Patent Publication Number: US-7717230-B2

Title: Device and method for amplifying suction noise

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application Serial No. 2006-155944 filed Jun. 5, 2006, the disclosure of which, including its specification, drawings and claims, are incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure pertains to a type of device for improving the sound quality of a suction noise generated by an intake system of an automobile or the like. 
     BACKGROUND 
     Japanese Patent No. 3613665 describes a known device that boosts suction noise. The device described therein is for amplifying suction noise and has plural intake ducts having resonance frequencies that are different from each other, so that it is possible to boost the suction noise at different frequencies, and permits introduction of suction noise into the vehicle passenger compartment. 
     However, the device for amplifying suction noise described in Japanese Patent No. 3613665 has some disadvantages. First, because the device is constituted with plural intake ducts, there is no leeway in the space required inside the engine compartment. Thus, there are restrictions on the layout, and the device is difficult to install in the engine compartment. 
     SUMMARY 
     The present disclosure provides a device to boost the suction noise of a vehicle characterized by the fact that resonance of an elastic membrane, due to variation in pressure of air transmitted into an engine intake port, is allowed to occur at least two different frequencies. 
     According to the present disclosure, it is possible to boost suction noise at plural frequencies without the need of plural intake ducts, so that it is possible to generate impressive suction noise, and at the same time to improve the freedom of design layout. 
     One embodiment of the disclosure includes a device for amplifying the suction noise of a vehicle. The embodiment of the device comprises an intake duct, a connecting pipe and a composite membrane. The intake duct is for feeding air to an engine intake port. A connecting pipe is connected to an interior of the intake duct. The composite membrane is positioned within the connecting pipe. The composite member blocks an interior passage formed in the connecting pipe. The composite member further includes at least two elastic membranes with one of masses and rigidities that different from each other. A method is also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other features and advantages of the present disclosure will be apparent from the ensuing description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a side elevational view of a vehicle equipped with a device for amplifying a suction noise of a vehicle. 
         FIG. 1B  is a top plan view of the vehicle of  FIG. 1A . 
         FIG. 1C  is a front elevational view of the vehicle of  FIG. 1A . 
         FIG. 2  is a diagram illustrating the structure of the device for amplifying suction noise according to a first embodiment. 
         FIG. 3  is a diagram illustrating in detail the structure of a composite membrane. 
         FIG. 4  is a diagram illustrating a vibration state of each elastic membrane in an out-of-plane direction of the composite membrane during a first acceleration mode. 
         FIG. 5  is a diagram illustrating a vibration state of each elastic membrane in the out-of-plane direction of the composite membrane during a second acceleration mode. 
         FIG. 6  is a diagram illustrating the vibration state of each elastic membrane in an out-of-plane direction of the composite membrane during a third acceleration mode. 
         FIG. 7  is a diagram illustrating the structure of a composite membrane of the device for amplifying the suction noise of a vehicle in a second embodiment. 
         FIG. 8  is a diagram illustrating the structure a composite membrane of the device for amplifying the suction noise of a vehicle in a third embodiment 
         FIG. 9  is a cross section of the composite membrane taken across X-Y in  FIG. 8 . 
         FIGS. 10A-10C  are diagrams illustrating modified examples of the composite membrane of the device for amplifying the suction noise of a vehicle in the third embodiment. 
         FIG. 11A-11D  are diagrams illustrating modified examples of the composite membrane of the device for amplifying the suction noise of a vehicle in the third embodiment. 
         FIG. 12  is a diagram illustrating the structure of the composite membrane of the device for amplifying the suction noise of a vehicle in a forth embodiment. 
         FIG. 13  is a cross section of the composite membrane taken across Y-Y in  FIG. 12 . 
         FIGS. 14A-14C  are diagrams illustrating modified examples of the composite membrane of the device for amplifying the suction noise of a vehicle the fourth embodiment. 
         FIGS. 15A-15D  are diagrams illustrating modified examples of the composite membrane of the device for amplifying the suction noise of a vehicle in the fourth embodiment. 
         FIG. 16  is a diagram illustrating the structure of a composite membrane for the device for amplifying the suction noise of a vehicle in a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     While the claims are not limited to the illustrated embodiments, an appreciation of various aspects of the apparatus is best gained through a discussion of various examples thereof. Referring now to the drawings, illustrative embodiments are shown in detail. Although the drawings represent the embodiments, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain an innovative aspect of an embodiment. Further, the embodiments described herein are not intended to be exhaustive or otherwise limiting or restricting to the precise form and configuration shown in the drawings and disclosed in the following detailed description. Exemplary embodiments of the present invention are described in detail by referring to the drawings as follows. 
     Embodiment 1 
       FIGS. 1A-1C  includes diagrams illustrating a vehicle C carrying a device  1  for amplifying suction noise according to a first embodiment.  FIG. 1A  is a side view of a vehicle C.  FIG. 1B  is a top view of vehicle C. And  FIG. 1C  is a front view of vehicle C. 
     As can be seen from  FIG. 1 , device  1  that boosts suction noise in the first embodiment is arranged in front of a vehicle passenger compartment  2 . Indeed, device  1  is arranged in an engine compartment  6  that is separated from vehicle passenger compartment  2  by a dash panel  4 . Further, device  1  is arranged on an intake duct  10  that is connected to an engine  8 . 
     The resonant vibration of air in intake duct  10  takes place in air intake duct  10 . When resonance occurs, pressure variations develop in air in intake duct  10 , and these pressure variations in the air are perceived by humans as noise. The noise accompanying intake is called suction noise. The frequency of the suction noise depends on the frequency of the pressure variations generated due to the resonance phenomenon. The frequency of the pressure variation that takes place due to the resonance phenomenon is determined by the resonance frequency, which depends on the length of the intake duct, etc. 
       FIG. 2  is a diagram illustrating the structure of device  1  that amplifies the suction noise in the first embodiment. As shown in  FIG. 2 , device  1  that amplifies the suction noise in the first embodiment comprises a connecting pipe  12 , an additional pipe  14 , and a composite membrane  16  (represented by dashed lines in  FIG. 2 ). 
     In the embodiment shown, connecting pipe  12  is generally cylindrical, and is attached to an outer peripheral surface of intake duct  10 , which may be formed of a draft tube with air inside it. Connecting pipe  12  communicates with intake duct  10 . 
     Similar to connecting pipe  12 , additional pipe  14  may also be generally cylindrical. A first opening at one end of additional pipe  14  is connected to connecting pipe  12 , and a second opening at the other end of additional pipe  14  opens to external air. 
     Composite membrane  16  is generally disk-shaped and may be made of, for example, rubber or another elastic material. Composite member  16  is attached on an inner peripheral surface of connecting pipe  12  and extends across an interior of connecting pipe  12  so as to close connecting pipe  12 . Composite membrane  16  undergoes elastic deformation during intake by engine  8 , corresponding to variation in an intake vacuum generated in air inside intake duct  10 , so that vibration of composite membrane  16  occurs in an out-of-plane direction. The detailed structure of composite membrane  16  will be explained later. 
     The structure of intake duct  10  and the parts related to thereto will now be explained. 
     Intake duct  10  forms an intake path from the external air to engine  8 , and is comprised of a dust side intake duct  20  and a clean side intake duct  18 . 
     A first opening at one end of dust side intake duct  20  is connected to an air cleaner  22 , and a second opening at the other end of dust side intake duct  20  opens to the external air. 
     Clean side intake duct  18  includes a throttle chamber  24 . A first opening at one end of clean side intake duct  18  is connected to air cleaner  22 , and a second opening at the other end of clean side intake duct  18  is connected via a surge tank  26  to various portions of an intake manifold  28  to the various cylinders (not shown in the figure) of engine  8 . 
     For example, air cleaner  22  includes an oiled filter or other filter part for cleaning air flowing from the second opening at one end of dust side intake duct  10  as it passes through the filter portion. 
     Throttle chamber  24  is installed between air cleaner  22  and surge tank  26 , and is connected to an accelerator pedal (not shown in the figure). Throttle chamber  24  adjusts the airflow rate from air cleaner  22  to surge tank  26  corresponding to the amount of accelerator pedal depression. When the amount of accelerator pedal depression is reduced, the airflow rate from air cleaner  22  to surge tank  26  is decreased, so that the rotational velocity of engine  8  falls, and at the same time the intake vacuum generated in the air inside intake duct  10  is reduced. On the other hand, when the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased, so that the rotational velocity of engine  8  rises, and at the same time, the intake vacuum generated in the air in intake duct  10  is increased. 
     During intake, engine  8  draws air that has flowed from the opening at the second end of dust side intake duct  20  and is present inside clean side intake duct  18  into the various cylinders via surge tank  26  and intake manifold  28 . 
     Also, in conjunction with the intake operation, engine  8  becomes a pressure source that generates intake pulsation in the air inside intake duct  10 , and this intake pulsation results in suction noise. 
     Here, the intake pulsation that occurs in conjunction with the intake operation of engine  8  is a pressure variation generated in the air inside intake duct  10 . This pressure variation is composed of plural pressure variations at different frequencies. That is, the intake pulsation that occurs in conjunction with the intake operation of engine  8  is composed of plural intake pulsations at different frequencies. In the first embodiment, engine  8  is assumed to be a 6-cylinder in-line engine. However, engine  8  is not limited to this construction. 
       FIG. 3  is a diagram illustrating the detailed structure of composite membrane  16 . 
     Viewed in the thickness direction of composite membrane  16 , as may be seen, composite membrane  16  includes three elastic membranes  30   a - 30   c . Elastic membranes  30   a - 30   c  are separated from each other by slots  32  formed in the surface on an intake duct side of composite membrane  16 . In the embodiment shown, and slots  32  are formed in shapes having different areas. More specifically, area Sa of elastic membrane  30   a  is larger than area Sb of elastic membrane  30   b , and area Sb of said elastic membrane  30   b  is larger than area Sc of elastic membrane  30   c . That is, elastic membranes  30   a - 30   c  are formed to satisfy the relationship Sa&gt;Sb&gt;Sc. 
     Here, because elastic membranes  30   a - 30   c  have different areas from each other, their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. 
     The resonance frequency is that for vibration at a prescribed frequency detected when an object is allowed to vibration freely. Any object has a natural resonance frequency. Usually, an object has plural resonance frequencies. The resonance frequency depends on the rigidity and mass of the object. More specifically, the higher the rigidity, the higher the resonance frequency, while the larger the mass, the lower the resonance frequency. Here, rigidity refers to the proportionality coefficient between a bending or twisting force applied to the structural body and the deflection of the structural body as a whole. 
     Consequently, because elastic membranes  30   a - 30   c  have different areas, they differ from each other in rigidity and mass. As a result, they have different resonance frequencies. 
     Compared with elastic membrane  30   c  with a smaller area, elastic membrane  30   a  with a larger area has a lower resonance frequency for vibration in the out-of-plane direction. Consequently, for said elastic membranes  30   a - 30   c , assuming the resonance frequency of elastic membrane  30   a  to be first resonance frequency f 1 , the resonance frequency of elastic membrane  30   b  to be second resonance frequency f 2 , and the resonance frequency of elastic membrane  30   c  to be third resonance frequency f 3 , the following conditional relationship among them applies: f 1 &lt;f 2 &lt;f 3 . 
     Also, elastic membranes  30   a - 30   c  are appropriately formed such that their resonance frequencies correspond to intake pulsation at a first frequency, intake pulsation at a second frequency and intake pulsation at a third frequency selected from among the intake pulsations at plural frequencies that form the intake pulsation generated in conjunction with the intake operation of engine  8 . More specifically, first resonance frequency f 1  of elastic membrane  30   a  matches the first intake pulsation frequency, second resonance frequency f 2  of elastic membrane  30   b  matches the second intake pulsation frequency, and third resonance frequency f 3  of elastic membrane  30   c  matches the third intake pulsation frequency. 
     Here, the first frequency is lower than the second frequency and the second frequency is lower than the third frequency. That is, the first frequency, second frequency and third frequency satisfy the following relationship: first frequency&lt;second frequency&lt;third frequency. 
     The first frequency is the frequency of the intake pulsation generated when the engine rotates at a prescribed rotational velocity R 1 , the second frequency is the frequency of the intake pulsation generated at a prescribed rotational velocity R 2 , and the third frequency is the frequency of the intake pulsation generated at a prescribed rotational velocity R 3 . 
     Here, R 1  is a rotational velocity lower than R 2  and R 2  is a rotational velocity lower than R 3 . That is, rotational velocities R 1 , R 2 , R 3  satisfy the following relationship: R 1 &lt;R 2 &lt;R 3 . 
     In addition, each of slots  32  is formed between two adjacent elastic membranes, and they form rigidity changing portions having different rigidities from those of elastic membranes  30   a - 30   c.    
     The operation of the first embodiment of device  1  that amplifies the suction noise will now be explained. 
     When engine  8  is started, the intake pulsation generated in conjunction with the intake operation of engine  8  is propagated via intake manifold  28  and surge tank  26  into the air inside intake duct  10  (see  FIG. 2 ). 
     While engine  8  is running, as the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased (hereinafter to be referred to as acceleration mode). As a result, while the rotational velocity of engine  8  is increased, the intake vacuum generated for the air in intake duct  10  rises (see  FIG. 2 ). 
     In the following, the operation of elastic membranes  30   a - 30   c  in the acceleration mode will be explained in more detail with reference to  FIGS. 4-6 . 
       FIGS. 4-6  are diagrams illustrating the vibration of elastic membranes  30   a - 30   c  in the out-of-plane direction of the composite membrane  16  during the acceleration mode.  FIG. 4  is a diagram illustrating the state when the rotational velocity of the engine is R 1 ;  FIG. 5  is a diagram illustrating the state when the rotational velocity of the engine is R 2 ; and  FIG. 6  is a diagram illustrating the state when the rotational velocity of the engine is R 3 . 
     When the rotational velocity of the engine is R 1 , among the plural intake pulsations at different frequencies that form the intake pulsation generated in conjunction with the intake operation of the engine, an intake pulsation at the first frequency is propagated via connecting pipe  12  to composite membrane  16 . 
     As illustrated in  FIG. 4 , because in this case the frequency of the intake pulsation at the first frequency matches first resonance frequency f 1  of elastic membrane  30   a , only elastic membrane  30   a  among the elastic membranes  30   a - 30   c  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   a  vibrates in the out-of-plane direction of composite membrane  16 , it causes pressure variations in the air in additional pipe  14  on the side of composite membrane  16  that is open to the external air. There, air pressure variations become noise that is emitted to an external air side, and the suction noise is thereby amplified. 
     When the amount of accelerator pedal depression is further increased, that is, when the rotational velocity of the engine is at R 2 , among the plural intake pulsations at different frequencies that form the intake pulsation in conjunction with the intake operation of the engine, the intake pulsation at the second frequency is propagated via connecting pipe  12  to composite membrane  16 . 
     As shown in  FIG. 5 , because in this case the frequency of the intake pulsation at the second frequency matches second resonance frequency f 2  of elastic membrane  30   b , only elastic membrane  30   b  among elastic membranes  30   a - 30   c  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   b  vibrates in the out-of-plane direction of composite membrane  16 , it causes pressure variations in the air between composite membrane  16  and the second opening of additional pipe  14 , and said air pressure variations become noise that is emitted to the external air side, and the suction noise is thereby amplified. 
     When the amount of accelerator pedal depression is further increased, that is, when the rotational velocity of the engine is at R 3 , among the plural intake pulsations at different frequencies that form the intake pulsation in conjunction with the intake operation of the engine, the intake pulsation at the third frequency is propagated via connecting pipe  12  to composite membrane  16 . 
     As shown in  FIG. 6 , because in this case the frequency of the intake pulsation at the third frequency matches third resonance frequency f 3  of elastic membrane  30   c , only elastic membrane  30   c  among elastic membranes  30   a - 30   c  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   c  vibrates in the out-of-plane direction of composite membrane  16 , it causes pressure variations in the air in additional pipe  14  on the side of composite membrane  16  that is open to the external air, and air pressure variations become noise that is emitted to the external air side, and therefore the suction noise is amplified. 
     Consequently, in the acceleration mode, elastic membranes  30   a - 30   c  with different resonance frequencies vibrate in the out-of-plane direction of the composite membrane according to variation in the rotational velocity of the engine. As a result, the suction noise at the first frequency, the suction noise at the second frequency and the suction noise at the third frequency are amplified, and the amplified suction noise is emitted to the external air side from the second opening at the additional pipe  14  (see  FIG. 2 ). 
     When the amplified suction noise is emitted to the external air side from the second opening of additional pipe  14 , the emitted suction noise is propagated via the air into vehicle passenger compartment  2  such that an impressive suction noise is transmitted into vehicle passenger compartment  2  (see  FIG. 1 ). 
     Variations of Embodiment 1 
     For device  1  that amplifies the suction noise in the first embodiment, three elastic membranes  30   a - 30   c  are formed to have different resonance frequencies for vibration in the out-of-plane direction of composite membrane  16 . However, it is understood that the present embodiment is not limited to this scheme. Indeed, a scheme may also be adopted in which among three elastic membranes  30   a - 30   c , at least two elastic membranes have resonance frequencies for vibration in the out-of-plane direction of the composite membrane that are different from each other. 
     Also, for device  1  that amplifies the suction noise in the first embodiment, three elastic membranes  30   a - 30   c  are formed to have different resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  by virtue of having different areas. The present embodiment is not limited to this scheme, however. That is, a scheme may also be adopted in which three elastic membranes  30   a - 30   c  are formed with the same area, and at the same time, they are formed different from each other with respect to rigidity and/or mass, so that the resonance frequencies for vibration in the out-of-plane direction of the composite membrane are different from each other. Here, to form an elastic membrane  30  having increased rigidity and/or mass, a core member may be arranged inside it, or a processed mass body for forming ribs on elastic membrane  30  may be attached, or the thickness of elastic membrane  30  may be increased. As a result, although elastic membrane  30  has the same area as the other elastic membranes, elastic membrane  30  nevertheless has higher rigidity and/or larger mass than the others. In this case, by selecting the rigidity and/or mass of each elastic membrane  30   a ,  30   b ,  30   c  to meet the required resonance frequency conditions for vibration in the out-of-plane direction of composite membrane  16 , it is possible to set each elastic membrane  30   a ,  30   b ,  30   c  at a desired resonance frequency. 
     In the first embodiment, device  1  that amplifies the suction noise has a composite membrane  16  composed of three elastic membranes  30   a - 30   c . The present embodiment is not limited to this scheme, however. A scheme can also be adopted in which composite membrane  16  is composed of two elastic membranes  30  or more than three elastic membranes  30 . 
     Also, in the structure for device  1  that amplifies the suction noise of the present embodiment, device  1  that amplifies the suction noise is set in engine compartment  6  in front of vehicle passenger compartment  2 . However, other locations for device  1  that amplifies the suction noise are contemplated. That is, for example, when vehicle C has an engine compartment  6  arranged behind vehicle passenger compartment  2 , the location for device  1  that amplifies the suction noise can be in engine compartment  6  located behind vehicle passenger compartment  2 . Also, for example, when vehicle C has an engine compartment  6  beneath vehicle passenger compartment  2 , the location for device  1  that amplifies the suction noise can be within engine compartment  6  set beneath vehicle passenger compartment  2 . In any case, the location of device  1  that amplifies the suction noise can be adjusted appropriately according to the configuration of vehicle C, that is, the position of engine compartment  6 . 
     Viewing the device  1  for amplifying suction noise of the first embodiment in the thickness direction of composite membrane  16 , the composite membrane  16  is composed of three elastic membranes  30   a ,  30   b ,  30   c . Elastic membranes  30   a ,  30   b ,  30   c  have resonance frequencies for vibrations in the out-of-plane direction of composite membrane  16  that differ from each other. 
     As a result, in the acceleration mode, the various elastic membranes  30   a ,  30   b ,  30   c  vibrate in the out-of-plane direction of composite membrane  16  corresponding to variation in the rotational velocity of the engine. 
     Consequently, the intake pulsation at the first frequency, and the suction noises at the second frequency and third frequency are amplified corresponding to variation in the rotational velocity of the engine, and the amplified suction noise is emitted from the second opening of additional pipe  14  on the external air side. The emitted suction noise is propagated via the air into the vehicle passenger compartment, so that an impressive suction noise is transmitted into vehicle passenger compartment  2 . 
     As a result, it is possible to generate the suction noise at plural frequencies by via composite membrane  16 , and it is possible to generate an impressive suction noise without a requirement of plural intake ducts. Because there is no need for plural intake ducts in this embodiment, freedom of layout is improved, allowing device  1  to be adopted on a variety of vehicles with different constructions, such as vehicles having different body sizes. 
     Also, viewing the device for amplifying suction noise of the present embodiment in the thickness direction, composite membrane  16  is comprised of three elastic membranes, and these elastic membranes are formed with different areas, so that they have different vibration frequencies in the out-of-plane direction of composite membrane  16 . 
     Consequently, by selecting the areas of the respective elastic membranes corresponding to resonance frequencies for vibration in the out-of-plane direction of composite membrane  16 , it is possible to set the resonance frequencies of the elastic membranes at the respective desired resonance frequencies. 
     As a result, it is possible to set the resonance frequencies for vibration in the out-of-plane direction of the various elastic membranes comprising composite membrane  16  at the plural desired frequencies, and it is possible to expand the frequency band range where amplifying the suction noise can be realized. As a result, it is possible to improve the sound quality of the suction noise directed into the vehicle passenger compartment. 
     Second Embodiment 
     Turning to  FIG. 7 , a second embodiment will be explained.  FIG. 7  is a diagram illustrating the structure of composite membrane  16  for device  1  for amplifying the suction noise of a vehicle. 
     As can be seen from  FIG. 7 , the structure of device  1  for amplifying the suction noise of a vehicle C in the second embodiment is the same as that of the first embodiment, except for the structure of composite membrane  16 . That is, composite membrane  16  in the second embodiment is divided by rigidity changing portions  34  formed between every pair of adjacent elastic membranes and having rigidities different from those of said elastic membranes  30   a - 30   d . Viewed in the thickness direction, composite membrane  16  has four elastic membranes  30   a - 30   d.    
     Rigidity changing portions  34  include an annular rigidity changing portion  36  and radial rigidity changing portions  38   a - 38   c.    
     Annular rigidity changing portion  36  is formed as a slot arranged in the surface of composite membrane  16  on an intake duct side of composite membrane  16 . Annular rigidity changing portion  36  is shaped to surround a portion of composite membrane  16  that includes the center of composite membrane  16 , and it has an overall circular or elliptical shape. In the second embodiment, the center portion surrounded with annular rigidity changing portion  36  is referred to as elastic membrane  30   d  in the following description. 
     Similar to annular rigidity changing portion  36 , radial rigidity changing portions  38   a - 38   c  are formed as slots in the surface of composite membrane  16  on the intake duct side of composite member  16 , and annular rigidity changing portions  38   a - 38   d  extend from annular rigidity changing portion  36  towards an outer periphery of composite membrane  16 , so that they divide the portions other than that surrounded by annular rigidity changing portion  36  into plural portions. With regard to radial rigidity changing portions  38   a - 38   c  in the second embodiment, an example is explained in which three radial rigidity changing portions  38   a - 38   c  are formed extending from annular rigidity changing portion  36  towards the outer periphery of composite membrane  16 . Also, in explanation of the second embodiment, the three elastic membranes  30  divided by said three radial rigidity changing portions  38   a - 38   c  are described as elastic membranes  30   a - 30   c , respectively. 
     Elastic membranes  30   a - 30   d  are formed into shapes with different areas by means of rigidity changing portions  34 . More specifically, area Sa of elastic membrane  30   a  is larger than area Sb of elastic membrane  30   b ; area Sb of elastic membrane  30   b  is larger than area Sc of elastic membrane  30   c ; and area Sc of elastic membrane  30   c  is larger than area Sd of elastic membrane  30   d . That is, elastic membranes  30   a - 30   d  are formed to satisfy the following relationship: Sa&gt;Sb&gt;Sc&gt;Sd. 
     Also, because elastic membranes  30   a - 30   d  have different areas, their resonance frequencies in the out-of-plane direction of composite membrane  16  are different from each other. More specifically, assuming the resonance frequency of elastic membrane  30   a  to be first resonance frequency f 1 , the resonance frequency of elastic membrane  30   b  to be second resonance frequency f 2 , the resonance frequency of elastic membrane  30   c  to be third resonance frequency f 3 , and the resonance frequency of elastic membrane  30   d  to be fourth resonance frequency f 4 , the following relationship is established: f 1 &lt;f 2 &lt;f 3 &lt;f 4 . 
     Also, elastic membranes  30   a - 30   d  are appropriately shaped such that their resonance frequencies match those of the intake pulsations at the first frequency, the second frequency, the third frequency and the fourth frequency, selected from among the intake pulsations at plural frequencies that form the intake pulsation generated in conjunction with the intake operation of engine  8 . More specifically, first resonance frequency f 1  of elastic membrane  30   a  matches the frequency of the intake pulsation at the first frequency, second resonance frequency f 2  of elastic membrane  30   b  matches the frequency of the intake pulsation at the second frequency, third resonance frequency f 3  of elastic membrane  30   c  matches the frequency of the intake pulsation at the third frequency, and fourth resonance frequency f 4  of elastic membrane  30   d  matches the frequency of the intake pulsation at the fourth frequency. 
     Here, the first frequency is lower than the second frequency, the second frequency is lower than the third frequency, and the third frequency is lower than the fourth frequency. That is, the first frequency, second frequency, third frequency and fourth frequency satisfy the following relationship: first frequency&lt;second frequency&lt;third frequency&lt;fourth frequency. 
     The first frequency is the frequency of the intake pulsation generated when the engine rotates at a prescribed rotational velocity R 1 , the second frequency is the frequency of the intake pulsation generated at a prescribed rotational velocity R 2 , the third frequency is the frequency of the intake pulsation generated at a prescribed rotational velocity R 3 , and the fourth frequency is the frequency of the intake pulsation generated at a prescribed rotational velocity R 4 . 
     Here, R 1  is a rotational velocity lower than R 2 , R 2  is a rotational velocity lower than R 3 , and R 3  is a rotational velocity lower than R 4 . That is, rotational velocities R 1 , R 2 , R 3 , R 4  satisfy the following relationship: R 1 &lt;R 2 &lt;R 3 &lt;R 4 . 
     The remaining structure of composite member  16  and device  1  is substantially the same as that of in the first embodiment. 
     The operation of device  1  that amplifies the suction noise according to the second embodiment will now be described. In the following description, because the structure of everything besides composite membrane  16  is substantially the same as in the first embodiment, only the operation of different parts will be explained. 
     When engine  8  is started, the intake pulsation generated in conjunction with the intake operation of engine  8  is propagated via intake manifold  28  and surge tank  26  into the air inside clean-side intake duct  18  (see  FIG. 2 ). 
     While engine  8  is running, as the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased (hereinafter to be referred to as the acceleration mode). As a result, while the rotational velocity of engine  8  is increased, the intake vacuum generated in the air inside intake duct  10  rises (see  FIG. 2 ). 
     When the engine is accelerating and the rotational velocity is R 1 , the intake pulsation at the first frequency, among the plural intake pulsations at different frequencies that form the intake pulsation generated in conjunction with the intake operation of engine  8 , is propagated via connecting pipe  12  to the composite membrane  16 . 
     Because the frequency of the intake pulsation at the first frequency matches first resonance frequency f 1  of elastic membrane  30   a , only elastic membrane  30   a  among elastic membranes  30   a - 30   d  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   a  vibrates in the out-of-plane direction of composite membrane  16 , it causes pressure variations in the air in additional pipe  14  on the side of composite membrane  16  that is open to the external air, and these air pressure variations become noise that is emitted to the external air side, such that the suction noise is amplified. 
     When the amount of accelerator pedal depression is further increased, that is, when the rotational velocity of the engine is at R 2 , from among the plural intake pulsations at different frequencies that form the intake pulsation in conjunction with the intake operation of engine  8 , the intake pulsation at the second frequency is propagated via connecting pipe  12  to the composite membrane  16  (elastic membrane). 
     Because the frequency of the intake pulsation at the second frequency matches second resonance frequency f 2  of elastic membrane  30   b  only elastic membrane  30   b  among elastic membranes  30   a - 30   d  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   b  vibrates in the out-of-plane direction of composite membrane  16 , pressure variations result in the air in the region between composite membrane  16  and the end of additional pipe  14  that is open to the external air, and air pressure variations become noise that is emitted to the external air side, thereby amplifying the suction noise. 
     When the amount of accelerator pedal depression is further increased, that is, when the rotational velocity of the engine is at R 3 , among the plural intake pulsations at different frequencies that form the intake pulsation in conjunction with the intake operation of engine  8 , the intake pulsation at the third frequency is propagated via connecting pipe  12  to composite membrane  16 . 
     Because the frequency of the intake pulsation at the third frequency matches third resonance frequency f 3  of elastic membrane  30   c , only elastic membrane  30   c  among elastic membranes  30   a - 30   d  vibrates in the out-of-plane direction of composite membrane  16 . When elastic membrane  30   c  vibrates in the out-of-plane direction of composite membrane  16 , pressure variations result in the air in the region between composite membrane  16  and the end of additional pipe  14  that is open to the external air, and said air pressure variations become noise that is emitted to the external air side, such that suction noise is amplified. 
     When the amount of accelerator pedal depression is further increased, that is, when the rotational velocity of the engine is at R 4 , among the plural intake pulsations at different frequencies that form the intake pulsation in conjunction with the intake operation of engine  8 , the intake pulsation at the fourth frequency is propagated via connecting pipe  12  to composite membrane  16  (elastic membrane member). 
     Because the frequency of the intake pulsation at the fourth frequency matches fourth resonance frequency f 4  of elastic membrane  30   d , only elastic membrane  30   d  among elastic membranes  30   a - 30   d  vibrates in the out-of-plane direction of composite membrane  16  (elastic membrane member). When elastic membrane  30   d  vibrates in the out-of-plane direction of composite membrane  16 , pressure variations result in the air in the region between composite membrane  16  and the end of additional pipe  14  that is open to the external air, and the air pressure variations become noise that is emitted to the external air side, thereby amplifying the suction noise. 
     Consequently, in the acceleration mode, elastic membranes  30   a - 30   d  with different resonance frequencies vibrate in the out-of-plane direction of composite membrane  16  corresponding to the variation in rotational velocity of engine  8 . As a result, the suction noise at the first frequency, the suction noise at the second frequency, the suction noise at the third frequency and the suction noise at the fourth frequency are amplified, and the amplified suction noise is emitted to the external air side from the opening at the second end of additional pipe  14  (see  FIG. 2 ). 
     When the amplified suction noise is emitted to the external air side from the second opening of additional pipe  14 , the emitted suction noise is propagated via the air into vehicle passenger compartment  2 , so that an impressive suction noise is transmitted into vehicle passenger compartment  2  (see  FIG. 1 ). 
     Variations of the Second Embodiments 
     Viewing device  1  that amplifies the suction noise in the second embodiment, in the thickness direction of composite membrane  16 , it may be seen that composite membrane  16  is composed of four elastic membranes  30   a - 30   d . However, the second embodiment is not limited to this scheme. That is, viewing in the thickness direction of composite membrane  16 , composite membrane  16  may be composed of five or more elastic membranes. In this case, composite membrane  16  may work with frequencies over a wider range than composite membrane  16  with just four elastic membranes  30   a - 30   d  as viewed in the thickness direction of composite membrane  16 . 
     Viewing the device  1  for amplifying suction noise in the second embodiment in the thickness direction of composite membrane  16 , composite membrane  16  is comprised of four elastic membranes  30   a - 30   d . Elastic membranes  30   a - 30   d  are formed with different areas, and their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. 
     As a result, by selecting the different areas of elastic membranes  30   a - 30   d  according to resonance frequencies of vibration in the out-of-plane direction of composite membrane  16 , it is possible to set the respective resonance frequencies of elastic membranes  30   a - 30   d  at the desired resonance frequencies. 
     Consequently, compared with the device for amplifying the suction noise of a vehicle in the first embodiment, that is, the device for amplifying the suction noise of a vehicle having three elastic membranes as viewed in the thickness direction, it is possible to further expand the frequency range where the suction noise can be amplified, and it is possible to improve the sound quality of the suction noise transmitted into vehicle passenger compartment  2 . 
     Third Embodiment 
     Referring to  FIGS. 8 and 9 , a third embodiment will be explained.  FIGS. 8 and 9  are diagrams illustrating the structure of device  1  that amplifies suction noise in the third embodiment.  FIG. 8  is a diagram illustrating the structure of composite membrane  16 , and  FIG. 9  is a cross section taken across X-Y in  FIG. 8 . 
     As shown in  FIGS. 8 and 9 , the structure of device  1  that amplifies suction noise in the third embodiment is substantially the same as that of the first embodiment except for the structure of composite membrane  16 . That is, the rigidity changing portion for composite membrane  16  in the third embodiment, is formed of convex portions  40  formed on the surface of composite membrane  16  on the intake duct side. 
     Viewed in the radial direction of composite membrane  16 , convex portions  40  are each generally V-shaped and project toward the intake duct side when composite member  16  is installed in connecting pipe  12 . The thickness of composite membrane  16  where convex portions  40  are formed is substantially equal to the thickness of the remaining portions. That is, composite membrane  16  is formed with a generally uniform thickness throughout. Composite membrane  16  with convex portions  40  formed thereon, may be formed by integral molding using dies. 
     The remainder of the structure of device  1  is generally the same as that of the first embodiment. 
     In the following, the operation of device  1  that amplifies the suction noise in the third embodiment will now be explained. Because the structure of everything besides composite membrane  16  is substantially the same as in the first embodiment, only the operation of the different portions will be explained in detail. 
     When engine  8  is started, the intake pulsation generated in conjunction with the intake operation of engine  8  is propagated via intake manifold  28  and surge tank  26  into the air inside clean-side intake duct  18  (see  FIG. 2 ). 
     While engine  8  is running, as the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased (hereinafter to be referred to as acceleration mode). As a result, while the rotational velocity of engine  8  is increased, the intake vacuum generated for the air in intake duct  10  rises (see  FIG. 2 ). 
     In the acceleration mode, when the amount of accelerator pedal depression is changed, the rotational velocity of the engine is changed. As a result, elastic membranes  30   a - 30   c  with different resonance frequencies vibrate in the out-of-plane direction of composite membrane  16  corresponding to the change in rotational velocity of engine  8 . As a result, pressure variations occur in the air in the region between composite membrane  16  and the end of additional pipe  14  that is open to the external air. The air pressure variations are emitted as noise to the external air side, so that the suction noise corresponding to the first frequency, the suction noise corresponding to the second frequency, and the suction noise corresponding to the third frequency are amplified (see  FIG. 2 ). 
     When the amplified suction noise is emitted to the external air side from the opening at the second end of additional pipe  14 , the emitted suction noise is propagated via the air into vehicle passenger compartment  2 , so that an impressive suction noise is transmitted into vehicle passenger compartment  2  (see  FIG. 1 ). 
     Variations of the Third Embodiment 
     As viewed in the radial direction of composite membrane  16 , device  1  that amplifies the suction noise in the third embodiment has convex portions  40  formed on composite membrane  16 , each being V-shaped and projecting to the intake duct side, and the thickness of composite membrane  16  is substantially uniform throughout when the shape is formed. However, the third embodiment is not limited to this scheme. 
     For example, as shown in  FIG. 10A , a scheme may also be adopted in which the thickness of the portions of composite membrane  16  where convex portions  40  are positioned is thicker than the remaining portions. Also, as shown in  FIG. 10B , a scheme may also be adopted in which convex portions  40  are each generally U-shaped as viewed in the radial direction of composite membrane  16 , and the thickness of composite membrane  16  is substantially uniform throughout. In addition, for example, as shown in  FIG. 10C , a scheme may be adopted in which convex portions  40  are each U-shaped projecting toward the intake duct side as viewed in the radial direction of composite membrane  16 , and the thickness of composite membrane  16  where convex portions  40  are formed is thicker than the remaining portions. 
     The rigidity changing portions in device  1  that amplifies suction noise in the present embodiment consist of convex portions  40  formed on the surface of composite membrane  16  on the intake duct side. The third embodiment is not limited to this scheme, however. For example, as shown in  FIGS. 11A and 11C , the rigidity changing portions may also comprise generally concave portions  42  formed in the surface of composite membrane (elastic membrane member)  16  on the intake duct side. And, as shown in  FIGS. 11B and 11D , a scheme may also be adopted in which the rigidity changing portions comprise generally convex portions  40  formed on the surface of composite membrane  16  on the external air side. 
     The device  1  for amplifying the suction noise of a vehicle in the third embodiment has rigidity changing portions that divide composite membrane  16  into plural elastic membranes by convex or concave portions  40 ,  42  formed on the surface of composite membrane  16  on the intake duct side. As a result, composite membrane  16  may be formed with plural elastic membranes by means of a simple structure. 
     As a result, it is possible to prevent increased manufacturing costs for composite membrane  16 , to prevent increased manufacturing costs for the device  1  for amplifying the suction noise of a vehicle, and to improve the producibility of the device for amplifying the suction noise of a vehicle. 
     Fourth Embodiment 
     Referring to  FIGS. 12 and 13 , a fourth embodiment will be explained.  FIGS. 12 and 13  are diagrams illustrating the structure of composite membrane  16  for device  1  that amplifies suction noise in the fourth embodiment.  FIG. 13  is a cross section of composite member  16  taken across Y-Y in  FIG. 12 . 
     As shown in  FIGS. 12 and 13 , the structure of device  1  that amplifies suction noise in the fourth embodiment is generally the same as that of the first embodiment except for the structure of composite membrane  16 . That is, the rigidity changing portion of composite membrane  16  in the fourth embodiment is formed of convex portions  40  formed on the surface of composite membrane  16  on the intake duct side, and each convex portion  40  has a core member  44 . 
     Viewed in the radial direction of composite membrane  16 , each convex portion  40  is generally nV-shaped and projects toward the intake duct side. The thickness of the portions of composite membrane  16  where convex portions  40  are formed is substantially equal to the thickness of the remaining portions. That is, thickness of composite membrane  16  is substantially uniform throughout. 
     Core member  44  is made of a wire material more rigid than composite membrane  16 , and it is arranged on the surface of composite membrane  16  on the external air side. 
     The remainder of the structure of device  1  is generally the same as that of the first embodiment 1. 
     In the following description, the operation of device  1  that amplifies suction noise in the fourth embodiment will be explained. Because the structure of everything besides composite membrane  16  is generally the same as in the first embodiment, only the operation of the different portions will be explained in detail. 
     When engine  8  is started, the intake pulsation generated in conjunction with the intake operation of engine  8  is propagated via intake manifold  28  and surge tank  26  into the air inside clean-side intake duct  18  (see  FIG. 2 ). 
     While engine  8  is running, as the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased (hereinafter to be referred to as acceleration mode). As a result, while the rotational velocity of engine  8  is increased, the intake vacuum generated in the air inside intake duct  10  rises (see  FIG. 2 ). 
     In the acceleration mode, when the amount of accelerator pedal depression is changed, the rotational velocity of the engine is changed. As a result, elastic membranes  30   a - 30   c  with different resonance frequencies vibrate in the out-of-plane direction of composite membrane  16  corresponding to changes in the rotational velocity of engine  8 . As a result, pressure variations develop in the air in the region between composite membrane  16  and the end of additional pipe  14  open to the external air. The air pressure variations become noise emitted to the external air side, so that the suction noise corresponding to the first frequency, the suction noise corresponding to the second frequency, and the suction noise corresponding to the third frequency are amplified, and the amplified suction noise is emitted to the external air side from the second opening of additional pipe  14  (see  FIG. 2 ). 
     When the amplified suction noise is emitted to the external air side from the second opening of additional pipe  14 , the emitted suction noise is propagated via the air into vehicle passenger compartment  2 , so that an impressive suction noise is transmitted into vehicle passenger compartment  2  via dash panel  4  (see  FIG. 1 ). 
     Variations of the Fourth Embodiment 
     Convex portions  40  formed on composite membrane  16  of device  1  that amplifies the suction noise in the present embodiment are each generally V-shaped and project to the intake duct side as viewed in the radial direction of composite membrane  16 . The thickness of composite membrane  16  is substantially uniform throughout when the shape is formed, and core member  44  is arranged on the surface of composite membrane  16  on the external air side. However, the fourth embodiment is not limited to this scheme. For example, as shown in  FIG. 14A , a scheme may also be adopted in which the thickness of composite film  16  where convex portions  40  are set is greater than in the remaining portions, with core member  44  being arranged inside convex portions  40  set on composite membrane  16 . Also, as shown in  FIG. 14B , a scheme may also be adopted in which each convex portion  40  is generally U-shaped as viewed in the radial direction of composite membrane  16 . In addition, for example, as shown in  FIG. 14C , a scheme may also be adopted in which each convex portion  40  of composite film  16  is generally U-shaped and projects toward the intake duct side as viewed from the radial direction of composite membrane  16 , and the composite membrane  16  is formed thicker where convex portions  40  are set than in the remaining portions, with core member  44  being arranged inside the convex portions  40 . 
     The rigidity changing portions of device  1  that amplifies suction noise in the fourth embodiment comprise convex portions  40  formed on the surface of composite membrane  16  on the intake duct side. However, the fourth embodiment is not limited to this scheme. For example, as shown in  FIGS. 15A and 15C , the rigidity changing portions can also comprise concave portions  42  formed on the surface of the composite membrane  16  on the intake duct side, and as shown in  FIGS. 15B and 15D , a scheme may also be adopted in which the rigidity changing portions consist of convex portions  40  formed on the surface of composite membrane  16  on the external air side. 
     The device  1  for amplifying the suction noise of a vehicle in the fourth embodiment has rigidity changing portions that divide composite membrane  16  into plural elastic membranes by convex portions  40  formed on the surface of the composite membrane on the intake duct side, and the convex portions each have a core member. 
     Thus composite membrane  16  may be formed with plural elastic membranes with a simple structure, and at the same time, the strength of the convex portions  40  may be increased. 
     As a result, it is possible to increase the producibility of the device  1  for amplifying the suction noise of a vehicle, and at the same time, the strength of composite membrane  16  may be increased compared to that in the device for amplifying the suction noise of a vehicle in the third embodiment, so that the durability of composite membrane  16  may be improved. 
     Fifth Embodiment 
     Referring to  FIG. 16 , a fifth embodiment will be explained.  FIG. 16  is a diagram illustrating the structure of composite member  16  of device  1  that amplifies suction noise in the present embodiment. 
     As shown in  FIG. 16 , the structure of device  1  that amplifies suction noise in the fifth embodiment is substantially the same as that of the first embodiment except for the structure of composite membrane  16 . That is, elastic membranes  30   a - 30   c  of composite membrane  16  in the fifth embodiment are made of materials having different modulus values. Here, the modulus refers to the property representing resistance to deformation of the object per unit volume. When the deformation and stress are proportional to each other, the modulus is the proportionality coefficient, and it depends on the material. Also, rigidity refers to the proportionality coefficient between a bending and twisting force applied to a structural body and the overall change in the structural body. The factors determining rigidity include the modulus of the material, the dimensions, and the shape of the structure. For example, when a material with a higher modulus is used, the rigidity is higher. When a single material is used, the thicker the sheet, the higher the rigidity. Also, the rigidity changes depending on the three-dimensional shape of the member that is obtained by pressing processes. 
     The modulus of elastic membrane  30   a  is lower than the modulus of elastic membrane  30   b , and the modulus of elastic membrane  30   b  is lower than the modulus of elastic membrane  30   c . Consequently, rigidity Ra of elastic membrane  30   a  is lower than rigidity Rb of elastic membrane  30   b , and rigidity Rb of elastic membrane  30   b  is lower than rigidity Rc of elastic membrane  30   c.    
     That is, the following relationship is established for elastic membranes  30   a - 30   c : Ra&gt;Rb&gt;Rc. 
     Here, because elastic membranes  30   a - 30   c  have different rigidities, their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. Also, elastic membrane  30  with a higher rigidity has a lower resonance frequency for vibration in the out-of-plane direction than does elastic membrane  30  with a lower rigidity. Consequently, for elastic membranes  30   a - 30   c , assuming the resonance frequency of elastic membrane  30   a  to be first resonance frequency f 1 , the resonance frequency of elastic membrane  30   b  to be second resonance frequency f 2 , and the resonance frequency of elastic membrane  30   c  to be third resonance frequency f 3 , the relationship f 1 &lt;f 2 &lt;f 3  is established. 
     In composite membrane  16  of the fifth embodiment, elastic membranes  30   a - 30   c  are made of materials having different modulus values. As a result, the structure is divided into three elastic membranes  30   a - 30   c  without providing slots or other rigidity changing portions on the intake duct side of composite membrane  16 . 
     The remaining features of the structure of the device  1  are substantially the same as those in the first embodiment. 
     In the following, the operation of device  1  that amplifies the suction noise in the fifth embodiment will be explained. Because the structure of everything besides composite membrane  16  is substantially the same as that in the first embodiment, only the operation of the different portions will be explained in detail. 
     When engine  8  is started, the intake pulsation generated in conjunction with the intake operation of engine  8  is propagated via intake manifold  28  and surge tank  26  into the air inside clean-side intake duct  18  (see  FIG. 2 ). 
     While engine  8  is running, as the amount of accelerator pedal depression is increased, the airflow rate from air cleaner  22  to surge tank  26  is increased (hereinafter to be referred to as acceleration mode). As a result, while the rotational velocity of engine  8  is increased, the intake vacuum generated in the air inside intake duct  10  rises (see  FIG. 2 ). 
     In this case, because said elastic membranes  30   a - 30   c  have different rigidity values, their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. 
     As a result, in the acceleration mode, as the amount of accelerator pedal depression is changed, the rotational velocity of the engine is changed. As a result, elastic membranes  30   a - 30   c  with different resonance frequencies vibrate in the out-of-plane direction of composite membrane  16  corresponding to changes in the rotational velocity of engine  8 . 
     As a result, the intake pulsation at the first frequency, the intake pulsation at the second frequency and the intake pulsation at the third frequency are amplified, and the amplified suction noise is emitted to the external air side from additional pipe  14  (see  FIG. 2 ). 
     When the amplified suction noise is emitted to the external air side from the opening at the other end of additional pipe  14 , the emitted suction noise is propagated via the air into vehicle passenger compartment  2 , so that an impressive suction noise is transmitted into vehicle passenger compartment  2  via dash panel  4  (see  FIG. 1 ). 
     Variations of the Fifth Embodiment 
     In the fifth embodiment, elastic membranes  30   a - 30   c  of device  1  that amplifies the suction noise have rigidities different from each other, so that their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. However, the fifth embodiment is not limited to this scheme. That is, a scheme may also be adopted in which elastic membranes  30   a - 30   c  are made of materials having different mass values, so that they have different resonance frequencies for vibration in the out-of-plane direction of composite membrane  16 . Also, one may adopt a scheme in which elastic membranes  30   a - 30   c  are made of materials different from each other with respect to their modulus and/or mass, so that they have different resonance frequencies for vibration in the out-of-plane direction of composite membrane  16 . 
     For composite membrane  16  in the fifth embodiment, elastic membranes  30   a - 30   c  are made of materials having different modulus values. As a result, the structure is provided with three divided elastic membranes  30   a - 30   c  without setting slots or other rigidity changing portions on the intake duct side of composite membrane  16 . However, the fifth embodiment is not limited to this scheme. For example, a scheme may also be adopted in which composite membrane  16  is composed of three separated elastic membranes  30   a - 30   c  by forming slots or other rigidity changing portions on the surface of composite membrane (elastic membrane member)  16  on the intake duct side, just as in any of the previous embodiments. 
     Viewed in the thickness direction of composite membrane  16 , composite membrane  16  of the device  1  for amplifying the suction noise of a vehicle in the fifth embodiment is composed of three elastic membranes. Because the elastic membranes have different rigidity values, their resonance frequencies for vibration in the out-of-plane direction of composite membrane  16  are different from each other. 
     As a result, in the acceleration mode, the various elastic membranes vibrate in the out-of-plane direction of composite membrane  16  corresponding to changes in the rotational velocity of engine  8 . 
     Consequently, the intake pulsation at the first frequency, the intake pulsation at the second frequency and the intake pulsation at the third frequency are amplified corresponding to changes in the rotational velocity of engine  8 , and the amplified suction noise is emitted to the external air side from the second opening of the additional pipe. The emitted suction noise is propagated via dash panel  4  into vehicle passenger compartment  2 , and an impressive suction noise is transmitted into vehicle passenger compartment  2 . 
     As a result, it is possible to generate plural resonance frequencies with a single composite membrane  16 , and an impressive suction noise may be generated without the need of plural intake ducts. Also, because the structure does not need plural intake ducts, the freedom of layout design may be improved, and device  1  may be adopted for vehicles with different body sizes or different structures. 
     Also, as viewed in the thickness direction, composite membrane  16  of the device  1  for amplifying suction noise in the fifth embodiment is composed of three elastic membranes, and these elastic membranes are made of materials with different modulus values, so that they have different frequencies for vibration in the out-of-plane direction of composite membrane  16 . 
     Consequently, by selecting the modulus values of the elastic membranes corresponding to the respective resonance frequencies for vibration in the out-of-plane direction of composite membrane  16 , it is possible to set the resonance frequencies of the elastic membranes at the respective desired resonance frequencies. 
     As a result, it is possible to set the resonance frequencies of the various elastic membranes for vibration in the out-of-plane direction of composite membrane  16  at plural desired frequencies, and it is possible to expand the range of frequency bands where amplification of the suction noise may be realized. As a result, it is possible to improve the sound quality of the suction noise transmitted into vehicle passenger compartment  2 . 
     Also, because composite membrane  16  in the device  1  for amplifying the suction noise of a vehicle in the fifth embodiment has elastic membranes made of materials having different modulus values, composite membrane  16  is constituted as three separated elastic membranes without the provision of slots or other rigidity changing portions on the surface of composite membrane  16  on the intake duct side. 
     Consequently, the durability of composite membrane  16  is improved due to the lack of rigidity changing portions with thicknesses different from other portions set at the boundaries between adjacent elastic membranes of composite membrane  16 . 
     The preceding description has been presented only to illustrate and describe exemplary embodiments of the oil return device according to the claimed invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope. The scope of the invention is limited solely by the following claims.