/16 inch diameter spaced apart at # inch between their centres. Disturbance of the air in the resonance chambers by the flow of fluid through the central passage is further prevented by providing (a) flanges around the perforations, (b) baffles in the central passage on the upstream side of the perforations, (c) increasing the thickness of the walls of the central passage through which the perforations extend, or (d) providing the double-walled passage described above. The perforations may also be of varying size and spacing at different points in the length of the central passage. The resonance chambers responding to the fundamental or harmonics of the tube constituted by the central passage are preferably placed at or near the nodal points of the fundamental or the corresponding harmonics. The two resonance units 27-44-43 and 33-48-46 are dimensioned so as to resonate to the third and fifth harmonics respectively, and are situated along the central passage at one of the nodal points of the third harmonic and at the adjacent nodal point of the fifth harmonic. The chambers 26-40, 28-41, 29-42, and 30 respond to those frequencies higher than the third and fifth harmonics and yet relatively low, while the chambers 36, 37 and 38 respond to the relatively high frequencies. Other forms of apparatus of similar type are described, each comprising series of resonance chambers arranged around and along a central perforated passage. Specification 391,180 is referred to.