Abstract:
A muffler is constructed to have a tubular perforated plate within its cavity so as to divide the cavity into an outer resonant cavity and an inner resonant cavity, to thereby provide two resonant frequencies. The tubular perforated plate is formed in the shape of a funnel.

Description:
TECHNICAL FIELD 
     The present invention relates to a muffler and more particularly to a resonant type muffler. 
     BACKGROUND OF THE INVENTION 
     In order to attenuate noise, a muffler is employed and its type is generally classified as a sound absorbing type or a resonant type, etc. A sound absorbing type muffler is provided with a sound absorbing material filled within a muffler tube. Glass fiber, rock wool or mineral wool is selected for a tube filling. However, these materials cannot be used in a sound absorbing type muffler if pollution due to a sound absorbing material is to be avoided. 
     On the other hand, in the case of a resonant type muffler, attenuation of sound may be insufficient if the dimension of the muffler in the fluid flowing direction is such that it restricts the length of its resonant cavity. Also, in the conventional resonant type mufflers, attenuation can only be effected within only a narrow frequency band due to the provision of a single resonant cavity. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a resonant type muffler, which can effectively attenuate sound over a wide frequency band. 
     The above object is accomplished by a muffler constructed according to the present invention. According to the present invention, a muffler is constructed to have a tubular perforated plate within a cavity of a barrel which is formed in the shape of a funnel and is in communication with inlet and outlet tubes of the muffler. The tubular plate divides the cavity into an outer resonant cavity and an inner resonant cavity so that the muffler possesses two resonant frequencies and attenuation is effected over a relatively wide frequency band. 
     Further details will be given in the detailed description of the embodiment following the explanation of the drawings given below. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 schematically shows an example of a conventional muffler; 
     FIG. 2 is a drawing showing a longitudinal sectional view of an embodiment according to the present invention; 
     FIG. 3 is a cross sectional view taken along the line X--X in FIG. 2; 
     FIG. 4 is a graph showing a comparison of the attenuation provided a muffler according to the present invention and that provided by a conventional muffler; 
     FIG. 5 is a cross sectional view similar to FIG. 3, but showing a different embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before explaining the present invention, a conventional resonant type muffler will briefly be explained referring to FIG. 1. 
     An example of a conventional resonant type muffler is shown in FIG. 1 and comprises a cylindrical or square column cavity 20 and a tube 21 extending through the cavity 20. Within the cavity 20, the tube 21 is provided with multiple perforations 22 in the wall thereof so that the tube 21 constitutes a perforated plate within the cavity. In this type of muffler, if a sufficient length of the muffler in the fluid flowing direction is not present within the cavity, the surface area of the perforated plate is small, such that the sound attenuation is insufficient. Also, in the construction of a conventional muffler, there is only a single resonant cavity. Thus, attenuation is limited to a narrow frequency band around the resonant frequency. 
     Now, referring to FIGS. 2 and 3, a muffler according to the present invention is shown schematically. In the drawings, numeral 7 designates an outer barrel which in this embodiment is shown as a square column type. However, the muffler may be in the form of a cylinder 7&#39; (as shown in FIG. 5). A cavity is defined within the barrel 7, 7&#39; by side plates 6, 8 and the barrel 7, 7&#39;. To the side plates 6 and 8, an intake tube 1 and an outlet tube 5 are attached, respectively. Within the barrel 7, 7&#39; a perforated plate 2 is installed. The plate 2 is formed in a cylindrical shape which expands from the side of the inlet tube 1 towards the side of the outlet tube 5 in a manner resembling a funnel shape. The minimum cross sectional area of the perforated plate 2 relative to the fluid flowing direction is equivalent to, or more than, that of the intake tube and the maximum cross sectional area thereof is arranged to be larger than that of the outlet tube. It is to be noted that the direction of expansion of the perforation plate 2 may be opposite to that shown in FIG. 2. By the tubular perforated plate 2, the cavity of the barrel 7, 7&#39; is divided into an outer resonant cavity 3 and an inner resonant cavity 4. 
     In the muffler constructed as above, a sound wave entering from the inlet tube 1 passes through the perforated plate 2 and propagates within the resonant cavity 3 wherein it is attenuated by resonance in the cavity 3. Further, the sound wave also causes resonance in the resonant cavity 4 and is attenuated. This muffler, thus, possesses two kinds of resonant cavities and has a resonant frequency at two points as indicated by the curve A (solid line) in FIG. 4. In FIG. 4, the curve B (dotted line) represents a conventional muffler which has only resonant frequency. So, in the conventional muffler, attenuation is effected only around this one resonant frequency. However, in the muffler according to the present invention, attenuation is effected in a relatively wide frequency band around the two resonant frequencies whereby attenuation is greatly enhanced compared to attenuation provided by a conventional muffler.