Patent Publication Number: US-2011056763-A1

Title: Acoustic resonance device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to acoustic resonance devices which reduce sounds/noises in cabins/compartments of vehicles. 
     The present application claims priority on Japanese Patent Application No. 2009-206496, the content of which is incorporated herein by reference. 
     2. Description of the Related Art 
     Conventionally, various technologies have been developed to improve quietness/noiselessness in cabins/compartments of vehicles by use of sound-absorbing materials. Patent Document 1 discloses a sound-absorbing material (e.g. a felt material) attached to a duct inside a dash panel in a cabin of a vehicle. Patent Document 2 discloses a panel/diaphragm sound-absorbing structure in which a panel/diaphragm vibrator and a rear cavity (or an air space in the rear of the vibrator) cooperate together to absorb sound. 
     Even when the technology of Patent Document 1 adopts the sound-absorbing structure of Patent Document 2, it is difficult to sufficiently reduce low-frequency sound owing to engine sound of a vehicle and frictional noise (which occurs due to friction between tires and roads while a vehicle is running). The technology of Patent Document 1 is unable to demonstrate a high sound-absorbing effect at positions of seats at which a driver and/or passengers may actually hear sound/noise inside a vehicle. 
     Patent Document 1: Japanese Patent Application Publication No. 2001-97020 
     Patent Document 2: Japanese Patent Application Publication No. 2006-11412 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an acoustic resonance device which reduces low-frequency sound so as to demonstrate a sound-absorbing effect at positions of seats at which a driver and/or passengers actually hear sound/noise inside a vehicle. 
     An acoustic resonance device according to the present invention is installed in a compartment of a vehicle and constituted of at least one resonator having an inner space and an opening. The resonator is arranged in the compartment of a vehicle such that the inner space communicates with the compartment via the opening. The resonator reduces sound pressure at a specific natural frequency corresponding to an antinode of a natural vibration emerging in a driver/passenger space inside the compartment of a vehicle. 
     Preferably, the resonator is positioned to reduce sound pressure at an antinode of natural vibration whose position is closest to the driver/passenger space among a plurality of antinodes of natural frequency occurring in the compartment of a vehicle. 
     In addition, the resonator increases a particle velocity at a specific natural frequency corresponding to an antinode of natural vibration emerging in the driver/passenger space inside the compartment of a vehicle. 
     Furthermore, the resonator reduces sound pressure at an excitation frequency which occurs due to an external condition of the vehicle and which differs from the specific natural frequency. 
     In the above, the natural vibration is a primary mode of vibration spreading sound pressure in the width direction of a vehicle. Alternatively, the natural vibration is a secondary mode of vibration spreading sound pressure in the forward-backward direction of a vehicle. 
     The resonator can be installed in a seat in connection with the driver/passenger space in the compartment of a vehicle. The resonator can be installed in a roof of a vehicle. The resonator can be installed in a pillar supporting the roof of a vehicle. The resonator can be installed in a door of a vehicle. 
     Generally speaking, a low-frequency sound which a driver/passenger may distinctively sense as noise has a strong dependency on a natural vibration occurring in the compartment of a vehicle. Considering a natural vibration which occurs in a height equivalent to the position of a driver/passenger&#39;s head on a front seat, the wavelength is approximately twice the width of a vehicle so that sound pressure spreads in the width direction of a vehicle. Antinodes of sound pressure owing to this natural vibration emerge in proximity to side windows fixed above front doors of a vehicle. For this reason, an acoustic resonance device is positioned to reduce sound pressure or to increase particle velocity at an antinode of sound pressure which is closest to the driver/passenger space among a plurality of antinodes of sound pressure owing to a natural vibration, thus achieving mode suppression. That is, the present invention is able to reduce a low-frequency sound pressure and to thereby improve a noise reduction effect at a driver/passenger&#39;s position at which a driver/passenger actually suffers from noise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, aspects, and embodiments of the present invention will be described in more detail with reference to the following drawings. 
         FIG. 1  is a perspective view showing the exterior appearance of a vehicle equipped with an acoustic resonance device according to a first embodiment of the present invention. 
         FIG. 2  is a side view partly in section showing a compartment and its related mechanical parts in the vehicle. 
         FIG. 3  is a perspective view showing the exterior appearance of a panel/diaphragm resonator serving as the acoustic resonance device of the first embodiment. 
         FIG. 4  is a cross-sectional view taken along line II-II in  FIG. 3 . 
         FIG. 5A  is a perspective view of the rear sides of front seats in a diagonal direction inside the compartment of the vehicle. 
         FIG. 5B  is a graph showing natural vibration in which sound pressure is spread in a width direction of the compartment. 
         FIG. 6  diagrammatically shows the details of measurement testing regarding a two-dimensional mode of vibration. 
         FIG. 7  is a graph showing the result of the measurement testing representing sound pressure at each evaluation position. 
         FIG. 8  is a perspective exploded view of the vehicle whose roof is equipped with panel/diaphragm resonators. 
         FIG. 9  is a perspective view showing the rear sides of front seats in a diagonal direction inside the compartment of the vehicle equipped with panel/diaphragm resonators. 
         FIG. 10  is a cross-sectional view taken along line VIII-VIII in  FIG. 2 , showing the constitution of a roof equipped with panel/diaphragm resonators. 
         FIG. 11  is a cross-sectional view of a center pillar which is cut in a plane perpendicular to its longitudinal direction. 
         FIG. 12A  is a lateral sectional view taken along line A-A on a front seat in FIG.  9 . 
         FIG. 12B  is a vertical sectional view taken along line B-B on the front seat in  FIG. 9 . 
         FIG. 13  is a vertical sectional view of a front door which is cut in a plane perpendicular to the width direction of the vehicle. 
         FIG. 14A  is a front view of a resonance pipe unit constituted of plural resonance pipes attenuating sound pressure. 
         FIG. 14B  is across-sectional view illustrating the internal structure of two adjacent resonance pipes which are combined together to cause coupled oscillation therebetween. 
         FIG. 15  is a perspective exploded view of the vehicle whose roof is equipped with resonance pipe units. 
         FIG. 16  is a perspective view of the rear sides of front seats in a diagonal direction inside the compartment of the vehicle equipped with resonance pipe units. 
         FIG. 17  is across-sectional view taken along line VIII-VIII in  FIG. 2 , showing the constitution of a roof equipped with resonance pipe units. 
         FIG. 18  is a cross-sectional view of the front pillar taken along line D-D in  FIG. 16 , illustrating installation of the resonance pipe unit. 
         FIG. 19  is a perspective view partly in section, showing the front door equipped with the resonance pipe unit. 
         FIG. 20A  is a perspective view showing a Helmholtz resonator serving as an acoustic resonance device according to a third embodiment of the present invention. 
         FIG. 20B  is a cross-sectional view of the Helmholtz resonator constituted of a body and a tubular portion. 
         FIG. 21  is a cross-sectional view taken along line VIII-VIII in  FIG. 2 , showing the constitution of a roof equipped with Helmholtz resonators in addition to panel/diaphragm resonators. 
         FIG. 22  is a graph showing frequency characteristics on sound pressure, indicating a noise reduction effect owing to resonators in terms of A-characteristic sound pressure. 
         FIG. 23A  is a cross-sectional view of a tubular portion according to a second variation in the Helmholtz resonator. 
         FIG. 23B  is a plan view of the tubular portion constituted of an outer tube, an inner tube, and an opening. 
         FIG. 24A  is a perspective exploded view of a lattice member attached to a roof inner panel of the vehicle. 
         FIG. 24B  is a side view of the lattice member in a direction F in  FIG. 24A . 
         FIG. 25  is a graph showing the simulation result on the panel/diaphragm resonator by using different surface densities of the vibrator, thus calculating sound absorption coefficients in connection with frequencies. 
         FIG. 26  is a perspective view of a corrugated panel according to a fifth variation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in further detail by way of embodiments and variations with reference to the accompanying drawings. 
     1. First Embodiment 
       FIG. 1  is a perspective view showing the exterior appearance of a vehicle  100 , i.e. a four-door sedan, equipped with an acoustic resonance device according to a first embodiment of the present invention.  FIG. 2  diagrammatically shows a cabin/compartment  105  of the vehicle  100 . Herein, a bonnet (hood)  101 , four doors  150  (which serve as entrance doors of the compartment  105 ), and a trunk door  103  are fixed to a chassis (serving as a skeletal structure of the vehicle  100 ) in an open/close manner. The chassis of the vehicle  100  includes a base  104 , center pillars  120  (which extend upwards from the base  104 ), front pillars  130 , and rear pillars  180  as well as a part of a roof  110  (which is supported by those pillars). The compartment  105  is circumscribed by the doors  150  in the vehicle  100 . The compartment  105  is a room space accommodating a driver/passenger who gets into the vehicle  100 . A rear package tray  220  is installed in the rear side of the vehicle  100 . The rear package tray  220  covers a partition (not shown) between the compartment  105  and a trunk. As shown in  FIG. 2 , the front side of the vehicle  100  corresponds to the forward direction (or running direction) of the vehicle  100 , while the rear side of the vehicle  100  corresponds to the backward direction of the vehicle  100 . 
     The compartment  105  includes a driver/passenger space in which a driver and/or passengers reside in the vehicle  100 . Similar to the conventionally-known interior structure of a car, the driver/passenger space of the compartment  105  includes front seats  140  and rear seats  190 . Specifically, the front seats  140  include a driver&#39;s seat  140 A and its adjacent seat  140 B. The driver/passenger space of the compartment  105  accommodating a driver and/or passengers is determined in advance in the design phase. The four doors  150  include two front doors  150 A fixed adjacent to the front seats  140  and two rear doors  150 B fixed adjacent to the rear seats  190 . The doors  150  are equipped with side windows  153 . When the side windows  153  are closed as shown in  FIG. 1 , they are disposed at top positions of the front doors  150 A. 
     An acoustic resonance device is installed in the vehicle  100  so as to reduce low-frequency sound in the compartment  105 . The acoustic resonance device includes a resonator which resonates to attenuate sound in the compartment  105 . The first embodiment adopts a panel/diaphragm resonator for use in the acoustic resonance device. 
       FIG. 3  diagrammatically shows the exterior appearance of the panel/diaphragm resonator  1 .  FIG. 4  is a cross-sectional view taken along line in  FIG. 3 , showing the inside of the panel/diaphragm resonator  1 . 
     The panel/diaphragm resonator  1  is essentially divided into a housing  10  and a vibrator  15 . The housing  10  is a rectangular parallelepiped member whose upper section is opened as an opening  12 . The housing  10  is constituted of the opening  12  and a rectangular parallelepiped cavity  13 , i.e. a hollow space communicating with the opening  12 . The housing  10  is made of woods; but this is not a restriction. That is, the housing  10  can be made of hard materials, such as a synthetic resin and metal, which are harder than the material of the vibrator  15 . The vibrator  15  is a rectangular member having elasticity, such as a panel or a diaphragm. For example, the vibrator  15  is a panel made of an elastic material causing elastic vibration, such as a synthetic resin, metal, and fiber board, or the vibrator  15  is a diaphragm made of an elastic material or a polymer compound. The edge of one surface of the vibrator  15  is supported by the housing  10  so that the vibrator  15  closes the opening  12  of the housing  10 . Since the opening  12  of the housing  10  is covered with the vibrator  15 , the cavity  13  is formed inside the panel/diaphragm resonator  1 . The cavity  13  serves as a layer composed of gaseous particles, practically, an air space including air molecules. 
     The panel/diaphragm resonator  1  is arranged in the compartment  105  such that it communicates with a space subjected to sound attenuation. In other words, the cavity  13  of the panel/diaphragm resonator  1  is positioned in a space experiencing a sound pressure which should be attenuated. When sound occurs in this space, the panel/diaphragm resonator  1  resonates to the sound pressure. Owing to resonance, a pressure difference occurs between the sound pressure of the space and the internal pressure of the cavity  13  of the panel/diaphragm resonator  1 . The pressure difference causes the vibrator  15  to vibrate so that acoustic energy is being consumed; subsequently, acoustic energy is radiated again. This operation works on the surface of the panel/diaphragm resonator  1  so that sound pressure is reduced in a space in proximity to the vibrator  15 . 
     The frequency at which sound pressure is reduced by way of resonance of the panel/diaphragm resonator  1  depends upon a resonance frequency of a spring-mass system based on a mass component (i.e. the weight of the vibrator  15 ) and a spring component of the cavity  13 . The vibration of the spring-mass system refers to “piston oscillation”. Since the vibrator  15  having elasticity has a small area, the property of a bending system additionally emerges due to elastic vibration at a part of the vibrator which is constrained by being supported by the housing  10 . That is, the panel/diaphragm resonator  1  possesses the vibrator  15  experiencing “bending oscillation” and the cavity  13  disposed in the backside of the vibrator  15 . 
     Next, setup conditions of the panel/diaphragm resonator  1  will be described with respect to a resonance frequency of piston oscillation and a resonance frequency of bending oscillation. 
     The resonance frequency f of the piston oscillation is expressed via equation (1), wherein ρ 0  [g/m 3 ] denotes a density of a gaseous medium, i.e. an air density; c 0  [m/s] denotes sound velocity; ρ [kg/m 3 ] denotes a density of a vibrator; t [m] denotes the thickness of the vibrator; and L [m] denotes the thickness of an air layer. 
     
       
         
           
             
               
                 
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     The resonance frequency f of both the piston oscillation and the bending oscillation is expressed via equation (2), wherein the vibrator has a rectangular shape whose one length is “a” [m] and whose other length is “b” [m]; E [Pa] denotes Young&#39;s modulus of the vibrator; σ [-] denotes the Poisson ratio of the vibrator; and p, q are positive integers representing mode degrees. This resonance frequency f is occasionally employed in architectural acoustic design. 
     
       
         
           
             
               
                 
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     As described above, the panel/diaphragm resonator  1  causes resonance owing to the piston oscillation and resonance owing to the bending oscillation. Herein, the piston oscillation and the bending oscillation do not occur independently of each other. When their resonance frequencies are close to each other, the resonance of the spring-mass system and the resonance of the bending system cooperate to determine the overall resonance frequency of the panel/diaphragm resonator  1 . When the resonance frequency of the spring-mass system differs from the resonance frequency of the bending system, they operate independently of each other while they may partially affect each other. For this reason, the fundamental oscillation of the bending system cooperates with the spring component of the cavity in the backside, so that a large amplitude oscillation is driven in a frequency band between the resonance frequency of the spring-mass system and the fundamental frequency of the bending system, thus increasing the attenuation of sound pressure. 
     The panel/diaphragm resonator  1  of the first embodiment satisfactorily works to reduce sound pressure in a frequency band whose center frequency is set to a relatively low resonance frequency. We (i.e. inventors of the present invention) made various experiment in which a fundamental frequency fa of the bending system is expressed via equation (3) while a resonance frequency fb of the spring-mass system is expressed via equation (1). We found that the panel/diaphragm resonator  1  whose parameters are adjusted according to equation (4) is able to adequately reduce sound pressure. 
     
       
         
           
             
               
                 
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     Since the fundamental oscillation of the bending system cooperates with the spring component of the air space in the backside so that a large amplitude vibration is driven in a frequency band between the fundamental frequency of the piston oscillation and the fundamental frequency of the bending oscillation, thus causing a resonance phenomenon where (fundamental frequency of bending oscillation) fa&lt;(peak frequency of attenuation of sound pressure) f&lt;(fundamental frequency of piston oscillation) fb. This causes the panel/diaphragm resonator  1  to radiate an anti-phase reflected wave, thus reducing sound pressure on the surface of the vibrator  15 . 
     When the above parameters of the panel/diaphragm resonator  1  are adjusted to meet equation (5), the peak frequency of the attenuation of sound pressure further decreases in comparison to the fundamental frequency of the piston oscillation. 
     
       
         
           
             
               
                 
                   0.05 
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     In order to adequately reduce sound pressure in a frequency range from 160 Hz to 315 Hz (corresponding to a center frequency of one-third octave), the panel/diaphragm resonator  1  needs to set the above parameters, such as ρ 0 =1.225 [kg/m 3 ], c 0 =340 [m/s], ρ=940 [kg/m 3 ], t=0.0017 [m], L=0.03 [m], a=b=0.1 [m], E=1.0 [GPa], σ=0.4, and p=q=1. 
     Next, a method for determining an installation location of the panel/diaphragm resonator  1  will be described in detail.  FIG. 5A  is a perspective view of the front seats  140 A whose center position is observed in a diagonal-rear direction in the compartment  105 , and  FIG. 5B  is a graph showing natural vibration (or a normal mode of vibration) spread in a width direction (corresponding to the lateral width of the vehicle  100 ) in the compartment  105 . The graph of  FIG. 5B  shows distribution of sound pressure in a one-dimensional mode of vibration regarding standing waves (or axial waves) in which sound pressure is spread in the width direction of the vehicle  100 . The wavelength of an axial wave is roughly twice the width of the compartment  105 . The natural vibration depends upon the width of the compartment  105 , wherein the natural frequency thereof is relatively low at 167 Hz, for example. The horizontal axis of the graph of  FIG. 5B  represents the position of a dotted line H in  FIG. 5A , i.e. the height of a head (or ears) of a person seated at the front seat  140 , while the vertical axis represents sound pressure in the one-dimensional mode of vibration. 
     We, the inventors, presumed that a low frequency of 170 Hz or so, which a person residing in the compartment  105  can recognize as noise, greatly depends upon the natural vibration. In general, numerous modes of vibration condense in an audio frequency range in a diffuse sound field; hence, sound pressure is spread uniformly in the sound field, explicitly indicating the uniform distribution of sound field at each position in the sound field on the frequency axis. In contrast, some modes of vibration which are difficult to be attenuated occur in a sound field of a small space such as the compartment  105  of the vehicle  100 . In other words, some modes of vibration are isolated from each other on the frequency axis in the compartment  105 . At low natural frequencies, antinodes of sound pressure are spread in a rarefactional manner in the compartment  105 ; hence, an antinode of sound pressure may emerge at a specific position of the sound field at which sound pressure is significantly increased compared to other positions. The isolated mode of vibration corresponds to the one-dimensional mode of vibration (causing axial waves), presenting high acoustic energy which is difficult to be attenuated. Compared to other modes of vibration, the one-dimensional mode of vibration leads to a small number of incidence of sound waves at a wall surface per unit time, so that acoustic energy is rarely absorbed by the wall surface. 
     We found a simple solution to attenuate sound at a specific natural frequency in a small sound field, wherein a position corresponding to an antinode of sound pressure of the specific natural frequency needs to be specified and reduced in sound pressure. This solution effectively reduces low-frequency sound in a small sound field. In other words, when a position corresponding to an antinode of natural vibration is specified and subjected to attenuation of sound pressure, it is possible to weaken natural vibration in a sound field. A resonator can be employed as a constituent element for reducing sound pressure, wherein the opening of the resonator is positioned in proximity to the position of an antinode of sound pressure. Herein, the term “proximity” refers to a distance which suffices the need of reducing sound pressure at an antinode. A small distance may reduce sound pressure at a specific natural frequency, wherein it may be set to one-sixth of a wavelength or less, for example. 
     We performed the following measurement testing in order to confirm whether or not a low-frequency sound pressure is actually reduced in a small sound field by way of the above working principle. 
       FIG. 6  shows the details of the measurement testing, which diagrammatically shows natural vibration in which sound pressure is spread in a height direction of a rectangular parallelepiped sound field. Specifically,  FIG. 6  shows a two-dimensional mode of vibration in which a low natural frequency of 158 Hz occurs owing to the height dimension of the sound field. Hatching portions with slashes indicate positions at which maximum sound pressure emerges or at which sound pressure increases maximally; hence, these positions matches antinodes of sound pressure. Other hatching portions with vertical lines indicate positions at which minimum sound pressure occurs or at which sound pressure decreases minimally; hence, these positions correspond to nodes of sound pressure. Other blank portions (or non-hatching portions) indicate positions at which sound pressure remains in an intermediate level. The two-dimensional mode of vibration of  FIG. 6  is computed by way of sound-field simulation adopting a finite element method (FEM). 
     We designated fifteen circular marks serving as evaluation positions, at which sound pressure is being measured, with numerals “1” through “15”. A microphone is positioned at each evaluation position. Evaluation positions  1  through  9  are laid along an edge line extending in the height direction of the sound field, wherein they are positioned with equal spacing therebetween. Evaluation positions  9  through  15  are laid along an edge line extending in the width direction or a baseline of the sound field, wherein they are positioned with equal spacing therebetween. The evaluation position  9  is set at a corner of the sound field, while a sound source is positioned at a farthest corner from the evaluation position  9 . 
       FIG. 7  is a graph showing the results of the measurement testing, which are produced in such a way that microphones disposed at respective evaluation positions receive sound emitted from the sound source so as to detect sound pressure. In  FIG. 7 , the horizontal line represents the evaluation positions  1  through  15 , and the vertical line represents sound pressure in units of decibels (dB) in a 160 Hz band corresponding to a one-third octave band whose center frequency is 160 Hz. Herein, a solid line represents a measurement result A in which sound pressure is attenuated at an antinode of natural vibration by way of resonance, wherein four resonators are arranged at four corners of a horizontal plane whose height corresponds to the evaluation position  5  in the sound field. Specifically, four resonators, each of which has one open end (communicating with its cavity) and the other closed end, are arranged such that the open ends thereof are positioned at four corners. Each resonator resonates at a predetermined resonance frequency in the 160 Hz band. A dotted line represents a measurement result B in which sound pressure is attenuated at a node of natural vibration by way of resonance, wherein four resonators are arranged at four corners of a horizontal plane whose height corresponds to the evaluation position  7 . A dashed line represents a measurement result C in which no resonator is arranged. 
     The measurement result A (see solid line in  FIG. 7 ) clearly shows that sound pressure significantly decreases at the evaluation position  5 , corresponding to an antinode of natural vibration, which is closest to the open end of a resonator, wherein sound pressure is reduced to approximately 62 dB. At the evaluation positions  3 ,  4 ,  6  and  7  which are close to the evaluation position  5 , sound pressure is around 90 dB; hence, sound pressure is lower than the measurement result C (see dotted line in  FIG. 7 ) produced without using resonators. At the evaluation positions  8  through  15  which are distant from the positions of resonators, sound pressure is lower than the measurement result C produced without using resonators, by approximately 20 dB. The above measurement result A shows that sound pressure at an antinode of natural vibration can be greatly reduced by use of resonators, while sound pressure at other positions, which are distant from positions of resonators, can be reduced as well. This proves that resonators can improve the quietness/noiselessness in a sound field occurring in the compartment of a vehicle. This effect will be referred to as a “mode suppression effect” in the following description. 
     The measurement result B (see dotted line in  FIG. 7 ) clearly shows that sound pressure significantly decreases at the evaluation position  7 , corresponding to a node of natural vibration, which is closest to the open end of a resonator, wherein sound pressure is reduced to approximately 76 dB. In contrast, no mode suppression effect is found at other evaluation positions in comparison with the measurement result C. The measurement result C shows that sound pressure cannot be sufficiently reduced by use of resonators which are arranged at a node of natural vibration; hence, it is very difficult to obtain a mode suppression effect in the sound field. 
     Referring back to  FIG. 5A  illustrating the one-dimensional mode of vibration in which sound pressure is spread in the width direction of the compartment  105 , we found that an antinode of sound pressure emerges around the height of a head (or ears) of a driver/passenger seated at the front seat  140 A/ 140 B. Antinodes may emerge around opposite ends of the width direction of the compartment  105 , wherein they may be proximate to the side windows  153  of the front doors  150 A. The wavelength of a natural frequency corresponding to natural vibration may be twice the width of the compartment  105 . For example, the wavelength of a natural frequency of 167 Hz is about 2.0 m, while the width of the compartment  105  ranges from 0.8 m to 1.5 m in a general size of a vehicle. 
     An antinode of sound pressure, which emerges around the side window  153  of the compartment  105  undergoing a normal mode of vibration, does not necessarily occur via a standing wave (or an axial wave) propagating in the width direction. We could estimate that an antinode of a sound wave may occur via an axial wave spreading sound pressure in the width direction dependent upon a one-dimensional mode of vibration. Strictly speaking, an antinode of sound pressure is connected with an axial wave spreading sound pressure in a forward-backward direction of the vehicle  100  (see  FIG. 2 ); in other words, an antinode of sound pressure possibly emerges due to a tangential wave dependent upon a two-dimensional mode of vibration. In addition, an antinode of sound pressure possibly emerge due to a slanted wave dependent upon a three-dimensional mode of vibration in connection with an axial wave spreading sound pressure in the height direction of the compartment  105 . Regardless of which type of vibration being involved, we found a fact that an antinode of sound pressure owing to a natural vibration emerges around the front seats  140  and the side windows  153 . 
     In the compartment  105 , plural isolated modes of vibration occur on the frequency axis in a low-frequency range, particularly, in a 160 Hz band, so that sound pressure is actually spread according to those modes of vibration in a sound field. We found that, among those isolated modes of vibration, a certain mode of vibration (see  FIG. 5B ) spreading sound pressure in the width direction needs to be suppressed in order to achieve a high noise reducing effect in the compartment  105 . That is, it is possible to significantly improve a noise reducing effect at a driver/passenger&#39;s position of hearing sound when an antinode of natural vibration, which is determined based on the driver/passenger&#39;s position and which emerges around the front seat  140  and the side window  153 , is selectively controlled in sound field. Considering the interior specification of the vehicle  100  having a generally-known structure and dimensions, it is necessary to embed a resonator in the front seat  140  or at a selected position of an interior surface close to the front seat  140  in the compartment  105 . 
     The position of an antinode of natural vibration depends upon the material and shape of the compartment  105 ; hence, it can be easily measured without actually driving the vehicle  100 . In actuality, a measurement sound having a predetermined frequency is emitted from a speaker which is installed in a generally-known compartment of a vehicle and is received by a microphones disposed at predetermined positions in the compartment; subsequently, sound pressure is detected based on the result of measuring the measurement sound, thus producing the sound pressure distribution shown in  FIG. 5B . Generally speaking, conventional vehicles adopt similar shapes and dimensions of compartments; hence, it can be said that an antinode of sound pressure emerges around front seats in a driver/passenger&#39; space undergoing a natural vibration. 
     Next, the positioning of the panel/diaphragm resonator  1  arranged in the compartment  105  will be described in detail. The positioning of the panel/diaphragm resonator  1  is set up to realize mode suppression with respect to a 160 Hz band based on a natural vibration. Specifically, the resonance frequency of the panel/diaphragm resonator  1  is roughly set up in conformity with the natural frequency. 
       FIG. 8  is a perspective exploded view of the vehicle, and  FIG. 10  is a perspective view whose center shows the backside of front seats  140 A in a slanted direction in the compartment  105 . In the present embodiment, numerous panel/diaphragm resonators  1  are attached to respective positions inside the vehicle  100 . Specifically, the panel/diaphragm resonators  1  are attached to the roof  110 , the center pillars  120 , the front pillars  130 , the front seats  140 , and the front doors  150 A. Among them, the roof  110 , the center pillars  120 , the front pillars  130 , and the front doors  150 A are constituent parts of the compartment  105 . The panel/diaphragm resonators  1  are positioned in proximity to reduce sound pressure at an antinode which emerges in the compartment  105  undergoing a natural vibration. Specifically, they are positioned within a small distance (e.g. 30 cm) roughly corresponding to the wavelength of a natural frequency from the antinode of natural vibration. The panel/diaphragm resonators  1  are each positioned such that the cavity  13  communicates with the compartment  105  via the opening  12 . 
     In the following description, the term “upper side” indicates a higher position in a height direction of the compartment  105 , while the “lower side” indicates a lower position in the height direction of the compartment  105 . The term “left side” indicates a left portion of the vehicle  100  in a running direction, while the “right side” indicates a right portion of the vehicle  100  in the running direction. In addition, the position of the side window  153  refers to the upper end of the side window  153  which is closed, wherein the height thereof roughly matches the height of a driver/passenger who is actually seated in the front seat  140 . 
     Next, the constitution of the roof  110  will be described in detail.  FIG. 10  is a cross-sectional view taken along line VIII-VIII in  FIG. 2 , showing the roof  110  of the vehicle  100 . 
     As shown in  FIGS. 8 and 10 , the panel/diaphragm resonators  1  are positioned on the ceiling, just above the driver/passenger&#39;s space on the front seats  140 A and  140 B, within the root  110 . Specifically, two sets of three panel/diaphragm resonators  1  linearly aligned in the forward-backward direction of the vehicle  100  are embedded in the roof  110 . The panel/diaphragm resonators  1  need to be arranged in the roof  110  since an antinode of sound pressure emerges at a high position close to the side window  153  of the front door  150 A. In the width direction of the vehicle  100 , the panel/diaphragm resonators  1  are each positioned such that the vibrator  15  is positioned close to the side window  153 . 
     The roof  110  is attached to a part of the chassis serving as the basic structure of the vehicle  100 , wherein the roof  110  includes a roof inner panel  114  composed of a polypropylene resin. The roof inner panel  114  has a base material  111  made of a wooden fiber board. Surface materials  112 , which are composed of cloth materials allowing sound pressure to transmit therethrough, are arranged in the roof inner panel  114  in proximity to the compartment  105 . The panel/diaphragm resonators  1  are embedded inside recesses  113  formed on the upper surface of the base material  111 , wherein the panel/diaphragm resonators  1  are fixed to the roof inner panel  114  via the adhesive. The fixing measure for fixing the panel/diaphragm resonators  1  to the roof inner panel  114  is not necessarily limited to the adhesive; hence, it is possible to adopt other fixing measures such as screws and nuts, and belts. In short, it is possible to adopt any fixing measures adaptive to the fixation between the panel/diaphragm resonator  1  and the roof inner panel  114 . The panel/diaphragm resonators  1  are each attached to the base material  111  such that the vibrator  115  communicates with the inner space of the compartment  105  via the surface material  112 . In addition, the other panel/diaphragm resonators  1  are embedded and fixed inside recesses  115  which are formed at opposite ends of the width direction in the roof inner panel  114 , wherein they are embedded in slanted portions of the ceiling in proximity to the side windows  153 . Specifically, the panel/diaphragm resonators  1  are positioned in proximity to an assist grip  200 , which is attached to a side portion of the ceiling close to the driver/passenger&#39;s position in the vehicle  100  having a generally-known structure. When the panel/diaphragm resonators  1  are positioned in proximity to the assist grip  200 , the vibrators  15  are positioned very close to the side window  153 , thus further improving a sound attenuating effect with respect to an antinode of sound pressure. The panel/diaphragm resonators  1  embedded in the roof  110  effectively achieve mode suppression with respect to natural vibration. 
     Next, the constitution of the center pillar  120  and the constitution of the front pillar  130  will be described in detail.  FIG. 11  is a cross-sectional view of the center pillar  120  which is cut in a plane perpendicular to the longitudinal direction. The center pillar  120  is constituted of a center pillar outer panel  121  (serving as a part of the chassis) and a center pillar inner panel  122  (which is fixed to the center pillar outer panel  121  via pins  122 A). A surface material  123  composed of a cloth material allowing sound pressure to transmit therethrough is attached to the inner portion of the center pillar inner panel  122  in proximity to the compartment  105 . 
     The panel/diaphragm resonator  1  is embedded in the center pillar  120  such that the vibrator  15  faces the center pillar inner panel  122 . Plural holes  124  are formed on the inner portion of the center pillar inner panel  122 . Sound occurring in the compartment  105  reaches the vibrator  15  of the panel/diaphragm resonator  1  via the holes  124 . As shown in  FIG. 9 , the center pillar  120  is positioned in proximity to the front seat  140 A and the front door  150 A. The panel/diaphragm resonator  1  embedded in the center pillar  120  effectively achieve mode suppression in the compartment  105 . 
       FIG. 9  shows that another panel/diaphragm resonator  1  is embedded in the front pillar  130 , wherein the fixing structure thereof is similar to the above structure for fixing the panel/diaphragm resonator  1  inside the center pillar  120 . Since the front pillar  130  is positioned in proximity to the front seat  140 A and the front door  150 A, the panel/diaphragm resonator  1  embedded in the front pillar  130  effectively achieve mode suppression in the compartment  105 . 
     Next, the constitution of the front seat  140  will be described in detail. FIG.  12 A is a lateral sectional view taken along line A-A on the front seat  140 A in  FIG. 9 , and  FIG. 12B  is a vertical sectional view taken along line B-B on the front seat  140 A in  FIG. 9 . The surrounding space of the front seat  140  constitutes the compartment  105 . 
     The front seat  140  is divided into a head rest  141  and a seat back  142 . The head rest  141  is attached to the seat back  142  via legs (not shown) which are inserted into the seat back  142 . The head rest  141  is adjusted in position with an occipital region of a head of a driver/passenger seated in the front seat  140 . The front rest  141  supports a driver/passenger&#39;s head. A head rest bag  143  made of leather or synthetic leather is filled with stuffing such as low-resilience urethane foam. The head rest bag  143  is covered with a head rest cover  144 . In the seat back  142 , a seat back bag  145  is filled with stuffing such as urethane foam. The surface of the seat back bag  145  is covered with a seat back cover  146 . Both the head rest cover  144  and the seat back cover  146  are made of a cloth material allowing sound pressure to transmit therethrough. 
     As shown in  FIG. 12A , two panel/diaphragm resonators  1  are embedded in the heat rest  141  such that the vibrators  15  are directed to opposite ends of the width direction of the compartment  105 . Since an antinode of sound pressure owing to a natural vibration emerges in proximity to the side window  153 , the vibrators  15  of the panel/diaphragm resonators  1  need to be positioned close to the antinode of sound pressure. Since the vibrators  15  are arranged inwardly of the heat rest cover  144  to communicate with the compartment  105 , they are not visualized by a driver/passenger with ease. In addition, the panel/diaphragm resonators  1  are fixed in position with respect to the head rest  141 , wherein the panel/diaphragm resonators  1  are fixed to legs or a frame (not shown) via which the heat rest  141  is attached to the seat back  142 . 
     As shown in  FIG. 12B , the panel/diaphragm resonators  1  are embedded in the sear back  142  such that the vibrators  15  are positioned at an upper surface and left/right-side surfaces (preferably, at a height close to the side window  153 ). The vibrators  15  of the panel/diaphragm resonators  1  are arranged inwardly of the seat back cover  146  to communicate with the compartment  105 , wherein they are not visualized by a driver/passenger with ease. The panel/diaphragm resonators  1  are fixed to a frame (not shown) of the seat back  142  and are thus fixed in position. Since the vibrators  15  of the panel/diaphragm resonators  1  are embedded in the front seats  140  in proximity to the side windows  153 , it is possible to effectively achieve mode suppression in the compartment  105 . In particular, the front seats  140  occupy the driver/passenger space in the compartment  105 , thus making it possible to position the vibrators  15  of the panel/diaphragm resonators  1 . Thus, an improved sound attenuating effect at a driver/passenger&#39;s position at which a driver/passenger actually hears sound is highly expected. 
     Even when the head rest  141  and the seat back  142  are integrally unified as a single seat, it is possible to arrange the panel/diaphragm resonators  1  in a manner similar to the separate type of the front seat  140 . 
     Next, the constitution of the front door  150 A will be described in detail. In the present embodiment, the panel/diaphragm resonators  1  are installed in the front doors  150 A, while no panel/diaphragm resonator is installed in each of the rear doors  150 B. 
       FIG. 13  is a vertical sectional view of the front door  150 A which is cut in a plane perpendicular to the width direction of the compartment  105 . 
     The front door  150 A has a base material  151 , which installs the side window  153  to be movable in a vertical direction of the front door  150 A. A surface material  154  made of a cloth material allowing sound pressure to be transmitted therethrough is attached to the inner surface of the base material  151  defining the compartment  105 . The base material  151  of the front door  150 A has a glass storage  151 A storing the side window  153  when being opened. The base material  151  of the front door  150 A has an inner space S arranged internally of the glass storage  151 A. The panel/diaphragm resonators  1  are installed in the front door  150 A such that the vibrators  15  communicate with the inner space S. Plural holes  155  are formed on the inner portion of the base material  151  of the front door  150 A, so that the inner space S communicates with the compartment  105  via the holes  155 . 
     Since sound occurring in the compartment  105  enters into the inner space S via the holes  155 , it is possible to reduce sound pressure at an antinode of natural vibration by way of resonance of the panel/diaphragm resonators  1 . The panel/diaphragm resonators  1  are set up in position such that the vibrators  15  are positioned close to the side window  153  which is installed in the upper side of the front door  150 A. In the present embodiment, the panel/diaphragm resonators  1  are positioned in the upper side of the inner space S, while no panel/diaphragm resonator is positioned in the lower side of the inner space S close to the floor of the compartment  105 . 
     The acoustic resonance device of the first embodiment is constituted of the panel/diaphragm resonators  1  which are attached to the front seats  140  (located within the driver/passenger space of the compartment  105 ) as well as the roof  110 , the center pillars  120 , the front pillars  130 , and the front doors  150 A which serve as surrounding walls of the compartment  105  in proximity to the driver/passenger space. In the compartment  105 , an antinode of sound pressure corresponding to a low natural frequency is located in proximity to the front seats  140  (particularly, at a height of a driver/passenger&#39;s head) as well as the side windows  153  of the front doors  150 A. For this reason, the panel/diaphragm resonators  1  each positioned close to an antinode of sound pressure can effectively achieve mode suppression in the compartment  105 . As described above, the first embodiment is designed to reduce sound at an antinode of sound pressure, which is closest to the driver/passenger space, among antinodes of sound pressure owing to a natural vibration. 
     The first embodiment is designed to reduce low-frequency sound in the compartment  105 , thus improving quietness/noiselessness thereof, particularly, at a driver/passenger&#39;s position at which a driver/passenger actually hears sound. 
     2. Second Embodiment 
     Next, a second embodiment of the present invention will be described with respect to an acoustic resonance device installed in the vehicle  100 , which is constituted of a resonance pipe unit  2  including resonance pipes. 
       FIGS. 14A and 14B  show the constitution of the resonance pipe unit  2 .  FIG. 14A  shows the exterior appearance of the resonance pipe unit  2 , which is constituted of five resonance pipes  21  (i.e.  21 - 1  through  21 - 5 ). The resonance pipes  21  are aligned in an alignment direction perpendicular to their longitudinal direction, wherein they are unified together via fixing measures or adhesive. The resonance pipes  21  are each composed of a metal or a synthetic resin, which is shaped in a pipe form. Each of the resonance pipes  21  is constituted of a closed end  22 , an open end  23  and a hollow space  25 ; hence, it is a closed pipe having one open end and one closed end. The open ends  23  of the resonance pipes  21  are linearly aligned so as to adjoin together. A neck portion of the open end  23  of the resonance pipe  21  can be closed with a flow resistor  24  having a flow resistance, i.e. an air-permeable material such as a glass wool, a cloth, or gauze. It is preferable to select an appropriate material as the flow resistor  24  when reducing sound pressure in the hollow space  25 . 
     Next, a sound attenuation effect of the resonance pipe unit  2  will be described with reference to  FIG. 14B . 
       FIG. 14B  shows two adjacent resonance pipes  21 - j  and  21 - k  (where k=j+1) having closed ends  22 - j ,  22 - k , open ends  23 - j ,  23 - k , and flow resistors  24 - j ,  24 - k  within the resonance pipe unit  2 . Herein, L 1  denotes the length of the hollow space  25  of the resonance pipe  21 - j , and L 2  denotes the length of the hollow space  25  of the resonance pipe  21 - k , where L 1 =L 2  since all the resonance pipes  21  have the same length of the hollow space  25 . When a sound wave is incident at the open ends  23 - j ,  23 - k  of the resonance pipe  21 - j ,  21 - k  from the compartment  105  of the vehicle  100 , they are introduced into the hollow spaces  25  and reflected at the closed ends  22 - j ,  22 - k , so that they are emitted from the open ends  23 - j ,  23 - k . At this time, sound waves, whose wavelength λc (i.e. L 1 =L 2 =λ/4) is four times longer than the lengths L 1 , L 2  of the hollow spaces  25 , causes standing waves S 1 , S 2 . During the repetition of oscillation of sound waves inside the resonance pipes  21 - j ,  21 - k , acoustic energy is consumed due to friction on interior walls of the resonance pipes  21 - j ,  21 - k  and due to viscosity of air molecules at the open ends  23 - j ,  23 - k , so that sound pressure is being reduced in proximity to the open ends  23 - j ,  23 - k  with respect to a center frequency corresponding to the wavelength λc. In  FIG. 14B , the length L 1 =L 2  is equal to 0.53 m so that the wavelength λc is equal to 2.12 m, for example. 
     The resonance pipe unit  2  is arranged such that the hollow spaces  25  of the resonance pipes  21  communicate with a predetermined space subjected to sound attenuation. Upon reception of sound, the resonance pipes  21  resonate to sound entering into the open ends  23 , thus reducing sound pressure in proximity to the open ends  23 . Herein, resonance frequency f is set to reduce sound pressure in the 160 Hz band; hence, the length of the hollow space  25  of the resonance pipe  21  is set to a quarter of the wavelength corresponding to the frequency of 160 Hz. That is, the length of the hollow space  25  of the resonance pipe  21  may range from 40 cm to 80 cm, for example. 
     Sound waves reflected at the closed ends  22 - j ,  22 - k  and emitted from the open ends  23 - j ,  23 - k  are diffracted at the open ends  23 - j ,  23 - k , thus emitting sound energy. A part of sound energy is emitted from the open end  23  of one resonance pipe  21  and re-entered into the open end  23  of the other adjacent resonance pipe  21 . That is, coupled oscillation occurs mutually between the adjacent resonance pipes  23 - j ,  23 - k , thus interchanging sound energy with each other. During couples oscillation, sound energy is consumed due to friction on the interior walls of the hollow spaces  25  and due to viscosity of air molecules at the open ends  23 - j ,  23 - k  of the resonance pipes  21 - j ,  21 - k , thus reducing sound pressure. This coupled oscillation can be presumed as an opposite-end closed pipe mode in which the two adjacent resonance pipes  21 - j ,  21 - k  are unified together, wherein it is possible to reduce sound pressure with respect to a center frequency corresponding to the wavelength depends upon the total length of L 1 +L 2 . 
     The resonance pipe unit  2  of  FIG. 14A  is constituted of the five resonance pipes  21 ; but this is not a restriction. It is possible to arbitrarily set the number of resonance pipes included in the resonance pipe unit  2 . In the following description, the second embodiment is designed to use the resonance pipe unit  2  constituted of the five resonance pipes  21  or the resonance pipe unit  2  constituted of only one resonance pipe  21 . 
     Next, the structure for arranging the resonance pipe unit  2  in the compartment  105  will be described with reference to  FIG. 15 . The resonance pipe unit  2  installed in the compartment  105  is set up to achieve mode suppression at a specific frequency, i.e. a natural frequency of a 160 Hz band. That is, the resonance frequency of the resonance pipe unit  2  is approximately set to the natural frequency of natural vibration. 
       FIG. 15  is a perspective exploded view of the vehicle  100  equipped with the resonance pipe units  2 .  FIG. 16  is a perspective view showing the rear sides of the front seats  140  in a diagonal direction, whose center position matches the center of the rear side of the front seat  140 A, in the compartment  105  of the vehicle  100 . As shown in  FIGS. 15 and 16 , a plurality of resonance pipe units  2  is installed in the compartment  105  at various positions, which are similar to the installation positions of the panel/diaphragm resonators  1  of the first embodiment (see  FIG. 9 ). That is, the resonance pipe units  2  are embedded in a roof  110   a , center pillars  120   a , and front pillars  130   a  as well as the front seats  140  and the front doors  150 A. The resonance pipe units  2  are arranged such that the open ends  23  of the hollow spaces  25  of the resonance pipes  21  communicate with the compartment  105 . The effect of the resonance pipe unit  2  is similar to the effect of the panel/diaphragm resonator  1 . 
     Next, the structure of the roof  110   a  for installing the resonance pipe units  2  will be described with reference to  FIG. 17 .  FIG. 17  is a cross-sectional view taken along line VIII-VIII in  FIG. 2 . 
     As shown in  FIGS. 15 and 17 , the resonance pipe units  2  are embedded in the ceiling portion of the roof  110   a  just above the driver/passenger space on the front seats  140 A and  140 B. Each of the resonance pipe units  2  includes five resonance pipes  21 , whose alignment direction is the forward-backward direction of the vehicle  100 . Since an antinode of sound pressure emerges in proximity to the side window  153  of the front door  150 A, the open ends  23  of the resonance pipes  21  are directed to the side window  153 , thus improving a sound attenuation effect at the side window  153 . In other words, the resonance pipe units  2  are arranged inside the compartment  105  such that the open ends  23  of the resonance pipes  21  are directed toward proximate interior walls of the compartment  105  in the width direction. 
     As shown in  FIG. 17 , a plurality of holes  116  is formed in a roof inner panel  114   a  of the roof  110   a  in proximity to the open ends  23  of the resonance pipes  21  of the resonance pipe units  2 . These holes  116  communicate with an upper space above the roof inner panel  114   a . Sound occurring in the compartment  105  enters into the open ends  23  of the resonance pipes  21  via the holes  116 . Other resonance pipe units  2  are attached to the side portions of the roof inner panel  114   a  just behind the assist grips  200 , wherein they are each constituted of a single resonance pipe  21  whose longitudinal direction lies along the forward-backward direction of the vehicle  100 . Other holes  117  are formed in the side portions of the roof inner panel  114   a  so as to introduce sound occurring in the compartment  105  into the open ends  23  of the resonance pipes  21 . Similar to the first embodiment, the second embodiment can effectively achieve mode suppression in the compartment  105 . 
     Next, the constitution of the center pillar  120   a  and the constitution of the front pillar  130   a  will be described with reference to  FIG. 18 .  FIG. 18  is a cross-sectional view of the center pillar  120   a  taken in line D-D in  FIG. 16 . The center pillar  120   a  includes a center pillar inner panel  122   a  attached to the center pillar outer panel  121  (see  FIG. 11 ). The front material  123  is attached to the interior surface of the center pillar inner panel  122   a  in proximity to the compartment  105 . An inner space Sa is formed between the center panel inner panel  122   a  and the surface material  123 . In order to form the inner space Sa, a recess  125  is formed in the upper side of the center pillar inner panel  122   a  such that the back portion thereof project toward the outside of the vehicle  100  in the width direction. A hole  126  is formed in the lower end of the recess  125 . The resonance pipe unit  2  is embedded in the center pillar  120   a  such that the open end  123  of the resonance pipe  21  is engaged in the hole  126  of the recess  125 . Sound occurring in the compartment  105  enters into the open end  13  of the resonance pipe  21  via the surface material  123  and the inner space Sa. The resonance pipe unit  2  resonates to sound occurring in the compartment  105  so as to reduce sound pressure at an antinode of natural vibration, thus achieving mode suppression in the compartment  105 . 
     As shown in  FIG. 16 , the resonance pipe unit  2  is embedded in the front pillar  130   a  as well. The structure for arranging the resonance pipe unit  2  in the front pillar  130   a  is similar to the structure for arranging the resonance pipe unit  2  in the center pillar  120   a ; hence, the front pillar  130   a  equipped with the resonance pipe unit  2  can achieve mode suppression, and a detailed explanation thereof will be omitted. 
     Next, the constitution of the front seat  140  equipped with the resonance pipe units  2  will be described with reference to  FIG. 16 . 
     The resonance pipe units  2  are arranged in the head rest  141  and the seat back  142  in the front seat  140 . Specifically, two resonance pipes  21  are vertically arranged in the seat back  142  of the front seat  140  such that the openings  23  are each directed toward the upper surface of the seat back  142 . That is, the resonance pipe units  2  are arranged in the seat back  142  of the front seat  140  such that the hollow spaces  25  of the resonance pipes  21  communicate with the compartment  105  via the open ends  23 , wherein sound occurring in the compartment  105  enters into the open ends  23  of the resonance pipes  21  via the seat back cover  146 . Another resonance pipe unit  2  is arranged in the heat rest  141  of the front seat  140 , wherein in order to adequately secure the overall length, the resonance pipe  21  is folded in the head rest  141 . The open end  23  of the resonance pipe  21  embedded in the head rest  141  of the front set  140  is directed towards the interior wall of the compartment  105 . 
     Next, the constitution of the front door  150 A equipped with the resonance pipe unit  2  will be described with reference to  FIG. 19 . In this connection, no resonance pipe unit is arranged in the rear door  150 B. 
     As shown in  FIG. 19 , the resonance pipe unit  2  is vertically arranged in the base material  151  of the front door  150 A such that the open ends  23  of the resonance pipes  21  are directed toward the side window  153 . The open ends  23  of the resonance pipes  21  is positioned in the upper side of the base material  151  of the front door  150 A so as to improve a sound attenuation effect at an antinode of sound pressure which emerges in proximity to the side window  153 . An opening is formed in the upper side of the base material  151  of the front door  150 A, thus allowing sound to easily enter into the open ends  23  of the resonance pipes  21 . It is preferable that the opening of the base material  151  of the front door  150 A be processed not to be visualized by a driver/passenger with ease. 
     Since an antinode of sound pressure at a specific frequency, i.e. a natural frequency in a 160 Hz band, emerges in proximity to the driver/passenger space, the second embodiment adopting the above arrangement of the resonance pipe units  2  can achieve mode suppression similarly to the first embodiment. That is, the second embodiment attenuates sound pressure at an antinode closest to the driver/passenger space among antinodes of sound pressure which may emerge in proximity to the front seats  140  and the side windows  153 . 
     When the resonance pipe unit  2  is constituted of plural resonance pipes  21  causing coupled oscillation, it is possible to reduce sound pressure at other frequencies different from a natural frequency, thus further improving quietness/noiselessness in the compartment  105 . 
     3. Third Embodiment 
     A third embodiment of the present invention is characterized by using a Helmholtz resonator  3 , which is installed in the compartment  105 . The third embodiment is similar to the first embodiment with respect to the overall structure of the vehicle  100  and the installation positions of resonators in the compartment  105 , wherein the same constituent elements are designated the same numerals accompanied with a subscript “b”; hence, a detailed description thereof will be omitted. 
     The Helmholtz resonator  3  is employed as an acoustic resonance device of the third embodiment.  FIG. 20A  is a perspective view showing the exterior appearance of the Helmholtz resonator  3 , and  FIG. 20B  is a cross-sectional view taken along line E-E in  FIG. 20A . The Helmholtz resonator  3  is constituted of a body  31  and a tubular portion  32 . A hollow space is formed inside the body  31  and the tubular portion  32  and communicates with an opening  33  of the tubular portion  32 . 
     A cavity is formed inside the body  31 , which is made of fiber-reinforced plastics (FRP) and which is formed in a cylindrical shape. The tubular portion  32  is an open tube made of vinyl chloride, whose opposite ends are opened. The tubular portion  32  is unified with the body  31  such that the tubular portion  32  is inserted into a center hole of the body  31 . The Helmholtz resonator  3  is arranged such that the hollow space formed inside the body  31  and the tubular portion  32  communicates with the space of the compartment  105  subjected to sound attenuation. When sound enters into the opening  33 , the Helmholtz resonator  3  resonates to sound so as to reduce sound pressure in proximity to the opening  33 . Specifically, the Helmholtz resonator  3  is a spring-mass system in which a mass component corresponds to an air (or a gaseous body) disposed inside the tubular portion  32 , and a spring component corresponds to a cavity of the body  31 . Sound energy is converted into thermal energy due to friction of air on the internal wall of the tubular portion  32 , thus reducing sound pressure while increasing particle speed in proximity to the opening  33 . A resonance frequency f of the spring-mass system corresponding to the Helmholtz resonator  3  meets equation (5), in which Le denotes an effective length of the tubular portion  32 . As shown in  FIG. 20B , the effective length Le is calculated by measuring the length of a cavity of the tubular portion  32  (ranging between opposite ends) and by correcting it with an open end correction value. In addition, V denotes the volume of the cavity formed in the body  31 , and Sc denotes an area of the opening  33 . 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     
                       
                         c 
                         0 
                       
                       
                         2 
                          
                         π 
                       
                     
                     · 
                     
                       
                         ( 
                         
                           S 
                           LeV 
                         
                         ) 
                       
                       
                         1 
                         / 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In this connection, the Helmholtz resonator  3  is not necessarily equipped with a single tubular portion  32 ; but it is possible to unify two tubular portions  32  with the body  31 . In addition, it is possible to close the opening  33  of the tubular portion  32  with a flow resistance material having air permeability, such as a glass wool, a cloth, and gauze. 
     Next, the structure for arranging the Helmholtz resonator  3  in the compartment  105  of the vehicle  100  will be described in detail. Similar to the first embodiment, a plurality of Helmholtz resonators  3  is installed in a roof  110   b , center pillars  120   b  and front pillars  130   b  as well as the front seats  140  and the front doors  150 A. Since the third embodiment can adopt the same installation positions as the first embodiment, the Helmholtz resonators  3  are arranged such that the hollow spaces communicate with the compartment  105  via the openings  33 . The Helmholtz resonators  3  can demonstrate similar effects as the panel/diaphragm resonators  1  employed in the first embodiment. As an example of the installation position, the roof  110   b  will be described below. 
       FIG. 21  is a cross-sectional view of the roof  110   b  taken along line VIII-VIII in  FIG. 2 . A plurality of panel/diaphragm resonators  1  is arranged on a planar portion of a roof inner panel  114   b  of the roof  110   b , while a plurality of Helmholtz resonators  3  is arranged on inclined side portions (whose areas are smaller than the area of the planar surface and which are positioned just behind the assist grips  200 ) of the roof inner panel  114   b . Thus, it is possible to effectively arrange different types of resonators on the roof  110   b.    
     The third embodiment can demonstrate a similar effect as the first embodiment. 
     4. Noise Reduction Effect 
       FIG. 22  is a graph showing the result of the measurement testing on a noise reduction effect owing to the resonators of the foregoing embodiments installed in the compartment  105  of the vehicle  100 . This graph shows frequency characteristics of sound pressure (or noise level) measured in a driver&#39;s seat, wherein a solid line represents the measurement result using resonators, and a dotted line represents the measurement result using no resonator. The measurement testing is performed by actually running a vehicle at a speed of 60 km/h, wherein the measurement testing is performed in terms of audibility characteristics (or A-characteristics) sound pressure by use of a one-third octave band-pass filter, thus precisely detecting frequency characteristics close to actual auditory sensation. 
       FIG. 22  clearly shows that a noise level is reduce in a frequency range between 125 Hz and 200 Hz. In particular, a noise reduction of 5 dB or more is detected at a 160 Hz band, indicating a significant reduction of sound pressure in a low-frequency range. Compared with the technology which reduces sound pressure at other positions different from antinodes of natural vibration, the foregoing embodiments adopting various resonators directed towards antinodes of sound pressure are able to improve quietness/noiselessness in the compartment  105 , thus demonstrating an outstanding noise reduction effect at the driver/passenger position at which a driver/passenger suffers from noise. In addition, the foregoing embodiments are designed to carefully select the “effective” installation positions of resonators dedicated to a noise reduction effect; this prevents excessive resonators from being arranged in insignificant positions. 
     5. Variations 
     The present invention is not necessarily limited to the foregoing embodiments, which can be appropriately combined together or which can be further modified in various ways as follows. 
     (1) First Variation 
     The first embodiment adopts the panel/diaphragm resonators  1 ; the second embodiment adopts the resonance pipe units  2 ; and the third embodiment adopts the Helmholtz resonators  3 . Of course, it is possible to combine the panel/diaphragm resonators  1 , the resonance pipe units  2 , and the Helmholtz resonators  3 , which are selectively installed in the compartment  105  of the vehicle  100 . The types of acoustic resonance devices are not necessarily limited to them, since the present invention simply requires that acoustic resonance devices have hollow spaces communicating with the compartment  105  via openings. It is preferable that acoustic resonance devices be able to reduce sound pressure at an antinode closest to a driver/passenger space by positioning openings in proximity to an antinode of sound pressure. It is further preferable that openings of acoustic resonance devices be directed toward the driver/passenger space. 
     The foregoing embodiments are each designed such that acoustic resonance devices are arranged in the roof  110 , the center pillars  120 , the front pillars  130 , the front seats  140 , and the front doors  150 A. It is possible to limit the installation positions of acoustic resonance devices among them. In the compartment  105  shown in  FIG. 5A , acoustic resonance devices are arranged only in the driver&#39;s seat  140 B while no acoustic resonance device devices are arranged in the next passenger&#39;s seat  140 A. In addition, an acoustic resonance device having a large size can be installed across a plurality of installation positions in the compartment  105 . 
     (2) Second Variation 
     The foregoing embodiments are designed to control sound pressure at an antinode of natural vibration, because a one-dimensional mode of vibration dominates an antinode of sound pressure spreading in the width direction, which may presumably emerge in proximity to the side window. Without targeting on a specific mode of vibration, it is possible to reduce sound pressure at any antinode actually emerging in the driver/passenger space. This may also demonstrate a similar sound attenuation effect as the foregoing embodiments. Regardless of antinodes of sound pressure depending upon different modes of vibration, it is possible to achieve an outstanding mode suppression effect in any types of compartments suffering from low-frequency sounds. 
     It is possible to focus on a natural vibration occurring in the compartment  105  in connection with a front-rear length of the vehicle  100  in a forward-backward direction. Since the length of the vehicle  100  is longer than the width of the vehicle  100 , a low-frequency antinode of sound pressure may emerge in the compartment  105  undergoing a secondary one-dimensional mode of vibration spreading sound pressure in the forward-backward direction of the vehicle. Specifically, antinodes of sound pressure may occur at end portions of the front-rear length of the vehicle  100 , such as the center pillars  120 , the rear pillars  180 , and the rear package tray  220  (see  FIGS. 2  and  5 A) in the compartment  105  undergoing a natural vibration. It is possible to arrange resonators in proximity to end portions of the front-rear length of the vehicle  100 , thus achieving mode suppression. In the case of a secondary one-dimensional mode of vibration, an antinode of sound pressure may be located around the center position of the compartment  105  in the forward-backward direction of the vehicle  100 ; hence, it is necessary to arrange resonators at the front seats  140 A,  140 B, thus achieving mode suppression. 
     The foregoing embodiments are designed to arrange acoustic resonance devices reducing sound pressure at an antinode of sound pressure closest to the driver/passenger space; but this is not a restriction. It is possible to arrange acoustic resonance devices at other positions. Regarding natural vibrations different from a primary mode of vibration, antinodes of sound pressure are positioned very close to the driver/passenger space, whereas they are not necessarily the one closest to the driver/passenger space. The measurement results of  FIGS. 6 and 7  clearly show that, regardless of modes of vibration (e.g. a one-dimensional mode, a two-dimensional mode, and a three-dimensional mode) and orders of vibration (e.g. a primary order, and a secondary order), the present embodiment is able to reduce low-frequency sound pressure of the compartment  105  and to improve a noise reduction effect at the driver/passenger&#39;s position at which a driver/passenger actually hears noise because an acoustic resonance device is arranged to suppress sound pressure at an antinode of sound pressure emerging in proximity to the driver/passenger space. 
     (3) Third Variation 
     It is possible to modify the third embodiment in such a way that the tubular portion  32  of the Helmholtz resonator  3  can be freely varied in length.  FIGS. 23A and 23B  show a modified example of the Helmholtz resonator  3  having a variable-length tubular portion  32   a .  FIG. 23A  is a cross-sectional view of the tubular portion  32   a , and  FIG. 23B  is a front view of the tubular portion  32   a  having an opening  323 . 
     The tubular portion  32   a  is constituted of an outer tube  321  and an inner tube  322 . The inner tube  322  is a tube-shaped member having an external thread on the external periphery thereof. The inner tube  322  of the tubular portion  32   a  is rotated and fixed to the body  31 . The outer tube  321  is a tube-shaped member whose inner diameter is larger than the diameter of the inner tube  322  and which has an internal thread on the interior surface thereof. The tubular portion  32   a  is assembled in such a way that the inner tube  322  is screwed into the outer tube  321 . The length L of the tubular portion  32   a  depends upon what length the inner tube  322  is screwed into the outer tube  321 . As shown in  FIG. 23B , the outline shape of the outer tube  321  is a hexagonal shape; hence, the user can adjust the screwed length by use of a mechanical tool such as a wrench, thus freely changing the length L of the tubular portion  32   a . Since the resonance frequency of the Helmholtz resonator  3  depends upon the length L of the tubular portion  32   a , it is possible to adjust the resonance frequency as necessary. 
       FIGS. 23A and 23B  show that the tubular portion  32   a  adopts a screw structure in the outer tube  321  and the inner tube  322 , thus freely adjust the length L; but this is not a restriction. The tubular portion  32   a  can be composed of three or more screw elements. Alternatively, it is possible to provide a corniced tube as the tubular portion  32   a  of the Helmholtz resonator  32 . That is, the tubular portion  32   a  can adopt various structures having flexibility of expansion and contraction. The outline shape of the outer tube  321  is not necessarily formed in a hexagonal shape; but it is preferable that the shape of the tubular portion  32   a  allows the user to easily adjust the length thereof. 
     The third variation allows the user to easily select a frequency greatly suppressing sound pressure even when the selected frequency dedicated to an improvement of quietness/noiselessness differs dependent upon different materials and structures adapted to the compartment  105  and different types of vehicles. 
     (4) Fourth Variation 
     It is possible to employ a lattice member  4  instead of the resonance pipe unit  2 .  FIGS. 24A and 24B  show the lattice member  4  serving as an acoustic resonance device according to a fourth embodiment.  FIG. 24A  is a perspective exploded view of the lattice member  4 , and  FIG. 24B  is a side view of the lattice member  4  in a direction F shown in  FIG. 24A . 
     As shown in  FIG. 24A , the lattice member  4  is constituted of a single partition  4 A (which is elongated in a single direction) and six crossed partitions  4 B (which are crossed with the partition  4 A and elongated perpendicularly to the partition  4 A). The lattice member  4  is attached to the upper surface of the roof inner panel  114  such that the partition  4 A lies in the forward-backward direction of the vehicle  100  while the crossed partitions  4 B lie in the width direction perpendicular to the forward-backward direction of the vehicle  100 . Thus, the lower end of the lattice member  4  is closed with the roof inner panel  114 , while the upper end of the lattice member  4  is covered with a part of the chassis serving as the skeletal structure of the vehicle, such as a roof outer panel  160  which is combined with the roof inner panel  114 . 
     The lattice member  4  has ten hollow spaces which are defined between the adjacent crossed partitions  4 B and which have openings directed in the width direction, thus realizing an acoustic resonance device whose constitution and functionality may roughly resemble those of the resonance pipe unit  2 . In this connection, it is possible to close both the upper end and the lower end of the lattice member  4  or either the upper end or the lower end of the lattice member  4 . The lattice member  4  replaces the resonance tube unit  2  attached to the ceiling (i.e. the roof  110 ) of the vehicle  100 . It is possible to arbitrarily change the number of the crossed partitions  4 B in light of a desired number of hollow spaces. 
     (5) Fifth Variation 
       FIG. 26  shows a corrugated panel  5  composed of a flexible material such as a resin, which has a plurality of recesses virtually serving as a plurality of resonators. The corrugated panel  5  can be attached to the ceiling portion or the interior wall of the compartment  105 , so that a plurality of resonators absorbs sound at the ceiling portion or the interior wall of the compartment  105 . The corrugated panel  5  having flexibility can be easily deformed in conformity with the curved surface; hence, it is possible to facilitate a plurality of resonators at a desired position of the compartment  105  with ease. 
     (6) Sixth Variation 
     The panel/diaphragm resonator  1  of the first embodiment is constituted of the rectangular housing  10 , the vibrator  15  closing the opening  12  of the housing  10 , and the cavity  13  formed inside the housing  10 . The outline shape of the housing is not necessarily limited to a rectangular shape, which can be replaced with a circular shape or a polygonal shape. Irrespective of the outline shape of the housing  10 , it is preferable that a concentrated mass altering a condition of vibrating the vibrator  15  be located at a center portion of the vibrator  15 . 
     The panel/diaphragm resonator  1  has a sound absorption mechanism constituted of a spring-mass system and a bending system. We performed experiments on sound absorption coefficients at resonance frequencies by changing the surface density of the vibrator  15 . 
     Specifically, we prepared a sample of the housing  10 , in which the cavity  13  has a length of 100 mm, a width of 100 mm, and a thickness of 10 mm, and a sample of the vibrator  15  has a length of 100 mm, a width of 100 mm, and a thickness of 0.85 mm, wherein the center portion (which has a length of 20 mm, a width of 20 mm, and a thickness of 0.85 mm) of the vibrator  15  is varied in terms of the surface density.  FIG. 25  is a graph showing the simulation result of the panel/diaphragm resonator  1  in terms of a normal incidence sound absorption coefficient. We adopt a simulation method according to the Japanese Industrial Standard, JIS A 1405-2 (titled “Acoustics—Determination of sound absorption coefficient and impedance in impedance tubes—Part 2: Transfer-function method”), in which the panel/diaphragm resonator  1  is arranged in a sound chamber so that its sound field is determined via the finite element method, thereafter, sound absorption characteristics are calculated using a transfer function. Specifically, the graph of  FIG. 25  shows five characteristic curves ( 1 ) through ( 5 ) based on different surface densities (at the center portion of the vibrator  15 ) such as ( 1 ) 399.5 [g/m 2 ], ( 2 ) 799 [g/m 2 ], ( 3 ) 1199 [g/m 2 ], ( 4 ) 1598 [g/m 2 ], and ( 5 ) 2297 [g/m 2 ]; the same surface density of 799 [g/m 2 ] at the peripheral portion of the vibrator  15 ; and different average densities of the vibrator  15  such as ( 1 ) 783 [g/m 2 ], ( 2 ) 799 [g/m 2 ], ( 3 ) 815 [g/m 2 ], ( 4 ) 831 [g/m 2 ], and ( 5 ) 863 [g/m 2 ]. This simulation result clearly shows that peaks significantly appear in frequencies of 300 Hz through 500 Hz and around a frequency of 700 Hz in terms of the sound absorption coefficient. 
     The sound absorption coefficient is locally maximized around the frequency of 700 Hz owing to the resonance of the spring-mass system composed of the mass component of the vibrator  15  and the spring component of the cavity  13 . The panel/diaphragm resonator  1  absorbs sound with a peak sound absorption coefficient at a resonance frequency of the spring-mass system, wherein even when the surface density of the center portion of the vibrator  15  increases, the total mass of the vibrator  15  is not significantly changed; this indicates that the resonance frequency of the spring-mass system is not greatly varied irrespective of the surface density of the center portion of the vibrator  15 . The sound absorption coefficient is maximized in the frequency range between 300 Hz and 500 Hz owing to the resonance of the bending system caused by a bending oscillation of the vibrator  15 . The panel/diaphragm resonator  1  causes a peaked sound absorption coefficient of the low-frequency side at a resonance frequency of the bending system, which becomes lower as the surface density of the center portion of the vibrator  15  increases. In general, the resonance frequency of the bending system is determined by an equation of motion dominating an elastic oscillation of the vibrator  15 , wherein it is inversely proportional to the density (or the surface density) of the vibrator  15 . The resonance frequency of the bending system is greatly affected by the density of antinodes of natural vibration (at which amplitude becomes maximal). That is, the resonance frequency of the bending system is varied because the above simulation varies an antinode region of 1×1 eigenmode with a different surface density of the center portion of the vibrator  15 . 
     The above simulation result indicates that within peaked sound absorption coefficients, a peaked sound absorption coefficient of the low-frequency side moves to a lower frequency as the surface density of the center portion of the vibrator  15  is increased larger than the surface density of the peripheral portion of the vibrator  15 . This indicates that a peaked sound absorption coefficient can be moved to a lower frequency or a higher frequency by changing the surface density of the center portion of the vibrator  15 . Compared to the technology in which the vibrator  15  is formed in a panel shape composed of the same material as the panel/diaphragm resonator  15  so that a sound absorption frequency is changed by increasing a total mass of the panel/diaphragm resonator  1 , the present technology can freely lower the sound absorption frequency without changing the total mass of the panel/diaphragm resonator  1  because the sound absorption frequency corresponding to a peaked sound absorption coefficient can be changed by simply changing the surface density of the center portion of the vibrator  15  in the panel/diaphragm resonator  1 . 
     It is possible to change the resonance characteristics by filling the cavity  13  of the panel/diaphragm resonator  1  with a porous sound absorbing material (e.g. a foaming resin, a felt material, and a cotton fiber such as polyester wool). This modification is able to cope with variations of noise characteristics in the compartment  105  due to changes of modes of vibration (e.g. changed numbers and shapes of persons and baggage) and changes of noise (e.g. changed tires and variances of road conditions). 
     (7) Seventh Variation 
     The first embodiment adopts the same shape to all the panel/diaphragm resonators  1 ; but it is possible to adopt different shapes as the panel/diaphragm resonators  1 . Thus, it is possible to broaden a frequency range reducing sound pressure because the resonance frequency of the panel/diaphragm resonator  1  differs dependent upon the dimensions of the housing  10 . It is possible to set different resonance frequencies to the front seats  140 A and  140 B. In addition, it is possible to set different resonance frequencies to the roof  110  and the center pillars  120 . This decreases sound pressure in a broader frequency range, wherein it is possible to reduce sound pressure at an optimum frequency suiting to each position. That is, various groups are each formed using a single resonator or a plurality of resonators, wherein each group of resonator(s) has a different resonance frequency. Similarly, it is possible to employ various types of resonance pipe units  2  having different resonance frequencies and various types of Helmholtz resonators  3  having different resonance frequencies. 
     The second embodiment employs a closed pipe as the resonance pipe  21  having one open end  23  and one closed end  22 . It is possible to use an open pipe whose opposite ends are opened as the resonance pipe  21 . Alternatively, it is possible to mix closed pipes and open pipes in the resonance pipe unit  2 . 
     (8) Eighth Variation 
     The foregoing embodiments applies acoustic resonance devices to the vehicle  100  such as an automobile; but it is possible to apply acoustic resonance devices to other types of vehicles such as trains, ships, aircrafts, space stations, and gondolas. The term “vehicle” includes transportation devices which carry people and/or baggage. In addition, the term includes other non-transport carriages and equipment used in amusement parks, such as Ferris wheels. The application of acoustic resonance devices is not necessarily limited to compartments in which people reside in vehicles; hence, acoustic resonance devices can be applied to machinery rooms and luggage rooms which are separated from compartments of vehicles. There is a possibility that a person may enter into a machinery room or a luggage room. Some vehicles are not equipped with seats in the driver/passenger space of a compartment. Many passengers do not use seats in gondola cars, busses, and trains. In those vehicles which include passenger spaces, it is possible to achieve mode suppression at antinodes of sound pressure suited to passenger spaces, thus achieving similar effects as the foregoing embodiments. 
     The automobile-type vehicle is equipped with installation structures, which are able to install acoustic resonance devices, such as the roof, center pillars, front pillars, front seats, and front doors, while other types of vehicles are not always equipped with those installation structures. However, it is possible to attach acoustic resonance devices to counterpart structures of other vehicles which may be comparable to installation structures of the automobile-type vehicles. Acoustic resonance devices are not necessarily attached to installation structures which are already incorporated into the vehicle and the compartment  105 . Before installation structures are unified with the vehicle  100  and the compartment  105 , it is possible to install acoustic resonance devices in the vehicle  100  and the compartment  105 . 
     The present invention does not matter installation positions of acoustic resonance devices, which are not necessarily limited to surrounding walls of the compartment  105  and seats of the driver/passenger space. It is possible to attach acoustic resonance devices to any positions dedicated to a reduction of sound pressure at an antinode of natural vibration. 
     (9) Ninth Variation 
     It is possible to modify the third embodiment such that the tubular portion  32   a  of the Helmholtz resonator  3  is automatically adjusted. This requires an automatic adjustment device including a microphone, a frequency analyzer, a controller, and a driver. In this automatic adjustment device, the microphone receives sound, and subsequently, the frequency analyzer analyzes received sound so as to specify a frequency significantly increasing noise. The controller calculates the length of the tubular portion  32   a  of the Helmholtz resonator  3  based on the specified frequency; then, it outputs a drive signal representing the calculated length to the driver such as a solenoid. The driver adjusts the length of the tubular portion  32   a  of the Helmholtz resonator  3  in response to the drive signal, thus reducing sound pressure particularly at the specified frequency significantly increasing noise. In this connection, it is possible to apply feedback control to the controller driving the tubular portion  32   a.    
     It is possible to apply an expansion/contraction mechanism to the Helmholtz resonator  3 , thus varying the dimensions of the body  31 . This varies the volume of the cavity of the body  31  so as to change the resonance frequency of the Helmholtz resonator  3 . Similarly, it is possible to apply an expansion/contraction mechanism to the resonance pipe unit  2  so as to adjust the length of the resonance pipes  21 . 
     (10) Tenth Variation 
     The foregoing embodiments arrange acoustic resonance devices at selected positions corresponding to antinodes of sound pressure at specific natural frequencies of natural vibration, wherein resonance frequencies are set to increase reduction values of sound pressure at antinodes of natural frequencies. It is possible to set resonance frequencies attenuating sound pressure at other frequencies different from natural frequencies. 
     During a running mode of the vehicle  100 , tires serve as an excitation source (causing a vibration in the compartment  105 ) so as to have the vehicle  100  undergo a vibration at a certain frequency (hereinafter, referred to as an excitation frequency), which in turn causes noise in the compartment  105 . Even when the compartment  105  has a natural frequency of 167 Hz, sound pressure is maximized at a frequency of 155 Hz in the compartment  105 ; hence, those frequencies may belong to the same frequency range subjected mode suppression, but they slightly differ from each other. Herein, the frequency range subjected to mode suppression is equal to a 160 Hz band, for example. The foregoing embodiments select the positions of acoustic resonance devices in light of a natural vibration, wherein they can determine resonance frequencies in light of sound caused by an excitation source. That is, when a specific natural frequency differs from an excitation frequency which is applied to the compartment  105  due to external conditions (e.g. friction of tires running on roads), resonance frequencies can be determined to achieve a sound attenuation effect at a high frequency which is increased due to an excitation at the excitation frequency. In actuality, acoustic resonance devices are positioned in proximity to antinodes of sound pressure in a 160 Hz band based on a natural vibration, thus having acoustic resonance devices resonate to a frequency of 155 Hz. Since both the excitation frequency and the natural frequency may belong to the same frequency range, acoustic resonance devices need to be adjusted at resonance frequencies reducing sound pressure at those frequencies; hence, resonance frequencies are not necessarily limited to the 160 Hz band. 
     It is possible to implement an automatic control of the ninth variation in the tenth variation in such a way that resonance frequencies are each set to a peak frequency caused by excitation at an excitation frequency in a running mode of the vehicle  100 . In general, automobile-type vehicles undergo variations of excitation frequencies while running on roads so that excitation frequency characteristics are varied from time to time while natural frequency characteristics are unique to each sound field; hence, natural frequency characteristics do not always emerge in compartments. Therefore, a microcontroller such as a microcomputer automatically controls acoustic resonance devices to adjust resonance frequencies in response to excitation frequencies, thus effectively reducing noise in a compartment. Herein, the microcomputer is able to calculate excitation frequency based on various parameters such as a running speed, an engine speed, an accelerator opening, and a gear position. 
     (11) Eleventh Variation 
     Resonance frequencies of acoustic resonance devices do not need to be fixed to natural frequencies; instead, acoustic resonance devices reduce sound pressure at natural frequencies by way of an interaction due to coupled oscillation between the space arranging an acoustic resonance device and the internal space of the housing of an acoustic resonance device. In a broad interpretation, the structure arranging an acoustic resonance device can be regarded as a secondary resonator connected to the acoustic resonance device. Owing to coupled oscillation via a correlation between an acoustic resonance device and a secondary resonator, sound energy is interchanged between a compartment and the acoustic resonance device plus the secondary resonator, thus achieving an additional sound attenuation effect in another frequency range. 
     (12) Twelfth Variation 
     The resonance pipe unit  2  consumes sound energy by way of viscous resistance and friction between the interior wall and air molecules. Consumption of sound energy increases at the position undergoing a high particle velocity of a sound wave. For this reason, it is possible to effectively reduce sound pressure when the resonance pipe unit  2  is arranged at the position undergoing a high particle velocity. It is possible to specify the position of a high particle velocity by way of the measurement testing which measures a particle velocity in addition to antinodes of sound pressure. 
     (13) Thirteenth Variation 
     The foregoing embodiments arrange acoustic resonance devices to reduce sound pressure at antinodes of a natural vibration; but it is possible to arrange acoustic resonance devices only for the purpose of increasing a motion velocity of medium such as particles (i.e. a particle velocity). Specifically, the motion velocity of particles is the speed at which particles vibrate. 
     At an antinode of sound pressure in the compartment  105  undergoing a natural vibration, sound pressure is maximized while a particle velocity is minimized. Increasing a particle velocity at an antinode of sound pressure may vary a natural vibration, thus improving the quietness/noiselessness in the compartment  105 . This causes resonance in the medium at an antinode of sound pressure in which sound pressure increases due to a natural vibration; thus, it is possible to achieve similar effects as the foregoing embodiments. 
     A resonance pipe may suffice the above acoustic resonance device according to the thirteenth variation. Even though a standing wave is resided in the hollow space of a resonance pipe in agreement with a boundary condition in which a particle velocity becomes zero at the opening of the resonance pipe, the particle velocity at the opening of the resonance pipe is maximized at a primary resonance frequency (namely, a minimum resonance frequency). That is, it is possible to increase the particle velocity at the opening of a resonance pipe which is identical to or proximate to an antinode of sound pressure owing to a natural vibration. When a resonance pipe is used to increase the particle velocity, it is preferable not to use a flow resistor. Because, a resonance pipe not using a flow resistor is able to cause a high particle velocity by way of resonance. Instead of a resonance pipe (or the resonance pipe unit  2 ), it is possible to employ the panel/diaphragm resonator  1  or the Helmholtz resonator  3 , wherein a particle velocity can be increased at the vibrator  15  of the panel/diaphragm resonator  1  or the opening  33  of the Helmholtz resonator  3 . 
     The above structure increasing a particle velocity is merely one example; hence, it is possible to employ other types of resonators having a capability of increasing a particle velocity by way of resonance. In short, the thirteenth variation is designed to determine the arrangement of resonators to thereby increase a particle velocity at an antinode of sound pressure owing to a natural vibration. 
     (14) Fourteenth Variation 
     Instead of improving quietness/noiselessness around the front seats  140 , it is possible to improve quietness/noiselessness in a passenger space on the rear seats  190  undergoing an antinode of sound pressure owing to a natural vibration. In this case, acoustic resonance devices are arranged to control sound pressure at antinodes of natural vibration which emerge in proximity to the back-ceiling portion just above the rear seats  190 , the rear doors  150 B, and the rear pillars  180 . The present invention is not necessarily limited to the arrangement of acoustic resonance devices improving quietness/noiselessness in the driver/passenger space on the front seats  140 ; hence, it is possible to control sound pressure at antinodes of natural vibration dependent upon any spaces of the compartment  105 . 
     Walls embedding acoustic resonance devices are not necessarily limited to partitions between the compartment  105  and the outside of the vehicle  100 ; hence, it is possible to install acoustic resonance devices in other walls communicating with the compartment  105 , such as doors and support members. 
     The foregoing embodiments focus on antinodes of sound pressure in a 160 Hz band based on a natural vibration; but it is possible to focus on other natural frequencies. 
     A region of a reduced sound pressure (via resonance of an acoustic resonance device) and a region of an increased particle velocity are dependent upon the position of an opening. Those regions are not necessarily located in the compartment  105  but at any positions of the vehicle  100 . 
     As described heretofore, the present invention is not necessarily limited to the foregoing embodiments and variations, which can be further modified in various ways within the scope of the invention as defined in the appended claim.