GB2276224A

GB2276224A - Sound shielding plate for underbody of engine compartment of automotive vehicle

Info

Publication number: GB2276224A
Application number: GB9405179A
Authority: GB
Inventors: Keijiro Iwao; Yasujuki Asahara
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1993-03-17
Filing date: 1994-03-16
Publication date: 1994-09-21
Anticipated expiration: 2014-03-16
Also published as: GB9405179D0; DE4409200C2; GB2276224B; DE4409200A1

Description

2276224 SOUND SHIELDING PLATE AND APPLICATION OF SOUND SHIELDING PLATE TO

UNDERBODY OF ENGINE ROOM OF AUTOMOTIVE VEHICLE The present invention relates to a sound shielding plate achieving both requirements of having an air ventilation characteristic and having a sound shielding characteristic and an application of the sound shielding plate to an underbody of an engine room (compartment) of an automotive vehicle.

Generally, in an automotive vehicle, an underbody is attached on a lower end of an engine room. This underbody serves as means for preventing engine parts such as an engine oil pan from a direct collision with projections and/or obstacles present on a road surface between both right and left tire wheels on which the vehicle runs and, in addition, serves as a sound shielding wall to suppress engine noises radiated from the vehicular engine toward an external of the vehicle.

It is natural that as the attached area of the underbody becomes wider, an achieved effect of preventing the radiation of the engine noise toward the external by the underbody become higher. However, as the attached area thereof becomes wider, a degree of sealing the engine room to the external of the vehicle by means of the underbody becomes larger so that a ventilation effect achieved by means of the underbody becomes worsened. Consequently, an atmospheric temperature of the engine room becomes increased and the increased atmospheric temperature gives an undesirable effect on the engine.

Japanese Utility Model Registration Applications First Publications No. Showa 59-188722 published on December 14. 1984 and No. Showa 60-131470 published on September 3, 1985 exemplify first previously proposed structures of underbodies applicable to the automotive vehicles (hereinafter referred to as a first reference).

In the above-identified two Japanese Utility Model Registration Applications First Publications (first reference), the attached area of the underbody to the oil pan is reduced so as to secure the required air ventilation characteristic, with the actual sound shielding characteristic of the underbody more or less sacrificed.

On the other hand, another Japanese Utility Model Registration Application First Publication No. Showa 59- 153598 published on October 15, 1984 exemplifies a second previously proposed sound absorbing member applicable to the underbody (hereinafter referred to as a second reference).

In the second reference, the sound absorbing member is provided with a plurality of stages of small chambers in the internal side of the sound absorbing member, each small chamber being communicated with one of the other small chambers via one of intermediate small holes.

When the material of the sound absorbing member in the second reference is deformed elastically due to a sound pressure change in the sound, an air is passed through the small holes to be moved to the small chambers so that a throttling action of the small holes during the movement of the air to the small chambers causes an acoustic energy to be attenuated, thus achieving the sound absorbing effect. 25 Anyway, as the above-described underbody, the vehicular industry demands the underbody not only to have the sound shielding characteristic but also to have the air ventilation characteristic. To meet these demands, an opening is installed on the underbody. However, in this case, the engine noise is leaked through the opening. Hence, no sound shielding means which satisfy both demands in a trade-off relationship is present. Conventionally. a more importance is placed on the underbody having a heat resistance characteristic and air ventilation characteristic and a sacrifice of the sound shielding characteristic to the underbody may possibly be given.

However, in recent years, an environmental demand to reduce the noises radiated from an internal of the vehicle to its external has been increased. Therefore, an improvement in the sound shielding characteristic for the underbody has been required to be achieved as well as the maintenance of the air ventilation characteristic for the underbody.

On the other hand, the sound absorbing member recited in the second reference is proposed to be applied to a vibration suppression of a panel and so on. Although it is effective to be applied to a dash lower panel of the automotive vehicle, the s ound shielding member cannot be applied to the underbody any more. This is because the ventilation characteristic cannot sufficiently be secured merely when the sound absorbing member is applied to the underbody.

A Japanese Patent Application First Publication No. Showa 60-85043 published on May 14, 1985 exemplifies a third previously proposed sound shielding wall applicable to the underbody of the vehicular engine room (hereinafter referred to as a third reference).

The sound shielding wall in the third reference can satisfy both demands of the ventilation characteristic and sound shielding characteristic for the underbody. In the sound shielding wall of the third reference, a plurality, of conduits are provided and their lengths are varied so that phases of the sounds radiated from a radiating surface are deviated in the sound shielding wall to cancel the sounds with each other, thus giving the sound shielding effect thereon.

The third reference teaches that the sound shielding wall can completely satisfy the conditions such that both ventilation characteristic and sound shielding characteristic are compatibly provided.

However, in a case where the sound shielding wall of the third reference is actually applied to the underbody of the automotive vehicle, its size is critical in mounting the underbody on the vehicle body, i.e., the engine room.

In details, in the case of the underbody of the engine room, the underbody is interposed between the parts within the engine room and the ground, i.e, a road surface. In this interposition of the underbody, it is also necessary to take a minimum road clearance into consideration.

Therefore, a space between the engine room of the vehicle body and road surface in which the underbody can actually be mounted is defined as a space ranging from the minimum road clearance to the parts within the engine room.

In addition, since the underbody is mounted within the above-defined space, a limitation is placed on its size, particularly, its thickness of the underbody.

Furthermore, since, in the sound shielding wall of the third reference, the sounds are mutually canceled to shield the sounds due to the phase differences caused by the differences in the lengths of the conduits through which the sounds are passed, it is necessary to producethe differences in the lengths of the conduits corresponding to respective half wavelengths of the sounds to be canceled.

Therefore, in a case where the sound shielding wall in the third reference is provided with the sound shielding effect against the sounds having the relatively long wavelengths which are problematic for the external noises of the vehicles, the differences in the conduit lengths need to be elongated and the sound shielding wall becomes too thick to be used for the underbody of the engine room.

It would be desirable to be able to provide a sound shielding plate and its application of the sound shielding plate to an underbody of an engine roj5w of an automotive vehicle which can satisfy both requirements of having an air ventilation characteristic and sound shielding characteristic with an appropriate thickness of the whole sound shielding plate.

According to one aspect of the present invention, there is provided a sound shielding plate, comprising: a) at least two sheets of aperture penetrated plates opposing to each other with an interval of distance; and b) a plurality of openings penetrated through each of said two sheets of aperture penetrated plates so as to face against each other.

According to another aspect of the present invention, there is provided a sound shielding plate applicable to an underbody located at a lower end of an engine room of an automotive vehicle, comprising: a) a plurality of sheets of aperture penetrated plates disposed between the lower end of the engine room of the vehicle and an external side of the engine room and approximately juxtaposed to one another; and b) a plurality of openings penetrated through the respective sheets of aperture penetrated plates and provided for an air ventilation purpose, wherein at least two kinds of vibration systems constituted by air masses formed in the respective openings of the respective sheets of aperture penetrated plates and pneumatic springs are present in the sound shielding plate.

According to a still aspect of the present invention, there is provided a sound shielding plate comprising: a) at least two sheets of sound shielding plates, one of the two sheets of the sound shielding plates being faced against the other sheet of the sound shielding plate with a spatial interval of distance; and b) a plurality of openings penetrated through the respective sheets of the sound shielding plates, each one of the openings penerated through one of the sheets of the sound shielding plates being faced against each of the other openings penerated through the other sheet of the sound shielding plate, and wherein a vibration system is provided from among the respective openings which is constituted by an air mass created within a predetermined one of the openings and a pneumatic spring created by an aerial layer between the mutually opposing sheets of the sound shielding plates and wherein at least one of the openings other than those constituting said vibration system is linked to the opposite opening via a cylindrical member having an inner surface whose cross sectional shape is approximately the same as that of the opposite opening.

According to still another aspect of the present invention, there is provided a sound shielding plate comprising: a) at least two sheets of aperture penetrated plates opposing to each other with an interval of distance; b) a plurality of openings penetrated through each of said two sheets of aperture penetrated plates so as to face against one another; and c) means for combining both sheets of aperture penetrated plates to provide the interval of distance between the respective sheets of aperture penetrated plates so as to form at least one kind of vibration system in the sound shielding plate, a resonance frequency of the vibration system being at least lower than a minimum frequency of a sound to be shielded by the sound shielding plate.

According to a further aspect of the present invention, there is provided with a method for providing a sound shielding plate structure for at least two sheets of aperture penetrated plates having a plurality of openings and interposed between a first aerial space and a second aerial space, wherein at least one resonance frequency of at least one vibration system created by air in the openings and aerial layers in an interplate interval of distance between the opposing openings is set lower than a frequency of a sound wave generated from the first aerial space to be shielded so that almost all transmitted waves through the two sheets of the aperture penetrated plates are not present in the second aerial space.

Fig. 1 is a schematic perspective view of a structure of a sound shielding plate of a first preferred embodiment according to the present invention.

Fig. 2 is a side view of a front part of an automotive vehicle to which the sound shielding plate in the first embodiment shown in Fig. 1 is applicable as an underbody of an engine room.

Fig. 3 is a bottom, view of the automotive i-ehicle to which the sound shielding plate in the first embodiment shown in Fig. 1 is applicable as the underbody.

Fig. 4 is an explanatory view of the sound shielding plate used for explaining a performance analysis of the sound shielding plate in the first preferred embodiment.

Fig. 5 is an explanatory view of a model (dynamic damper) used for the performance analysis of the sound shielding plate of the first embodiment shown in Fig. 1.

Figs. 6 (A) and 6 (B) are explanatory views of the sound shielding plate used to explain the performance analysis of the sound shielding plate in the first preferred embodiment according to the present invention, Fig. 6 (A) being a perspective view of the sound shielding plate in the first embodiment and Fig. 6 (B) being a plan view of the sound shielding plate in the first embodiment.

Fig. 7 is a characteristic graph for evaluating the performance of the sound shielding plate in the first embodiment shown in Figs. 1 through 6 (B).

Fig. 8 is a schematic explanatory view of a vibration model used for explaining the performance analysis of the sound shielding plate in the first embodiment shown in Figs. 1 through 7.

Figs. 9 (A), 9 (B), and 9 (C) are characteristic graphs of the sound shielding plate of the first embodiment, Fig. 9 (A) being a result of a theoretical analysis of a performance (transmission loss: TL) of the sound shielding plate in the first embodiment according to the present invention, Fig. 9 (A) being a result of a change in the transmission loss when a plate thickness of a penetrated aperture plate of the sound shielding plate in the first embodiment is varied, Fig. 9 (B) being a result of a change in a transmission loss when a rate of apertures of the penetrated aperture plate is varied, and Fig. 9 (C) being a result of a change in the transmission loss when an interval between the penetrated aperture plates of the sound shielding plate is varied.

Fig. 10 is a side cross sectional view of an experiment apparatus used to verify the performance of the sound shielding plate in the first preferred embodiment according to the present.invention.

Figs. 11 (A) and 11 (B) are characteristic graphs for explaining the evaluation of the performance of the sound shielding plate in the first preferred embodiment according to the present invention, Fig. 11 (A) being a characteristic graph representing a result of verified experiment and Fig. 11 (B) being a characteristic graph representing a theoretical solution. Fig. 12 is an enlarged cross sectional view of an essential part of the sound shielding plate in a second preferred embodiment according to the present invention. 30 Fig. 13 is an enlarged cross sectional view of an essential part of the sound shielding plate in a third preferred embodiment according to the present invention. Fig. 14 is an enlarged cross sectional view of an essential part of the sound shielding plate in a fourtli preferred embodiment according to the present invention.

Fig. 15 is an explanatory view of the sound shielding plate for explaining a sound shielding mechanism of the sound shielding plate in either or all of fifth, sixth, eighth, ninth, and tenth preferred embodiment according to the present invention.

Figs. 16 (A) and 16 (B) are schematic view of the automotive vehicle to which the sound shielding plate in the fifth embodiment is applicable as the underbody of the engine room.

Fig. 16 (C) is a detailed perspective and partially cut view of the sound shielding plate of the fifth embodiment.

Fig. 16 (D) is an explanatory view for explaining a sound shielding mechanism in the case of the fifth embodiment.

Fig. 16 (E) is a characteristic graph presenting an effect achieved by the sound shielding plate in the fifth embodiment.

Fig. 17 (A) is a detailed perspective and partially cut view of the sound shielding plate in a sixth embodiment. Fig. 17 (B) is an explanatory view for explaining the sound shielding mechanism of the sound shielding plate in the sixth embodiment. 25 Fig. 17 (C) is a characteristic graph of an effect achieved by the sound shielding plate in the sixth embodiment. Fig. 18 (A) is a detailed perspective and partially cut view of the sound shielding plate in a seventh embodiment.

Fig. 18 (B) is an explanatory view of the sound shield plate for explaining a sound shielding mechanism of the sound shielding plate in the seventh embodiment shown in Fig. 18 (A).

Fig. 18 (C) is a characteristic graph representing an effect achieved by the sound shielding plate in the seventh embodiment shown in Fig. 18 (B).

Fig. 19 is an explanatory and cross sectional view of the sound shielding plate in an eighth embodiment.

Fig. 20 is an explanatory and cross sectional view of the sound shielding plate in a ninth embodiment.

Reference will hereinafter be made to the drawings in order to facilitate a better understanding of the present invention.

(First Embodiment) Fig. 1 shows a sound shielding plate of a first preferred embodiment according to the present invention.

The sound shielding plate generally denoted by 5 serves to acoustically separate a first space A from a second space B and is so constructed as to enable for an air to be freely communicated between both spaces A and B therethrough.

Two sheets of first and second aperture penetrated plates 5A and 5B are interposed between the spaces A and B and juxtaposed to each other so as to face against each other with a predetermined spatial interval of distance. The first and second aperture penetrated plates 5A and 5B are provided with a multiple number of circular, penetrated, first and second openings (penetrated holes) 6a and 6b, respectively, and are made of the similar metals having the same profiles (for example, aluminum or steel).

The multiple number of the first and second openings 6a and 6b are penetrated so as to allow the air to freely be communicated between the spaces A and B, have the mutually same dimensions. and are disposed in the respective sheets of aperture Penetrated plates 5A and 5B at the same positions so as to face against the opposite openings Ba and 6b.

Figs. 2 and 3 show a part of the automotive vehicle on which the sound shielding plate 5 in the first embodiment is mounted.

The shielding plate 5 is integrally mounted onto an underbody UC portion defining a bottom surface of an engine room (engine compartment) 12 of the automotive vehicle 11. That is to say, the shielding plate 5 serves as a part of the underbody UC. It is noted that UC is an abbreviation for an undercover in Japanese but exactly corresponds to the underbody in English.

The portion of the underbody UC provides a passage through which an air stream 16 generated by a cooling of the engine 15 ( it is noted that an engine noise is transmitted via the air stream to an external side of the vehicle 11) flows.

At this portion, the sound shielding plate 5 having the two sheets of the first and second aperture penetrated plates 5A and 5B is positioned.

Naturally, when the sound shielding plate 5 is actually mounted on the portion of the underbody UC of the vehicle 11, profiles (shapes), dimensions, and the number of first and second penetrated openings 6a and 6b are specified and selected according to a model or kind of the vehicle 11.

Therefore, a slight difference is actually present in the sound shielding plate 5 shown in Fig. 1.

In this case, each of plate thicknesses t, rate of apertures a, and interplate interval d of the first and second aperture penetrated plates 5A and 5B is set to an appropriate value by which the sounds or noises of the engine 15 can effectively be shielded.

Particularly, in order to hold the interplate interval d at a constant value, the two sheets of the first and second aperture penetrated plates 5A and 5B are mutually attached to each other by means of spacers 5a at respective positions thereof except peripheral edge portions and first and second penetrated openings 6a and 6b.

Both of the sheets of first and second aperture penetrated plates 5A and 5B constitute one sheet of a structure of shielding plate 5. The shielding plate 5 has the sheet of lower second aperture penetrated plate 5B constituting a rear part of the underbody UC and being integrated with a front part 17a and a middle part 17b to form the whole underbody. It is noted that a window 17c is installed on the middle part 17b of the underbody UC so that a maximum bottom part of an oil pan 15a is exposed to the external of the vehicle 11.

Next. an action of the sound shielding plate 5 in the first embodiment will be described below.

As shown in Figs. 1 through 3, the sound propagated from the engine room 12 toward the lower portion of the vehicle 11 is shielded by means of the underbody UC so constructed as described above so that a leakage of the sound to the external of the vehicle is limited.

Particularly, the sound shielding plate 5 constituting the rear part of the underbody UC effectively attenuates the sound and the effective sound shielding is carried out by means of the sound shielding plate 5.

In addition, a heat generated in the engine room 12 is exhausted via the first and second penetrated openings 6a and 6b to the external of the vehicle 11 and, thus, an air ventilation effect can also be assured.

Generally, persons skilled in the art may consider that presences of the first and second penetrated openings Ga and 6b abruptly reduce a sound shielding performance thereof.

However, when. as in the embodiments according to the present invention. a plurality of sheets of plates having openings 6a and 6b (aperture penetrated plates) (in the first embodiment, two sheets) are disposed as the underbody UC so as to face against each other with the predetermined interplate interval of distance, another acoustic attenuating mechanism is newly provided therein.

The other acoustic attenuating mechanism will be explained below with reference to Fig. 4 through 6 (B).

Fig. 4 shows a general configuration of a sound shielding wall structure to explain the other acoustic attenuating mechanism.

In a model of Fig. 4, an incident wave, as the noise, impinges on a wall W, a part of the incident wave is reflected as a reflected wave, and another part of the incident wave is transmitted through the wall W as a transmitted wave.

Suppose, as shown in Fig. 4, that sound pressures of the incident wave, reflected wave, and transmitted wave are denoted by Pi, Pr, and Pt and their particle volocities are denoted by Vi, Vr, and Vt, respectively.

If the waves denoted by arrow marks shown in Fig. 4 are applied to the noise radiated from the engine shown in Figs. 2 and 3, a region (I) corresponds to an internal of the engine room 12, a wall of a region (II) corresponds to the sound shielding plate 5, and a region (III) corresponds to an external of the engine room 12.

Furthermore, if the wall W is replaced with the sound shielding plate 5 having the openings 6a and 6b described in the first embodiment, a vibration system created by the sound shielding plate 5 corresponds to a vibration system model (mass-spring model) as a dynamic damper having two degrees of freedom shown in Fig. 5. That is to say, air masses MA and mB at the respective openings 6 of the two sheets of the first and second aperture penetrated plates 5A and 5B and air (pneumatic) springs 8 constituted by an intermediate aerial layer between the two sheets of first and second aperture penetrated plates 5A and 5B constitute the vibration model having the two degrees of freedom.

As viewed from the vibration model shown in Fig.

- 14 5, when the engine noise causes one aerial (pneumatic) mass 7A of the first penetrated opening 6a of the first aperture penetrated plate 5A (namely, the air mass virtually present within the first opening 6a) to be excited and vibrated, this vibration of the mass 7A being propagated to another aerial mass 7B of the second opening 6b of the second aperture penetrated plate 5B located at the external air side via the pneumatic spring 8. The vibration of the other aerial mass 7B causes the external air to be excited and lo vibrated so as to be radiated toward the external of the vehicle as the engine noise.

Attention needs to be paid herein to a vibration transmittivity of the air masses 7A and 7B. The present invention is based on the vibration transmittivity in the air masses.

The vibration system having the two degree of freedom has, generally, the vibration transmittivity which becomes lower than a given value, namely, 1 when its vibration frequency is above a resonance frequency (point), namely, the vibration system falls in a vibration suppression region.

Thus, in the case of the model of Fig. 5, the model falls in the vibration suppression region (in other words, an acoustically sound shielding region) above the resonance frequency point so that the sound shielding plate 5 functions as the sound shielding wall.

Hence, in the sound shielding plate 5 of the first embodiment according to the present invention, the resonance frequency of the massspring system constituted by the two sheets of first and second aperture penetrated plates 5A and 5B is set to a value lower than the frequency of sound to be shielded by adjusting their individual plate thicknesses t, rates of apertures a, and interplate interval of distance d between the two sheets of the respective aperture penetrated plates 5A and 5B. Consequently, the sound shielding plate 5 functions as the sound shielding wall against the sound having the frequency region exceeding the resonance frequency described above.

Factors (parameters) to determine the resonance frequency will more specifically be described with reference to Figs. 6 (A) and 6 (B).

As shown in Figs. 6 (A) and 6 (B), the factors include the rates of apertures a (a = na2/L2, if a distance from a center of one circular penetrated opening 6a or 6b to lo a center of another adjacent circular penetrated opening 6a or 6b is L and a radius of either of the individual openings 6a or 6b is a), the plate thickness t of either plate 5A or 5B, the profile of each of the openings 6a or Gb, area of each of the openings 6a or 6b, an interplate interval of distance d between the two sheets of the first and second aperture penetrated plates 5A and 5B. Although these factors are combined together so that the air masses 7A and 7B become large and the pneumatic spring 8 becomes smaller in order to lower the resonance frequency, a particular attention should be paid to the magnitude of the pneumatic spring 8 since the pneumatic spring 8 is varied toward both a large value and a smaller value if the parameters of t, a, and a are varied. For example, if a becomes large, the magnitude of the pneumatic spring 8 becomes excessively larger.

Consequently, the resonance frequency, namely, the frequency above which the sound shielding plate can function as the sound shielding wall can be set lower and the sound shielding performance as the sound shielding wall can be improved.

That is to say, Fig. 7 shows an experiment result which confirmed the effect of sound shielding in a case where the plate thicknesses t of the sound shielding plate in the first embodiment is varied.

In Fig. 7. a longitudinal axis denotes a - is - transmission loss (TL, in dB) of the sound shielding plate and lateral axis denotes the frequency (in Hz). Furthermore, a) denotes a case where the plate thickness t is large (thickest), c denotes a case where the plate thickness t is middle (a second thickest) (rather thick), and G3 denotes a case where the plate thickness t is small (thinnest). The every case indicated that the transmission loss (TL) became increased in a frequency region ab ove the resonance frequency determined by the respective factors described above, thus the shielding plate functioning as the sound shielding wall.

The engine noise of the automotive vehicle has a nisedominant frequency region of about 500 Hz through about 2. 5 KHz. If the resonance frequency is set below about 500 Hz through 1 KHz so that the sound shielding platecan function as a perfect sound shielding plate against the engine noise.

Next, a relationship between the frequency and transmission loss will be discussed below in terms of a theoretical analysis and a practical analysis.

Referring back to Fig. 4, if the spaces partitioned by the sound shieldingwall W are divided into the regions (I), (II), and (III). The regions (I) and (III) are air and region (II) is the wall W. Suppose that an acoustic impedance of the air is pC (p denotes an air density and C denotes a sound velocity). The following relationship is introduced:

Pi Pr Pt - = -- = PC (1) vi Vr Vt Next, suppose that the region (II) is replaced with the two sheets of the first and second aperture penetrated plates 5A and 5B shown in Figs. 6 (A) and 6 (13). A resonance of mutual aperture penetrated plates 5A and 5B is neglected. The reason is that since, when the interplate interval of distance between the two sheets of plates SA and 5b is set to a practical value (the interval of distance d s 40 mm), its f irst-order resonance frequency ( 4. 25 KHz) is considerably higher than the frequency region (1 KHz through 3 KHz) to be shielded and even if the resonance described above is neglected, no problem occurs as a practical matter of fact.

Suppose that the multiple number of equally spaced first and second openings 6a and 6b are formed in the first and second aperture penetrated plates 5A and 5B, respectively, as shown in Figs. 6 (A) and 6 (B).

Assume that disturbances of sound fields due to the radiated waves at the first and second penetrated openings 6a and Gb of the two sheets of the first and second aperture penetrated plates SA and 5B are limited to surfaces adjacent to the respective openings 6a and 6b and such incident and transmitted waves as described in Fig. 5 are plane waves.

In this case, a sound pressure P(,) of the region (I) is given by:

P(I) = Pi + Pr... (2).

On the other hand, a sound pressure P (III) of the region (III) is given by:

P (III) = Pt (3).

Suppose a motion acted upon the vibration system of the model. Fig. 8 shows the motion acted upon the vibration system model.

The motion shown in Fig. 8 will be analyzed below.

Suppose that MA = MB = m. In addition, suppose that fA and fB denote forces acted upon a unit of area.

X, ((k - MW 2)fA + kfBI MW 2(2k m. 2) 18 - X2 {kfA + (k - mw2) fB} MC02(2k Múo2) (4).

In the equation (4), k denotes a constant of the pneumatic spring 8.

Next, suppose that w02 k/m and the mass m per unit area is m = pat (wherein a (rate of apertures) a2/S, S = L x L).

X, = RW02 W2)fA + 6002 fBI p a tco2 (2w02 (02) X2 = R(O0 2 fB + (c002 602)fA} p a teo 2 (2co 02 C02) (5).

Since P (I) is acted upon MA from a left side viewed from Fig. 8 and P (III) is acted upon mB from a right side viewed from Fig. 8, the following equations are established:

fA = aP(I) (6) fB = - aP(III) Rearranging the equation (5) with the equations of (2) and (3) taken into consideration, - 1 X,= (COO 2 - ú02) (pi + Pr) - c002pt} Ptú02(2co02- &)2) - 1 X2= ---{ (COO 2(pi + Pr) - (c002 - w2)pt} Ptc02(2ca02- W2) (7).

Next, suppose a particle velocity of air.

The particle velocity of air is expressed as: Vi + Vr = Al j&)a 2-W 2pt} {(WO 1) (Pi+Pr) -WO Ptw2(2w02 - W2) 0 Vt = aX2 jwa UW02(Pi+Pr)-(WO2(02)pt} Ptw2(2w02-W2) (8) From the equation of (1), 1 Vi + Vr -(Pi-Pr) pc Vt Pt pc Using the equations of (9), the above-described equations of (6) and (7) are rearranged as:

- JwCa Pi-Pr = { (wo 2-W2) (Pi+Pr)-a)02pt) t(02(2o)02-W2) jwCa Pt (W02(Pi,Pr) - (W02-&,2) pt} Pt602 (2a)02-a)2) (10).

If both sides of the equations of (10) are divided by Pi and if wo2-W2 = A, w02= B, -jwca and - = G tw2(2(002-,02) - 20 Pr Pr Pt -- = G{A(1 + -) B -} Pi Pi Pi Pr Pt = GA + GA -- - GB - Pi Pi Pr Pt (1 - GA) = (1 + GA) - GB -- Pi Pi Pt Pr Pt G {B (1 + -) - A -} Pi Pi Pi Pr Pt GB + GB - - GA Pi Pi Pr Pt GB GB - - (1 + GA) Pi Pi (12).

The equations of (12) are expressed in a matrix form as: - P r 1+GA -GB 1 - G A Pi Pt -GB 1 L. G A Pi GB (13).

The matrix formula of (13) can be rearranged as:

Pr, 11 + G (A - B) 1, 4. G (A1 - B) P,, P. G A L X 1-GA GB GB 1 -' CA GB j (14).

Therefore, the following e-quation (15) is established: 2 G B Pi (1+G G (A-B) -2jwCacoo 2 tW2 (2wo 2 w. 2 i co C a j wc a 2 + 2- 2 (2 coo W 2. W C 2 j 2 2 2 2 j j cj C C W2 t (2wo 2 _W2 Next, the transmission loss TL is expressed as:

Pi 2 TL = 10 logi 1 lpt If the above equation (15) is transformed and arranged, the following equation (3.7) is established.

- 2 2 2 2 p i t W (2w 0 - CO 2co C a co 0 2 X 2 2 - 2 p t 2 w C a c-.,, CO (2wo W 2 C a) + j 1 2 2 2 2 t W (2 wo 2 2 2 2 j t (2 c) o w to C a 2 C 0 2 2 t w 0 (17).

If the equation (17) is substituted for the previous equation (15), the following equation (18) is established and the following equation (18) is a theoretical formula to derive the transmission loss TL.

p TL-100'0g - p t (2 coo CO C -2 -102 0 C a CO o 9 2 t cog (18).

Using the above theoretical formula (18), trial calculations for several samples were carried out.

In the samples of the sound shielding plate in this case, other parameters were changed with these parameters of a = 0. 2, t = 20 mm, d = 20 mm, a = 20 mm fixed.

Consequently. the data as shown in Figs. 9 (A), 9 (B), and 9 (C) were obtained.

As viewed from the data shown in Figs. 9 (A), 9 (B), and 9 (C), both vibration suppression effect and transmission loss became larger as t becomes larger, a becomes smaller, and d became larger and correspondingly the 5 sound shielding performance was improved. Particularly, as the interplate distance d becomes d - large, the sound shielding performance was improved beginning from the lower frequency of the sound.

Next, a verification of the above-described theory was carried out.

An experiment apparatus is shown in Fig. 10.

A cubic-shaped box 21 having a longitudinal length, a lateral length, and a height, each of 600 mm. and having an open upper end, was made of wood. A constant sound source 22 was arranged in an inside of the cubic-shaped box 21, the sound shield plate 5 of the first embodiment as one of the samples was arranged over the upper open end of the box 21, and a microphone 23 disposed at an evaluation point located at a height above 1,400 mm from the upper end of the sound shielding plate 5 was used to measure a transmission rate of the sound. In the experiment described above, the parameters were set as follows: t = 10 MM, a = 0. 2, d = 0 through 40 mm (intermittently as 0, 10 mm, 20 mm, and 40 mm). 25 Fig. 11 (A) shows the result of the experiment using the apparatus described above with reference to Fig. 10. Fig. 11 (B) shows a result of theoretical analysis for the same sample condition as the case of Fig. 11 (A). 30 As viewed from both results shown in Figs. 13- (A) and 11 (B) and compared to each other, frequencies (risiii,,,frequencies) at which the sound shielding effect started to appear were approximately coincident with each other of both cases of the experimental result (actual case) and the theoretical analyzed result and were in the vicinity to 1 - 24 KHz. Naturally, the experimental result indicated that a drop of the effect of sound shielding appeared due to an interplate resonance which was neglected when the theoretical derivation of the sound shielding effect was carried out.

As appreciated from above, it is preferable to experimentally determine the resonance frequency, when determining the resonance frequency, due to various types of mutual actions such as the interplate resonance phenomenon as a practical matter of fact, although it is possible to determine the resonance frequency using the calculation from the model.

(Second Embodiment) Fig. 12 shows a second preferred embodiment of the sound shielding plate.

As shown in Fig. 12, in the second embodiment, peripheral walls 61a and 61b of the first and second openings 6a and 6b penetrated through the respective sheets of first and second aperture penetrated plates 35A and 35B are projected cylindrically toward the opposing first and second openings 6a and 6b, respectively.

Thus. only one of the parameters, namely, the plate thicknesses t of the respective sheets of aperture penetrated plates 35A and 35B is increased so that the quantity of air masses constituted by the openings 6 can be enlarged.

In the second embodiment, since the air masses mA and mB can only be enlarged without variation of the whole plate thickness, the sound shielding plate can, accordingly, be light weighted and compacted, maintaining the constant sound shielding performance.

(Third Embodiment) Fig. 13 shows a third preferred embodiment of the sound shielding plate.

As shown in Fig. 1.3, in the third embodiment, the dimensions (inner diameters) of the first and second openings 6a and 6b penetrated through the first and second aperture penetrated plates 45A and 45B are mutually different from each other. In the third embodiment, the dimension of the first opening 6a of the first aperture penetrated plate 45A is larger than that of the second aperture penetrated plate 45B.

The peripheral walls 61a and 61b are projected cylindrically toward the side surface of the opposite openings 6a and 6b, respectively, in the same way as the second embodiment. Thus, maintaining the performance as the sound shielding wall, the ventilation performance is improved.

(Fourth Embodiment) Fig. 14 shows a fourth preferred embodiment of the sound shielding plate.

A plurality of or a single sound absorbing material 37 is interposed between the two sheets of first and second aperture penetrated plates 35A and 35B except the first and second openings 6a and 6b. In this way, the resonance of the respective sheets of first andsecond aperture penetrated plates 35A and 35B can also be limited.

Thus, maintaining the ventilation performance, the sound shielding performance can be improved.

In the first, second. t h i r d and fourth embodiments, the two sheets of the first and second aperture penetrated plates are faced against each other.

It is possible to combine (overlap or laminate) a number, i.e., three or more of the sheets of aperture penetrated plates exceeding the two sheets of plates.

In the first, second. third, and fourth embodiments, the openings 6 have circularly penetrated through the respective sheets of first and second aperture penetrated plates.

The openings may alternatively be of any other - 26 shapes such as n-sided polygons.

In addition, although. in the first, second, third, and fourth embodiments, all of the shapes and radii of the openings 6 are the same for each of the aperture penetrated plates, the shapes and radii of the respective sheets of aperture penetrated plates may appropriately be changed.

The plate thicknesses t and interplate interval d between the aperture penetrated plates may be varied at the positions along the surfaces of the respective sheets of plates.

Furthermore, although, in the first, second, third, and fourth embodiments, the sound shielding plate according to the present invention is attached to the vehicle of an engine front-mounted type, the position of the engine is not limited to the front portion of the vehicle. However, it is more effective in the sound shielding standpoint of view for the sound shielding plate to be mounted on an exit side of the engine cooling air stream. 20 (Fifth Embodiment) Before explaining a fifth preferred embodiment, a general concept of a mechanism of sound shielding by means of the sound shielding plate will be described below with reference to Fig. 15. 25 Fig. 15 shows the general concept of the sound shielding plate applicable to fifth, sixth, seventh, eighth, ninth, and tenth embodiments to be described later. As shown in Fig. 15, a plurality of spaced apart linkage rods or interposing walls 60 (hereinafter, referred simply to as walls but the walls do not exactly correspond to the spacers 5a described in the first embodiment shown in Fig. 2) are extended within the internal space between the two sheets of aperture penetrated plates 5AA and 5BB. The walls 60 are deviated (offset) from centers of predetermined parts of the respective sheets of aperture penetrated plates 5AA and 5BB except positions of the openings constituting air masses, so that a volume of an aerial layer per each opening can be formed into two kinds of large and small aerial layers. The large volumes of aerial layers correspond to small spring constants of the pneumatic springs kl. Therefore, the resonance frequency becomes lower. On the other hand, the small volumes of aerial layers correspond to the large spring constants of the pneumatic springs k2. Therefore, the resonance frequency becomes higher.

In this way, when the single sound shielding wall shown in Fig. 15 is provided with two kinds of vibration systems having different resonance frequencies, in a frequency region between the two higher and lower resonance frequencies, the hases of the transmitted waves from the respective vibration systems are deviated from each other by 180 degrees, i.e., the transmitted waves are in mutually opposite phases so that the transmitted waves are mutually canceled to achieve the sound shielding effect in the sound shielding plate of Fig. 15.

In addition, in the frequency region in which the transmitted waves have mutually opposite phases and in a frequency at which the sound pressures of the transmitted waves become equal. the sound pressures are perfectly canceled to give zero so that the sounds are not transmitted any more through the two sheets of aperture penetrated plates 5AA and 5BB.

In this way, when the single sound shielding wall shown in Fig. 15 is provided with the two kinds of vibration systems, the sound shielding effect can more remarkably be achieved even if the sounds have very low frequencies.

Next, Figs. 16 (A) through 16 (C) show the f if th embodiment of the sound shielding plate.

In Figs. 16 (A) and 16 (B) the underbody UC attached onto the lower portion of the engine room 12 of the - 28 vehicle 11 is provided with the penetrated openings 66 for the ventilation. As shown in Fig. 16 (B), the openings 66 have the circular shapes and a part of the underbody UC at which the openings 66 are disposed provide a double structure of the two sheets of the aperture penetrated plates 5AA and SBB. Parts 7AA of the penetrated openings 66 which face against the opposite openings 66 are linked together by means of cylindrically shaped walls 60.

Fig. 16 (D) shows the sound shielding mechanism of the sound shielding plate in the case of the fifth embodiment shown in Figs. 16 (A) through 16 (C).

In the openings 7AA which are linked via the cylindrical walls 60 from among the plurality of the openings 66, the air present within cylindrically shaped ends of the respective openings 7AA functions as the air mass and only the air mass thereof constitutes the vibration system having one degree of freedom shown in Fig. 16 (D).

On the other hand, parts of the openings 66 in Fig. 16 (C) which are not linked via the cylindrical walls 60 function as the air masses and parts of aerial layers between the penetrated openings which are not partitioned by means of the cylindrical walls 60 function as the pneumatic springs, as shown in Fig. 16 (D). Thus, these air masses and pneumatic springs constitute the mass-spring vibration system having two degrees of freedom.

In Fig. 16 (D), the vibration systems having the one degree of freedom constituted by the parts 7AA of openings 60 linked by means of the cylindrical walls 60 have no resonance frequency (point) so that the phases of the incident waves are always the same as those of the transmitted waves. On the contrary, each of the other vibration systems having the two degrees of freedom constituted by parts 66 of the openings 60 which are not linked via the cylindrical walls 60 have a resonance frequency fl as established below:

v -cf.B 1+ 1.6l) c I (18).

In the equation (18), c denotes the sound velocity, a denotes the rate of apertures (the rate of apertures means herein a rate of occupied areas of the openings 66 to the whole area of each sheet of the aperture penetrated plate 5AA or 5BB, the areas of the openings inclusive), t denotes the plate thickness (the plate thickness means herein a thickness of each sheet of the aperture penetrated plate 5AA or 5BB), d denotes the interplate distance between the penetrated aperture plates 5AA and 5BB, a denotes a radius of each opening, and 31 denotes a ratio of the number of the openings 7AA linked via the cylindrical walls 60 to the number of all of the openings 66.

In a frequency region exceeding the resonance frequency defined in the equation (18). the phases of the incident waves and transmitted waves are mutually opposite to each other. Hence, at the frequencies exceeding rhe resonance frequency of the vibration system having the rwo degree of freedom, the phases of these two incident and transmitted waves are mutually opposite and mutually canceled. thus the sound shielding plate in the fif-th embodiment achieving the sound shielding performance.

Fig. 16 (E) shows an effect of the sound shielding mechanism by the sound shielding plate in the case of the fifth embodiment shown in Figs. 16 (A) through 16 (D).

That is to say, Fig. 16 (E) shows the transmission loss of the whole sound shielding plate (wall) when the ratio 01 of the number of the openings 7AA which are linked via the cylindrical walls 60 to the number of all openings 66 on the sound shielding plate (wall) is varied as.81 = 1 to is, = 0.

In the case of the engine noise of the vehicle 11, its dominating frequency band ranges from about 1 KHz to about 2. 5 KHz. Therefore, when the resonance frequency fl of the equation (18) is set equal to or lower than 1 KHz, the sound shielding plate in the fifth embodiment gives the remarkable sound shielding effect on the engine noise.

The volume of the aerial layer per the opening (which is not linked to the opposing opening via the cylindrical wall 60 and which corresponds to the pneumatic spring of the vibration system having the two degrees of freedom) becomes larger than that of the aerial layer per opening which is faced against the opposite opening and no cylindrical wall 60 in the case of the fifth embodiment is present. Hence, since the resonance frequency becomes lower, the sound shielding plate in the fifth embodiment can provide the sound shielding effect starting from the lower frequency than that of the sound shielding plate with no cylindrical walls 60. In the case where the sound shielding effect is achieved starting from the same frequency, a more compact sound shielding plate can be achieved. 30 (Sixth Embodiment) Fig. 17 (A) shows a sixth preferred embodiment of the sound shielding plate. As shown in Fig. 17 (A), in the si.xtii embodiment, the parts of the sound shielding plate through which the ventilation openings 66 are penetrated provide the double structure of the two sheets of aperture penetrated plates SAA and 5BB. In addition, the cylindrical walls 60 as in the case of the fifth embodiment and partitioning walls 60A are extended in the space between the two sheets of the aperture penetrated plates 5AA and 5BB so that two kinds of large and small volumes of the aerial layers between the mutually opposing openings 66 are formed. Furthermore, the openings having two kinds of large and small radii are formed at the sound shielding plate in the sixth embodiment.

Fig. 17 (B) shows the sound shielding mechanism in the case of the sixth embodiment shown in Fig. 17 (A).

In the sixth embodiment shown in Fig. 17 (B), two kinds of the vibration systems having two degrees of freedom constituted by the air masses in the openings 66 and pneumatic springs of the interplate aerial layers can be formed and the resonance frequencies fl and f2 of these two vibration systems are expressed as:

Supposing, in the sixth embodiment and the equation (19), that the whole vibration system is divided into respective vibration systems; vibration 1 and vibration 2, fl denotes the resonance frequency of the vibration system 1, f2 denotes the resonance frequency of the vibration system -9, a, denotes each radius of the openings in the vibration system 1. a2 denotes each radius of the openings in the vibration system 2, o' 1 denotes a ratio of each area of the openings in the vibration system 1 to all areas of the openings 66, and 0 2 denotes a ratio of the volume of the (interplate) aerial layer between the two sheets of aperture penetrated plates 5AA and 5BB in the vibration system 1 to all volumes of the interplate aerial layers between all of the openings 66.

In the frequency region between the two resonance frequencies fl and f2 in the equation (19), the phases of the transmitted waves from the respective vibration systems are mutually opposite and mutually canceled, thus achieving the sound shielding effect.

Fig. 17 (C) shows the sound shielding effect in the case of the sixth embodiment.

That is to say, Fig. 17 (C) shows the transmission loss M) when the ratio between the respective volumes of the aerial layers corresponding to the pneumatic springs of the two kinds of vibration systems 1 and 2 when the positions B20f the cylindrical and partitioning walls 60 and 60A between the respective sheets of aperture penetrated plates 5AA and 5BB is varied as 82 = 0. 2 to 32 0. 5.

Hence, when the two resonance frequencies of fl and f2 expressed in the equation (24) are set so that the dominating frequency band of about 1 to 2. 5 KHz of the engine noise is included in-between the two resonance frequencies fl and f2, the sound shielding wall (plate) in the sixth embodiment can give the sound shielding effect on the engine noise. 30 Since the sound shielding effect can be achieved starting from the lower frequency of the sound with the same height as the sound shielding plate which lacks neither cylindrical walls 60 nor partitioning walls 60A, the more compact sound shielding plate can be achieved. (Seventh Embodiment) Figs. 18 (A) and 18 (B) show a seventh preferred embodiment of the sound shielding plate.

The sound shielding plate in the seventh preferred embodiment provides a triple structure of three sheets of the penetrated aperture plates SAA, 5BB, and SCC, namely. provide another sheet of the aperture penetrated plates 5CC in addition to the double structure of the two sheets of the aperture penetrated plates 5AA and SBB in the fifth embodiment.

The cylindrical walls 60 are extended along the peripheral ends of the respective Openings 7AA to integrally form the respective longitudinally cylindrical walls 60. The cylindrical walls 60 have the same inner radii as the corresponding openings 7AA, as shown in Figs. 18 (A) and 18 (B).

Fig. 18 (B) shows the sound shielding mechanism in the case of the sixth embodiment.

It is noted that the sound shielding plate shown in Fig. 18 (A) is applied to the underbody UC shown in Fig.

16 (A).

From among the openings 66 of Fig. 18 (A), the parts 7AA of the openings 66 which are linked via the cylindrical walls 60 function as the air masses as in the case of the fifth embodiment so as to constitute the vibration system having one degree of freedom only by the air mass.

In addition, the other parts of the penetrated openings 66 which are not linked via the cylindrical walls 60 function as the air masses in the corresponding parts of the openings 66 and parts of the aerial layers between the respective three sheets of aperture penetrated plates 5AA, 5BB, and 5CC which are not partitioned by means of the cylindrical walls 60 function as pneumatic springs. Thus, these masses and springs constitute the vibration systems having three degrees of freedom.

In the same way as in the fifth embodiment, the vibration systems having the one degree of freedom which are constituted by the parts of the openings 66 which are linked via the cylindrical walls 60 has no resonance frequency point so that the incident waves always have the same phases as those of the transmitted waves.

On the contrary, the vibration systems having the three degrees of freedom which are constituted by the parts 7AA of the openings 60 which are not linked via the cylindrical walls 60 have the two resonance frequencies fl and f2 as expressed as follows:

2_ f3 is fl C T-C-V (1 -a.p 1Xt I+ 1.6a)d f2 - 4_ 0 c 2n V (1-9.PIXI 1+ (Y P 1 X 12+1.6a)d .. (20).

In the equation (20), fl denotes the first-order resonance f requency, f 2 denotes the second-order resonance frequency, 13 1 denotes the ratio of the number of openings linked via the cylindrical walls to the number of all openings, t, denotes each thickness of the first (highest) and third (lowest) end aperture penetrated plates 5AA and SCC, and t2 denotes the thickness of the second (middle) aperture penetrated plate 5BB.

In the frequency region between these two resonance frequencies fl and f2 in the equation (20), the incident waves have the opposite phase to those of the transmitted waves. llence, in the frequency region between the two resonance frequencies fl and f2, these two transmitted waves are mutually Opposite and mutually - 35 canceled to achieve the remarkable sound shielding effect.

Fig. 18 (C) shows a sound shielding effect of the sound shielding plate (wall) in the case of the seventh embodiment.

That is to say, Fig. 18 (C) shows the transmission loss M) of the sound shielding plate in the seventh embodiment when the ratio fil in the equation (20) of the number of the openings 7AA linked via the cylindrical walls to the number of all of the openings 66 is varied as 81 0. 60 to 81 = 0. 75.

Hence, if the main (dominating) frequency-band (about 1 through 2. 5 KHz) of the engine noise is included in the frequency region between the two resonance frequencies fl and f2 in the equation (20), the sound shielding plate (wall) in the seventh embodiment gives the remarkable sound shielding effect on the engine noise.

In addition, the more compact soundshielding plate can be achieved since the sound shielding effect can be achieved starting fr ' om the lower frequency than the frequency above which the sound shielding effect can be achieved by the sound shielding plate having the same thickness as in the seventh embodiment in which no cylindrical wall 60 as in the case of the seventh embodiment is extended. 25 (Eighth Embodiment) Fig. 19 shows a cross sectional view of the sound shielding plate in an eighth preferred embodiment. As shown in Fig. 19, in the eighth embodiment, the projected walls 61a and 61b as in the same way as the second embodiment are extended toward each other from the peripheral ends of the openings 66 of the penetrated aperture plates 5AA and 5BB at which no cylindrical wall 60 is present to provide the large air masses of the corresponding openings 60 and the distance between the two aperture penetrated plates SAA and 5BB is elongated to - 36 provide the small pneumatic springs. Thus, the more compact sound shielding plate becomes possible.

(Ninth Embodiment) Fig. 20 shows a cross sectional view of the sound shielding plate in a ninth preferred embodiment according to the present invention.

As shown in Fig. 20, in the ninth embodiment, the plurality of the sound absorbing materials 37 are inserted into the spaces defined by the wall parts of the respective sheets of aperture penetrated plates 5AA and 5BB except the openings 66 in the case of the eighth embodiment. As described hereinabove, since, the sound shielding plate according to the present invention, the sound shielding plate has at least two spaced-apart aperture 15 penetrated plates 5A and 5B; 5AA and 5BB; and SAA, 5BB, and 5CC each having the penetrated openings. both ventilation and sound shielding characteristics required for the underbody can be achieved with the appropriate thickness of the whole sound shielding plate. 20 In addition. since the interplate thickness defined as d is appropriately set, the sound shielding plate can be applied to the part of the underbody of the engine room of the automotive vehicle. Furthermore, since the resonance frequency(ies) can be set lower than the dominating frequency band of the engine noise with the parameter of the interplate d fixedlyset, the compact sound shielding plate can be achieved so as to be applicable to the underbody of the vehicle 11.

Other various effects can be achieved by the sound shielding plate according to the present invention.

Claims

CLAIMS,

A sound shielding plate, comprising:

a) at least two sheets of aperture penetrated plates opposing to each other with an interval of distance; and, b) a plurality of openings penetrated through each of said two sheets of aperture penetrated plates so as to face against each other.
2. A sound shielding plate as set forth in claim 1, wherein parameters of an opening depth of each sheet of the penetrated aperture plates, a rate of apertures of each sheet thereof, and the interplate interval of distance are set such that a resonance frequency of a vibration system constituted by an air mass of each opening penetrated through each sheet of the aperture penetrated plates and a pneumatic spring created by an aerial layer between the two sheets of the aperture penetrated plates becomes lower than a dominating frequency band of the sound to be shielded.
3. A sound shielding plate as set forth in either claim 1 or claim 2, wherein a position of each opening penetrated through one sheet of the aperture penetrated plates is the same as that of each opening penetrated through the opposing other sheet of the aperture penetrated plates.
4. A sound shielding plate as set forth in claim 3, which further comprises a plurality, of sound absorbing materials disposed in the interplate space between the two sheets of the aperture penetrated plates except the positions of the respective openings.
5. A sound shielding plate as set forth in any. one of claims 1 through 4, wherein shapes of the respective openings are approximately circular.
6. A sound shielding plate as set forth in any one of claims 1 through 4, wherein shapes of the respective openings are approximately n-sided polygons.
7. A sound shielding plate as set forth in any one -of claims 1 through 6, wherein the sound shielding plate is applied to an underbody to partition a lower part of an engine room of an automotive vehicle from an external side of the vehicle.
8. A sound shielding plate as set forth in any one of claims 1 through 7, wherein said two sheets of the aperture penetrated plates are made of rigid bodies and at least one spacer is extended between the interplate space to set the interplate interval of distance.
9. A sound shielding plate as set forth in any one of claims 1 through 8, wherein a resonance frequency of the sound shielding plate is set equal to or lower than about lKHz of a dominating frequency band of an engine noise radiated from an engine of the vehicle.
10. A sound shielding plate as set forth in claim 1, wherein at least two kinds of vibration systems constituted by pneumatic masses formed in the respective openings and pneumatic springs formed in aerial layers between the respective sheets of the aperture penetrated plates are present in the sound shielding plate.
11. A sound shielding plate as set forth in claim 10, which further comprises a plurality of walls disposed in the space between the two sheets of the aperture penetrated plates so as to partition the aerial layers formed between the respective sheets of the penetrated aperture plates from the other aerial layers formed between the respective sheets of the aperture penetrated plates, one or more of such group of the openings being present in the sound shielding plate.
12. A sound shielding plate comprising:

a) at least two sheets of sound shielding plates. one of the two sheets of the sound shielding plates being faced against the other sheet of the sound shielding plate with a spatial interval of distance; and b) a plurality of openings penetrated through the respective sheets of the sound shielding plates, each one of the openings penerated through one of the sheets of the sound shielding plates being faced against each of the other openings penerated through the other sheet of the sound shielding plate, and wherein a vibration system is provided from among the respective openings which is constituted by an air mass created within a predetermined one of the openings and a pneumatic spring created by an aerial layer between the mutually opposing sheets of the sound shielding plates and wherein at least one of the openings other than those constituting said vibration system is linked to the opposite opening via a cylindrical member having an inner surface whose cross sectional shape is approximately the same as that of the opposite opening.
13. A sound shielding plate as set forth in claim 11, wherein cross sectional shapes of the walls at all cross sections cut away along lines in parallel to the respective sheets of the penetrated aperture plates at internal surface of the walls to partition the aerial layers between the respective openings are larger than those of the respective openings and wherein resonance frequencies of all of vibration systems constituted by air masses formed in the - 40 respective openings and pneumatic springs formed in the aerial layers between the sheets of the aperture penetrated plate are roughly divided into at least two kinds of resonance frequencies so that a dominating frequency of the sound to be shielded is present in a frequency band between a maximum of the resonance frequencies and a minimum of the resonance frequencies.
14. A sound shielding plate as set forth in claim 12, which further comprises a further aperture penetrated plate so that the sound shielding plate has a triple structure of the three sheets of the aperture penetrated plates, wherein cross sectional shapes of the walls at all cross sections cut away along lines in parallel to the respective sheets of the aperture penetrated plates at internal surface of the walls to partition the aer ial layers between the respective openings are the same as those of the respective openings, and wherein a dominating frequency of the sound to be shielded is present in a frequency band -between a first lowest resonance frequency and a second lowest resonance frequency from among resonance frequencies of the vibration systems constituted by air masses formed in the openings which are other than the remaining openings which are partitioned by said walls and pneumatic springs formed in aerial layers between the respective sheets of the aperture penetrated plates other than the aerial layers partitioned by said walls.
15. A sound shielding plate as set forth in any one of claims 10, 12, 13, and 14, wherein said sound shielding plate is provided with openings for ventilation based on the openings to be applied to an underbody of an engine room of an automotive vehicle.
16. A sound shielding plate as set forth in any one of claims 1 through 15, wherein shapes of the openings of the respective aperture openings located at both upper and lower ends of the sound shielding plate are such as to be projected toward an inner side of the sound shielding plate.
17. A sound shielding plate as set forth in claim any one of claims 1 through 16, wherein a plurality of sound absorbing materials are disposed in a space defined by the respective aperture plates except the positions of the openings.
18. A sound shielding plate as set forth in claim 12.

wherein said resonance frequencies are expressed as fl defined below:

& 2n V wherein c denotes a sound velocity, a denotes a rate of apertures, t denotes each thickness of the respective sheets of aperture penetrated plates, a denotes a radius of each opening, and 61 denotes a ratio of the number of openings linked via the cylindrical walls to the number of all of the openings and d denotes the spatial interplate interval of distance.
19. A sound shielding plate as set forth in claim 13, wherein said maximum and minimum resonance frequencies are expressed as fl and f2 defined below:

27 2 2,'11 wherein, supposing the two kinds of vibration systems are denoted by the vibration system 1 and vibration system 2, fi denotes the resonance frequency of the vibration system 1, f2 denotes the resonance frequency of the vibration system 2, a, denotes a radius of each opening in the vibration system 1, a2 denotes a radius of each opening in the vibration system 2, A', denotes a ratio of each area of the openings in the vibration system 1 to the areas of all of the openings, and 82 denotes a ratio of the volumes of the interplate aerial layers in the vibtation system 1 to the volumes of the interplate aerial layers of all openings.
20. A sound shielding plate as set forth in claim 14, wherein said first and second lowest resonance frequencies are expressed as fl and f2 defined below:

fl = c 2nV (1-aplXtl+1.6i)d f2.= C, c '1 - p ' 2x wherein fi denotes a first-order resonance frequency, f2 denotes a second- order resonance frequency, 6 1 denotes a ratio of the number of the openings linked via the cylindrical walls to the number of all openings, t, denotes each plate thickness of highest and lowest sheets of the aperture penetrated plates, and t2 denotes a plate thickness of a middle sheet of aperture penetrated plate.
21. A sound shielding plate applicable to an underbody located at a lower end of an engine room of an aLitomotive vehicle, comprising:

43 - a) a plurality of sheets of aperture penetrated plates disposed between the lower end of the engine of the vehicle and an external side of the engine room and approximately juxtaposed to one another; and b) a plurality of openings penetrated through the respective sheets of aperture penetrated plates and provided for an air ventilation purpose, wherein at least two kinds of vibration systems constituted by air masses formed in the respective openings of the respective sheets of aperture penetrated plates and pneumatic springs are present in the sound shielding plate.
22. A sound shielding plate applicable to an underbody located at a lower end of an engine room of an automotive vehicle as set forth in claim 21, wherein said openings are formed on the basis of the openings defined and recited in any one of the claims 12, 13, and 14.
23. A sound shielding plate comprising:

a) at least two sheets of aperture penetrated plates opposing to each other with an interval of distance; b) a plurality of openings penetrated through each of said two sheets of aperture penetrated plates so as to face against one another; and c) means for combining both sheets of aperture penetrated plates to provide the interval of distance between the respective sheets of aperture penetrated plates so as to form at least one kind of vibration system in the sound shielding plate, a resonance frequency of the vibration system being at least lower than a minimum frequency of a sound to be shielded by the sound shielding plate.
24. A method for providing a sound shielding plate structure for at least two sheets of aperture penetrated plates having a plurality of openings and interposed between a f irst aerial space and a second aerial space, wherein at least one resonance frequency of at least one vibration system created by air in the openings and aerial layers in an interplate interval of distance between the opposing openings is set lower than a frequency of a sound wave generated from the first aerial space to be shielded so that almost all transmitted waves through the two sheets of the aperture penetrated plates are not present in the second aerial space.
25. A sound shielding plate as described hereinbefore with reference to and as in the accompanying drawings.
26. A method for providing a sound shielding plate structure for at least two sheets of aperture penetrated plates having a plurality of openings and interposed between a first aerial space and a second aerial space as described hereinbefore with reference to and as in the accompanying drawings.