From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Patent specification for a sound wave attenuating unit, such as an exhaust silencer, for an internal combustion engine.
| Identifier | ExFiles\Box 147\1\ scan0076 | |
| Date | 10th April 1933 guessed | |
| 2 417,935 over a sound wave attenuating unit in which only one of the mentioned types of resonance units is included. 5 The invention accordingly consists in an apparatus for attenuating sound waves set up by forced vibrations in a sound wave passage, including a unit of at least 10 one chamber for attenuating a lower characteristic (fundamental or lower harmonic) of the passage, the said unit being characterized in that (1) it communicates with the passage at or rear a nodal point 15 of the characteristic to be attenuated, and/or (2) it responds to and attenuates by resonance sound waves the frequency of which is equal to the fundamental frequency of the passage or a lower har- 20 monic. By the term " lower harmonic " as used herein is meant a harmonic the wave length of which is at least twice the length of the zone of communication between the sound wave passage and the 25 attenuating unit under consideration. Best results are obtained by having the chamber, or a resonance unit comprising several chambers, connected to the sound wave passage exactly at the nodal point 30 of the fundamental or one of the lower harmonics of the passage and by designing it to attenuate sound waves of the frequency of the fundamental or the harmonic at the nodal point of which the 35 chamber or unit is located. The advantages of the invention are however in part achieved by locating the chamber or unit near, though not exactly at, the nodal point and also by locating a 40 chamber or unit at or near a nodal point of the fundamental or one of the lower harmonics whether or not it is especially designed to attenuate sound waves of that frequency and, conversely, 45 by designing it to attenuate sound waves of the frequency of the fundamental or one of the lower harmonics of the passage whether or not it is located at a particular point along the passage. 50 For a better understanding of the nature and objects of the present invention, reference is made to the following specification in which are described the preferred embodiments of our invention 55 which are illustrated in the accompanying drawing. In the accompanying drawing: Fig. 1 is a view showing the exhaust system of the internal combustion propel- 60 ling engine of an automotive vehicle. Fig. 2 is a longitudinal section through the sound wave attenuating unit of the exhaust system illustrated in Fig. 1. Fig. 3 is a section taken on the line 65 3—3 of Fig. 2 and drawn to an enlarged scale. Figs. 4, 5, 6 and 7 are longitudinal sections through modified forms of sound wave attenuating units. 70 In Fig. 1 of the drawing there is shown the chassis 10 of an automotive vehicle on which there is installed an internal combustion propelling engine 11 from which the exhaust gases are discharged 75 through an exhaust manifold 12 into the exhaust pipe 13 whence they pass, successively, through the sound wave attenuating unit 14 and the tail pipe 15 to the atmosphere. 80 The sound wave attenuating unit 14 which is shown in Figures 1, 2 and 3 consists of an imperforate tubular shell 16 whose opposite ends are closed by heads 17 and 18. Through the head 17, there 85 extends an inlet opening 19 surrounded by collar 20 which is adapted to be connected to the exhaust pipe 13 and through the head 18, there extends a discharge opening 21 surrounded by collar 22 which 90 is adapted to be connected to the tail pipe 15. The heads 17 and 18 and the openings 19 and 21 are connected by an unobstructed double-walled tube 23—52 whose outer wall 52 throughout 95 its length there extend a multitude of small, closely spaced perforations and through whose inner wall 23 throughout its length there extend a plurality of larger, more distantly spaced 100 perforations 24. An intermediate portion of the tube 23—52 is surrounded by a tube 25 which is imperforate except for openings connecting chambers 28 and 29 and chambers 29 and 42 and is circum- 105 ferentially spaced from and secured to the tube 23—52 by annular elements 31 in conjunction with the tubes 105 and 25, define chambers 28, 29. The end of the tube 25 nearest 110 the head 17 is surrounded by a tubular element 54 which is circumferentially spaced therefrom and mounted on the annular wall 55 and which, in conjunction with the adjacent end of the tube 25, 115 and the adjacent element 31, the tube and the wall 55, defines a chamber 26 which communicates with a chamber 40 through the annular space between the element 54 and the tube 25. The portion 120 of the tube 23—52 adjacent the discharge end of the attenuating unit is surrounded by an imperforate tube 32 which is mounted on the head 18 and circumferentially spaced from the tube 23—52. The 125 closed ends of the tubes 25 and 32, which are spaced apart axially of the attenuating unit, are surrounded by an imperforate tube 34 which is circumferentially spaced therefrom and is 130 secured to the tube 23—52 by an annular spacer 35. The tube 34, in conjunction with the adjacent end of the tube 25, the 417,935 3 element 31, the spacer 35 and adjacent element 31, the spacer 35 defines a chamber 27 which communicates with a chamber 44 5 through the annular space between the tubes 25 and 34 and, in conjunction with the tube 32, the head 18, the spacer 35 and the tube 23—52, defines a chamber 30 which communicates with a chamber 10 45 through the annular space between the tubes 32 and 34. The space between the portions of the tube 23—52 and the shell 16 between the head 17 and the wall 55 is divided by 15 imperforate annular walls 36, 37 and 38 into annular chambers 26—40, 27—44, and 28—41, respectively. The space between the portions of the tube 23—52 and the shell 16 is divided into annular chambers 40, 41, 42, 20 43, 44, 45 and 46 by imperforate annular walls 47 and 48 and annular walls 50 and 51. Each of the chambers 28, 29, 30, 36, 37, 38, 40, 41, 42, 43, 44, 45 and 46 constitutes a resonance chamber, and 25 the chambers 26, 27 and 33 may be considered as constituting parts of the passages through which the interior of the tube 23—52 is acoustically connected to the chambers 40, 44 and 45, 30 respectively, they are preferably considered as resonance chambers which constitute elements of the series-compound resonance units 26—40, 27—44 and 33—45—46, respectively. 35 An inspection of the drawing will show that, in the sound wave attenuating unit shown in Figures 1, 2 and 3, the several resonance chambers are so arranged that the sound wave attenu- 40 ating unit may be considered to consist of two sections, A and B. The section A includes the simple resonance units 36, 37 and 38, and constitutes a multiple-compound resonance unit. The section 45 B includes the simple resonance unit 30 and the series-compound resonance units 26—40, 28—41, 29—42, 27—44, 43 and 33—45—46. Either section B, as a whole, or the sound wave attenuating 50 unit, as a whole, may be considered as constituting a complex multiple-compound resonance unit. Each of the several simple and series-compound resonance units referred to 55 above is so proportioned and dimensioned that it will respond to and attenuate certain of the sound waves which occur in the exhaust of the engine 11. The multiple-compound resonance unit 36— 60 37—38 is so proportioned and dimensioned that it responds to and attenuates other sound waves which occur in the exhaust. The sound waves of the higher frequencies are attenuated, primarily by 65 the section A, and secondarily by the sound wave attenuating unit as a whole. functioning as a complex multiple-compound resonance unit. The sound waves 70 of the lower frequencies are attenuated by the section B. While both simple and series-compound resonance units and multiple-compound 75 resonance units attenuate sound waves by resonance, the mechanics of the process by which a multiple-compound resonance unit attenuates sound waves whose frequencies are higher or lower than that to 80 which it is designed to respond appears to differ from the mechanics of the process by which a simple or series-compound resonance unit attenuates such sound waves. Since a simple or series-com- 85 pound resonance unit will attenuate almost completely sound waves whose frequency or frequencies are equal to that to which it is designed to respond and increasingly less completely sound 90 waves of higher and lower frequencies, it is to be expected that a multiple-compound resonance unit of which the several constituent resonance units are similar will almost completely attenuate sound 95 waves whose frequency or frequencies is equal to that to which the several constituent resonance units, individually, are designed to respond and increasingly less completely sound waves of higher 100 and lower frequencies. However, a multiple-compound resonance unit more completely than one of its constituent resonance units attenuates sound waves whose frequencies are higher or lower 105 than that or those to which its several constituent resonance units, individually, are designed to respond and an increase in the number of constituent resonance units increases the completeness with 110 which it attenuates such sound waves. This appears to indicate that complete attenuation by a multiple-compound resonance unit of sound waves whose frequency or frequencies are higher or lower 115 than that or those to which its several constituent resonance units, individually, are designed to respond is the result of successive partial attenuation of such sound waves by each of the several con- 120 stituent resonance units. While, theoretically, a sound wave of any frequency may be attenuated by any multiple-compound resonance unit of the type under consideration, irrespective of the propor- 125 tions and dimensions of the several constituent resonance units, if a sufficiently large number of the constituent resonance units are incorporated therein, it is generally preferable, in order to reduce 130 the number of constituent resonance units to a minimum, so to proportion and dimension the several constituent resonance units that, individually, they | ||