Abstract:
A noise-reducing device for water vehicles including: a rigid housing having a fluid-tightly sealed chamber formed therein, and fixable to a vibrating part of the water vehicle to be displaced with the vibrating part; a mass member housed in the rigid housing with a given gap therebetween so that the mass member is freely movable in the vibrating direction relative to the rigid housing, the mass member being adapted to come into collision with a contact part of the housing during input of vibration; and an elastic element formed on at least one surface of contact on the contact part or mass member.

Description:
INCORPORATED BY REFERENCE  
       [0001]     The disclosure of Japanese Patent Application No. 2003-419127 filed on Dec. 17, 2003 including the specification, drawings and abstract is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a noise-reducing device for water vehicles or other small vessels in which thrust is provided when water taken in by a jet pump is pressurized and accelerated, and is blasted to the rear by means of a jet nozzle.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, there has been more widespread recreational use and the like of water vehicles in which thrust is provided when water taken in by a jet pump is pressurized and accelerated, and is blasted to the rear by means of a jet nozzle. The noise produced by such water vehicles when in use has been pointed out as a problem.  
         [0006]     To cope with this problem, several attempts have been made for reducing the noise by elastically supporting an engine on a hull of the water vehicles, as disclosed in JP-2000-255491, for example, in which the engine, i.e., a source of vibration in a water vehicle, is elastically supported by engine mounts that include rubber mounts. As noted above, thrust is provided in water vehicles when water taken in by a jet pump is pressurized and accelerated, and is blasted to the rear by means of a jet nozzle. However, these conventional attempts are not still sufficient to reduce noises produced by such water vehicles.  
         [0007]     An extensive study on water vehicles conducted by the present inventors has revealed that the jet pump can also act as a source of vibration, and these vibrations can be transmitted to the hull, particularly the deck, where the hull acts as a speaker, producing noise.  
         [0008]     As one means of solution of this problem, it would be conceivable to affix a conventionally available mount rubber or dynamic damper. However, these conventional vibration-damping devices may suffer from inherent problem for use in water vehicles. Namely, in the mount rubbers and the dynamic dampers, rubber members as major components are exposed to the outside area, so that these rubber members will be exposed to sea or flesh water, resulting in a problem of deterioration of the rubber members, such as rubber member separation by salt spray. In particular, the water pump when in use is buried in fresh or sea water or at least exposed to water or salt spray, it is impossible to affix the rubber mounts or the dynamic damper to the water pump.  
       SUMMARY OF THE INVENTION  
       [0009]     It is therefore one object of this invention to provide a noise-reducing device for water vehicles which provide effective noise reducing performance against noise originating primarily from jet pump vibration (as well as against noise originating from other vibrative member).  
         [0010]     The above and/or optional objects of this invention may be attained according to at least one of the following modes of the invention. Each of these modes of the invention is numbered like the appended claims and depending from the other mode or modes, where appropriate, to indicate possible combinations of elements or technical features of the invention. It is to be understood that the principle of the invention is not limited to these modes of the invention and combinations of the technical features, but may otherwise be recognized based on the teachings of the present invention disclosed in the entire specification and drawings or that may be recognized by those skilled in the art in the light of the present disclosure in its entirety.  
         [0011]     A first mode of the present invention provides a noise-reducing device for water vehicles in which thrust is provided when water taken in by a jet pump is pressurized and accelerated, and is blasted to the rear by means of a jet nozzle, the noise-reducing device comprising: a rigid housing having a fluid-tightly sealed chamber formed therein, and fixable to a vibrating part of the water vehicle to be displaced with the vibrating part; a mass member housed in the rigid housing with a given gap therebetween so that the mass member is freely movable in the vibrating direction relative to the rigid housing, the mass member being adapted to brought into collision with a contact part of the housing during input of vibration; and an elastic element formed on at least one surface of contact on the contact part or mass member.  
         [0012]     The noise-reducing device operates in the following manner. When the vibrating part of the water vehicle begins to vibrate, the contact part of the housing is displaced with it and begins to vibrate in synchronization with the vibrating part. Meanwhile, since the mass member is in a freely moving state in the vibrating direction relative to the contact part, that is, since it independently moves freely in the same direction relative to the contact part, it collides with the contact part while the contact part is vibrating, and counteracts the vibration of the contact part, that is, the vibration of the vibrating part in the water vehicle.  
         [0013]     At that time, kinetic energy is imparted in the opposite direction by the contact part to the mass member colliding with the contact part (thus, some of the vibrating energy from the contact part is absorbed as kinetic energy of the mass member), and the mass member moves in the opposite direction. The mass member then again collides with a contact surface in a location opposite the first contact surface, again acting to counteract the vibration of the contact part, that is, the vibrating part in the water vehicle.  
         [0014]     The mass member subsequently repeats the same movement in opposite or different phases from that of the contact part. Therefore, the mass member absorbs the vibrating energy of the contact part at each collision, and converts the vibrating energy to its own kinetic energy, thereby repeatedly colliding with the contact part. Thus, the vibrating energy of the vibrating part in the water vehicle is thus absorbed, and the excited vibrations are damped, resulting in highly effective vibration control.  
         [0015]     When both the contact part and mass member are made of a rigid element, a loud, harsh noise (noise from impact) will be produced. In the noise-reducing device of the present invention, however, an elastic element such as resin or rubber is formed on at least one contact surface of the contact part or mass member, thereby avoiding the problem of loud, harsh noise during impact. The sliding friction and the viscous behavior of the elastic element during impact allow the vibration energy to be converted to heat and absorbed. That is, the vibration damping by the elastic element helps to attenuate the vibration of the vibrating part.  
         [0016]     The noise-reducing device of the invention is more suitable to damp vibration excited in water vehicles in comparison with the case where a dynamic damper is mounted for the vibrating part in the water vehicle to damp the vibration, in the following points, for example. The dynamic damper adds damper mass to the vibrating part by means of a spring. The natural frequency of the supplemental vibration system comprising the damper mass and spring can be tuned to the natural frequency of the primary vibration system (natural frequency of the vibration part) so as to lower the resonance magnification of the vibrating part and control its vibration.  
         [0017]     However, vibration damping by means of a dynamic damper is effective only for the vibration of a single resonance frequency, and cannot effectively prevent the resonance of other frequencies. That is, it is ineffective against resonance of multiple frequencies. Another problem with dynamic dampers is that separate resonance is produced anew before and after the resonance point. In addition, when a rubber elastic element is used as the spring, the spring constant of the rubber elastic element is variable according to temperature, making its damping properties (vibration damping properties) highly temperature-dependent, with a decrease of vibration controlling efficiency at elevated or lower temperatures.  
         [0018]     Additional problems are that considerable mass is required for the damper mass in dynamic dampers (the mass must be about 10% of the vibrating part), resulting in a heavier device as a whole and the need for greater installation space, and that the direction in which vibrations can be controlled is also fixed, making this an ineffective option for multidirectional vibrations.  
         [0019]     In the noise-reducing device of the present invention, the mass member is independently moveable, allowing it to move and collide with the contact part in opposite phases or different phases from that of the vibrating part of the water vehicle where vibration is to be controlled, thereby absorbing and damping the vibration energy. Vibrations can therefore be controlled over a wide range of frequencies, without any particular frequency dependency. Multidirectional collisions are also a simple and easy matter, allowing multidirectional vibrations to be controlled. In addition, the minimal temperature dependency results in good vibration control over a wide temperature range, from high to low temperatures. The required mass of the mass member is also lighter (only about 5% of the vibrating part), allowing the space needed for the device as a whole to be reduced and made more compact, so that it can be readily mounted on the vibrating part, among a variety of other advantages.  
         [0020]      FIG. 1  schematically compares an embodiment of the noise-reducing device of the invention to a dynamic damper, where  1  is the noise-reducing device, and  2  is the dynamic damper. The numeral  3  is the mass member,  4  is the elastic element formed on the surface of the mass member  3 , that is, the contact surface, and  5  is the housing forming the contact part.  6  is the damper mass in the dynamic damper  2 , and  7  is the spring (rubber).  8  is the vibrating part.  
         [0021]     When the dynamic damper  2  in  FIG. 1B  is mounted, it can control resonance in a specific frequency range, yet separate resonance is produced anew before and after the resonance point. The noise-reducing device of  FIG. 1C , on the other hand, can control vibrations over a wide frequency range, from high to low frequencies, and does not new resonance before or after the resonance point.  
         [0022]     As is evident in view of the above, attaching the noise-reducing device according to the invention to the vibrating part of a water vehicle allows the vibration in the vibrating part to be well controlled, and can thus effectively control noise originating from the vibrations of the vibrating part.  
         [0023]     Moreover, the noise-reducing device of the present invention has the rigid housing with the fluid-tightly sealed chamber, and the mass member is housed within the sealed chamber. Thus, the noise-reducing device is able to provide desired vibration damping effect based on collisions of the mass member against the contact part with high stability and without being adversely influenced by water flows or sprays. Especially, the elastic element is held inside the sealed chamber, thereby eliminating conventional problem as discussed above, namely deterioration of rubber members due to salt spray or ambient water.  
         [0024]     The rigid housing including the contact part in the invention may be made of a metal such as iron or an aluminum alloy, a hard resin, or another hard material. Preferably, employed is a rigid material having high resistance to salt, such as stainless steel and titanium. The mass member may itself be made of an elastic element such as rubber or a resin. It is preferably made of a rubber, resin, or the like having a high specific gravity, comprising an admixture of metal powder or the like in the interior. When the mass member is made of an elastic element such as rubber or resin, the mass member itself can constitute the elastic element of rubber, resin, or the like formed on at least one contact surface of the rigid contact part or mass member. The mass member can also be made of a foamed elastic element of rubber, resin, or the like.  
         [0025]     The mass member is preferably made of a metal such as iron, aluminum alloy, or lead to ensure more effective vibration control.  
         [0026]     In such cases, an elastic element of rubber, resin, or the like is formed on at least one contact surface of the rigid contact part or mass member.  
         [0027]     The gap in the present invention means the gap that is necessary to allow the mass member to most effectively collide with the contact part. In that sense, the gap distance D should be 0.05 to 2.0 mm, and more preferably 0.2 to 1.0 mm (see  FIG. 5A ).  
         [0028]     The elastic member in the invention may be made of rubber or a resin. Examples of rubbers which can be used include NR, SBR, and BR, and other diene rubbers, EPDM rubbers, urethane rubbers, silicone rubbers, and the like. Examples of resins which can be used include thermoplastic resins such as polypropylene and polyamide, thermoplastic elastomers such as polyolefins, urethane, and polyesters, and the like.  
         [0029]     The elastic element may be adhesively fixed through vulcanization of a rubber material for forming thereof to a contact surface of the mass member or contact part, or alternatively it may be formed on a contact surface in an unbonded state.  
         [0030]     Further, the rigid housing is formed separately from the vibrating part in the water vehicle, and can be fixed to the vibrating part along with the mass member. This will allow the noise-reducing device to be easily attached and fixed to the vibrating part in the water vehicle.  
         [0031]     A second mode of the present invention provides a noise-reducing device according to the above described first mode, wherein the noise-reducing device is formed so as to be attached to the jet pump acting as the vibration source, thereby controlling the vibration of the jet pump. This will allow the vibrations of the water vehicle originating from the jet pump to be effectively controlled so as to reduce noise.  
         [0032]     A third mode of the present invention provides a noise-reducing device according to the above described second mode, wherein the noise-reducing device is formed so as to be attached to an upper portion of the jet pump. With this arrangement, the noise-reducing device is less likely influenced by water flow energy or other energy created by water existing around the device, making it enable for the noise-reducing device to exhibit desired damping effect with high stability.  
         [0033]     A fourth mode of the present invention provides a noise-reducing device according to the above described first mode, wherein the noise-reducing device is formed so as to be attached to a part by which the jet pump is fixed to a hull of the water vehicle, thereby controlling the vibration of the jet pump. According to a fifth mode of the present invention, the noise-reducing device is attached to the jet pump using a fixing hole where the jet pump is fixed to the hole with the fixing part. With this arrangement, the noise-reducing device is preferably attached to a part by which the jet pump is fixed to the hull. In this case, it is also desirable to attach the noise-reducing device to the jet pump using a fixing hole by which the fixing part is fixed to the hull.  
         [0034]     A sixth mode of the present invention provides a noise-reducing device according to the above described first mode, wherein the noise-reducing device is formed so as to be attached to a deck of a hull of the water vehicle, where is subjected to vibration from the jet pump, thereby controlling the vibration of the deck. Since the deck undergoes vibrations transmitted from the jet pump, this arrangement will control the vibration of the deck, serving as the source of resonance, to allow noise in the water vehicle to be more effectively reduced.  
         [0035]     A seventh mode of the present invention provides a noise reducing device according to any one of the above described first through sixth modes, wherein the rigid housing has a cantilever bar protruding in a direction remote from the fluid-tightly sealed chamber, the cantilever bar being fixable at a protruding end portion thereof to the vibrating part of the water vehicle so that the rigid housing is fixed to the vibrating part in a cantilevered state. With this arrangement, since the fluid-tightly sealed chamber is supported on the vibrating part of the water vehicle by the cantilever bar in a cantilevered state or one end supported manner, the sealed chamber is more likely displaced or oscillated with the help of spring of the cantilever bar. This arrangement is effective to excite displacement of the mass member within the chamber and resultant collision or impact of the mass member against the contact part of the housing. Therefore, the present noise-reducing device is capable of efficiently providing desired damping effect owing to collision of the mass member against the contact part of the housing, even if the displacement of the housing is somewhat reduced by resistance of water. A plurality of cantilever bars may be employed.  
         [0036]     A eighth mode of the present invention provides a noise-reducing device assembly for water vehicles in which thrust is provided when water taken in by a jet pump is pressurized and accelerated, and is blasted to the rear by means of a jet nozzle, the noise-reducing device comprising: a plurality of noise-reducing device each including a rigid housing having a fluid-tightly sealed chamber formed therein, and fixable to a vibrating part of the water vehicle to be displaced with the vibrating part; a mass member housed in the rigid housing with a given gap therebetween so that the mass member is freely movable in the vibrating direction relative to the rigid housing, the mass member being adapted to come into collision with a contact part of the housing during input of vibration; and an elastic element formed on at least one surface of contact on the contact part or mass member; an attachment bracket having a body portion to which the plurality of noise-reducing device are fixed, and a cantilever bar protruding in a direction remote from the body portion, the cantilever bar being fixable at a protruding end portion thereof to the vibrating part of the water vehicle so that the attachment bracket is fixed to the vibrating part in a cantilevered state.  
         [0037]     This noise-reducing device assembly facilitate installation of the plurality of noise-reducing devices on the water vehicle. Like in the seventh mode of the invention, the plurality of noise-reducing devices and their fluid-tightly sealing chambers are supported on the vibrating part of the water vehicle via the cantilever bar of the attachment bracket in a cantilevered state or one end supported manner, whereby the sealed chambers are more likely displaced or oscillated with the help of spring of the cantilever bar of the attachment bracket, resulting in further excited collisions of the mass member against the housing. Thus, the present noise-reducing device assembly is capable of efficiently providing desired damping effect owing to excited collision of the mass members against the contact parts of the housings. A plurality of cantilever bars may be employed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     The foregoing and/or other objects features and advantages of the invention will become more apparent from the following description of a preferred embodiment with reference to the accompanying drawings in which like reference numerals designate like elements and wherein:  
         [0039]      FIG. 1  is a view schematically illustrating a noise-reducing device of the present invention, together with conventional dynamic dampers;  
         [0040]      FIG. 2  is a fragmentary sectional view showing a noise-reducing device assembly of construction according to one embodiment of the present invention, which is installed on a water vehicle;  
         [0041]      FIG. 3  is a fragmentary enlarged view of the noise-reducing device assembly of  FIG. 2  and ambient components;  
         [0042]      FIG. 4  is a fragmentary enlarged front view of the noise-reducing device assembly of  FIG. 2  and ambient components;  
         [0043]      FIGS. 5A-5C  are views showing a noise-reducing device of the noise-reducing device assembly of the invention;  
         [0044]      FIG. 6  is a fragmentary enlarged front view of the noise-reducing device assembly of construction according to another embodiment of the present invention and ambient components.  
         [0045]      FIGS. 7A and 7B  are views showing a noise-reducing device of the noise-reducing device assembly of the invention of  FIG. 6 ;  
         [0046]      FIG. 8  is a graph showing results of measurements of vibration at oscillating point P of  FIG. 10 , for the case where the present noise-reducing device assembly is installed and the case where no device is installed;  
         [0047]      FIG. 9A  is a graph showing results of measurements of vibration at upper surface of a jet pump of  FIG. 10 , for the case where the present noise-reducing device assembly is installed and the case where no device is installed, and  FIG. 9B  is a graph showing results of measurements of vibration at upper surface of a deck of  FIG. 10 , for the case where the present noise-reducing device assembly is installed and the case where no device is installed;  
         [0048]      FIG. 10  is a view showing designated measuring points for vibration measuring tests; and  
         [0049]      FIG. 11  is a fragmentary sectional view showing a noise-reducing device of construction according to yet another embodiment of the present invention, which is installed on a water vehicle.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0050]     Referring first to  FIGS. 2 and 10 , there is shown a water vehicle  10  which glides on water. The water vehicle includes a hull  12  and a steering handle  14  with a pair of handle grips  16 .  
         [0051]     This water vehicle  10  is equipped with an engine  18  and jet pump  20 . The jet pump  20  is equipped with an impeller  24  in the interior of a pump housing  22 . The impeller  24  is operationally connected to the engine  18  by a drive shaft  26 .  
         [0052]     In the water vehicle  10 , the impeller  24  in the jet pump  20  is rotated and driven by the drive shaft  26  through the output from the engine  18 . Thus, water that has been taken up through the intake  28  in the bottom of the hull is pressurized and accelerated in the pump housing  22  of the jet pump  20 , and is blasted from the end jet nozzle  30  to the rear in the form of a rushing jet flow. The water vehicle  10  glides on the water on the basis of the thrust thus provided.  
         [0053]     As illustrated in  FIGS. 3 and 4 , the jet pump  20  has a fixing flange  32  at the outer periphery of the pump housing  22 , and is fixed to the hull  12  side by means of fixing bolts  36  threaded into fixing holes  34  (sea  FIG. 3 ) in the fixing flange  32 . A noise-reducing device assembly  38  of construction according to a first embodiment of the invention is attached and fixed to the fixing flange  32  by which the jet pump  20  is fixed to the hull  12 .  
         [0054]     As illustrated in  FIG. 4 , the noise-reducing device assembly  38  in this embodiment has an attachment bracket  40 , and a plurality (in this case, three) noise-reducing devices  42  fixed to a body portion  40   a  of the attachment bracket  40 . The attachment bracket  40 , which is rigid as a whole, is made of a member in the form of a plate, and is attached and fixed with its cantilever bars  40   b,    40   b  to the fixing flange  32  by means of the fixing bolts  36  threaded into the fixing holes  34  in the fixing flange  32 .  
         [0055]     The plurality of noise-reducing devices  42  all have the same structure in this embodiment. The specific structure is shown in detail in  FIG. 5 . As shown in that figures, each noise-reducing device  42  has a pair of rigid columnar housings  44 ,  44  located side by side, with free movement chambers  46 ,  46  formed inside. A pair of mass members  48 ,  48  of solid cylinder shapes are housed in a freely moving state, by means of a certain gap, and are covered on the surface with elastic elements  50 .  
         [0056]     In this embodiment, a gap is formed between the elastic element  50  closely covering the mass member  48  and the inner surface of the housing  44  to allow the mass member  48  to move freely in either the perpendicular or axial directions. In other words, the mass members  48  can move freely in either the perpendicular or axial directions in the free movement chambers  46 . Namely, with each mass member  48  situated concentrically with the corresponding columnar housing  44 , the mass member  48  is completely separate from the inner surface of the housing  44  with a gap therebetween in all-radial directions thereof, thereby permitting freely movement or displacement of the mass member  48  within the columnar housing  44 . The vibrations of the jet pump  20  are primarily vibrations in the perpendicular and axial rotating directions.  
         [0057]     Each noise-reducing device  42  has a pair of cantilever bars in the form of arms  52 , and are fixed to the attachment bracket  40  by means of the fixing bolts  56  threaded into the fixing holes  54  at the tips of the arms  52  (see  FIGS. 4 ).  
         [0058]     As illustrated in  FIG. 5A , the main body  58  of the noise-reducing device  42  is formed by placing a first member  58 - 1  and a second member  58 - 2  on top of each other, and then inserting a sealing member  60  at the surface where they are placed on top of each other, so as to entirely encompass the pair of housings  44 ,  44 . That is, the first member  58 - 1  and the second member  58 - 2  are fixed by welding or the like on top of each other, with the sealing member  60  sandwiched between them. The sealing member  60  keeps the interior of the housings  44 , that is, the free movement chambers  46 , water-tight, thus preventing water from penetrating into the free movement chambers  46 .  
         [0059]     In the noise-reducing device assembly  38  in this embodiment, the housings  44  of the noise-reducing devices  42  are displaced with the jet pump  20  as it begins to vibrate, and they begin to vibrate in synchronization with the jet pump  20 , or more exactly, the pump housing  22 , but since the mass members  48  housed in the housings  44  independently move freely in a free moving state in the direction of vibration relative to the housings  44 , the mass members  48  come into collision or impact in the direction of vibration against the housings  44  when the housings  44  are vibrating, and counteract the vibrations of the housings  44 , that is, the vibrations of the jet pump  20 . With this regards, since the attachment bracket  40  is affixed to the flange  32  of the jet pump  20  in a cantilevered manner, the vibrations of the housings  44  are also excited owing to the spring of the cantilever bars  40   b.    
         [0060]     At that time, kinetic energy in the opposite direction is imparted by the housings  44  to the mass members, and the mass members  48  move in the opposite direction. The mass members  48  then again collides with a contact surface in a location opposite the first contact surface, acting to counteract the vibration of the housings  44 , that is, the jet pump  20 .  
         [0061]     The mass members  48  subsequently repeat the same movement in opposite or different phases than the housings  44  to absorb the vibrating energy of the housings  44 , that is, the jet pump  20 , at each collision, and converts it to its own kinetic energy. In this way, and also as a result of the sliding friction at each collision, the vibrating energy of the jet pump  20  is absorbed, and the vibrations are damped, resulting in highly effective vibration control.  
         [0062]      FIGS. 6 and 7  illustrate a second embodiment of the noise-reducing device assembly  38 . As illustrated in  FIG. 6 , a plurality of (three in this case) noise-reducing devices  68  in this noise-reducing device assembly  38  as well are attached and fixed to the fixing flange  32  of the jet pump  20  by the attachment bracket  40  shared in common.  
         [0063]      FIGS. 7A and 7B  illustrate the specific structure of the noise-reducing device  68  in this noise-reducing device assembly  38 . As illustrated, each noise-reducing device  68  in this embodiment has a dome-shaped rigid housing  70 , inside of which a mass member  74  consisting of a ball coated on the surface with an elastic element  76  is housed freely moveable in three directions, by means of a certain gap therebetween.  
         [0064]     As illustrated in  FIGS. 6 and 7 , each noise-reducing device  68  in this embodiment is equipped with a pair of arms  80  at the main body  78 , and is fixed to the attachment bracket  40  shared in common by means of fixing bolts  56  at the arms  80 . In the noise-reducing device assembly  38  in  FIGS. 6 and 7 , the mass members  74  move freely in the free movement chambers  72  formed in the rigid housings  70 , and collide with the rigid housings  70  to absorb energy and damp vibrations in the same manner as described above with respect to the first embodiment.  
         [0065]      FIGS. 8 and 9  show a result of measurements when the noise-reducing device assembly  38  in  FIGS. 4 and 5  was set up, and vibrations were produced at oscillating point P in  FIG. 10  to measure the vibrations at that oscillating point P as well as vibration receiving points Q 1  and Q 2 . In this case, oscillating point P was a location at the lower surface of the jet pump  20 , vibration receiving point Q 1  was a location at the upper surface of the jet pump  20 , and vibration receiving point Q 2  was a location at the upper surface of the deck  82  aft of the hull  12 .  
         [0066]      FIG. 8  gives the results for the vibration measured at oscillating point P,  FIG. 9A  gives the results for the vibration measured at the upper surface of the jet pump  20 , which was vibration receiving point Q 1 , and  FIG. 9B  gives the results for the vibration measured at the upper surface of the aft deck  82 , which was vibration receiving point Q 2 . In each of the figures, the dashed lines show the results for vibration measured when the noise-reducing device assembly  38  of the embodiment was not used, and the solid lines show the results for vibration measured when the noise-reducing device assembly  38  of the embodiment was used. The horizontal axis indicates frequency, and the vertical axis indicates vibration level.  
         [0067]     As is evident in the figures, the noise-reducing device assembly  38  of the embodiment controlled vibrations over a wide frequency range.  
         [0068]     Since an elastic element  50  is formed on the surface of the mass members  48  in the noise-reducing device assembly  38  of the invention, no loud, harsh noise is produced by the collision of the mass members  48  against the housings  44 , and sliding friction and the viscous behavior of the elastic element during impact allow the vibration energy to be converted to heat and absorbed. That is, the vibration damping by the elastic element  50  helps to attenuate the vibration of the jet pump  20 .  
         [0069]     In the noise-reducing device assembly  38  of the present invention, the mass members  48  are independently moveable, allowing them to move and collide with the contact part in opposite phases or different phases than the jet pump  20  of the water vehicle  10 , thereby absorbing and damping the vibration energy. Vibrations can therefore be controlled over a wide range of frequencies, without any particular frequency dependency. Multidirectional collisions are also a simple and easy matter, allowing multidirectional vibrations to be controlled. In addition, the minimal temperature dependency results in good vibration control over a wide temperature range, from high to low temperatures. The required mass of the mass members  48  is also lighter, allowing the space needed for the device as a whole to be reduced and made more compact, so that the noise-reducing device assembly  38  can be readily mounted on the jet pump  20 .  
         [0070]     The noise-reducing device assembly  38  in this example is attached to the jet pump  20  of the water vehicle  10 , allowing the vibrations in the jet pump  20  to be controlled better and thus allowing better control of noise originating from the vibrations of the jet pump  20 .  
         [0071]     In this embodiment, the noise-reducing device assembly  38  is equipped with the rigid housings  44  to form the contact parts. The mass members  48  are housed in a freely moving state in the free movement chambers  46  of the housings  44 . The housings  44  are formed separately from the jet pump  20  in the water vehicle  10 , allowing the noise-reducing device assembly  38  to be easily attached and fixed to the jet pump  20  of the water vehicle.  
         [0072]     Embodiments of the invention were described in detail for the illustrative purpose only, the present invention may otherwise be embodied. For instance, as illustrated in  FIG. 11 , embodiments of the noise-reducing device of the invention other than the noise-reducing device assembly  38  illustrated above may be mounted on certain parts of the hull  12  other than the aft deck  82  where vibrations are transmitted from the jet pump  20  in order to control the vibrations of the hull  12  itself, allowing noise produced by the water vehicle  10  to be controlled.  
         [0073]     While the noise-reducing device assembly of the present invention has been described in detail, the principle of the present invention may be embodied in a single noise reducing device. For instance, the noise-reducing device may be directly attached to the vibrating part of the water vehicle, preferably in a cantilevered manner with its cantilever bar, thereby providing the same vibration damping effect as described above with respect to the noise-reducing device assembly  38 . In this case, the noise-reducing device may be attached to the vibrating part via a suitable bracket. Alternatively, a plurality of noise-reducing devices may be directly affixed to the vibrating part of the water vehicle.  
         [0074]     It is also to be understood that the present invention may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the following claims.