Patent Publication Number: US-6705429-B2

Title: Muffler

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates in general to mufflers and more particularly to mufflers of a type that is suitable for use with an internal combustion engine to muffle or silence exhaust noise produced by the engine. 
     2. Description of Related Art 
     Hitherto, various types of mufflers have been proposed and put into practical use particularly in the field of automotive internal combustion engines. 
     One of such mufflers is shown in Laid-open Japanese Patent Application (Tokkaihei) 9-125930. In FIGS. 15 and 16 of the accompanying drawings, there is shown the muffler of the Laid-open Application. FIG. 15 is a sectional view of the muffler and FIG. 16 is a sectional view taken along the line “XVI—XVI” of FIG.  15 . As is seen from the drawings, the muffler comprises a semi-cylindrical outer case  301  having longitudinal ends  301   a  and  301   b  closed, and a regularly cylindrical inner case  302  installed in outer case  301  to extend longitudinally. As is seen from FIG. 15, inner case  302  has a gas inlet part “IN” and a gas outlet part “OUT” at longitudinal ends. Within inner case  302 , there are arranged front and rear inner sections  303  and  304  each having a cylindrical wall  302   a  with a plurality of small openings  303   a  and  304   a . Between front and rear inner sections  303  and  304 , there is arranged an intermediate space section  306  that comprises a partition wall  305  formed with a plurality of small openings  306   a . Within an annular space defined between outer and inner cases  301  and  302 , there are arranged front and rear outer sections  403  and  404  that are separated by an intermediate space section  308  that comprises a partition wall  307  formed with a plurality of small openings  308   a . In operation, exhaust gas from an associated engine enters front inner section  303  through gas inlet part “IN”. One part of gas in front inner section  303  is led to rear inner section  304  through intermediate space section  306  and discharged to open air through gas outlet part “OUT”, while the remaining part of gas in front inner section  303  is led into front outer section  403  through small openings  303   a , into rear outer section  404  through small openings  308   a , into rear inner section  304  through small openings  304   a  and discharged to open air through gas outlet part “OUT”. During such flow in the muffler, acoustic energy or noise of the exhaust gas is reduced or damped due to expansion/resonance effect possessed by the gas flow passages. 
     While, Laid-open Japanese Patent Applications (Tokkaihei) 7-13573 and 7-175485 show a sound insulating structure that is used as an under cover of an engine room of a motor vehicle for blocking noises of exhaust system of the engine from being emitted to open air. That is, for blocking noises of exhaust system, the sound insulting structure employs an acoustically improved mechanism. 
     Laid-open Japanese Patent Application (Tokkaihei) 11-132024 shows a muffler that is produced by practically employing the acoustically improved mechanism of the above-mentioned publications 7-13573 and 7-175485. 
     SUMMARY OF INVENTION 
     However, due to inherent construction, the above-mentioned known mufflers have failed to provide users with a satisfaction. That is, some are poor in muffling performance, some are high in cost or some are difficult to assemble. 
     It is therefore an object of the present invention to provide a muffler that is high in muffling performance, low in cost and easy to assemble. 
     According to a first aspect of the present invention, there is provided a muffler which comprises a case having opposed ends closed; a partition structure installed in the case to constitute a sound silencing path; inlet and outlet pipes incorporated with the sound silencing path; and a sound shielding wall structure installed in the case to constitute a part of the sound silencing path, wherein the sound shielding wall structure comprises first and second partition plates; a positioning structure that puts the first and second partition plates together to keep a given distance therebetween; a first group of projections defined by the first partition plate and projected toward the second partition plate, each projection of the first group having a first opening formed therethrough; and a second group of projections defined by the second partition plate and projected toward the first partition plate, each projection of the second group having a second opening formed therethrough, the projections of the second group respectively facing the projections of the first group having a given clearance kept therebetween. 
     According to a second aspect of the present invention, there is provided a muffler which comprises a case having opposed ends closed; at least one partition plate installed in the case to divide an interior of the same into two chambers; a sound shielding wall structure installed in one of the two chambers to divide the same into first and second sound chambers allowing the other one of the two chambers to serve as a third sound chamber; and pipe members projected into the case passing through the partition plate and the sound shielding wall structure to define in the case a sound silencing path including the first, second and third sound chambers, wherein the sound shielding wall structure comprises first and second partition plates; a positioning structure that puts the first and second partition plates together to keep a given distance therebetween; a first group of projections defined by the first partition plate and projected toward the second partition plate, each projection of the first group having a first opening formed therethrough; and a second group of projections defined by the second partition plate and projected toward the first partition plate, each projection of the second group having a second opening formed therethrough, the projections of the second group respectively facing the projections of the first group having a given clearance kept therebetween. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a sectional view of a muffler that is an embodiment of the present invention; 
     FIG. 2 is a sectional view taken along the line “II—II” of FIG. 1; 
     FIG. 3 is an enlarged sectional view of a portion of a sound shielding wall structure employed by the muffler of the invention; 
     FIG. 4 is an enlarged sectional view of a positioning structure employed by the sound shielding wall structure; 
     FIG. 5 is a sectional view of the sound shielding wall structure; 
     FIG. 6 is an enlarged sectional view of a part of the sound shielding wall structure where spot-welding is used; 
     FIG. 7 is a view similar to FIG. 2, but showing a modification of the sound shielding wall structure; 
     FIG. 8 is a view schematically showing two partition plates employed in the sound shielding wall structure; 
     FIG. 9 is a view of a vibration model for explaining an acoustic mechanism established by the sound shielding wall structure; 
     FIG. 10 is a view similar to FIG. 9, but showing a modification of the sound shielding wall structure; 
     FIG. 11 is a view similar to FIG. 9, but showing an acoustic mechanism established by the modification of FIG. 10; 
     FIGS. 12,  13  and  14  are views similar to FIG. 4, but showing modifications of the positioning structure; 
     FIG. 15 is a sectional view of a known muffler; and 
     FIG. 16 is a sectional view taken along the line “XVI—XVI” of FIG.  15 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, the present invention will be described in detail with reference to the accompanying drawings. 
     Referring to FIGS. 1 to  6 , there is shown a muffler  50  according to the present invention. 
     As is best seen from FIGS. 1 and 2, muffler  50  comprises a cylindrical outer case  2  that has longitudinal ends  3  and  4  closed. In practice, front and rear plates are welded to longitudinal open ends of outer case  2  to constitute the closed ends  3  and  4 . 
     Hereinafter, the closed ends  3  and  4  will be referred to front and rear plates for ease of description. Furthermore, for easy understanding, the portion of outer case  2  where front plate  3  is provided will be referred to a front portion of muffler  50 , and the portion of outer case  2  where rear plate  4  is provided will be referred to a rear portion of muffler  50 . 
     As is seen from FIG. 1, within outer case  2 , there are arranged a sound shielding wall structure  5  and a partition plate  6  which are axially spaced. With provision of such wall structure  5  and partition plate  6 , there are defined first, second and third sound chambers  7 ,  8  and  9  in outer case  2 . That is, first and third sound chambers  7  and  9  are respectively provided at the front and rear portions of muffler  50 , and second sound chamber  8  is placed between first and third sound chambers  7  and  9 . 
     Sound shielding wall structure  5  generally comprises first and second circular partition plates  10  and  20  which are put on each other in an after-mentioned manner. 
     An inlet pipe  31  is inserted into outer case  2  from the front portion in a manner to pass through front plate  3  and sound shielding wall structure  5 . As shown, inlet pipe  31  extends along a given axis eccentric to a center axis of outer case  2  and has an open inner end  31   a  exposed to second sound chamber  8 . 
     Although not shown in the drawings, inlet pipe  31  is connected to an exhaust part of an associated engine through an exhaust pipe. 
     A shorter resonator pipe  32  is held by partition plate  6  to extend along the given axis of inlet pipe  31 . Resonator pipe  32  has front and rear open ends exposed to second and third sound chambers  8  and  9 , so that second and third sound chambers  8  and  9  are communicated through resonator pipe  32 . 
     An outlet pipe  33  is inserted into outer case  2  from the rear portion in a manner to pass through rear plate  4 , partition plate  6  and sound shielding wall structure  5 . 
     As is best seen from FIG. 2, outlet pipe  33  extends along another given axis eccentric to the center axis “0” of outer case  2 . More specifically, inlet pipe  31  and outlet pipe  33  are arranged symmetrically with respect to an imaginary plane “X1” that includes the center axis “0” and extends along the same. 
     As is seen from FIG. 1, outlet pipe  33  has an open inner end  33   a  exposed to first sound chamber  7  and has an open outer end exposed to open air. 
     In the following, the detail of sound shielding wall structure  5  will be described with the aid of the drawings. 
     As has been described hereinabove, the wall structure  5  comprises generally first and second circular partition plates  10  and  20  that are put on each other. 
     As is seen from FIG. 1, first partition plate  10  has an outer diameter identical to an inner diameter of outer case  2 . As is seen from FIGS. 1 and 5, first partition plate  10  comprises a main portion  11  that has a cylindrical peripheral edge  12  that is directed forward to be neatly received in outer case  2 . 
     As is seen from FIG. 5, main portion  11  is formed with supporting portions  13  and  14  for supporting inlet and outlet pipes  31  and  33  respectively. Furthermore, main portion  11  is formed with a plurality of openings  15   a  and a positioning projection (no numeral). As will be described hereinafter, the positioning projection is used for achieving a relative positioning between first and second partition plates  10  and  20 . As is seen from FIG. 3, each opening  15   a  of main portion  11  is defined in a rearward projected portion formed on main portion  11 . 
     As is seen from FIGS. 1 and 6, cylindrical peripheral edge  12  of first partition plate  10  is spot-welded to the inner wall of outer case  2 . 
     As is seen from FIGS. 1 and 5, second partition plate  20  of sound shielding wall structure  5  has a diameter that is smaller is than that of first partition plate  10 . A main portion  21  of second partition plate  20  is formed with supporting portions  23  and  24  for supporting inlet and outlet pipes  31  and  33  respectively. Furthermore, main portion  21  is formed with a plurality of openings  25   a  and a positioning projection  26 . As will be described hereinafter, the positioning projection  26  is used for achieving the relative positioning between first and second partition plates  10  and  20 . As is seen from FIG. 3, each opening  25   a  of main portion  21  is defined in a forward projected portion formed on main portion  21 . 
     As is seen from FIGS. 5 and 6, a circular peripheral edge  22  of main portion  21  is spot-welded to a peripheral portion of a rear surface of main portion  11  of first partition plate  10 . 
     Accordingly, as is seen from FIG. 6, respectively main portions  21  and  11  of second and first partition plates  20  and  10  are spaced apart by a certain distance. More specifically, as is seen from FIG. 3, a leading edge of each rearward projected portion ( 15 ) of first partition plate  10  and that of each forward projected portion ( 25 ) of second partition plate  20  are spaced apart by a predetermined distance of “L”. 
     In the following, the detail of openings  15   a  and  25   a  of first and second partition plates  10  and  20  will be described with reference to FIG. 3 that is an enlarged, partial and sectional view taken along the line “III—III” of FIG.  2 . 
     As is seen from FIG. 3, first and second partition plates  10  and  20  are formed with first and second groups of projections  15  and  25  each having an opening  15   a  or  25   a . That is, each of first group of projections  15  of first partition plate  10  and that of second group of projections  25  of second partition plate  20  are arranged to face each other keeping the given distance “L” therebetween. As is understood from FIG. 2, projections  15  and  25  (or openings  15   a  and  25   a ) formed in first and second partition plates  10  and  20  are arranged to have a given distribution suitable for obtaining a desired performance. 
     As is described hereinabove, each rearward projection  15  of first partition plate  10  and corresponding forward projection  25  of second partition plate  20  faces each other with a space of “L” kept therebetween. The space is denoted by numeral  40  in FIG.  3 . It is to be noted that the opening  15   a  of each rearward projection  15  of first partition plate  10  is coaxial with the opening  25   a  of the corresponding forward projection  25  of second partition plate  20 . That is, the openings  15   a  and  25   a  of first and second partition plates  10  and  20  are communicated through the space  40 . 
     As is understood from FIG. 3, each opening  15   a  or  25   a  of first or second partition plate  10  or  20  is of a cylindrical shape having a smoothly curved inner wall  11   b  or  21   b . A radius of curvature possessed by each opening  15   a  or  25   a  is denoted by “R” in the drawing. Due to provision of such smoothly curved inner walls  11   b  and  21   b , exhaust gas flow from second sound chamber  8  to first sound chamber  7  is smoothly made, which suppresses or at least minimizes any noise produced when the gas flows in openings  25   a  and  15   a . Furthermore, by the same reason, undesired separation of gas flow and construction flow, such as those described in Laid-open Japanese Patent Application (Tokkaihei) 11-132024, are suppressed or at least minimized. 
     In the following, positioning openings  16   a  and  26   a  respectively formed in first and second partition plates  10  and  20  for achieving a relative positioning between the two plates  10  and  20  will be described with reference to FIG. 4 that is an enlarged, partial and sectional view taken along the line “IV—IV” of FIG.  2 . 
     As is seen from FIG. 4, first partition plate  10  is formed with a positioning projection  16  that projects rearward. Positioning projection  16  has an opening  16   a  formed therethrough. Second partition plate  20  is formed with a positioning projection  26  that projects forward. Projection  26  has an opening  26  formed therethrough. As shown, Projection  26  of second partition plate  20  is press-fitted into opening  16   a  of positioning projection  16  of first partition plate  10 . That is, positioning projection  16  of first partition plate  10  projects rearward from a rear surface  11   a  of mail portion  11  of the same, and projection  26  of second partition plate  20  projects forward from a front surface  21   a  of main portion  21  thereof. More specifically, as is seen from the drawing, positioning projection  16  is constructed to have a rounded bank portion  16   b  that projects rearward. It is to be noted that opening  16   a  of positioning projection  16  has an oval shape. 
     Furthermore, as is seen from FIG. 4, projection  26  of second partition plate  20  has an oval cross section and has an oval opening  26   a  formed therethrough. That is, due to matching in shape, projection  26  of second partition plate  20  is intimately fitted in opening  16   a  of first partition plate  10  thereby to achieve a relative positioning between first and second partition plates  10  and  20 . Upon coupling, the front surface  21   a  of second partition plate  20  abuts against a top  16   c  of rounded bank portion  16   b  of first partition plate  10 , as shown. Due to provision of the mutually engaged projections  16  and  26 , openings  15   a  and  25   a  of first and second partition plates  10  and  20  precisely face to one another. Because of the oval shape of projection  26  and opening  16   a , a play between first and second partition plates  10  and  20  is suppressed once they are tightly mated. Furthermore, manual work for coupling the positioning projection  26  with the positioning opening  16   a  is easily made because the shape of opening  16   a  is different from that of openings  15   a.    
     With the above-mentioned openings  15   a ,  25   a  and  26   a  possessed by sound shielding wall structure  5 , there is provided a fluid communication between first and second sound chambers  7  and  8 . 
     It is now to be noted that the positioning projections  16  and  26  are provided at given portions of first and second partition plates  10  and  20  where the plates  10  and  20  are subjected to a primary vibration of resonance. With this measure, undesired noise caused by the resonance is suppressed or at least minimized. 
     More specifically, as is seen from FIG. 2, in muffler  50  of the invention, positioning projections  16  and  26  are placed on an imaginary line “X1” at a position ( 26 ,  16 ) that is opposite to a position where the line “X1” and another imaginary line “Y1” passing through central portions of supporting portions  23  and  24  that support inlet and outlet pipes  31  and  33  intersect at right angles, the line “X1” being a line that passes through the center axis “0” of outer case  2  and is perpendicular to the imaginary line “Y1”. 
     In the following, assembling steps for installing sound shielding wall structure  5  in outer case  2  will be described. 
     First, as is understood from FIG.  5  and FIG. 4, first and second partition plates  10  and  20  are coupled together having positioning projection  26  of second plate  20  press-fitted into the positioning opening  16   a  of first plate  10 . Then, as is seen from FIG. 6, the circular peripheral edge  22  of second plate  20  is spot-welded to the peripheral portion of the rear surface of first plate  10 . With these steps, sound shielding wall structure  5  is produced. In the produced structure  5 , as is understood from FIG. 3, each opening  15   a  of first plate  10  faces the corresponding opening  25   a  of second plate  20  keeping a certain distance “L” therebetween. 
     Then, as is seen from FIG. 6, the sound shielding wall structure  5  thus produced is put into outer case  2  and the cylindrical peripheral edge  12  of first plate  10  is spot-welded to the inner wall of outer case  2 . 
     In the following, operation of muffler  50  will be described with reference to FIG.  1 . 
     As has been described hereinabove, inlet pipe  31  is connected to an exhaust part of an associated internal combustion engine through an exhaust pipe, and outlet pipe  33  has the open end exposed to the open air. 
     Under operation of the engine, exhaust gas is led into muffler  2  through inlet pipe  31 . Thus, in inlet pipe  31 , there is produced a pulsation flow of exhaust gas. The exhaust gas is led into second sound chamber  8  at first. Then, a part of the gas is led into third sound chamber  9  through resonator pipe  32 . 
     While, a greater part of the gas in second sound chamber  8  is led into first sound chamber  7  through the openings  15   a ,  25   a  and  26   a  of sound insulating wall structure  5 , and led into the open air through outlet pipe  33 . 
     It is to be noted that under flowing of the exhaust gas from second sound chamber  8  to first sound chamber  7  through the openings  15   a ,  25   a  and  26   a , a suitable sound shielding effect is carried out by sound insulating wall structure  5  and thus muffler  50  can effectively shield the noise of the exhaust gas. Acoustic mechanism for damping the noise will be described hereinafter. 
     If desired, the following modification  5 A of sound shielding wall structure  5  may be employed in muffler  50  of the invention. 
     That is, as is seen from FIG. 7, about a half of openings  15   a  and  25   a  of first and second partition plates  10  and  20  may be directly connected without producing a clearance “L” therebetween. In this drawing, openings  25   a  (or  15   a ) illustrated by hatched circles are those that leave the clearance “L”, while openings  25   a  (or  15   a ) illustrated by blank circles are those that have no clearance “L”. As is seen from this drawing, these two types of openings  25   a  (or  15   a ) are uniformly distributed. 
     Furthermore, if desired, the openings  25   a  (or  15   a ) that have no clearance “L” may have the same construction as the above-mentioned positioning opening  26   a  (or  16   a ) of positioning projection  26  (or  16 ). That is, a so-called male-female connection is made between the openings  25   a  and  15   a . Thus, in this case, because of provision of the male-female connection, there is no need of providing the above-mentioned positioning projections  16  and  26 . 
     In the following, acoustic mechanism for damping or silencing the exhaust noise by muffler  50  of the invention will be described with the aid of disclosure of Laid-open Japanese Patent Application (Tokkaihei) 7-175485. 
     Referring to FIGS. 8 and 9, there is diagrammatically shown sound shielding wall structure  5  provided by muffler  50  of the invention. FIG. 9 shows a vibration model for explaining the acoustic mechanism established by the sound shielding wall structure  5 . 
     As is seen from FIG. 8, the sound shielding wall structure  5  comprises first and second partition plates  101  and  102  that correspond to the above-mentioned first and second partition plates  20  and  10  respectively. These two plates  101  and  102  are spaced from each other by the distance “L”. Each plate  101  or  102  has a plurality of openings  101   a  or  102   a , which correspond to  25   a  or  15   a  of the above-mentioned plates  20  and  10 . As shown, the openings  101   a  are arranged to face the openings  102   a  respectively. 
     When, as is seen from FIG. 9, it is assumed that the mass of air in openings  101   a  and  102   a  is “m” and air put between first and second partition plates  101  and  102  serves as an air spring  105  of spring constant “k”, a given vibration system with two factors (viz., “m” and “k”) is established. In FIG. 9, denoted by reference I.W. is an incident wave, R.W. is a reflected wave and T.W. is a transmitted wave. 
     With the vibration system thus established, the following consideration would be provided. 
     That is, when air  103  of mass “m1” in openings  101   a  of first partition plate  101  is vibrated by the open air (viz., exhaust gas led into second sound chamber  8 ), the vibration is transmitted through the air spring  105  to air  103  of mass “m2” in openings  102   a  of second partition plate  102 . The vibration of air  103  of mass “m2” then vibrates the open air (viz., exhaust gas in first sound chamber  7 ). The vibration of the open air produces the noise of exhaust gas. 
     In such acoustic mechanism, attention is paid on a transmission rate of vibration between air  103  of mass “m1” and air  103  of mass “m2”. That is, in the vibration system with two factors (viz., “m” and “k”), a certain sound shielding effect is obtained when, with the vibration kept above a resonance point, the vibration transmission rate is smaller than 1 (one). That is, in the acoustic system of FIG. 9, first and second partition plates  101  and  102  can exhibit a sound shielding effect when they are vibrated at a frequency higher than a resonance frequency. The resonance frequency of the acoustic system of FIG. 9 can be controlled by varying the thickness of first and second partition plates  101  and  102 , the number of openings  101   a  and  102   a  and the distance between the two plates  101  and  102 . By practically employing this fact, the acoustic system can be adjusted to shield a noise having a specified frequency. That is, in the present invention, sound shielding wall structure  5  installed in outer case  2  practically uses the acoustic mechanism of FIG.  9 . 
     In the following, description will be directed to the modification of sound shielding wall structure  5  wherein some or about a half of openings  15   a  and  25   a  of first and second partition is plates  10  and  20  are directly connected without leaving the clearance “L” therebetween. 
     Referring to FIGS. 10 and 11, there is schematically shown the modification of the shielding wall structure  5 . FIG. 11 shows a vibration model for explaining the acoustic mechanism established in the modification. 
     As is seen from FIG. 10, in this modification, like in the above-mentioned wall structure  5 , two partition plates  151  and  152  having respective openings  151   a ,  151   b ,  152   a  and  152   b  are provided. However, as is seen, some  151   b  of the openings of first plate  151  are connected to corresponding openings  152   b  of second plate  152  through tubular portions  153 . 
     As is seen from FIG. 11, due to provision of such tubular portions  153 , first and second partition plates  151  and  152  constitute a single structure. 
     When it is assumed that the mass of air in openings  151   a  and  152   a  is “m” and air put between first and second partition plates  151  and  152  serves as an air spring  155  of spring constant “k”, a given vibration system with two factors (viz., “m” and “k”) is established. For ease of description, this vibration system will be referred to “double factor vibration system” hereinafter. 
     In addition to the above-mentioned double factor vibration system, another vibration system is also provided in the modification, in which air  156  of mass “m” received in tubular portions  153  forms one factor of the vibration system. This vibration system has no resonance point and thus incident wave and transmitted wave are in the same phase. For ease of description, this vibration system will be referred to “single factor vibration system” hereinafter. 
     In the modification having the above-mentioned two, viz., single and double factor vibration systems, incident wave is separately treated by these two vibration systems. That is, one part of incident wave entering the single factor vibration system provides a transmitted wave having the same phase as the incident wave. While, the other part of incident wave entering the double factor vibration system provides a transmitted wave having a phase differing from that of the incident wave by 180 degrees. This means that the transmitted wave from the single factor vibration system and that from the double factor vibration system cancel out each other, and thus an appropriate sound shielding effect is obtained from the modification. 
     Referring to FIGS. 12,  13  and  14 , there are shown other modifications  5 B,  5 C and  5 D of sound shielding wall structure  5  which may be employed in muffler  50  of the invention. As will become apparent from the following description, these modifications  5 B,  5 C and  5 D have different structures on positioning projections  16  and  26  of first and second partition plates  10  and  20 . 
     In modification  5 B of FIG. 12, a positioning projection  55  possessed by second partition plate  20  is substantially the same as the above-mentioned positioning projection  26  of second partition plate  20  (see FIG.  4 ). While, a positioning projection  51  possessed by first partition plate  10  is different from the above-mentioned positioning projection  16  of first partition plate  10  (see FIG.  4 ). That is, in this modification  5 B, positioning projection  51  has an annular leading end  51   b  that is directed toward second partition plate  20 . Upon assembly, positioning projection  55  is intimately thrust into an opening  51   a  of positioning projection  51 , as shown. With this, an opening  55   a  of positioning projection  55  provides a fluid communication between first and second sound chambers  7  and  8 . Upon insertion of positioning projection  55  into opening  51   a , the annular leading end  51   b  abuts against a root portion of positioning projection  55  thereby separating first and second partition plates  10  and  20  away from each other by a distance that is enough for keeping the predetermined distance “L” between the leading edge of each rearward projected portion  15  (see FIG. 3) of first partition plate  10  and that of corresponding forward projected portion  25  of second partition plate  20 . 
     In modification  5 C of FIG. 13, a positioning projection  65  possessed by second partition plate  20  has a stepped portion  65   b , and a positioning projection  61  possessed by first partition plate  10  has an annular leading end  61   b  that is directed away from second partition plate  20 . Upon assembly, an annular leading portion  65   a  of positioning projection  65  is intimately thrust into an opening  61   a  of positioning projection  61  from the back of first partition plate  10 , as shown. With this, an opening  65   c  of positioning projection  65  provides a fluid communication between first and second sound chambers  7  and  8 . Upon insertion of the annular leading portion  65   a  into opening  61   a , stepped portion  65   b  of positioning projection  65  abuts against the rear surface of first partition plate  10  thereby separating first and second partition plates  10  and  20  away from each other by a distance that is enough for keeping the predetermined distance “L” between the leading edge of each rearward projected portion  15  (see FIG. 3) of first partition plate  10  and that of corresponding forward projected portion  25  of second partition plate  20 . 
     In modification  5 D of FIG. 14, a positioning projection  65  possessed by second partition plate  20  is substantially the same as that shown in FIG.  13 . While, a positioning projection  61  is different from that shown in FIG.  13 . That is, in this modification  5 D, positioning projection  61  is provided with an annular raised portion  61   b  that is directed toward second partition plate  20 . Upon assembly, an annular leading portion  65   a  of positioning projection  65  is intimately thrust into an opening  61   a  of positioning projection  61  from the back of first partition plate  10 , as shown. With this, an opening  65   c  of positioning projection  65  provides a fluid communication between first and second sound chambers  7  and  8 . Upon insertion of the annular leading portion  65   a  into opening  61   a , stepped portion  65   b  of positioning portion  65  abuts against a top of the annular raised portion  61   b  of first partition plate  10  thereby separating first and second partition plates  10  and  20  away from each other by a distance that is enough for keeping the predetermined distance “L” between the leading edge of each rearward projected portion  15  (see FIG. 3) of first partition plate  10  and that of corresponding forward projected portion  25  of second partition plate  20 . 
     In the above-mentioned modifications  5 B,  5 C and  5 D of FIGS. 12 to  14 , description is directed to only the positioning projections  51 ,  55 ,  61  and  65  which are used for achieving a relative positioning between first and second partition plates  10  and  20 . However, if desired, the measures of such modifications  5 B,  5 C and  5 D may be applied to projections  15  and  25  of first and second partition plates  10  and  20 . 
     In the above-mentioned sound shielding wall structures  5 ,  5 A,  5 B,  5 C and  5 D, only one positioning structure is provided which comprises positioning projections  16  and  26 ,  51  and  55 , and  61  and  65 . However, if desired, two or more positioning structures may be employed for achieving much assured relative positioning between first and second partition plates  10  and  20 . Furthermore, the positioning opening  16   a  of positioning projection  16  of first partition plate  10  may have a triangular shape, rectangular shape or the like, that is, an angled shape other than the above-mentioned oval shape. 
     The entire contents of Japanese Patent Application 2002-040636 filed Feb. 18, 2002 are incorporated herein by reference. 
     Although the invention has been described above with reference to one embodiment and modifications of the embodiment, the invention is not limited to such embodiment and modifications as described above. More modifications and variations of such embodiment may be carried out by those skilled in the art, in light of the above description.