Structure

Abstract

To provide a structural body that can construct a double Helmholtz resonance structure while reducing the number of members.SOLUTION: The present invention provides a structural body having a resin molding with a front surface and a rear surface, and a rear member, in which the rear member is superimposed on the rear surface of the resin molding to provide a rear air layer on the rear surface side. The resin molding has an internal space and a through opening. The through opening extends through the front surface and the rear surface of the resin molding, and causes the internal space and the rear air layer to communicate with each other. Preferably, a plurality of the through openings is provided to be aligned in the front surface, and adjacent ones of the plurality of through openings are connected to each other via the internal space.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a structure, and more particularly to a structure having a sound absorbing function. [Background technology]

[0002] Patent Document 1 discloses an integrally molded body in which a sound absorbing member is fixed to a base material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-025784 A Summary of the Invention [Problem to be solved by the invention]

[0004] One type of sound-absorbing structure is called a double Helmholtz resonance structure. The conventional method for constructing a double Helmholtz resonance structure is to stack several plates, including two perforated plates and a back plate, and to use the gaps between each plate as the first and second air layers. This structure has the problem that it tends to require a large number of parts.

[0005] The present invention has been made in consideration of the above circumstances, and provides a structure that can construct a double Helmholtz resonance structure while reducing the number of parts. [Means for solving the problem]

[0006] According to the present invention, there is provided a structure having a resin molded body having a front and back surface, and a back surface member, wherein the back surface member is overlapped with the back surface of the resin molded body to provide a back surface air layer on the back surface side, and the resin molded body has an internal space and a through opening, which penetrates the front and back surface of the resin molded body and connects the internal space with the back surface air layer.

[0007] The inventors of the present invention discovered through experiments that it is possible to construct a double Helmholtz resonance structure by using a member with a through hole in a resin molded body having an internal space. By combining a resin molded body and a back member, it is possible to construct a double Helmholtz resonance structure while reducing the number of parts.

[0008] Various embodiments of the present invention will be described below. The embodiments described below can be combined with each other. Preferably, a plurality of the through openings are provided in a line on the surface, and adjacent ones of the through openings are connected via the internal space. Preferably, the resin molding is a foamed resin molding having a plurality of closed cells, the internal space is constructed by the closed cells becoming open cells, and the moisture content of the resin molding is 4% or more. Preferably, the resin molding is a sheet-shaped resin molding. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a perspective view of a structure according to an embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a structure according to an embodiment. [Diagram 3] 1A to 1C are diagrams for explaining a manufacturing method of a structure according to an embodiment. [Figure 4] 1A to 1C are diagrams for explaining a manufacturing method of a structure according to an embodiment. [Diagram 5] 1A to 1C are diagrams for explaining a manufacturing method of a structure according to an embodiment. [Figure 6] 6A and 6B are schematic diagrams of an experimental sample of a comparative example, and FIG. 6C is a schematic diagram of an experimental sample of an example. [Figure 7] 13 is a graph showing experimental results of a comparative example. [Figure 8] 1 is a graph showing experimental results of Example 1 and a comparative example. [Figure 9] 13 is a graph showing experimental results of a comparative example. [Figure 10]1 is a graph showing experimental results of Example 1 and a comparative example. [Figure 11] 1 is a graph showing the experimental results of Example 2. [Figure 12] 1 is an enlarged cross-sectional photograph of a sample (closed cells) of a comparative example. [Figure 13] 1 is an enlarged cross-sectional photograph of a sample (open-cell) of Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Also, each feature can be an invention independently.

[0011] 1. Structure composition 1-1. Overall structure As shown in FIG. 1, the structure 1 has a resin molded body 2 and a back member 3. One example of the resin molded body 2 is a sheet-shaped resin molded body. The surface exposed toward the top of the paper in FIG. 1 is the front surface of the resin molded body 2. The opposite side to this front surface is the back surface of the resin molded body 2, and a back member 3 is provided on the back surface. In comparison with the double Helmholtz resonance structure, the resin molded body 2 corresponds to a "perforated plate" and the back member 3 corresponds to a "member that forms an air layer (also called a back air layer)."

[0012] In the embodiment, the resin molded body 2 is a foamed resin molded body, for example. The material of the foamed resin molded body may be polypropylene (PP), for example. Through openings 2c are arranged at a constant pitch in the width direction and depth direction of the resin molded body 2. The through openings 2c penetrate the resin molded body 2 in its thickness direction.

[0013] The back member 3 has a ventilation member 3a and a back film 3b. The through opening 2c is connected to the ventilation member 3a directly below. The ventilation member 3a has continuous breathability in the width direction, thickness direction, and depth direction. In the embodiment, as an example, polyurethane (PU) foam is used as the ventilation member 3a. The back film 3b is attached to the back surface of the ventilation member 3a to block ventilation of the back surface.

[0014] Fig. 2 illustrates a cross section taken along the line XX in Fig. 1. The resin molded body 2 illustrated in Fig. 2 has an internal space 2b1 that spreads in the width direction and a plurality of through openings 2c that extend in the thickness direction. The internal space 2b1 will be described in detail later. The through openings 2c penetrate the front and back surfaces of the resin molded body 2. Adjacent through openings 2c are connected via the internal space 2b1.

[0015] A rear member 3 overlaps the rear surface of the resin molded body 2. The ventilation member 3a of the rear member 3 overlaps, thereby providing a rear air layer 3a1 on the rear surface side of the resin molded body 2. The through opening 2c reaches the surface of the ventilation member 3a, and connects the internal space 2b1 with the rear air layer 3a1. The through opening 2c creates ventilation as indicated by arrow Q in FIG. 2 between the outside of the resin molded body 2 (upper side in FIG. 2), the internal space 2b1, and the rear air layer 3a1. As a result, a double Helmholtz resonance structure is constructed.

[0016] 1-2. Configuration of resin molded body 2 As shown in Fig. 2, the resin molded body 2 is composed of a large number of closed cells 2a and a large number of wide spaces 2b. The wide spaces 2b are formed by adjacent closed cells 2a communicating with each other. Therefore, the wide spaces 2b are surrounded by the large number of closed cells 2a. The large number of wide spaces 2b are provided at the center of the resin molded body 2 in the thickness direction.

[0017] In the embodiment, as an example, the internal space 2b1 is constructed as follows. A plurality of wide spaces 2b are arranged in the width direction and the depth direction of the resin molded body 2, and the internal space 2b1 is formed by these wide spaces 2b communicating with each other. In other words, the wide space 2b is constructed by the communication of a plurality of closed cells 2a, and the internal space 2b1 is constructed by the communication of the wide spaces 2b with each other in the width direction and / or depth direction. The internal space 2b1 can also be said to be a space in which the closed cells 2a are made open (or interconnected).

[0018] A surface closed cell layer made of many closed cells 2a is provided on the surface side of the internal space 2b1. A back closed cell layer made of many closed cells 2a is also provided on the back side of the internal space 2b1. The two dense cell layers, the surface closed cell layer and the back closed cell layer, sandwich the internal space 2b1. The through opening 2c penetrates both the surface closed cell layer and the back closed cell layer. The inner wall surface of the through opening 2c is constructed by the cross section of the closed cells 2a, so it is difficult to form a smooth curved surface. For this reason, in FIG. 2, for convenience, the inner wall surface of the through opening 2c is illustrated by a broken line.

[0019] 1-3. Helmholtz resonance structure The Helmholtz resonance structure is a sound-absorbing structure that uses the principle of the Helmholtz resonator, and is also called the Helmholtz-type sound-absorbing structure or the perforated panel sound-absorbing structure. The Helmholtz resonator has a resonance system consisting of the "air in the openings (mass)" and the "air in the cavity (spring)." When resonating, the air in the openings vibrates violently, and a sound-absorbing effect is achieved through friction with the sides of the openings. A single Helmholtz resonator is one that has one set of "air in the openings (mass)" and "air in the cavity (spring)."

[0020] A "single Helmholtz resonance structure using a perforated plate and an air layer" will be described. As an example of the structure, a perforated plate with several through-openings is placed away from a wall. The through-openings are, for example, circular holes or slits. The gap between the perforated plate and the wall becomes the air layer of the Helmholtz resonator. Therefore, the illustrated structure can be regarded as a structure in which single Helmholtz resonators are lined up in a row equal to the number of holes in the perforated plate.

[0021] The following formula 1 is the theoretical formula for the resonance frequency in a "single Helmholtz resonance structure using a perforated plate and an air layer." c is the speed of sound, P is the aperture ratio of the perforated plate, L is the thickness of the air layer, t is the plate thickness of the perforated plate, and d is the hole diameter, and the resonance frequency f0 is calculated from these parameters. Note that if a layer of porous sound-absorbing material (breathable) is used instead of the air layer, a higher sound absorption effect can be obtained.

[0022]

number

[0023] The double Helmholtz resonance structure of the structure 1 of the embodiment corresponds to a double Helmholtz resonance structure using a plurality of perforated plates and an air layer. The double Helmholtz resonance structure is a structure in which the above-mentioned single Helmholtz resonance structure is provided twice in the thickness direction. Although there is a certain commonality between the single and double structures, there are also some clear differences. First, as a difference in frequency characteristics, the single Helmholtz resonance structure has one resonance frequency. In contrast, the double Helmholtz resonance structure has two resonance frequencies, and has sound absorption characteristics that show maximum peaks at each resonance frequency. This point will be explained later in the examples. In addition, when comparing structures of the same thickness, the double Helmholtz resonance structure is easier to shift the resonance frequency f0 to the low frequency side without a large decrease in the sound absorption coefficient than the single Helmholtz resonance structure. This is advantageous when it is desired to improve the sound absorption effect in the low frequency band. Since the structure 1 of the embodiment has a double Helmholtz resonance structure, the above advantages can be obtained.

[0024] 1-4. Examples of specifications for each component The thickness of the resin molding 2 is, for example, 1 to 15 mm, preferably 4 to 10 mm, specifically, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mm, and may be within a range between any two of the numerical values ​​exemplified here.

[0025] The size of each wide space 2b is a factor that determines the size of the internal space 2b1. As shown in Fig. 2, if the thickness of the resin molded body 2 at the position where the thickness of the wide space 2b is maximum is Ft and the width of the wide space 2b is Sw, Sw / Ft may be 0.5 or more. The width Sw is the width of the wide space 2b at the position where the length in the width direction of the wide space 2b is maximum, as shown in Fig. 2.

[0026] Assuming that the average width of the closed bubbles 2a adjacent to the wide space 2b in the thickness direction is Nw, then, for example, Sw / Nw≧4 may be satisfied. In other words, the wide space 2b may have a width equal to or greater than four closed bubbles 2a. Nw means the average width of the closed bubbles 2a adjacent to the wide space 2b in the thickness direction that have a width of 100 μm or more. Sw / Nw is, for example, 4 to 30, and specifically, for example, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, or 30, and may be within a range between any two of the numerical values ​​exemplified here.

[0027] Assuming that the average thickness of the closed cells 2a adjacent to the wide space 2b in the thickness direction is Nt, for example, St / Nt≧2 may be satisfied. In other words, the wide space 2b may have a thickness equal to or greater than two of the closed cells 2a. Nt means the average value of the thicknesses of the closed cells 2a adjacent to the wide space 2b in the thickness direction that are 100 μm or more. St / Nt is, for example, 2 to 10, specifically, for example, 2, 3, 4, 5, 6, 7, 8, 9, or 10, and may be within a range between any two of the numerical values ​​exemplified here.

[0028] The degree of open cells in the resin molded body 2 may be determined, for example, by the "moisture content (%)" in Example 3 described later. The moisture content may be, for example, 4% or more, 8% or more, 14% or more, 4% to 20%, or 8% to 14%. The moisture content may be, for example, 4 to 20%, specifically, for example, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, or 30%, or may be within a range between any two of the numerical values ​​exemplified here.

[0029] The through opening 2c may be a round hole, an elliptical hole, a square hole, or any other shape. The through opening 2c may be a shape other than a through hole, specifically, for example, a through slit extending in the width direction or depth direction of the resin molded body 2. The opening diameter of the through opening 2c is also arbitrary. However, as can be seen from the fact that the above-mentioned formula 1 includes the hole diameter d and the aperture ratio P, the opening diameter affects the resonance frequency, so it is preferable to set it taking this point into consideration.

[0030] The thickness of the rear air layer 3a1 may be, for example, 5 mm to 40 mm, or 10 mm to 20 mm. Specifically, the thickness of the rear air layer 3a1 may be, for example, 5, 10, 15, 20, 25, 30, 35, or 40 mm, or may be within a range between any two of the numerical values ​​exemplified here. When a ventilation member 3a is used as in the embodiment, the thickness of the ventilation member 3a may be regarded as the thickness of the rear air layer 3a1.

[0031] The rear air layer 3a1 may be thicker than the resin molded body 2, but is not necessarily limited to this. Just as the air layer L in Equation 1 is a factor that determines the resonance frequency in the single Helmholtz resonance frequency, the thickness of the internal space 2b1 and the thickness of the rear air layer 3a1 also determine the resonance frequency in the double Helmholtz structure, so they may be set arbitrarily taking this into consideration.

[0032] As an example of a combination of the specifications of each component, the ventilation member 3a may have a thickness of 10 to 20 mm, the through opening 2c may be a hole with a diameter Φ=3 mm provided in the resin molded body 2 with an opening rate of about 4.3%, and the resin molded body 2 may have a thickness of about 5 mm and be produced by the core-back method shown in Figures 3 to 5 described below. As another example of a combination, the ventilation member 3a may have a thickness of, for example, 6 to 30 mm, the through opening 2c may have a diameter of, for example, Φ=1.5 to 6 mm, the opening rate of the resin molded body 2 may be about, for example, 3 to 10%, and the resin molded body 2 may have a thickness of, for example, 3 to 15 mm.

[0033] 2. Manufacturing method of the structure A method for manufacturing the structure 1 of one embodiment of the present invention will be described below. This manufacturing method includes a step of preparing the resin molded body 2 (a placement step and an open-cell forming step), a hole making step, and a back member integration step.

[0034] 2.1 Placement process and open-cell process An example of a method for producing the resin molded body 2 will be described with reference to Figures 3 to 5. In the embodiment, as an example, a method for molding the resin molded body 2 using a mold device 11 shown in Figures 3 to 5 will be described. As shown in Figure 3, one foamed resin sheet FRS formed by extruding a molten foamed resin from a slit (not shown) of a T-die and letting it hang down is placed between molds 11a and 11b.

[0035] Next, as shown in Fig. 4, while the molds 11a, 11b are brought close to each other, the foamed resin sheet FRS is sucked under reduced pressure by both the molds 11a, 11b, thereby reducing the pressure in the space SP and the space surrounded by the pinch-off portions 23a, 23b. This applies a force to the foamed resin sheet FRS in the direction of the arrow in the figure, causing the foamed resin sheet FRS to expand until it fills the space between the mold 11b and the piece 12. In this way, when the foamed resin sheet FRS is sucked under reduced pressure by both the molds 11a, 11b, the foaming of the foamed resin sheet FRS is promoted, and the foamed resin sheet FRS expands.

[0036] Next, as shown in FIG. 5, the piece 12 is moved toward the mold 11a by the piece cylinder 24 to draw in the foamed resin FR. This is also called "core back." The moving distance of the piece 12 is not particularly limited, and can be 1 to 20 mm, preferably 2 to 10 mm, and more preferably 3 to 5 mm. This makes it possible to obtain a resin molded body 2 that is further expanded from the state shown in FIG. 4. During the expansion process, the multiple closed cells 2a inside the resin molded body 2 become interconnected cells, forming a wide space 2b and an internal space 2b1.

[0037] 2.2 Hole punching process and back component integration process Next, a plurality of through openings 2c are provided in the resin molded body 2 so as to have a preset opening ratio. There is no limitation on the method of forming the through openings 2c. For example, the through openings 2c may be provided by a drill, or a punching die or the like may be separately provided to punch out any through holes or slits.

[0038] Next, the resin molded body 2 and the back member 3 are integrated. There is no limitation on the method of integration, but as an example, they may be integrated via an adhesive layer. The adhesive layer may be an adhesive agent, an adhesive film, or the like. At this time, care should be taken not to block the area directly below the through opening 2c with the adhesive film, or the like. Alternatively, the resin molded body 2 and the ventilation member 3a may be fixed to each other by allowing the resin constituting the resin molded body 2 to penetrate into the surface pores of the ventilation member 3a.

[0039] According to the embodiment described above, a double Helmholtz resonance structure can be constructed while suppressing the number of parts. That is, the inventors of the present application discovered that it is possible to construct a double Helmholtz resonance structure using a member having a through-opening 2c in a resin molded body 2 having an internal space 2b1, and thus completed the present invention. Since the resin molded body 2 alone can provide a three-layer structure of a perforated plate / air layer / perforated plate, there is an advantage in that the number of parts is reduced. Using the resin molded body 2 and the back member 3, a double Helmholtz resonance structure can be constructed with a small number of parts.

[0040] Furthermore, the above manufacturing method has the advantage that it is easy to produce the sheet-like resin molding 2 with a relatively large area, which has the advantage that the structure 1 can easily be made into a large-area sound absorbing board.

[0041] 3. Variations The internal space 2b1 may extend over the entire surface of the resin molded body 2 by communicating most of the wide spaces 2b, but is not necessarily limited to this. A single resin molded body 2 may have a plurality of internal spaces 2b1, and a medium- or small-sized internal space 2b1 may exist in each of a plurality of regions in the surface of the resin molded body 2. The internal space 2b1 is not limited to a plurality of wide spaces 2b communicating with each other, and one wide space 2b may constitute one internal space 2b1. In either case, a double Helmholtz resonance structure is constructed by the through opening 2c, at least one wide space 2b as the internal space 2b1, and the rear air layer 3a1.

[0042] In the embodiment, the resin molded body 2 is made open-cell to provide the internal space 2b1, but the present invention is not limited to this. A resin molded body 2 having an internal space formed by another method may be used. Modified examples of the resin molded body 2 will be described below.

[0043] As one of the modified examples of the resin molded body 2, a resin molded body having an internal space formed by so-called "spring back" may be used. For example, a known non-patent document on spring back is "Development of Frequency-Specific Soundproofing Material for Automobiles (Second Report), 8 Evaluation of Sound Absorption Characteristics by Plastic Resonator" (Research Report of Western Industrial Technology Center, Hiroshima Prefectural General Technology Research Institute, No. 50, published in June 2007) by Hasegawa Koji et al. This non-patent document states that "The spring back of the core back of the mold used for injection molding and the glass fiber filled in the resin creates a void in the expansion layer of the plastic molded body. In the case of injection expansion molding, the voids inside the plastic molded body are continuously connected. The continuous voids were regarded as the cavity part of the resonator, and a hole leading from the surface of the skin layer to the expansion layer was opened to produce a resonator." Therefore, the internal void formed by this spring back may be substituted for the internal space 2b1 of the embodiment. Specifically, the resin molded body 2 according to the modified example may be obtained by providing the through opening 2c as in the embodiment in the resin molded body in which an internal void has been provided by this spring back technique.

[0044] As another modified example of the resin molded body 2, a resin molded body in which an internal space is formed by applying a core back to a short fiber-like resin body may be used. For example, Japanese Patent No. 5626875 discloses a manufacturing technique in which a core back is applied to a short fiber-like resin body to form an internal cavity. According to paragraph 0033 of the patent document, for example, it is described that a void (reference numeral 3b) is provided inside a thermoplastic resin foam molded body. Therefore, this internal void may be substituted for the internal space 2b1 of the embodiment. Specifically, the resin molded body 2 according to the modified example may be obtained by providing a through opening 2c as in the embodiment in a resin molded body in which an internal void is provided by the technique of this patent document.

[0045] As a modified example of the back member 3, a plate member having an abutment portion may be used. The abutment portion may be, for example, a rib, a protrusion, or a support. A plurality of abutment portions are arranged in the width direction and the depth direction of the plate member. By abutting the abutment portion against the back surface of the resin molded body 2, a "gap" can be created on the back surface side of the resin molded body 2. This gap may be used as the back air layer 3a1. By adjusting the rib position to position the gap directly below the through opening 2c, a double Helmholtz resonance structure can be constructed as in the embodiment. There is no limitation on the material or thickness of the plate member in this modified example, but as an example, it may be a resin plate.

[0046] The ventilation member 3a is not limited to polyurethane foam, and may be a plastic foam made of a material other than urethane. Any material having air permeability may be used as the ventilation member 3a, and a ventilation member other than a plastic foam may be used. Specifically, a porous body having a relatively large pore size may be used. There is no limitation on the structure and material of the porous body.

[0047] The material and shape of the resin molded body 2 may be modified in various ways. The material of the resin molded body 2 is not limited to polypropylene, and may be, for example, polyethylene. The resin molded body 2 is not limited to a sheet-shaped resin molded body as shown in FIG. 1. The structure 1 may be an uneven resin molded body including a trapezoidal cross-sectional convex portion, and accordingly, the resin molded body 2 may also be an uneven resin molded body. In that case, the cavity shape of the core back mold described in FIG. 3 to FIG. 5 may have unevenness. EXAMPLES

[0048] Example 1 To confirm the sound absorption effect, the normal incidence sound absorption coefficient was measured using an acoustic tube (φ29) on several samples.

[0049] 1. Test sample description As shown in Table 1, each test sample is a laminate of a perforated plate / rear air layer / back layer. The rear air layer and the back layer correspond to the ventilation member 3a and the back film 3b in the back member 3 of the embodiment. There were three types of perforated plate and two types of air layer, totaling six types of samples, and the number n for each sample was two.

[0050] [Table 1]

[0051] 1-1. Comparative Example: Sample A Group FIG. 6A shows a schematic cross-sectional structure of sample A group (A1-1, A1-2, A2-1, A2-2 in Table 1). Sample A group has a single Helmholtz resonance structure using a perforated plate 102. Perforated plate 102 is made of non-foamed PP (polypropylene) and has a plate thickness of about 5 mm. The ventilation member 3a (PU foam) has two thicknesses, 10 mm and 20 mm. This is the same for samples B and C groups described below.

[0052] 1-2. Comparative Example: Sample B FIG. 6B shows a schematic cross-sectional structure of sample B (B1-1, B1-2, B2-1, and B2-2 in Table 1). Sample B has a single Helmholtz resonance structure using a perforated plate 112 made of foamed polypropylene with closed bubbles. The thickness of the perforated plate 112 is about 6 mm. The structural difference is that in sample A, the inner wall surface of the through opening 2c is a smooth curved surface without any unevenness caused by bubbles, while in sample B, the inner wall surface of the through opening 2c is a bubble cross section (with some unevenness).

[0053] 1-3. Example: Sample C group FIG. 6C shows a schematic cross-sectional structure of sample C (C1-1, C1-2, C2-1, and C2-2 in Table 1). Sample C is an embodiment of the present invention. A resin molded body 2 of foamed polypropylene having the core-back open cell structure described in FIG. 3 to FIG. 5 was used as a perforated plate. The thickness of the resin molded body 2 in sample C is about 5 mm.

[0054] Four through openings 2c were provided in each of the perforated plates of the above samples A to C. The through openings 2c had a diameter of φ3 and were provided by drilling through holes. The rear air layer was PU foam (porous sound absorbing material), and two types, 10 mm and 20 mm, were used. The back layer was an adhesive film with a thickness of less than 0.1 mm. The perforated plate and PU foam were attached with thin double-sided tape so as not to block the through openings 2c. Each sample was cut to fit into a φ29 acoustic tube.

[0055] 2. Test Method The normal incidence sound absorption coefficient was measured using an acoustic tube (φ29). Specifically, each sample was placed in a thin tube, and sound was output from a speaker. Then, the sound pressure of the output sound and the reflected sound was measured with a microphone, and the sound absorption coefficient was measured from the attenuation of the sound pressure of the output sound and the reflected sound. This measurement method is a known method described in, for example, JP 2019-111737 A, and therefore further detailed description will be omitted.

[0056] 3. Explanation of sound absorption data measurement graph Figures 7 and 8 show the experimental results for samples A to C when the air layer was a 10 mm thick PU foam. In Figure 7, samples A1-1 and A1-2 had maximum sound absorption peaks at around 1500 Hz. This is the sound absorption effect due to Helmholtz resonance.

[0057] In FIG. 8, the measurement results of samples A1-1 and A1-2 and samples C1-1 and C1-2 (embodiment) are compared. In the embodiment samples C1-1 and C1-2, the resonance frequency f0 is 1050Hz, and the maximum sound absorption coefficient peak value is about 0.8. The resonance frequencies f0 are almost the same, but on the other hand, the maximum sound absorption coefficient peak value of sample C1-1 is 0.84, and the maximum sound absorption coefficient peak value of sample C1-2 is 0.78, so there is variation. Although it is considered that the variation in the processed hole diameter and hole cross-sectional shape is the cause, there is no variation in the resonance frequency f0. This is presumably because the inside of the base material is an open-cell layer (hollow), which affects the particularly large variation in the cross-sectional shape of the through opening. In addition, a second maximum sound absorption coefficient peak with a sound absorption coefficient of about 0.55 is observed at about 3200Hz. Even in the frequency range between the first and second maximum peaks, a sound absorption coefficient of at least about 0.45 is obtained. This can be said to be the effect of the embodiment having a double Helmholtz resonance structure.

[0058] Although the total thickness of the structure is the same, the resonance frequency f0 of samples C1-1 and C1-2 is about 400 Hz lower than that of samples A1-1 and A1-2. The difference in sound absorption coefficient between the samples of both groups is also less than 0.1, which is negligible. As a basic concept of sound absorption, there is a trade-off between reducing the total thickness of the sound absorbing material or sound absorbing structure and increasing the sound absorption coefficient in the low frequency range. From the test results of Example 1, it was confirmed that, compared to sample A group, sample C group (Example) is easier to position the resonance frequency f0 on the low frequency side without a large decrease in sound absorption coefficient with the same thickness.

[0059] Figures 9 and 10 show the experimental results for samples A to C when the air layer is a 20 mm thick PU foam. For A2-1, the resonance frequency f0 is 880 Hz and the sound absorption coefficient is approximately 0.78. Compared to the results for samples A1-1 and A1-2 in Figures 7 and 8, the resonance frequency f0 and sound absorption coefficient are lower for samples A2-1 and A2-2 in Figures 9 and 10, but this is in accordance with the theoretical values. The results for the two samples A2-1 and A2-2 are significantly different, but this is thought to be due to problems with the installation of sample A2-2.

[0060] In Example 1, due to the limitations of the measurement equipment (a φ29 acoustic tube), the sound absorption coefficient below 500 Hz could not be measured. Therefore, the resonance frequency f0 of samples B2-1 and B2-2 in Fig. 9, which is about 500 Hz, was not completely shown on the graph in Fig. 9. The first resonance frequency f0 of samples C2-1 and C2-2 in Fig. 10 is considered to be below 500 Hz, but this was also not shown on the graph in Fig. 10.

[0061] Table 2 below compares the theoretical and measured values ​​of the resonance frequency f0 in the single Helmholtz resonance structure (sample A group) under the test conditions of Example 1. The difference between the theoretical and measured values ​​is thought to be due to the dimensional accuracy of the sample and the gap between the sample and the acoustic tube.

[0062] [Table 2]

[0063] Example 2 Using samples with different positions of the through holes, the same verification as in Example 1 was carried out. Three types of samples with different configurations of the perforated plate were produced, and additional experiments were carried out for each type with the number of samples n=2. Table 3 shows a list of the samples.

[0064] [Table 3]

[0065] Samples Aa (Aa1, Aa2) are single Helmholtz resonance structures using non-foamed PP and correspond to Sample A in Example 1. Samples Cc (Cc1, Cc2) are examples of the present invention and are double Helmholtz resonance structures with open bubbles. Samples Cc correspond to Sample C in Example 1. Samples Dd (Dd1, Dd2) are comparative examples in which the foaming element has been removed from Sample Cc. Sample Dd is a perforated plate created by stacking two thin polypropylene plates (1 mm) with a gap (air layer) of 3 mm between them.

[0066] FIG. 11 is a graph showing the experimental results. From the results in FIG. 11, it can be seen that each of the embodiment samples Cc1 and Cc2 has two sound absorption coefficient peaks. The resonance frequency on the low frequency side of the sample Cc group is lower than the resonance frequency of the comparative sample Aa group. These characteristics are similar to the results of the embodiment 1, and reproducibility was confirmed.

[0067] In addition, the sound absorption coefficient of the sample Dd group at the resonance frequency on the low frequency side is high at about 0.9. When focusing on the thickness of the air layer in the internal space of each perforated plate, the air layer of the sample Dd group is thicker than that of the sample Cc group. This may be the reason for the higher sound absorption coefficient.

[0068] Example 3 Table 4 shows a quantitative evaluation of the presence and degree of open cells in foamed resin molded products, based on the moisture content. Specifically, the difference in moisture content was compared between sample E (n=5) made of closed cells and sample F (n=5) with open cells for foamed resin sheet molded products. Sample F with open cells is a sample in which the inner layer is open-celled by the manufacturing method according to the embodiment (Figs. 3 to 5, core-back).

[0069] [Table 4]

[0070] The test method is as follows. In step 1, the flat sample is cut into a 30mm square test piece so that air bubbles are exposed on each cross section. Next, in step 2, the test piece is completely submerged in a cup of water using tweezers or the like and gently shaken for 10 seconds. Next, in step 3, the test piece is removed from the cup and any water droplets on the surface are wiped off. Next, in step 4, the moisture content is calculated from the difference between the weight before and after hydration, and the moisture content relative to the volume of the test piece is calculated. The "moisture content" measured in steps 1 to 4 above is shown in Table 4.

[0071] As seen in Table 4, the moisture content of the closed-cell sample E was about 0.11 to 0.26% (excluding minus 11.60%), and the moisture content was generally less than 0.3%. On the other hand, the moisture content of the open-cell sample F was 8 to 14%, which was significantly higher. It was confirmed that the inner layer of the open-cell sample F had open cells, and water had penetrated to the inside. Based on the experimental results, it is considered that if the moisture content is 1% or more, open cells are formed at least in a part. It is presumed that if the moisture content is 8% or more, as in the case of sample F, a large number of open cells that communicate with the inner layer are obtained. It is presumed that in the intermediate state between sample E and sample F (moisture content of 1% to 8%), the degree of open cells increases, and the internal space 2b1 expands.

[0072] Fig. 12 is an enlarged cross-sectional photograph of sample E (closed cells) of the comparative example. Fig. 13 is an enlarged cross-sectional photograph of sample F (open cells) of the embodiment. No through openings 2c are provided in each sample. As can be seen from Fig. 13, it was confirmed that the inner layer of sample F was open cells. [Explanation of symbols]

[0073] 1 :Structure 2: Resin molded body (foamed resin molded body) 2a: Closed bubbles 2b: Wide space 2b1 :Inner space 2c: Through opening 3: Rear material 3a: Ventilation material 3a1: Rear air layer 3b: Rear film 11: Mold equipment 11a, 11b: Mold 12: Piece 23a, 23b: Pinch-off section 24: Cylinder for bridge 102, 112: Perforated plate FRS: Foam resin sheet

Claims

1. A structure having a resin molded body having a front surface and a back surface, and a back surface member, The back surface member is overlapped on the back surface of the resin molded body to provide a back air layer on the back surface side, The resin molded body has an internal space and a through opening, the through-opening penetrates the front surface and the back surface of the resin molded body and communicates the internal space with the back air layer; The back member has a ventilation member, The ventilation member is made of a porous body, The resin molded body is a foamed resin molded body having a plurality of closed cells, the internal space is constructed by converting the plurality of closed cells into open cells, a surface closed-cell layer constructed of the plurality of closed cells is provided on a surface side of the internal space so as to include the surface of the resin molded body; a rear closed-cell layer constructed of the plurality of closed cells is provided on a rear surface side of the internal space so as to include the rear surface of the resin molded body.

2. 2. The structure of claim 1, A plurality of the through openings are provided in line on the surface, A structure in which adjacent ones of the plurality of through openings are connected via the internal space.

3. The structure according to claim 1 or 2, The resin molded body has a moisture content of 4% or more.

4. The structure according to any one of claims 1 to 3, The resin molded body is a sheet-shaped resin molded body.

5. A structure according to any one of claims 1 to 4, the back surface member has the ventilation member and a back surface film, The back film is attached to the back surface of the ventilation member.

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