JP7664147B2 - Sound absorbing cover - Google Patents

Description

This disclosure relates to a sound-absorbing cover that is placed on a noise source such as a vehicle.

2. Description of the Related Art In vehicles such as automobiles, sound-absorbing covers are arranged around noise sources such as engines, motors, intake manifolds, and electric compressors in order to reduce noise generated from the noise sources.
For example, Patent Document 1 discloses a vehicle exterior sound absorbing structure that includes a cover body that is attached to the vehicle body and a sound absorbing board that is fixed to the cover body. The sound absorbing board is made of a resin foam and has a multi-layer structure that includes a foam layer in the center portion having an open cell structure, and an upper skin layer and a lower skin layer that are located on both sides of the foam layer in the thickness direction. The sound absorbing board also has a plurality of first through holes and second through holes that penetrate all of the layers in the thickness direction.

JP 2012-166717 A

The sound-absorbing structure described in Patent Document 1 has first and second through holes in a sound-absorbing board, which guides sound transmitted from the outside to a large number of interconnected air bubbles in the foam layer, diffusing the sound and causing a resonance phenomenon in the foam layer, and converts the sound energy into kinetic energy and thermal energy for dissipation.
However, the sound absorbing performance on the low frequency side, particularly in the 800 to 1000 Hz band, was insufficient, leaving room for improvement.

This disclosure was made in consideration of this situation, and aims to provide a sound-absorbing cover that can more effectively absorb noise on the low frequency side, particularly in the 800 to 1000 Hz band.

In order to solve the above problems, the present disclosure provides a sound-absorbing cover for covering a target part that is a noise source,
a first sound absorbing layer having a first foamed layer and a first skin layer integrally formed at the time of molding the first foamed layer, the first sound absorbing layer being disposed on the noise source side;
A second sound absorbing layer having a second foamed layer and a second skin layer integrally molded during molding of the second foamed layer, the second skin layer being laminated on the first sound absorbing layer with the second skin layer facing the first skin layer;
an air layer formed between the opposing surfaces of the first skin layer and the second skin layer;
the first sound absorbing layer and the second sound absorbing layer have a non-communicating structure having no communicating holes communicating in the stacking direction,
The stiffness of the first skin layer is smaller than the stiffness of the second skin layer.

The sound-absorbing cover disclosed herein can more effectively absorb noise on the low frequency side, particularly in the 800 to 1000 Hz band.

FIG. 2 is a plan view of the sound absorbing cover according to the first embodiment. This is a cross-sectional view of FIG. 1 taken along line II-II. FIG. 2 is a cross-sectional view taken along line III-III of FIG. FIG. 6 is a partial cross-sectional view of a sound absorbing cover according to a second embodiment. FIG. 11 is a partial cross-sectional view of a sound absorbing cover according to a third embodiment. FIG. 11 is a partial enlarged view of a sound absorbing cover according to a third embodiment. FIG. 11 is a partial cross-sectional view of a sound absorbing cover according to a fourth embodiment. FIG. 13 is a partial cross-sectional view of a sound absorbing cover according to a fifth embodiment. FIG. 11 is a graph comparing the frequency dependence of sound absorption coefficient in Examples 1 to 4. FIG. 11 is a graph comparing the frequency dependence of sound absorption coefficient in Comparative Examples 1 to 5.

<1. Application examples of sound absorbing covers>
The sound-absorbing cover of the present disclosure is applied to a noise source such as an automobile engine, a motor, an intake manifold, or an electric compressor, and is used to absorb noise generated from the noise source. It is particularly suitable for use with noise sources that generate low-frequency noise. These sound-absorbing covers are generally concave with a bottom that opens toward the noise source, and are fixed to a fastening member such as a bolt on the noise source side.

2. Structure of the sound absorbing cover
This sound-absorbing cover is constructed by laminating two foam sound-absorbing layers each having a skin layer, with the skin layers facing each other. One sound-absorbing layer is disposed on the noise source side, and the other sound-absorbing layer is disposed on the non-noise source side, forming a design surface. This sound-absorbing cover has an air layer between the skin layers that are laminated facing each other.

First Embodiment
The configuration of the sound absorbing cover 1 according to the first embodiment of the present disclosure will be described with reference to Figures 1-3. As shown in Figures 1 and 2, the sound absorbing cover 1 has a concave shape with a bottom that opens toward a noise source (not shown). The sound absorbing cover 1 can be molded into any shape depending on the outer shape of the target noise source.
In this embodiment, the cover body, which is a concave shape with a bottom that opens to the noise source side, is formed of the second sound absorbing layer 20, and the first sound absorbing layer 10 is arranged in the area facing the noise source within the concave portion.

As shown in FIG. 3, a first sound absorbing layer 10 and a second sound absorbing layer 20 are laminated in the area facing the noise source. The first sound absorbing layer 10 is a foamed sound absorbing layer having a first foamed layer 11 and a first skin layer 12 that is molded integrally with the first foamed layer 11 when the first foamed layer 11 is molded. In this embodiment, the first skin layers 12, 12 are provided on both sides in the lamination direction, but a configuration in which the first skin layer 12 is provided only on the side facing the second sound absorbing layer 20 can also be adopted.

The second sound absorbing layer 20 is a foamed sound absorbing layer having a second foamed layer 21 and a second skin layer 22 that is molded integrally with the second foamed layer 21. In this embodiment, the second skin layers 22, 22 are provided on both sides in the stacking direction, but a configuration in which the second skin layer 22 is provided only on the side facing the first sound absorbing layer 10 can also be adopted.

As shown in Figures 2 and 3, the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20 are laminated facing each other, and an air layer 30 is provided between the facing surfaces of the first skin layer 12 and the second skin layer 22.

As shown in Figures 1 and 3, the first sound absorbing layer 10 has a first region 40 in which the first skin layer 12 is formed, and a second region 41 surrounding the first region 40, in the surface direction of the surface layer on which the first skin layer 12 is formed. In this embodiment, the first region 40 is set to a rectangular shape in a plan view. Note that the first region 40 can be set to any shape depending on the shape of the sound absorption target that is the noise source.

As shown in Figures 2 and 3, in a cross-sectional view of the thickness direction of the first sound absorbing layer 10, the upper surface of the second region 41 is disposed closer to the second sound absorbing layer 20 than the upper surface of the first region 40. In other words, the first region 40 is a recessed region formed inside the second region 41, and the first region 40 is disposed below the second region 41.

As shown in FIG. 1, six first engagement recesses 13 are formed at intervals in the circumferential direction in the first region 40 of the first sound absorbing layer 10. In this embodiment, the first engagement recesses 13 are circular recesses in plan view that are connected to the outer edge of the first region 40 and recessed in the thickness direction of the first sound absorbing layer 10.

The first engagement recess 13 may have any shape, such as a rectangular recess in a plan view, or may have a stepped or inclined recess in a cross-sectional view in the thickness direction of the first sound absorbing layer 10.
In addition, the first engagement recess 13 may be a circumferentially continuous annular recess so as to surround the entire periphery of the first region 40, and can be set to any shape depending on the shape of the first region 40.

As shown in Figures 2 and 3, the second sound absorbing layer 20 has six first engaging protrusions 23 that protrude toward the first sound absorbing layer 10 at a predetermined interval in a region (second region 41) surrounding the second skin layer 22 that is arranged opposite the first skin layer 12 of the first sound absorbing layer 10.

The first engaging protrusion 23 is formed at a position opposite to the first engaging recess 13 of the opposing first sound absorbing layer 10, and in this embodiment is a circular (cylindrical) protrusion in a plan view. The outer diameter of the first engaging protrusion 23 is slightly larger than the inner diameter of the first engaging recess 13 of the first sound absorbing layer 10.

When a ring-shaped first engagement recess is provided in the first sound absorbing layer 10, the first engagement protrusion can be set to any shape according to the shape of the first engagement recess, such as being a ring-shaped protrusion corresponding to the first engagement recess.

The air layer 30 is formed by integrating the first sound absorbing layer 10 and the second sound absorbing layer 20. Specifically, when the first engaging recess 13 of the first sound absorbing layer 10 engages with the first engaging protrusion 23 of the second sound absorbing layer 20, the thickness of the first region 40 of the first sound absorbing layer 10 is smaller than the thickness of the second region 41, so that a gap is formed between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20, as shown in FIG. 3, and this gap becomes the air layer 30.

In this embodiment, as described below, the hardness of the second sound absorbing layer 20 is greater than the hardness of the first sound absorbing layer 10, and the outer diameter of the first engaging protrusion 23 of the second sound absorbing layer 20 is slightly greater than the inner diameter of the first engaging recess 13 of the first sound absorbing layer 10. Therefore, when engaged, the first engaging protrusion 23 is prevented from deforming and is pressed into and engaged with the first engaging recess 13 of the first sound absorbing layer 10, so that the first sound absorbing layer 10 and the second sound absorbing layer 20 are integrated in a manner that does not interfere with handling.

In this embodiment, the first sound absorbing layer 10 is provided with the first engaging recess 13, and the second sound absorbing layer 20 is provided with the first engaging protrusion 23. However, the first sound absorbing layer 10 may be provided with an engaging protrusion, and the second sound absorbing layer 20 may be provided with an engaging recess. In addition, the hardness of the second sound absorbing layer 20 may be made smaller than the hardness of the first sound absorbing layer 10, as long as it does not interfere with handling.

Second Embodiment
In the first embodiment, the first sound absorbing layer 10 is provided with a first region 40 having a height difference in the thickness direction, and a second region 41. In the second embodiment, the second sound absorbing layer 20 is provided with a third region 50 having a height difference in the thickness direction, and a fourth region 51.
The configuration of the sound absorbing cover 1a of the second embodiment will be described with reference to Fig. 4. Note that the same components as those in the first embodiment are designated by the same reference numerals as those in the first embodiment.

The second sound absorbing layer 20 has, in the planar direction, a third region 50 in which the second skin layer 22 is formed and which is arranged opposite the first skin layer 12 of the first sound absorbing layer 10, and a fourth region 51 surrounding the third region 50. In this embodiment, the third region 50 is set to a rectangular shape in a plan view. The first region 40 can be set to any shape depending on the shape of the sound absorption target that is the noise source.

As shown in FIG. 4, in a cross-sectional view of the thickness direction of the second sound absorbing layer 20, the lower surface of the fourth region 51 is positioned closer to the first sound absorbing layer 10 than the lower surface of the third region 50. In other words, the third region 50 is a recessed region formed inside the fourth region 51, and the third region 50 is positioned above the fourth region 51.

As shown in Figs. 1 and 4, six second engagement protrusions 24 are formed at intervals in the circumferential direction in the fourth region 51 of the second sound absorbing layer 20. In this embodiment, the second engagement protrusions 24 are convex portions that are connected to the outer edge of the third region 50 and have a circular (cylindrical) shape in a plan view and protrude in the thickness direction of the second sound absorbing layer 20.

The second engaging protrusions 24 may have any shape, such as a rectangular protrusion when viewed from above. In addition, the second engaging protrusions 24 may have a step or an incline when viewed in cross section in the thickness direction of the second sound absorbing layer 20.
In addition, the second engagement protrusion 24 may be a circumferentially continuous annular protrusion so as to surround the entire periphery of the third region 50, and can be set to any shape depending on the shape of the third region 50.

The first sound absorbing layer 10 has six second engagement recesses 14 that open toward the second sound absorbing layer 20 at predetermined intervals in a region (fourth region 51) surrounding the first skin layer 12 that is arranged opposite the second skin layer 22 of the second sound absorbing layer 20.

The second engagement recess 14 is formed at a position opposite to the second engagement protrusion 24 of the opposing second sound absorbing layer 20, and in this embodiment, is a circular recess in a plan view. In this embodiment, the outer diameter of the second engagement recess 14 is slightly smaller than the inner diameter of the second engagement protrusion 24 of the second sound absorbing layer 20.

When a ring-shaped second engagement protrusion is provided on the second sound absorbing layer 20, the second engagement recess can be set to any shape according to the shape of the second engagement protrusion, such as being a ring-shaped recess corresponding to the second engagement protrusion.

The air layer 30 is formed by integrating the first sound absorbing layer 10 and the second sound absorbing layer 20. Specifically, when the second engaging recess 14 of the first sound absorbing layer 10 engages with the second engaging protrusion 24 of the second sound absorbing layer 20, the thickness of the third region 50 of the second sound absorbing layer 20 is smaller than the thickness of the fourth region 51, so that a gap is formed between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20, as shown in FIG. 4, and this gap becomes the air layer 30.

In this embodiment, as described below, the hardness of the second sound absorbing layer 20 is greater than the hardness of the first sound absorbing layer 10, and the outer diameter of the second engaging protrusion 24 of the second sound absorbing layer 20 is slightly larger than the inner diameter of the second engaging recess 14 of the first sound absorbing layer 10. Therefore, when engaged, deformation of the second engaging protrusion 24 is suppressed, and the second engaging protrusion 24 is pressed into and engaged with the second engaging recess 14 of the first sound absorbing layer 10, resulting in an integrated state that is easy to handle.

In the second embodiment, the first sound absorbing layer 10 is provided with the second engaging recess 14, and the second sound absorbing layer 20 is provided with the second engaging protrusion 24. However, the first sound absorbing layer 10 may be provided with an engaging protrusion, and the second sound absorbing layer 20 may be provided with an engaging recess. In addition, the hardness of the second sound absorbing layer 20 may be smaller than the hardness of the first sound absorbing layer 10, as long as it does not interfere with handling.

Third Embodiment
In the third embodiment, the sound absorbing cover 1 according to the first embodiment is further configured to have a plurality of protrusions 15 on the first sound absorbing layer 10 or the second sound absorbing layer 20. The configuration of a sound absorbing cover 1b according to the third embodiment will be described with reference to Figures 5 and 6. Note that the same reference numerals as in the first embodiment are used for the same components as in the first embodiment.

The first sound absorbing layer 10 has a number of protrusions 15, 15... that protrude toward the second sound absorbing layer 20. The protrusions 15, 15... are integrally formed with the first skin layer 12. In this embodiment, as shown in FIG. 6, each cone-shaped protrusion 15 with a rounded tip is formed continuously.

The shape of each protrusion 15 is not important as long as it protrudes toward the second skin layer 22 of the opposing second sound absorbing layer 20, including an uneven shape, due to the nature of being integrally molded when the first foamed layer 11 is foamed. In addition, any interval may be provided between adjacent protrusions 15.

As in the first embodiment, the air layer 30 is formed by integrating the first sound absorbing layer 10 and the second sound absorbing layer 20. In this case, as shown in FIG. 6, the multiple protrusions 15, 15... of the first skin layer 12 are arranged in the gap formed between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20.

In this embodiment, the protrusion height of each protrusion 15 is slightly smaller than the length between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20, and each protrusion 15 is arranged with a small gap between the tip of the protrusion 15 and the second skin layer 22 of the second sound absorbing layer 20. With this configuration, the gaps between adjacent protrusions 15 and between each protrusion 15 and the second skin layer 22 form air layers 30.

By providing multiple protrusions 15, 15... on the first skin layer 12 of the first sound absorbing layer 10, sound passing through the first skin layer 12 is diffused and the sound absorbing area is increased, thereby improving the sound absorbing effect.

In this embodiment, the first sound absorbing layer 10 and the first skin layer 12 are provided with protrusions 15, but the second sound absorbing layer 20 and the second skin layer 22 may also be provided with protrusions, or both the first skin layer 12 and the second skin layer 22 may be provided with protrusions. In addition, the tips of the protrusions 15 of the first skin layer 12 may be arranged to contact the second skin layer 22 of the second sound absorbing layer 20.

(Fourth embodiment)
In the fourth embodiment, the sound absorbing cover 1 according to the first embodiment is configured without the first engaging recess in the first sound absorbing layer 10 and the first engaging protrusion in the second sound absorbing layer 20. The configuration of a sound absorbing cover 1c according to the fourth embodiment will be described with reference to Fig. 7. Note that the same reference numerals as in the first embodiment are used for the same components as in the first embodiment.

The first sound absorbing layer 10 is a plate-like member having a rectangular shape in a plan view. As shown in FIG. 7, the first sound absorbing layer 10 has a first recess 16 that opens from the inner edge of the second region 41 (the outer edge of the first region 40) toward the second sound absorbing layer 20.

In this embodiment, the first recess 16 is a recess that is rectangular in plan view. As shown in FIG. 7, the first recess 16 causes the top surface of the second region 41 of the first sound absorbing layer 10 to be positioned closer to the second sound absorbing layer 20 than the top surface of the first region 40 (the bottom surface of the first recess 16) in a cross-sectional view in the thickness direction. A first skin layer 12 is formed on the bottom surface of the first recess 16.

As shown in FIG. 7, the second sound absorbing layer 20 has a second recess 25 that opens from the outer edge of the second region 41, which is a plate-shaped portion, toward the first sound absorbing layer 10 in a cross-sectional view in the thickness direction of the second sound absorbing layer 20. In this embodiment, the second recess 25 is a recess that is rectangular in plan view and corresponds to the outer diameter shape of the first sound absorbing layer 10. The recess depth of the second recess 25 is approximately the same as the thickness of the first sound absorbing layer 10.

The second sound absorbing layer 20 has a second skin layer 22 formed in a region corresponding to the first region 40 on the bottom surface of the second recess 25. The second skin layer 22 may be formed on the entire bottom surface of the second recess 25, or may be provided at least on the surface facing the first skin layer 12 of the first sound absorbing layer 10.

The air layer 30 is formed by integrating the first sound absorbing layer 10 and the second sound absorbing layer 20. Specifically, when the first sound absorbing layer 10 is placed in the second recess 25 of the second sound absorbing layer 20, the thickness of the first region 40 of the first sound absorbing layer 10 is smaller than the thickness of the second region 41, so that as shown in FIG. 7, a gap is formed between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20, and this gap becomes the air layer 30.

In this embodiment, the outer diameter of the first sound absorbing layer 10 is slightly larger than the inner shape of the second recess 25 of the second sound absorbing layer 20, and the hardness of the second sound absorbing layer 20 is greater than the hardness of the first sound absorbing layer 10. Therefore, the first sound absorbing layer 10 is pressed into the second recess 25 of the second sound absorbing layer 20 and accommodated therein, so that the first sound absorbing layer 10 and the second sound absorbing layer 20 are integrated together in a manner that does not interfere with handling.

Other fixing methods may also be used, such as fixing the outer surface of the first sound absorbing layer 10 to the inner surface of the second recess 25 of the second sound absorbing layer 20 using an adhesive or double-sided tape, or welding the area excluding the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20. Also, as in the third embodiment, protrusions may be provided on the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20.

Fifth Embodiment
In the fifth embodiment, the sound absorbing cover 1a according to the second embodiment does not include the first engaging recess in the first sound absorbing layer 10 and the first engaging protrusion in the second sound absorbing layer 20. The structure of a sound absorbing cover 1d according to the fifth embodiment will be described with reference to Fig. 8. Note that the same reference numerals as in the second embodiment are used for the same components as in the second embodiment.

The first sound absorbing layer 10 is a plate-like member having a rectangular shape in a plan view. The surface of the first sound absorbing layer 10 facing the second sound absorbing layer 20 is a flat surface extending between the third region 50 and the fourth region 51. In this embodiment, the first skin layer 12 is formed on the surface facing the second sound absorbing layer 20 in the third region 50. The first skin layer 12 may be formed on the entire surface facing the second sound absorbing layer 20, including the fourth region.

As shown in FIG. 8, the second sound absorbing layer 20 has a second recess 25 that opens from the outer edge of the fourth region 51, which is a plate-shaped portion, toward the first sound absorbing layer 10 in a cross-sectional view in the thickness direction of the second sound absorbing layer 20. In this embodiment, the second recess 25 is a recess that is rectangular in plan view and corresponds to the outer diameter shape of the first sound absorbing layer 10. The recess depth of the second recess 25 is approximately the same as the thickness of the first sound absorbing layer 10.

A third recess 26 is formed on the bottom surface of the second recess 25, and opens from the inner edge of the fourth region 51 (the outer edge of the third region 50) toward the first sound absorbing layer 10. In this embodiment, the third recess 26 is a recess that is rectangular in plan view.

As shown in FIG. 8, in a cross-sectional view of the thickness direction of the second sound absorbing layer 20, the lower surface of the fourth region 51 is disposed closer to the first sound absorbing layer 10 than the lower surface of the third region 50 (the bottom surface of the third recess 26). A second skin layer 22 is formed on the bottom surface of the third recess 26 disposed in the third region 50.

The air layer 30 is formed by integrating the first sound absorbing layer 10 and the second sound absorbing layer 20. Specifically, when the first sound absorbing layer 10 is placed in the second recess 25 of the second sound absorbing layer 20, the thickness of the third region 50 of the second sound absorbing layer 20 is smaller than the thickness of the fourth region 51, so that a gap is formed between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20, as shown in FIG. 8, and this gap becomes the air layer 30.

In this embodiment, the outer diameter of the first sound absorbing layer 10 is slightly larger than the inner shape of the second recess 25 of the second sound absorbing layer 20, and the hardness of the second sound absorbing layer 20 is greater than the hardness of the first sound absorbing layer 10. Therefore, the first sound absorbing layer 10 is pressed into the second recess 25 of the second sound absorbing layer 20 and accommodated therein, so that the first sound absorbing layer 10 and the second sound absorbing layer 20 are integrated together in a manner that does not interfere with handling.

Other fixing methods may also be used, such as using an adhesive or double-sided tape to secure the outer surface of the first sound absorbing layer 10 to the inner surface of the second recess 25 of the second sound absorbing layer 20, or welding the area excluding the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20. Also, as in the third embodiment, protrusions may be provided on the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20.

Other Embodiments
In the above first to third embodiments, an engaging protrusion is provided on one of the first sound absorbing layer 10 or the second sound absorbing layer 20, and an engaging recess is provided on the other, and the first sound absorbing layer 10 and the second sound absorbing layer 20 are integrated by engaging them, but the engaging method is not limited to the above embodiments. For example, engaging recesses may be provided on both the first sound absorbing layer 10 and the second sound absorbing layer 20, and separate engaging members may be pressed into the engaging recesses via a spacer or the like that maintains a predetermined gap between the first sound absorbing layer 10 and the second sound absorbing layer 20 to engage them. Other known engaging methods may also be employed, such as thermal welding, or engagement using an adhesive, double-sided tape, clips, or the like.

<Configuration of sound absorbing layer>
(First sound absorbing layer)
The first sound absorbing layer 10 is made of urethane foam and has a first foamed layer 11 made of urethane foam and a first skin layer 12 integrally formed on the surface layer during foam molding. Note that the first sound absorbing layer 10 is not limited to urethane foam, and may be made of foamed resin such as silicone foam.

The first foamed layer 11 of the first sound absorbing layer 10 in the sound absorbing cover 1 according to the first embodiment of the present disclosure has a thickness of 5 mm, a density of 0.14 g/cm ³ , an airflow resistance of 378,000 Ns/m ⁴ , and an Asker C hardness of 22 degrees.

The airflow resistance was measured by cutting a sample with a diameter of 50 mm and a thickness of 4±2 mm from the first foamed layer 11 part of the first sound absorbing layer 10, excluding the first skin layer 12, and measuring it using an airflow resistance measuring device (model number: MFR-02) manufactured by Nippon Onkyo Engineering Co., Ltd. in accordance with the direct current method (DC method) specified in ISO 9053, and dividing the measured value by the thickness of the sample.

The Asker C hardness is a value measured in accordance with JIS K 7312 using a rubber hardness tester (model number: Asker rubber hardness tester type C) manufactured by Kobunshi Keiki Co., Ltd. on a 5 mm thick sample of the first sound absorbing layer 10 (including the first skin layer 12 on both sides).

The density was determined by cutting a sample 50 mm in diameter and 5 mm in thickness from the first sound absorbing layer 10 (including the first skin layer 12 on both sides), measuring the mass using a scale manufactured by Shimadzu Corporation (model number: TX3202N), and dividing the mass by the sample volume.

The first foamed layer 11 of the first sound absorbing layer 10 may be set to a thickness of 3 to 30 mm, a density of 0.06 to 0.2 g/cm ³ , an air resistance of 1000 to 1,000,000 Ns/m ⁴ , and an Asker C hardness of 5 to 40 degrees depending on the characteristics, shape, and placement space of the noise source.
Preferably, from the viewpoint of being able to effectively absorb low-frequency noise over a wider range, and from the viewpoint of achieving both space-saving and weight-saving of the soundproof cover, the thickness is set to within the range of 4 to 10 mm, the density to 0.10 to 0.16 g/cm ³ , and the airflow resistance to 100,000 to 1,000,000 Ns/m ^4. Furthermore, from the viewpoint of preventing sound leakage by closely contacting a noise source having an undulating surface, and reducing secondary radiation sound from the surface of the sound-absorbing cover due to vibration transmission from the noise source accompanied by vibration, it is preferable that the Asker C hardness is set to 10 to 30 degrees.

The first skin layer 12 of the first sound absorbing layer 10 in the sound absorbing cover 1 according to the first embodiment of the present disclosure has a thickness of 10 μm, an air flow resistance of 505,000 Ns/m ⁴ , and an Asker C hardness of 22 degrees.

The airflow resistance is calculated by cutting a sample with a diameter of 50 mm and a thickness of 2±1 mm from the surface side of the first sound absorbing layer 10 (including the first skin layers 12 on both sides), including one of the first skin layers 12, and measuring the airflow resistance from the side of one of the first skin layers 12 using an airflow resistance measuring device (model number: MFR-02) manufactured by Nippon Onkyo Engineering Co., Ltd. in accordance with the direct current method (DC method) specified in ISO 9053, and dividing the measured value by the thickness of the sample.

The Asker C hardness is a value measured in accordance with JIS K 7312 on a 5 mm thick sample of the first sound absorbing layer 10 (including the first skin layers 12 on both sides) from one of the first skin layers 12 using a rubber hardness tester (model number: Asker rubber hardness tester type C) manufactured by Kobunshi Keiki Co., Ltd.

The first skin layer 12 of the first sound absorbing layer 10 may be set to a thickness of 3 to 100 μm, an air resistance of 1000 to 10,000,000 Ns/m ⁴ , and an Asker C hardness of 5 to 40 degrees, depending on the frequency characteristics of the noise source.
Preferably, from the viewpoint of effectively absorbing noise in the low frequency band of 1000 Hz or less, the Asker C hardness is set in the range of 10 to 30 degrees and the airflow resistance is set in the range of 100,000 to 1,000,000 Ns/ ^m4 . The Asker C hardness can be used as an index of rigidity.

(Second sound absorbing layer)
The second sound absorbing layer 20 is made of urethane foam and has a second foamed layer 21 made of urethane foam and a second skin layer 22 integrally formed on the surface layer during foam molding. Note that the second foamed layer 21 is not limited to urethane foam, and may be made of a foamed resin such as silicone foam.

The second foamed layer 21 of the second sound absorbing layer 20 in the sound absorbing cover 1 according to the first embodiment of the present disclosure has a thickness of 5 mm, a density of 0.12 g/cm ³ , an airflow resistance of 153,000 Ns/m ⁴ , and an Asker C hardness of 76 degrees.

The airflow resistance is calculated by cutting a sample with a diameter of 50 mm and a thickness of 4±2 mm from the second foamed layer 21 of the second sound absorbing layer 20, excluding the second skin layer 22, and measuring it using an airflow resistance measuring device (model number: MFR-02) manufactured by Nippon Onkyo Engineering Co., Ltd. in accordance with the direct current method (DC method) specified in ISO 9053, and dividing the measured value by the thickness of the sample.

The Asker C hardness is a value measured in accordance with JIS K 7312 using a rubber hardness tester (model number: Asker rubber hardness tester type C) manufactured by Kobunshi Keiki Co., Ltd. on a 5 mm thick sample of the second sound absorbing layer 20 (including the second skin layer 22 on both sides).

The density was determined by cutting a sample 50 mm in diameter and 5 mm in thickness from the second sound absorbing layer 20 (including the second skin layers 22 on both sides), measuring the mass using a scale manufactured by Shimadzu Corporation (model number: TX3202N), and dividing the mass by the sample volume.

The second foam layer 21 of the second sound absorbing layer 20 may be set to a thickness of 3 to 30 mm, a density of 0.06 to 0.2 g/cm ³ , an air resistance of 1000 to 1,000,000 Ns/m ⁴ , and an Asker C hardness of 40 to 95 degrees depending on the characteristics, shape, and arrangement space of the noise source.
Preferably, from the viewpoint of being able to effectively absorb low-frequency noise over a wider range, and from the viewpoint of achieving both space saving and weight reduction of the soundproof cover, the thickness is set in the range of 4 to 10 mm, the density is set in the range of 0.10 to 0.16 g/cm ³ , and the airflow resistance is set in the range of 100,000 to 1,000,000 Ns/m ^4. Also, from the viewpoint of making it easier to fix the soundproof cover to an object, it is preferable that the Asker C hardness is set to 60 to 90 degrees.

The second skin layer 22 of the second sound absorbing layer 20 in the sound absorbing cover 1 according to the first embodiment of the present disclosure has a thickness of 10 μm and an air flow resistance of 584,000 Ns/m ⁴ .

The airflow resistance is calculated by cutting a sample with a diameter of 50 mm and a thickness of 2±1 mm from the surface side of the second sound absorbing layer 20 (including the second skin layers 22 on both sides), including one of the second skin layers 22, and measuring the airflow resistance from the second skin layer 22 side using an airflow resistance measuring device (model number: MFR-02) manufactured by Nippon Onkyo Engineering Co., Ltd. in accordance with the direct current method (DC method) specified in ISO 9053, and dividing the measured value by the thickness of the sample.

The Asker C hardness is a value measured in accordance with JIS K 7312 on a 5 mm thick sample of the second sound absorbing layer 20 (including the second skin layers 22 on both sides) from one of the second skin layers 22 using a rubber hardness meter (model number: Asker rubber hardness meter type C) manufactured by Kobunshi Keiki Co., Ltd.

The second skin layer 22 of the second sound absorbing layer 20 may be set to a thickness of 3 to 100 μm, an air resistance of 1000 to 1,000,000 Ns/m ⁴ , and an Asker C hardness of 40 to 95 degrees, depending on the frequency characteristics of the noise source.
Preferably, from the viewpoint of effectively absorbing noise in the low frequency band of 1000 Hz or less, the Asker C hardness is set in the range of 60 to 90 degrees and the airflow resistance is set in the range of 100,000 to 1,000,000 Ns/ ^m4 . The Asker C hardness can be used as an index of rigidity.

From the viewpoint of expanding the range of low-frequency absorption frequencies by the membrane vibration type sound absorbing structure described later, it is effective to make the characteristics of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20 different. In particular, it is preferable to set the hardness of the first skin layer 12 of the first sound absorbing layer 10 to 5 to 40 degrees and the hardness of the second skin layer 22 of the second sound absorbing layer 20 to 40 to 95 degrees according to the above-mentioned measurement method. It is more preferable to set the hardness of the first skin layer 12 of the first sound absorbing layer 10 to 10 to 30 degrees and the hardness of the second skin layer 22 of the second sound absorbing layer 20 to 60 to 90 degrees according to the above-mentioned measurement method. Depending on the required usage conditions, the first sound absorbing layer 10 can be a sound absorbing layer having the characteristics of the second sound absorbing layer 20 described above, and the second sound absorbing layer 20 can be a sound absorbing layer having the characteristics of the first sound absorbing layer 10 described above.

<Method of manufacturing sound absorbing cover>
To manufacture the sound absorbing cover 1 of the present disclosure, foam molding can be performed twice. First, a first foamed body (first sound absorbing layer 10) consisting of a first foamed layer 11 and a first skin layer 12 is formed. A release agent is applied to the inner surfaces of the upper and lower dies of the molding die. Next, the upper and lower dies are clamped together, and a urethane foam resin raw material is injected into the first cavity formed by the inner surfaces of the upper and lower dies to perform foam molding. At this time, the preferred temperature of the upper die where the first skin layer 12 is formed is 40 to 60°C. A wax-based release agent is preferably used, but the type of release agent is not important.

Next, a second foamed body (second sound absorbing layer 20) consisting of a second foamed layer 21 and a second skin layer 22 is formed. Similarly, a mold release agent is applied to the inner surfaces of the upper and lower dies of the molding die. Next, the upper and lower dies are clamped together, and a urethane foam resin raw material is injected into the first cavity formed by the inner surfaces of the upper and lower dies to perform foam molding. At this time, the suitable temperature of the upper die where the second skin layer 22 is formed is 40 to 60°C. A wax-based mold release agent is preferably used, but the type is not important.

When forming multiple protrusions 15 on the surface of the first skin layer 12 of the first sound-absorbing layer 10 or the second skin layer 22 of the second sound-absorbing layer 20, a mold with multiple recesses formed on its inner surface is used as the upper mold.

<Effects of sound absorbing covers>
The effect of the sound absorbing cover 1 of this embodiment will be described. In the sound absorbing cover 1 of the present disclosure, the first sound absorbing layer 10 and the second sound absorbing layer 20 are laminated with the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20 facing each other, and an air layer 30 is provided between the opposing surfaces of the first skin layer 12 of the first sound absorbing layer 10 and the second skin layer 22 of the second sound absorbing layer 20. Here, the first sound absorbing layer 10 and the second sound absorbing layer 20 are described as not having the first skin layer 12 and the second skin layer 22 on the non-opposing surfaces.

When the first sound absorbing layer 10 is placed on the noise source side, the sound generated from the noise source is first diffused inside the first foam layer 11 and converted into thermal energy. Next, the sound that penetrates the first foam layer 11 is released via the first skin layer 12. At this time, the first skin layer 12 and the air layer 30 form a first membrane vibration type sound absorbing structure. Since the first skin layer 12 closes multiple fine cells of the first foam layer 11 and the rigidity of each cell membrane formed by the first skin layer 12 is small, the natural frequency is set small and low frequency sounds are effectively absorbed.

Furthermore, sound transmitted through the air layer 30 is released via the second skin layer 22 of the second sound absorbing layer 20. The second skin layer 22 closes the multiple fine cells of the second foamed layer 21, and the cell membrane formed by the second skin layer 22 and the air within the cells form a second membrane vibration type sound absorbing structure. Similarly, since the rigidity of each cell membrane is small, the natural vibration is set small, and low frequency sounds are effectively absorbed. Furthermore, sound transmitted through the second skin layer 22 diffuses inside the second foamed layer 21 and is converted into thermal energy.

Since the first skin layer 12 and the second skin layer 22 have different rigidities, the natural frequency of the first membrane vibration type sound absorbing structure and the natural frequency of the second membrane vibration type sound absorbing structure are in different ranges, making it possible to absorb a wider range of low-frequency sounds.

In addition, by making the rigidity of the first skin layer 12 of the first sound absorbing layer 10 arranged on the noise source side smaller than the rigidity of the second skin layer 22 of the second sound absorbing layer 20 arranged on the non-noise source side, it is possible to absorb sounds with lower frequencies from the first skin layer 12 toward the second skin layer 22.

In particular, by setting the Asker C hardness of the first skin layer 12 of the first sound absorbing layer 10 to different values within the range of 5 to 40 degrees and the Asker C hardness of the second skin layer 22 of the second sound absorbing layer 20 to different values within the range of 40 to 95 degrees according to the above-mentioned measurement method, noise in the low frequency band of 630 to 1000 Hz can be effectively absorbed.

When the first sound absorbing layer 10 and the second sound absorbing layer 20 are provided with a first skin layer 12 and a second skin layer 22, respectively, on the non-opposing surfaces of the first sound absorbing layer 10 and the second sound absorbing layer 20, the membrane vibration type sound absorbing structure of the first skin layer 12 on the non-opposing surface side of the first sound absorbing layer 10 and the membrane vibration type sound absorbing structure of the second skin layer 22 on the non-opposing surface side of the second sound absorbing layer 20 provide the same low frequency sound absorption effect as described above.

<Sound absorbing cover evaluation results>
The effects of the present disclosure will be described with reference to examples.

Example 1
A polyether polyol for soft polyurethane (functional group number 3, molecular weight 6000), a crosslinking agent, water as a foaming agent, a catalyst, and a foam stabilizer were mixed to prepare 60 parts by weight of a polyol raw material, which was then mixed with 40 parts by weight of a polyisocyanate raw material to obtain a urethane foam resin raw material. Next, a wax-based mold release agent was applied to the inner surfaces of the upper and lower dies of the mold, and the upper and lower dies were clamped together. With the upper and lower dies held at 50°C, the urethane foam resin raw material was injected into the cavity of the mold to perform foam molding, and a first sound absorbing layer 10a having a first foam layer 11a (hereinafter, symbols a to d are given to distinguish between the examples) and a first skin layer 12a was formed. After foam molding was completed, the mold was removed to obtain a rectangular plate-shaped sample having a length of 500 mm, a width of 600 mm, and a thickness of 5 mm. After demolding, needle processing was performed to adjust the air flow resistance. The upper mold had a predetermined convex portion, and the first sound absorbing layer 10a had a predetermined engaging concave portion.

The first foamed layer 11a of this sample had a density of 0.14 g/cm ³ , an Asker C hardness of 22 degrees, and an airflow resistance of 378,000 Ns/m ^4. The airflow resistance of the surface layer side including the first skin layer 12a was 505,000 Ns/m ⁴ .

Next, a wax-based mold release agent was applied to the inner surfaces of the upper and lower dies of the mold, and the upper and lower dies were then clamped together. With the upper and lower dies held at 50°C, the urethane foam resin raw material was injected into the cavity of the mold to form a foamed layer 21a and a second skin layer 22a. The second foamed layer was prepared by mixing a polyether polyol for soft polyurethane (functionality 3, molecular weight 6000), a crosslinking agent, water as a foaming agent, a catalyst, and a foam stabilizer to prepare a polyol raw material, which was then mixed with 51 parts by weight of the polyol raw material and 49 parts by weight of the polyisocyanate raw material to form a urethane foam resin raw material. In order to increase the hardness of the second sound absorbing layer more than that of the first sound absorbing layer, a polyol raw material was used in which the polyether polyol (functionality 3, molecular weight 6000) used in the first sound absorbing layer was mixed with a low molecular weight polyether polyol (functionality 2, molecular weight 400) having a smaller molecular weight in a ratio of 70:30 parts by weight. After molding, the mold was removed to obtain a rectangular plate-shaped sample having a length of 500 mm, a width of 600 mm, and a thickness of 5 mm. After demolding, needle processing was performed to adjust the air resistance. In addition, a predetermined recess was provided in the lower mold, and a predetermined engagement protrusion was formed in the second sound absorbing layer 20a.

The hardness of the second sound absorbing layer can be adjusted by adjusting the mixing ratio of polyether polyol and low molecular weight polyether polyol. If mixing low molecular weight polyether polyol deteriorates the moldability of the urethane foam, this can be adjusted by adjusting the amount of crosslinking agent added.

The second foamed layer 21a of this sample had a density of 0.12 g/cm ³ , an Asker C hardness of 76 degrees, and an airflow resistance of 153,000 Ns/m ^4. The airflow resistance of the surface layer side including the second skin layer 22a was 584,000 Ns/m ⁴ .

Next, the engaging recess of the first sound absorbing layer 10a is engaged with the engaging protruding portion of the second sound absorbing layer 20a, and similarly to the first embodiment, the first sound absorbing layer 10a is placed below the second sound absorbing layer 20a, and a gap of 0.1 mm is formed between the opposing surfaces of the first skin layer 12a and the second skin layer 22a to form an air layer 30.

Example 2
The first sound absorbing layer 10b and the second sound absorbing layer 20b were molded in the same manner as in Example 1, except for the amount of the foamed urethane resin raw material added, and the first sound absorbing layer 10b was placed below the second sound absorbing layer 20b, and a gap of 0.1 mm was formed between the opposing surfaces of the first skin layer 12b and the second skin layer 22b to form an air layer 30. Specifically, the first sound absorbing layer 10b was molded using the same raw material as the foamed urethane resin raw material of the first sound absorbing layer 10a in Example 1, and when the amount of the foamed urethane resin raw material added in Example 1 was taken as the standard amount, 1.14 times the standard amount was added to mold the first sound absorbing layer 10b. Moreover, when the amount of the foamed urethane resin raw material added in the second sound absorbing layer 20a in Example 1 was taken as the standard amount, 0.83 times the standard amount was added to mold the second sound absorbing layer 20b.

The first foamed layer 11b of this sample had a density of 0.16 g/ ^cm3 , an Asker C hardness of 38 degrees, and an airflow resistance of 639,794 Ns/ ^m4 . The airflow resistance of the surface layer side including the first skin layer 12b was 130,850 Ns/ ^m4 . The second foamed layer 21b had a density of 0.1 g/ ^cm3 , an Asker C hardness of 59 degrees, and an airflow resistance of 103,000 Ns/ ^m4 . The airflow resistance of the surface layer side including the second skin layer 22b was 379,148 Ns/ ^m4 .

Example 3
The first sound absorbing layer 10c and the second sound absorbing layer 20c were molded in the same manner as in Example 1, except for the amount and blending ratio of the urethane foam resin raw material, and the first sound absorbing layer 10c was placed below the second sound absorbing layer 20c, and a gap of 0.1 mm was formed between the opposing surfaces of the first skin layer 12c and the second skin layer 22c to form an air layer 30. Specifically, the first sound absorbing layer 10c was molded using the same raw material as the urethane foam resin raw material of the first sound absorbing layer 10a in Example 1, and when the amount of the urethane foam resin raw material of the first sound absorbing layer 10a in Example 1 was taken as the standard amount, 0.85 times the standard amount was added to mold the first sound absorbing layer 10c. In addition, the second sound absorbing layer 20c was molded using 54 parts by weight of polyol raw material and 46 parts by weight of polyisocyanate raw material.

The first foamed layer 11c of this sample had a density of 0.12 g/ ^cm3 , an Asker C hardness of 23 degrees, and an airflow resistance of 201098 Ns/ ^m4 . The airflow resistance of the surface layer side including the first skin layer 12c was 438441 Ns/ ^m4 . The second foamed layer 21c had a density of 0.12 g/ ^cm3 , an Asker C hardness of 63 degrees, and an airflow resistance of 148029 Ns/ ^m4 . The airflow resistance of the surface layer side including the second skin layer 22c was 713111 Ns/ ^m4 .

Example 4
A first sound absorbing layer 10d and a second sound absorbing layer 20d were molded in the same manner as in Example 1, except for the compounding ratio of the polyol raw material and the polyisocyanate raw material and the post-processing method, and the first sound absorbing layer 10d was disposed below the second sound absorbing layer 20d, and a gap of 0.1 mm was formed between the opposing surfaces of the first skin layer 12d and the second skin layer 22d to form an air layer 30. Specifically, the first sound absorbing layer 10d was molded using 63 parts by weight of the polyol raw material and 37 parts by weight of the polyisocyanate raw material. In addition, the second sound absorbing layer 20d was molded using the same raw material as the urethane foam resin raw material of the second sound absorbing layer 20a in Example 1, and then needle processing was performed to adjust the amount of ventilation.

The first foamed layer 11d of this sample had a density of 0.14 g/ ^cm3 , an Asker C hardness of 21 degrees, and an airflow resistance of 281,978 Ns/ ^m4 . The airflow resistance of the surface layer side including the first skin layer 12d was 456,171 Ns/ ^m4 . The second foamed layer 21d had a density of 0.12 g/ ^cm3 , an Asker C hardness of 67 degrees, and an airflow resistance of 715,612 Ns/ ^m4 . The airflow resistance of the surface layer side including the second skin layer 22d was 504,229 Ns/ ^m4 .

(Comparative Example 1)
The first sound absorbing layer 10a and the second sound absorbing layer 20a were formed in the same manner as in Example 1, and the first skin layer 12a and the second skin layer 22a were bonded to each other by applying an adhesive to the entire surfaces thereof, i.e., no air layer was formed.

(Evaluation Method)
The sound absorption coefficient of the samples of Examples 1 to 4 and Comparative Example 1 was measured by a method conforming to the reverberation room sound absorption coefficient measurement method of JIS A 1409. In addition, the surface opposite to the sound source was placed on a hard floor for measurement.
In Example 1-4, the first sound absorbing layer 10 was disposed on the sound source side, while in Comparative Example 1, the first sound absorbing layer 10 was disposed on the sound source side.

As shown in Fig. 9, it was confirmed that Example 1-4 had a higher sound absorption coefficient in the low frequency band of 800 to 1000 Hz compared to Comparative Example 1. It is presumed that the above-mentioned two membrane vibration type sound absorbing structures were able to absorb a wider range of low frequency noise.
On the other hand, in Comparative Example 1, the first skin layer 12a and the second skin layer 22a are integrated to form a single skin layer with a relatively high rigidity, which is presumably why the frequency band on the low frequency side of the membrane vibration type sound absorbing structure is narrowed. Also, the adhesive clogs the cell membrane, increasing the air flow resistance of the first skin layer 12a and the second skin layer 22a and reducing the air permeability, making it difficult for noise to reach the second foamed layer 21a of the second sound absorbing layer 20a on the side away from the sound source (the rigid floor side), and the proportion of high frequency components absorbed by the porous type sound absorbing decreases, which is thought to result in a decrease in the high frequency sound absorption coefficient.
The sound absorption coefficient in the low frequency band of Example 2 was lower than that of Example 1. The reason for this is thought to be that, compared to the other Examples, Example 2 had a higher density, hardness, and air resistance of the first foamed layer of the first sound absorbing layer, so that the sound energy reaching the interface between the first skin layer and the second skin layer was reduced, and the sound absorbing effect of the membrane vibration type sound absorbing structure was not fully exhibited.

Furthermore, in Example 1-4, the first foam layer 11 of the first sound absorbing layer 10 and the second foam layer 21 of the second sound absorbing layer 20 exhibit noise absorbing performance at mid- and high-frequencies above 1000 Hz, and it was confirmed that the sound absorption rate is higher in a wide frequency band from 630 to 5000 Hz than in Comparative Example 1.

(Verification of sound absorption performance by through holes)
In the sample of Example 1, through holes were formed in the first sound absorbing layer 10 on the sound source side and the second sound absorbing layer 20 on the non-sound source side. The through holes were formed using a laser processing machine, with a hole diameter of 1 mm or 2 mm from a position 5 mm from the end of the sample, and a hole pitch of 3 mm or 10 mm from the end of the hole. As shown in Table 1, the through holes were set so that one or both of the hole diameter and hole pitch were different between the sound source side and the non-sound source side, so that the first sound absorbing layer 10 had a higher light transmittance and opening rate than the second sound absorbing layer 20, to obtain Comparative Example 2-5. The light transmittance was measured using a method conforming to the light blocking test of JIS L 1055.
Due to this setting (the light transmittance and opening rate of the first skin layer 12 are greater than the light transmittance and opening rate of the second skin layer 22), in comparison example 2-5, the rigidity of the first skin layer 12 of the first sound absorbing layer 10 is smaller than the rigidity of the second skin layer 22 of the second sound absorbing layer 20.

For Comparative Examples 2-5, the sound absorption coefficient was measured using a method conforming to the reverberation room sound absorption coefficient measurement method of JIS A 1409. The surface opposite to the sound source was placed on a hard floor for measurement. The change in sound absorption coefficient for each frequency for each Comparative Example is shown in Figure 10.
From the results shown in FIG. 10, it was confirmed that Comparative Example 2-5, in which through holes are formed and a structure is formed in the stacking direction, has a lower sound absorption coefficient in the low frequency band of 800 to 1000 Hz than Comparative Example 1, in which no through holes are formed and a structure is formed in the stacking direction that is not connected.
However, if the first sound-absorbing layer has a non-interconnected structure in which the light transmittance in the stacking direction is less than 4.68%, which is the minimum value in Comparative Example 2-5, and if the second sound-absorbing layer has a non-interconnected structure in which the light transmittance in the stacking direction is less than 0.36%, which is the minimum value in Comparative Example 2-5, then it is considered that the sound absorption coefficient in the low frequency band of 800 to 1000 Hz will be higher than that of Comparative Example 2-5.

Thus, the sound absorbing cover 1 of the above embodiment has a first foamed layer 11, a first skin layer 12 integrally molded when the first foamed layer 11 is molded, and a first sound absorbing layer 10 disposed on the noise source side. The sound absorbing cover 1 also has a second foamed layer 21, a second skin layer 22 integrally molded when the second foamed layer 21 is molded, and a second sound absorbing layer 20 laminated on the first sound absorbing layer 10 with the second skin layer 22 facing the first skin layer 12. The sound absorbing cover 1 further includes an air layer 30 formed between the opposing surfaces of the first skin layer 12 and the second skin layer 22. The first sound absorbing layer 10 and the second sound absorbing layer 20 have a non-communicating structure that does not have a communicating hole communicating in the stacking direction, and the rigidity of the first skin layer 12 is smaller than the rigidity of the second skin layer 22.
This configuration makes it possible to more effectively absorb noise on the low frequency side, particularly in the 800 to 1000 Hz band.

1, 1a, 1b, 1c, 1d: sound absorbing cover, 10: first sound absorbing layer, 11: first foam layer, 12: first skin layer, 13: first engagement recess, 14: second engagement recess, 15: protrusion, 16: first recess, 20: second sound absorbing layer, 21: second foam layer, 22: second skin layer, 23: first engagement protrusion, 24: second engagement protrusion, 25: second recess, 26: third recess, 30: air layer, 40: first region, 41: second region, 50: third region, 51: fourth region

Claims (12)

A sound-absorbing cover that covers a target part that is a noise source,
a first sound absorbing layer having a first foamed layer and a first skin layer integrally formed at the time of molding the first foamed layer, the first sound absorbing layer being disposed on the noise source side;
A second sound absorbing layer having a second foamed layer and a second skin layer integrally molded during molding of the second foamed layer, the second skin layer being laminated on the first sound absorbing layer with the second skin layer facing the first skin layer;
an air layer formed between the opposing surfaces of the first skin layer and the second skin layer;
the first sound absorbing layer and the second sound absorbing layer have a non-communicating structure having no communicating holes communicating in the stacking direction,
The sound absorbing cover, wherein the stiffness of the first skin layer is less than the stiffness of the second skin layer. The sound absorbing cover according to claim 1, wherein the first sound absorbing layer has a light transmittance in the stacking direction of less than 4.68%, and the second sound absorbing layer has a light transmittance in the stacking direction of less than 0.36%. The hardness of the first skin layer of the first sound absorbing layer is 5 to 40 degrees on the Asker C scale,
The sound absorbing cover according to claim 1 or 2, wherein the second skin layer of the second sound absorbing layer has an Asker C hardness of 40 to 95 degrees. The hardness of the first skin layer of the first sound absorbing layer is 10 to 30 degrees on the Asker C hardness scale;
The sound absorbing cover according to claim 1 or 2, wherein the second skin layer of the second sound absorbing layer has an Asker C hardness of 60 to 90 degrees. The density of the first foamed layer of the first sound absorbing layer is 0.06 to 0.2 g/cm ³ ;
5. The sound absorbing cover according to claim 1, wherein the second foamed layer of the second sound absorbing layer has a density of 0.06 to 0.2 g/ ^cm3 . The density of the first foamed layer of the first sound absorbing layer is 0.10 to 0.16 g/cm ³ ;
The sound absorbing cover according to any one of claims 1 to 4, wherein the second foamed layer of the second sound absorbing layer has a density of 0.10 to 0.16 g/ ^cm3 . The air flow resistance of the first sound absorbing layer is 100,000 to 1,000,000 Ns / ^m4 ,
The sound absorbing cover according to any one of claims 1 to 6, wherein the second sound absorbing layer has an air flow resistance of 100,000 to 1,000,000 Ns / ^m4 . the first sound absorbing layer includes a first region in which the first skin layer is formed, a second region surrounding the first region and disposed on the second sound absorbing layer side relative to the first region, and a plurality of first engagement recesses which are recesses in the second region that open to the second sound absorbing layer side and are disposed at predetermined intervals in the circumferential direction of the second region,
the second sound absorbing layer includes a plurality of first engaging protrusions that are engaged with the plurality of first engaging recesses of the first sound absorbing layer,
8. The sound-absorbing cover according to claim 1, wherein the air layer is a gap formed between opposing surfaces of the first skin layer of the first sound-absorbing layer and the second skin layer of the second sound-absorbing layer when the first engaging recesses of the first sound-absorbing layer and the first engaging protrusions of the second sound-absorbing layer are respectively engaged with each other. the second sound absorbing layer includes a third region in which the second skin layer is formed, a fourth region surrounding the third region and disposed on the first sound absorbing layer side relative to the third region, and a plurality of second engaging protrusions which are protrusions protruding toward the first sound absorbing layer side in the fourth region and are disposed at predetermined intervals in the circumferential direction surrounding the third region;
the first sound absorbing layer includes a plurality of second engaging recesses that engage with the plurality of second engaging protrusions of the second sound absorbing layer,
8. The sound-absorbing cover according to claim 1, wherein the air layer is a gap formed between opposing surfaces of the first skin layer of the first sound-absorbing layer and the second skin layer of the second sound-absorbing layer when the multiple second engagement recesses of the first sound-absorbing layer and the multiple second engagement protrusions of the second sound-absorbing layer are engaged with each other. the first sound absorbing layer has a first recess that opens to the second sound absorbing layer side,
The first skin layer is formed on a bottom surface of the first recess,
the second sound absorbing layer has a second recess that opens to the first sound absorbing layer side,
The second skin layer is formed on a bottom surface of the second recess,
the first sound absorbing layer has an outer diameter shape corresponding to an inner shape of the second recess of the second sound absorbing layer,
8. The sound-absorbing cover according to claim 1, wherein the air layer is a gap formed between opposing surfaces of the first skin layer of the first sound-absorbing layer and the second skin layer of the second sound-absorbing layer when the first sound-absorbing layer is accommodated in the second recess of the second sound-absorbing layer. the second sound absorbing layer includes a second recess that opens toward the first sound absorbing layer and a third recess that opens toward the first sound absorbing layer at a bottom surface of the second recess, the second skin layer being formed on the bottom surface of the third recess;
the first sound absorbing layer has an outer diameter shape corresponding to an inner shape of the second recess of the second sound absorbing layer,
8. The sound-absorbing cover according to claim 1, wherein the air layer is a gap formed between opposing surfaces of the first skin layer of the first sound-absorbing layer and the second skin layer of the second sound-absorbing layer when the first sound-absorbing layer is accommodated in the second recess of the second sound-absorbing layer. A sound absorbing cover according to any one of claims 1 to 11, wherein at least one of the first skin layer of the first sound absorbing layer and the second skin layer of the second sound absorbing layer has a plurality of protrusions protruding toward the other side that is disposed opposite to the first skin layer.

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