Sound absorption structure

Abstract

To make a sound absorption structure compact or thin without varying a frequency as an object of sound absorption nor lowering a sound absorption coefficient.SOLUTION: A sound absorption structure 1 comprises a housing 10 which has a cavity CV inside and a cylindrical member 20 which has a flow passage F formed to connect the cavity CV to the outside of the housing 10, and the cylindrical member 20 has a reduction part R, reducing the sectional area of the flow passage F toward the center axis of the cylindrical member 20, formed at an end on the side connected to the cavity CV.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sound absorbing structure. [Background technology]

[0002] A Helmholtz resonator is known as a sound-absorbing structure. In general, a Helmholtz resonator is composed of a neck and a cavity, and the frequency to be absorbed varies depending on the cross-sectional area of ​​the neck, the length of the neck, and the volume of the cavity. Patent Documents 1 and 2 disclose techniques related to sound absorption using a Helmholtz resonator. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5446018 [Patent Document 2] Japanese Patent Application Publication No. 7-18759 Summary of the Invention [Problem to be solved by the invention]

[0004] When making a Helmholtz resonator smaller or thinner, it is possible to shorten the length of the neck. However, shortening only the length of the neck results in a change in the frequency of the sound absorption target. On the other hand, when making a Helmholtz resonator smaller or thinner without changing the frequency of the sound absorption target, the sound absorption coefficient decreases. For this reason, there has been a demand for technology that can realize the miniaturization or thinning of sound-absorbing structures without changing the frequency of the sound absorption target and without decreasing the sound absorption coefficient.

[0005] The present invention has been made to solve at least some of the above-mentioned problems, and aims to provide a technology that can achieve a reduction in the size or thickness of a sound-absorbing structure without changing the frequency to be absorbed and without reducing the sound absorption coefficient. [Means for solving the problem]

[0006] The present invention has been made to solve at least part of the above-mentioned problems, and can be realized in the following aspects.

[0007] (1) According to one aspect of the present invention, there is provided a sound-absorbing structure comprising: a housing having an internal cavity; and a tubular member having an internal flow path connecting the cavity to the outside of the housing, wherein the end of the tubular member connected to the cavity is formed with a reducing portion that reduces the cross-sectional area of ​​the flow path toward the central axis of the tubular member.

[0008] With this configuration, even if the length of the tubular member is shorter than that of a sound absorbing structure designed to absorb sound of a certain frequency and that does not have a reduced section formed inside the tubular member, the reduced section allows the sound absorbing structure to absorb the same frequency as the target frequency and also improves the sound absorption coefficient at that frequency. Therefore, it is possible to make the sound absorbing structure smaller or thinner without changing the target frequency or decreasing the sound absorption coefficient.

[0009] (2) In the sound-absorbing structure of the above form, in the longitudinal cross section of the tubular member, the distance from an extension line passing over a portion of the inner surface of the tubular member where no reduction portion is formed, toward the side connecting to the cavity, to the tip of the reduction portion may be smaller than the value obtained by multiplying the distance from that portion to the central axis of the tubular member by 0.8. According to this configuration, the effect of improving the sound absorption coefficient of the reduced portion can be properly exhibited.

[0010] (3) In the sound-absorbing structure of the above form, in the longitudinal section of the tubular member, the cross-sectional shape of the reduction portion is an isosceles triangle, and when the part where the extension line and the reduction portion overlap is the base of the reduction portion, the length of the base may be less than or equal to five times the distance from the extension line to the tip of the reduction portion. According to this configuration, the reduced portion having an isosceles triangular cross section can fully exhibit the effect of improving the sound absorption coefficient.

[0011] (4) In the sound-absorbing structure of the above form, in the longitudinal section of the tubular member, the cross-sectional shape of the reduction portion is a right-angled triangle, and when the part where the extension line and the reduction portion overlap is the base of the reduction portion, the length of the base may be less than or equal to 2.5 times the distance from the extension line to the tip of the reduction portion. According to this configuration, the reduced portion having a right-angled triangular cross section can fully exhibit the effect of improving the sound absorption coefficient.

[0012] (5) In the sound-absorbing structure of the above form, in the longitudinal section of the tubular member, the cross-sectional shape of the reduction portion is rectangular, and when the part where the reduction portion overlaps with an extension line of a line passing over the part of the inner surface of the tubular member where the reduction portion is not formed, extended toward the side connected to the cavity, is defined as the bottom edge of the reduction portion, and the edge of the reduction portion closer to the central axis is defined as the top edge, the length of the top edge may be 2.0 mm or less, and the length of the top edge may be less than the length of the bottom edge. According to this configuration, the reduced portion having a rectangular cross section can exhibit the effect of improving the sound absorption coefficient.

[0013] The present invention can be realized in various forms, for example, in the form of a sound-absorbing device, a sound-absorbing panel, a sound-absorbing material, a structure including these, or a manufacturing method for manufacturing these. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic perspective view illustrating the configuration of a sound absorbing structure according to a first embodiment. FIG. [Figure 2] FIG. 2 is an explanatory diagram showing a cross section of an opening. [Figure 3] 10 is an explanatory diagram showing an enlarged longitudinal cross section of the end portion of the cylindrical member that is connected to the cavity. FIG. [Figure 4]FIG. 10 is a diagram illustrating the change in frequency and sound absorption coefficient when the distance is changed. [Figure 5] FIG. 10 is a diagram illustrating the change in frequency and sound absorption coefficient when the length is changed. [Figure 6] 1 is an explanatory diagram comparing the sound absorbing structure of the present embodiment with a sound absorbing structure of a comparative example. [Figure 7] FIG. 10 is a schematic perspective view of a sound absorbing structure of a comparative example. [Figure 8] FIG. 10 is a schematic perspective view of a sound absorbing structure of a comparative example. [Figure 9] 1 is an explanatory diagram comparing the sound absorbing structure of the present embodiment with a sound absorbing structure of a comparative example. [Figure 10] FIG. 10 is an explanatory diagram showing an enlarged view of a part of a vertical cross section of a sound absorbing structure according to a second embodiment. [Figure 11] FIG. 10 is a diagram illustrating the change in frequency and sound absorption coefficient when the length is changed. [Figure 12] 10A and 10B are explanatory diagrams of modified examples regarding the arrangement of the cylindrical members. [Figure 13] FIG. 2 is a simplified explanatory diagram of a sound absorbing structure. [Figure 14] 10 is an explanatory diagram illustrating an example of a cross-sectional shape of a reduction portion having a substantially right-angled triangular shape. FIG. [Figure 15] 10 is an explanatory diagram illustrating an example of a substantially rectangular cross-sectional shape of a reduction portion. FIG. [Figure 16] FIG. 10 is an explanatory diagram comparing the presence and absence of a reduction portion in a sound absorbing structure. [Figure 17] FIG. 10 is a schematic perspective view of a sound absorbing structure of a comparative example. [Figure 18] FIG. 1 is a schematic perspective view of a sound absorbing structure having a reduced portion formed therein. [Figure 19] 10 is an explanatory diagram showing an enlarged vertical cross section of an end portion of a cylindrical member that is connected to the other end portion; FIG. [Figure 20] FIG. 1 is a schematic perspective view of a sound absorbing structure having a reduced portion formed therein. [Figure 21] FIG. 1 is a schematic perspective view of a sound absorbing structure having a reduced portion formed therein. [Figure 22] FIG. 4 is an explanatory diagram showing an enlarged vertical cross section of an end portion of a cylindrical member. DETAILED DESCRIPTION OF THE INVENTION

[0015] First Embodiment FIG. 1 is a schematic perspective view illustrating the configuration of a sound-absorbing structure 1 according to a first embodiment of the present invention. FIG. 1(a) shows the external appearance of the sound-absorbing structure 1, and FIG. 1(b) shows a portion of a cross section of the sound-absorbing structure 1. FIG. 1 illustrates X, Y, and Z axes that are perpendicular to each other. The Z axis corresponds to the extension direction of a tubular member 20 (described later) and is also the height direction of the sound-absorbing structure 1. The X and Y axes are directions perpendicular to the Z axis. The sound-absorbing structure 1 is a so-called Helmholtz resonator. The sound-absorbing structure 1 includes a housing 10 and a tubular member 20.

[0016] The housing 10 is a cylindrical member having a cavity CV therein. The bottom surface of the housing 10 on the +Z-axis direction side and the bottom surface on the -Z-axis direction side are both circular surfaces extending along the XY plane. The tubular member 20 is connected to the bottom surface of the housing 10 on the +Z-axis direction side, and is a cylindrical member that connects the cavity CV with the outside of the housing 10. A flow path F that connects the cavity CV with the outside of the housing 10 is formed inside the tubular member 20. In this embodiment, the entire tubular member 20 is disposed inside the housing 10.

[0017] The central axis OL is an axis along the Z-axis direction and is a virtual axis connecting the centers of the cross section (cross section when cut on the XY plane) of the cylindrical member 20. In addition, the central axis OL also coincides with the central axis of the casing 10 and the central axis of the flow path F, and therefore can also be considered as the central axis of the casing 10 and the central axis of the flow path F.

[0018] The flow path F has an opening H1 at its end on the side that connects to the outside of the housing 10 (the +Z-axis direction side in FIG. 1(b)). The opening H1 corresponds to the boundary between the flow path F and the outside of the housing 10. In this embodiment, the opening H1 is located at the same position in the Z-axis direction as the top surface 10f of the housing 10. On the other hand, the flow path F has an opening H2 at its end on the side that connects to the cavity CV (the -Z-axis direction side in FIG. 1(b)). The opening H2 corresponds to the boundary between the flow path F and the cavity CV. The length L is the length between the opening H1 and the opening H2, and can also be said to be the length L of the tubular member 20.

[0019] FIG. 2(a) shows a cross-section of the opening H1. FIG. 2(b) shows a cross-section of the opening H2. In FIGS. 2(a) and 2(b), the hatched areas indicate the cross-sectional area of ​​the flow path F at the opening H1 and the cross-sectional area of ​​the flow path F at the opening H2, respectively. Here, the cross-sectional area of ​​the flow path F refers to the cross-sectional area of ​​the flow path F in the cross-section. As shown in FIGS. 1 and 2, a constricted portion R is formed around the periphery of the opening H2. In other words, the constricted portion R is formed at the end of the cylindrical member 20 that connects to the cavity CV. The constricted portion R reduces the cross-sectional area of ​​the flow path F toward the central axis OL of the cylindrical member 20. That is, due to the formation of the constricted portion R, the shape of the opening H2 is a similar circle to the opening H1, which has a circular shape centered on the central axis OL, reduced toward the central axis OL. Due to the presence of such a constricted portion R, the cross-sectional area of ​​the flow path F at the opening H2 is smaller than the cross-sectional area of ​​the flow path F at the opening H1, as indicated by the hatching. In detail, the cross-sectional area of ​​the flow path F at a position in the Z-axis direction where the contraction portion R is not formed is approximately constant, and the cross-sectional area of ​​the flow path F at a position in the Z-axis direction where the contraction portion R is formed is smaller than the cross-sectional area at that position. Here, "approximately constant" means that the cross-sectional area is roughly constant while allowing for variations due to manufacturing errors, etc.

[0020] FIG. 3 is an explanatory diagram showing an enlarged longitudinal cross section of the end of the cylindrical member 20 connected to the cavity CV. The longitudinal cross section here refers to a cross section cut along a plane including the central axis OL, and FIG. 3 shows a cross section cut along the XZ plane. As shown in FIG. 3 , in the longitudinal cross section of the cylindrical member 20, the cross-sectional shape of the reduced portion R is triangular. Here, a line LN is a line passing through a portion IN on the inner surface of the cylindrical member 20 where the reduced portion R is not formed, and an extension line EL is an extension of the line LN toward the side connected to the cavity CV. This extension line EL defines the base end of the reduced portion R. In this embodiment, the distance h from the extension line EL to the tip T of the reduced portion R is smaller than the distance a from the portion IN to the central axis OL multiplied by 0.8. The distance h is the shortest distance from the extension line EL to the tip T of the reduced portion R and can also be referred to as the height of the reduced portion R. The distance a can also be referred to as the radius of the portion IN of the flow path F. In this embodiment, the distance h is 1.3 mm and the distance a is 2.0 mm.

[0021] In this embodiment, the cross-sectional shape of the reduced portion R in the longitudinal section of the tubular member 20 is an isosceles triangle. Here, when the overlapping portion of the extension line EL and the reduced portion R is defined as the base B of the reduced portion R, the length w of the base B is equal to or less than five times the distance h. The term "isosceles triangle" in the cross-sectional shape of the reduced portion R includes not only a mathematically strict isosceles triangle but also an approximately isosceles triangle. The approximately isosceles triangle shape includes an isosceles triangle with a rounded tip T, an approximately trapezoidal shape with a missing tip T, and a shape with a rounded or C-chamfered tip of such an approximately trapezoidal shape. In other words, the isosceles triangle shape in the cross-sectional shape of the reduced portion R includes a shape that is roughly an isosceles triangle while allowing for deformation due to manufacturing errors, etc.

[0022] FIG. 4 is an explanatory diagram showing the change in frequency and sound absorption coefficient when the distance h is changed in the sound absorbing structure 1. The horizontal axis of FIG. 4 indicates the frequency at which the sound absorbing structure 1 absorbs sound when the distance h is changed. The vertical axis of FIG. 4 indicates the sound absorption coefficient of the sound absorbing structure 1 when the distance h is changed. FIG. 4 shows line segments that indicate the results when the distance h is changed from 0.0 mm to 1.8 mm for a sound absorbing structure 1 having a length L of 24 mm and a distance a (the radius of the portion IN of the flow path F) of 2.0 mm. Of the multiple line segments shown in FIG. 4, the rightmost line segment h 0.0 indicates the result when the distance h is 0.0 mm. That is, the line segment h 0.0 is a line segment that shows the result of the sound absorbing structure 1 that does not have the reduced portion R formed therein. Among the multiple line segments shown in FIG. 4, the line segment h 1.2 , line segment h 1.3 , line segment h 1.4 , line segment h 1.6 and line segment h 1.8 The results are shown for the cases where the distance h is 1.2 mm, 1.3 mm, 1.4 mm, 1.6 mm, and 1.8 mm, respectively. 0.0 From line segment h 1.2 Each line segment between indicates the results when the distance h is varied between 0.0 mm and 1.2 mm. Note that for the sound absorption coefficient shown on the vertical axis, when the distance h is 1.2 mm (line segment h 1.2 When the sound absorption coefficient is 1.0, the frequency at which sound is most absorbed (the frequency at the peak of the line segment) is 1.0.

[0023] Line segment h 0.0 From line segment h 1.8 As shown by the lines between h and h, it was confirmed that the frequency at which sound is most absorbed becomes smaller as the distance h increases. 0.0 From line segment h 1.2 As shown by the lines between h and h, it was confirmed that the longer the distance h is within the range of 1.2 mm or less, the higher the sound absorption coefficient at the most absorbed frequency becomes. 1.2 From line segment h 1.8As shown by the lines between 1.2 mm and 1.8 mm, it was confirmed that the longer the distance h, the lower the sound absorption coefficient at the most absorbed frequency. 1.4 The sound absorption coefficient of the most absorbed frequency is the line segment h 0.0 The absorption coefficient was still higher than that of the most absorbed frequency at line h 1.6 and line segment h 1.8 The sound absorption coefficient of the most absorbed frequency is the line segment h 0.0 Therefore, from the results shown in Figure 4, it was confirmed that when the distance a (the radius of the portion IN of the flow path F) is 2.0 mm and the distance h is 1.2 mm, the sound absorption coefficient of the frequency at which sound is most absorbed becomes maximum, and that it is preferable that the distance h be smaller than the value obtained by multiplying the distance a by 0.8.

[0024] FIG. 5 is an explanatory diagram showing the change in frequency and sound absorption coefficient when the length L (the length between the openings H1 and H2, see FIG. 1(b)) of the sound absorbing structure 1 is changed. The horizontal axis of FIG. 5 indicates the frequency at which the sound absorbing structure 1 absorbs sound when the length L is changed. The vertical axis of FIG. 5 indicates the sound absorption coefficient of the sound absorbing structure 1 when the length L is changed. FIG. 5 shows line segments that indicate the results when the length L is changed from 6 mm to 24 mm for a sound absorbing structure 1 in which the distance a is 2.0 mm and the distance h is 1.3 mm. Of the multiple line segments shown in FIG. 5, the leftmost line segment L 24 shows the results when the length L is 24 mm. 18 , line segment L 10 , line segment L8, and line segment L6 show the results when the length L is 18 mm, 10 mm, 8 mm, and 6 mm, respectively. 24 The lines from to line L6 show the results when the length L is varied between 24 mm and 6 mm. Note that, in the sound absorption coefficient shown on the vertical axis, when the length L is 24 mm (line L 24When the sound absorption coefficient is 1.0, the frequency at which sound is most absorbed (the frequency at the peak of the line segment) is 1.0.

[0025] Line segment L 24 As shown by the lines between L1 and L6, it was confirmed that the frequency at which sound is most absorbed increases as the length L is shortened. It was also confirmed that even when the length L is shortened, the sound absorption coefficient at the frequency at which sound is most absorbed remains at approximately 1.0.

[0026] Fig. 6 is an explanatory diagram comparing the sound absorbing structure 1 of this embodiment with the sound absorbing structures 1a and 1b of Comparative Examples 1 and 2. In the sound absorbing structure 1 used for comparison with the sound absorbing structures 1a and 1b of Comparative Examples 1 and 2, the length L is 18 mm, the distance h is 1.3 mm, and the distance a is 2.0 mm. Before explaining Fig. 6, the sound absorbing structures 1a and 1b of Comparative Examples 1 and 2 will be explained.

[0027] 7 is a schematic perspective view of a sound absorbing structure 1a of Comparative Example 1. The sound absorbing structure 1a of Comparative Example 1 is the same as the sound absorbing structure 1 of the present embodiment, except that it includes a tubular member 20a that is different from the tubular member 20 of the sound absorbing structure 1 of the present embodiment.

[0028] The cylindrical member 20a differs from the cylindrical member 20 in that a reduced portion R is not formed at the end of the channel F that is connected to the cavity CV. That is, a reduced portion R is not formed around the opening Ha2 at the end of the channel F that is connected to the cavity CV. Therefore, in the sound absorbing structure 1a of Comparative Example 1, the cross-sectional area of ​​the channel F at the opening Ha2 is the same as the cross-sectional area of ​​the channel F at the opening H1. That is, the cross-sectional area of ​​the channel F is approximately constant at any position between the opening H1 and the opening Ha2.

[0029] 8 is a schematic perspective view of a sound absorbing structure 1b of Comparative Example 2. The sound absorbing structure 1b of Comparative Example 2 is the same as the sound absorbing structure 1 of the present embodiment, except that it includes a tubular member 20b that is different from the tubular member 20 of the sound absorbing structure 1 of the present embodiment.

[0030] The cylindrical member 20b does not have a reduced portion R at the end connected to the cavity CV. That is, the reduced portion R is not formed around the opening Hb2, which is the end of the flow path Fb formed inside the cylindrical member 20b that connects to the cavity CV. Furthermore, compared to the cylindrical member 20, the length Lb of the cylindrical member 20b (between the opening H1 and the opening Hb2) is longer than the length L of the cylindrical member 20 (see FIG. 1(b)). Specifically, the length L is 18 mm, while the length Lb is 24 mm.

[0031] Returning to the explanation of Figure 6, the horizontal axis of Figure 6 represents the frequency at which each sound absorbing structure absorbs sound. The vertical axis of Figure 6 represents the sound absorption coefficient of each sound absorbing structure. The thick line HL represents the sound absorption coefficient at each frequency of the sound absorbing structure 1 of this embodiment. The dashed-dotted line CL represents the sound absorption coefficient at each frequency of the sound absorbing structure 1a of Comparative Example 1. The thin line TL represents the sound absorption coefficient at each frequency of the sound absorbing structure 1b of Comparative Example 2. Note that, in the sound absorption coefficient represented on the vertical axis, as shown by the thick line HL, the sound absorption coefficient at the frequency at which the sound is most absorbed by the sound absorbing structure 1 (the frequency at the peak of the line segment) represents 1.0.

[0032] As described above, the difference between the sound absorbing structure 1 of this embodiment (see FIG. 1(b)) and the sound absorbing structure 1a of Comparative Example 1 (see FIG. 7) is the presence or absence of the reduced portion R. As shown by comparing the thick line HL and the dashed-dotted line CL, in the sound absorbing structure 1, the frequency at which sound is most absorbed (the frequency at the peak of the line segment) is smaller than in the sound absorbing structure 1a, and the sound absorption coefficient at the frequency at which sound is most absorbed is higher due to the reduced portion R formed. This result is shown in the line segment h in FIG. 0.0 From line segment h 1.2 It can also be estimated from the difference between each line segment between

[0033] As described above, the sound absorbing structure 1 of this embodiment (see FIG. 1(b)) differs from the sound absorbing structure 1b of Comparative Example 2 (see FIG. 8) in the presence or absence of a reduced portion R and in the lengths of the tubular members (the length Lb of the tubular member 20b is different from the length L of the tubular member 20). As shown by comparing the thick line HL and the thin line TL, the frequency at which sound is most absorbed by the sound absorbing structure 1 and the frequency at which sound is most absorbed by the sound absorbing structure 1b are almost the same, but it was confirmed that the sound absorption coefficient at that frequency is 8% higher for the sound absorbing structure 1 than for the sound absorbing structure 1b. The structure of the sound absorbing structure 1 corresponds to the structure of the sound absorbing structure 1b, in which a reduced portion R is formed around the opening Hb2 and the length Lb of the tubular member 20b is shortened (from 24 mm to 18 mm). In other words, it was confirmed that a sound absorbing structure (sound absorbing structure 1) having a reduced portion R formed therein can absorb the same frequency as the target frequency of the sound absorbing structure (sound absorbing structure 1b) even if the length L of the tubular member 20 is shorter than the length Lb of the tubular member 20b of a sound absorbing structure (sound absorbing structure 1b) not having a reduced portion R formed therein, and can improve the sound absorption coefficient at that frequency.

[0034] 9 is an explanatory diagram comparing the sound absorbing structure 1 of this embodiment with another comparative sound absorbing structure 1c (not shown). In the sound absorbing structure 1 used for comparison with the comparative sound absorbing structure 1c, the length L is 25 mm, the distance h is 1.3 mm, and the distance a is 2.0 mm. The comparative sound absorbing structure 1c is the same as the sound absorbing structure 1 of this embodiment except that, like the sound absorbing structure 1b of Comparative Example 2, it does not have a reduced portion R and the length of the tubular member is different from the length L. The length of the tubular member in the other comparative sound absorbing structure 1c is 36 mm.

[0035] The horizontal and vertical axes in Fig. 9 are the same as those in Fig. 6. In Fig. 9, the thick line HL indicates the sound absorption coefficient at each frequency of the sound absorbing structure 1 of this embodiment. The dashed line DL indicates the sound absorption coefficient at each frequency of the sound absorbing structure 1c of another comparative example. Note that in Fig. 9, among the sound absorption coefficients indicated on the vertical axis, the sound absorption coefficient at the frequency at which the sound absorbing structure 1 is most absorbed (the frequency corresponding to the peak of the line segment) indicates 1.0. Comparing the thick line HL and dashed line DL, the frequency at which the sound absorbing structure 1 is most absorbed and the frequency at which the sound absorbing structure 1c of another comparative example is most absorbed are almost the same, but it was confirmed that the sound absorption coefficient at that frequency is 6.8% higher for the sound absorbing structure 1 than for the sound absorbing structure 1c of another comparative example.

[0036] According to the sound absorbing structure 1 of the first embodiment described above, even if the length L of the tubular member 20 is shorter than that of sound absorbing structures 1b, 1c that are designed to absorb sound of a certain frequency and do not have the reduced portion R, the reduced portion R makes it possible to absorb sound of the same frequency as that targeted by the sound absorbing structures 1b, 1c and also improve the sound absorption coefficient at that frequency. Therefore, it is possible to make the sound absorbing structure 1 smaller or thinner without changing the frequency targeted for sound absorption and without reducing the sound absorption coefficient.

[0037] Furthermore, in the sound absorbing structure 1 of the first embodiment, the distance h from the extension line EL to the tip T of the contracted portion R in the longitudinal cross section of the tubular member 20 is smaller than the distance a from the portion IN to the central axis OL multiplied by 0.8. This allows the contracted portion R to properly exhibit its effect of improving the sound absorption coefficient.

[0038] Furthermore, in the sound absorbing structure 1 of the first embodiment, the cross-sectional shape of the reduced portion R in the longitudinal section of the tubular member 20 is an isosceles triangle, and when the portion where the extension line EL and the reduced portion R overlap is defined as a base B of the reduced portion R, the length w of the base B is equal to or less than five times the distance h. Therefore, the reduced portion R, whose cross-sectional shape is an isosceles triangle, can fully exhibit the effect of improving the sound absorption coefficient.

[0039] Generally, the frequency f that a Helmholtz resonator absorbs sound is expressed by the following formula (1):

number

[0040] Second Embodiment 10 is an explanatory view showing an enlarged portion of a longitudinal cross section of a sound absorbing structure 1r according to the second embodiment. The sound absorbing structure 1r according to the second embodiment is the same as the sound absorbing structure 1 according to the first embodiment, except that it includes a tubular member 20r different from the tubular member 20 of the sound absorbing structure 1 according to the first embodiment.

[0041] Similar to the contraction portion R in the cylindrical member 20 of the first embodiment, the cylindrical member 20r has a contraction portion Rr formed at its end connected to the cavity CV. While the cross-sectional shape of the contraction portion R in the longitudinal section of the cylindrical member 20 is triangular (see FIG. 3), the cross-sectional shape of the contraction portion Rr in the longitudinal section of the cylindrical member 20r is rectangular. Here, when the overlapping portion of the extension line EL and the contraction portion Rr is defined as the base edge Br of the contraction portion Rr and the edge of the contraction portion Rr closer to the central axis OL is defined as the top edge C, the length u of the top edge C is 2.0 mm or less, and the length u of the top edge C is equal to or less than the length w of the base edge Br. In this embodiment, the cross-sectional shape of the contraction portion Rr is rectangular, so the length u of the top edge C is the same as the length w of the base edge Br. Furthermore, the distance hr (the distance from the extension line EL to the top edge C) is 1.3 mm, and the distance a (the radius of the portion IN of the flow path F) is 2.0 mm. The rectangular cross-sectional shape of the reduced portion R includes not only a mathematically strict rectangular shape but also an approximately rectangular shape. An approximately rectangular shape includes a shape in which the angles of all four corners are within the range of 85° to 95° and a shape in which some sides are curved. In other words, the rectangular cross-sectional shape of the reduced portion R includes a shape that is roughly rectangular while allowing for deformation due to manufacturing errors, etc.

[0042] FIG. 11 is an explanatory diagram showing the change in frequency and the change in sound absorption coefficient when the length of the cylindrical member 20r (corresponding to the length L of the cylindrical member 20 in the first embodiment, see FIG. 1(b)) and the length w(u) of the base side Br (top side C) are both changed in the sound absorbing structure 1r. The horizontal and vertical axes in FIG. 11 are the same as those in FIG. 5. FIG. 11 shows lines indicating the respective results when the length of the cylindrical member 20r is changed from 16 mm to 24 mm. Of the multiple lines shown in FIG. 11, line L 0.5 shows the results when the length w(u) of the base Br (top side C) is 0.5 mm and the length of the cylindrical member 20r is 24 mm. 0.5 It is arranged to the right from the line segment L 0.5Each line segment including a peak equivalent to represents the results when the length w(u) of the base side Br (top side C) is fixed at 0.5 mm and the length of the cylindrical member 20r is varied between 24 mm and 16 mm. For example, line segment Ls represents the results when the length w(u) of the base side Br (top side C) is 0.5 mm and the length of the cylindrical member 20r is 16 mm.

[0043] Line segment L 1.0 , line segment L 1.5 , line segment L 2.0 shows the results when the length of the cylindrical member 20r is 24 mm and the lengths w(u) of the base side Br (top side C) are 1.0 mm, 1.5 mm, and 2.0 mm, respectively. 1.0 , line segment L 1.5 and line segment L 2.0 are arranged to the right of each of the lines L 1.0 , line segment L 1.5 and line segment L 2.0 The lines containing the peaks equivalent to each of the lines show the results when the length w(u) of the base side Br (top side C) is fixed at 1.0 mm, 1.5 mm, and 2.0 mm, respectively, and the length of the cylindrical member 20r is varied between 24 mm and 16 mm. In the sound absorption coefficient shown on the vertical axis, the sound absorption coefficient of the frequency at which the most sound is absorbed (the frequency corresponding to the peak of the line segment) for line segment Ls is 1.0.

[0044] Line segment L 0.5 , line segment L 1.0 , line segment L 1.5 and line segment L 2.0 As shown by the line segments arranged to the right of each of the lines, when the length of the cylindrical member 20r is shortened, the frequency at which sound is most absorbed increases, and the sound absorption coefficient at that frequency is 0.5 , line segment L 1.0 , line segment L 1.5 and line segment L 2.0 It was confirmed that the line segment L 0.5 , line segment L 1.0 , line segment L 1.5 and line segment L 2.0As shown in the figure, it was confirmed that when the length w(u) of the base side Br (top side C) is increased, the sound absorption coefficient at the frequency where sound is most absorbed decreases. It was also confirmed that if you want to maintain the sound absorption coefficient at 0.8 or more, it is preferable to set the length w(u) of the base side Br (top side C) to 2.0 mm or less.

[0045] In the sound absorbing structure 1r of the second embodiment described above, the reduction portion Rr is formed, and even if the length of the tubular member 20r is shorter than that of a sound absorbing structure that does not have a reduction portion Rr, it is possible to absorb the same frequencies as those targeted by a sound absorbing structure that does not have a reduction portion Rr, and it is also possible to improve the sound absorption coefficient at those frequencies.

[0046] Furthermore, in the sound absorbing structure 1r of the second embodiment, in the longitudinal section of the tubular member 20r, the cross-sectional shape of the reduced portion Rr is rectangular, and the length u of the upper side C is 2.0 mm or less, and the length u of the upper side C is less than the length w of the base side Br. Therefore, the reduced portion Rr having a rectangular cross-sectional shape can exhibit an effect of improving the sound absorption coefficient. Note that, even in the reduced portion Rr having a rectangular cross-sectional shape, it has been confirmed that the distance hr from the extension line EL to the tip (upper side C) of the reduced portion Rr is preferably smaller than the value obtained by multiplying the distance a from the portion IN to the central axis OL by 0.8.

[0047] <Modification of this embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0048] In the first embodiment described above, the housing 10 is a cylindrical member, but is not limited to this. The housing 10 may have any shape as long as the volume of the cavity CV is maintained at a volume that can obtain the above-described effects due to the formation of the contracted portion R in relation to the length L of the cylindrical member 20, the cross-sectional area of ​​the flow path F, and the height (distance h) of the contracted portion R.

[0049] FIG. 12 is an explanatory diagram of a modified example regarding the arrangement of the tubular member 20. In the first embodiment described above, as shown in FIG. 12(a), the entire tubular member 20 is arranged inside the housing 10, but this is not limited thereto. The tubular member 20 may be arranged as shown in FIG. 12(b) and FIG. 12(c). Specifically, as shown in FIG. 12(b), one end of the tubular member 20 may be arranged inside the housing 10, while the other end of the tubular member 20 is arranged outside the housing 10. Furthermore, as shown in FIG. 12(c), the entire tubular member 20 may be arranged outside the housing 10. Even when the tubular member 20 is arranged in this manner, the same effects as those of the first embodiment can be obtained as long as the reduced portion R is formed.

[0050] In the first embodiment described above, the cross-sectional shape of the triangular reduced portion R in the longitudinal section of the tubular member 20 is an isosceles triangle, but this is not limited thereto. The cross-sectional shape of the triangular reduced portion R in the longitudinal section of the tubular member 20 may also be a right-angled triangle. In such a reduced portion R, the length w of the base B may be equal to or less than 2.5 times the distance h. The right-angled triangle in the cross-sectional shape of the reduced portion R includes not only a mathematically strict right-angled triangle but also an approximately right-angled triangle. The approximately right-angled triangle shape includes a shape in which the largest angle among the three corners is within the range of 85° to 95°, a right-angled triangle with a rounded tip T, an approximately trapezoidal shape in which the tip T is missing, and a shape in which the tip side of such an approximately trapezoidal shape is R-chamfered or C-chamfered. In other words, the right-angled triangle in the cross-sectional shape of the reduced portion R includes a shape that is approximately a right-angled triangle while allowing for deformation due to manufacturing errors, etc. Furthermore, the cross-sectional shape of the reduced portion R, which is triangular, does not have to be an isosceles triangle or a right triangle.

[0051] Fig. 16 is an explanatory diagram comparing sound absorbing structures 1A to 1C in which reduced portions having a cross-sectional shape of a right-angled triangle are formed, with the sound absorbing structure 1c of Comparative Example 3. Before explaining Fig. 16, the sound absorbing structure 1c and the sound absorbing structures 1A to 1C will be explained.

[0052] FIG. 17 is a schematic perspective view of a sound absorbing structure 1c of Comparative Example 3. The shape of the sound absorbing structure 1c of Comparative Example 3 is similar to that of the sound absorbing structure 1b of Comparative Example 2 described in FIG. 8. Like the cylindrical member 20b (see FIG. 8), the cylindrical member 20c in the sound absorbing structure 1c does not have a reduced portion R at the end connected to the cavity CV. That is, the reduced portion R is not formed at the end of the flow path Fc formed inside the cylindrical member 20c that connects to the cavity CV. The length Lc of the cylindrical member 20c (between the opening H1 and the opening Hc2) is 24 mm, and the height Mc of the cavity CV (the length along the Z-axis direction) is 27.4 mm.

[0053] 18 is a schematic perspective view of a sound absorbing structure 1A. A reduced portion RA is formed at the end of a tubular member 20A in the sound absorbing structure 1A that is connected to the cavity CV. The length LA of the tubular member 20A (between openings H1 and H2) is 15.5 mm, and the height MA of the cavity CV (the length along the Z-axis) is 26.6 mm.

[0054] Figure 19 is an explanatory diagram showing an enlarged longitudinal cross section of the end of cylindrical member 20A that connects to cavity CV. As shown in Figure 19, in the longitudinal cross section of cylindrical member 20A, the cross-sectional shape of reduced portion RA is a right-angled triangle. The length wA of base BA (corresponding to length w in Figure 3) is 2.1 mm, and the distance hA (corresponding to distance h in Figure 3) is 1.3 mm.

[0055] 20 is a schematic perspective view of a sound absorbing structure 1B. Similar to the sound absorbing structure 1A, a reduced portion RA is formed at the end of a tubular member 20B in the sound absorbing structure 1B that is connected to the cavity CV. The length LB of the tubular member 20B (between openings H1 and H2) is 19.93 mm, and the height MB of the cavity CV (the length along the Z-axis direction) is 23.16 mm. That is, while the sound absorbing structure 1B has the same reduced portion RA as the sound absorbing structure 1A, it differs from the sound absorbing structure 1A in that the length LB is longer than the length LA and the height MB is shorter than the height MA.

[0056] FIG. 21 is a schematic perspective view of the sound-absorbing structure 1C. In the sound-absorbing structure 1C, the contracted portion RC is formed in the cylindrical member 20C from the position on the side connected to the outside of the housing 10 to the end (opening H2) on the side connected to the cavity CV (see FIG. 22 for details). Here, the position on the side connected to the outside of the housing 10 within the range where the contracted portion RC is formed is a position on the -Z-axis side of opening H1, different from opening H1 (the end of the housing 10 connected to the outside). As shown in FIG. 21, the contracted portion RC reduces the cross-sectional area of ​​the flow path F toward the central axis (not shown) of the cylindrical member 20C as it moves toward opening H2. The length LC of the cylindrical member 20C (between openings H1 and H2) is 20.51 mm, and the height MC of the cavity CV (length along the Z-axis) is 17.9 mm.

[0057] Figure 22 is an explanatory diagram showing an enlarged longitudinal cross section of the end of tubular member 20C. As shown in Figure 22, in the longitudinal cross section of tubular member 20C, the cross-sectional shape of reduced portion RC is a right-angled triangle. The length wC of base BC (corresponding to length w in Figure 3 and length wA in Figure 19) is 18.3 mm, and the distance hC (corresponding to distance h in Figure 3 and distance hA in Figure 19) is 0.9 mm.

[0058] Returning to the explanation of FIG. 16, the horizontal axis of FIG. 16 indicates the frequency at which each sound-absorbing structure absorbs sound. The vertical axis of FIG. 16 indicates the sound absorption coefficient of each sound-absorbing structure. The dashed-dotted line cS indicates the sound absorption coefficient at each frequency of the sound-absorbing structure 1c of Comparative Example 3 (FIG. 17). The thin line AS indicates the sound absorption coefficient at each frequency of the sound-absorbing structure 1A (FIG. 18). The thick line BS indicates the sound absorption coefficient at each frequency of the sound-absorbing structure 1B (FIG. 20). The dashed line CS indicates the sound absorption coefficient at each frequency of the sound-absorbing structure 1C (FIG. 21). As indicated by the dashed-dotted line cS, thin line AS, thick line BS, and dashed line CS, it was confirmed that the frequencies at which sound is most absorbed by the sound-absorbing structure 1c and the sound-absorbing structures 1A to 1C are substantially the same. On the other hand, it was confirmed that the sound absorption coefficients at the frequencies at which sound is most absorbed are higher for the sound-absorbing structures 1A to 1C than for the sound-absorbing structure 1c.

[0059] Comparing the sound absorbing structure 1c of Comparative Example 3 (Figure 17) with the sound absorbing structure 1A (Figure 18), it is found that the sound absorbing structure 1A (Figure 18) has a reduced section RA, which allows the length LA (15.5 mm) of the tubular member 20A to be shorter than the length Lc (24 mm) of the tubular member 20c.

[0060] Comparing the sound absorbing structure 1c of Comparative Example 3 (Fig. 17) with the sound absorbing structure 1B (Fig. 20), the height MB (23.16 mm) of the cavity CV is 4.24 mm lower than the height Mc (27.4 mm) of the cavity CV. From the viewpoint of the volume of the cavity CV, the volume of the cavity CV in the sound absorbing structure 1c of Comparative Example 3 (Fig. 17) is 10378.7 mm. 2 In contrast, the volume of the cavity CV in the sound-absorbing structure 1B (Fig. 20) is 8821.7 mm 2 As a result, the volume of the hollow CV can be reduced by approximately 15%.

[0061] Comparing the sound absorbing structure 1c of Comparative Example 3 (Fig. 17) with the sound absorbing structure 1C (Fig. 21), the height MC (17.9 mm) of the cavity CV is 9.5 mm lower than the height Mc (27.4 mm) of the cavity CV. From the viewpoint of the volume of the cavity CV, the volume of the cavity CV in the sound absorbing structure 1c of Comparative Example 3 (Fig. 17) is 10378.7 mm. 2 In contrast, the volume of the cavity CV in the sound-absorbing structure 1C (Fig. 21) is 6607.0 mm 2 This means that the volume of the cavity CV can be reduced by approximately 36.3%. This result indicates that when the reduction portion RC is formed, the volume of the cavity CV can be further reduced compared to when the reduction portion RA is formed.

[0062] Compared to the sound absorbing structure 1c of Comparative Example 3 (FIG. 17), the sound absorbing structures 1A to 1C have reduced portions RA and RC, which enable them to absorb sounds of the same frequency as the sound absorbing structure 1c and improve the sound absorption coefficient at that frequency, as in the first and second embodiments. Furthermore, comparing the sound absorbing structures 1A to 1C, it was confirmed that it is also possible to reduce the overall size of the sound absorbing structure by adjusting the lengths LA to LC and the heights MA to MC to reduce the volume of the cavity CV.

[0063] FIG. 14 is an explanatory diagram illustrating an example of a substantially right-angled triangular shape in the cross section of the reduction portion R. As shown in FIG. 14, the substantially right-angled triangular shape may include a shape in which at least a portion of the hypotenuse is curved. FIGS. 14(a) to 14(c) show a substantially right-angled triangular shape in which at least a portion of the hypotenuse is curved. FIG. 14(a) shows a substantially right-angled triangular shape having a hypotenuse hy that is convexly curved in a direction away from the position where the right angle is formed. FIG. 14(b) shows a substantially right-angled triangular shape having a hypotenuse hy that is convexly curved in a direction approaching the position where the right angle is formed. FIG. 14(c) shows a substantially right-angled triangular shape having a hypotenuse hy that includes a portion that is convexly curved in a direction away from the position where the right angle is formed and a portion that is convexly curved in a direction approaching the position where the right angle is formed. The hypotenuse hy is not limited to the shapes shown in FIGS. 14(a) to 14(c) and may have any curved shape.

[0064] In the second embodiment described above, the cross-sectional shape of the quadrangular contracted portion Rr in the longitudinal section of the cylindrical member 20r is rectangular, but this is not limited thereto. The cross-sectional shape of the quadrangular contracted portion Rr in the longitudinal section of the cylindrical member 20r may also be trapezoidal. Even in such a contracted portion Rr, in order to fully exhibit the effect of improving the sound absorption coefficient, it is preferable that the length u of the upper side C is 2.0 mm or less and that the length u of the upper side C is shorter than the length w of the base side Br.

[0065] FIG. 15 is an explanatory diagram illustrating an example of a substantially rectangular cross-sectional shape of the reduced portion Rr. As shown in FIG. 15, the substantially rectangular cross-sectional shape of the reduced portion Rr may include a curved shape in at least a portion of the leg connecting the top side C and the bottom side Br. FIGS. 15(a) to 15(c) illustrate a substantially rectangular shape in which at least a portion of the leg is curved. FIG. 15(a) illustrates a substantially rectangular shape in which the leg Lg1 on the side opposite to the side facing the cavity CV is convexly curved in a direction away from the cavity CV. FIG. 15(b) illustrates a substantially rectangular shape in which the leg Lg1 is convexly curved in a direction approaching the cavity CV. FIG. 15(c) illustrates a substantially rectangular shape in which the leg Lg1 includes a portion convexly curved in a direction away from the cavity CV and a portion convexly curved in a direction approaching the cavity CV. The leg Lg1 is not limited to the shapes illustrated in FIGS. 15(a) to 15(c) and may have any curved shape.

[0066] In the above-described embodiments, the sound absorbing structures 1, 1r each include one tubular member 20 and one tubular member 20r. However, this is not limiting. For example, the sound absorbing structure may include two or more tubular members. FIG. 13(a) shows a simplified version of the sound absorbing structure 1 of the first embodiment. Here, the volume of the cavity CV is V, and the height of the sound absorbing structure 1 is e. FIG. 13(b) shows the sound absorbing structure 2. The sound absorbing structure 2 has a shape similar to four sound absorbing structures 1 connected together, and includes four tubular members 22. Like the tubular member 20, the tubular member 22 has a reduced portion R at the end connected to the cavity CV. In this case, when the sound absorbing structure 2 is intended to achieve the same effect as the sound absorbing structure 1 (the effect due to the reduced portion R), the cavity CV may have any shape as long as the volume of the cavity CV is 4V. The height of the sound absorbing structure 2 is e, similar to the sound absorbing structure 1. FIG. 13(c) shows the sound absorbing structure 3. The sound absorbing structure 3 includes four tubular members 23. Like the tubular members 20 and 22, the tubular members 23 have a reduced portion R formed at the end connected to the cavity CV. In this case, when trying to obtain the same effect as the sound absorbing structure 2 (the effect due to the formation of the reduced portion R), the cavity CV may have any shape, as with the sound absorbing structure 2, as long as the volume of the cavity CV is secured to 4V. The height e' of the cavity CV in the sound absorbing structure 3 is shorter than the height e of the cavity CV in the sound absorbing structure 2, and the length of the cavity CV in the horizontal direction (perpendicular to the height direction) in the sound absorbing structure 3 is longer than the length of the cavity CV in the horizontal direction in the sound absorbing structure 2. In the sound absorbing structure 3, the cavity CV has such a shape, so that the volume of the cavity CV is secured to 4V. In the sound absorbing structure 3, the volume of the cavity CV is the same as in the sound absorbing structure 2, but it is smaller in the height direction. The height e' can be adjusted to a length that is close to the length of the cylindrical member 23 in the height direction.

[0067] This aspect has been described above based on embodiments and modifications. However, the above-described embodiments are intended to facilitate understanding of this aspect and are not intended to limit this aspect. This aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents thereof are included in this aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

[0068] The present invention can also be realized in the following forms. [Application example 1] A sound absorbing structure, a housing having an internal cavity; a cylindrical member having a flow path formed therein that connects the cavity with the outside of the housing, A sound-absorbing structure, wherein a reducing portion that reduces the cross-sectional area of ​​the flow path toward the central axis of the cylindrical member is formed at the end of the cylindrical member that is connected to the cavity. [Application example 2] The sound absorbing structure according to Application Example 1, In a longitudinal section of the cylindrical member, A sound-absorbing structure in which the distance from a line passing over a portion of the inner surface of the tubular member where no reduced portion is formed and extending it toward the side connecting to the cavity to the tip of the reduced portion is smaller than the value obtained by multiplying the distance from that portion to the central axis of the tubular member by 0.8. [Application example 3] The sound absorbing structure according to Application Example 1 or Application Example 2, In a longitudinal section of the cylindrical member, The cross-sectional shape of the reduced portion is an isosceles triangle, A sound-absorbing structure, wherein when the overlapping portion of the extension line and the reduction portion is defined as the base of the reduction portion, the length of the base is not more than five times the distance from the extension line to the tip of the reduction portion. [Application example 4] The sound absorbing structure according to Application Example 2, In a longitudinal section of the cylindrical member, A sound-absorbing structure, wherein the cross-sectional shape of the reduced portion is a right-angled triangle. [Application example 5] The sound absorbing structure according to Application Example 4, In a longitudinal section of the cylindrical member, A sound-absorbing structure, wherein when the overlapping portion of the extension line and the reduction portion is defined as the base of the reduction portion, the length of the base is not more than 2.5 times the distance from the extension line to the tip of the reduction portion. [Application Example 6] The sound absorbing structure according to Application Example 4 or Application Example 5, The sound-absorbing structure, wherein the reduced portion is formed in a range from a position on the cylindrical member that is connected to the outside of the housing to an end portion on the side that is connected to the cavity. [Application Example 7] The sound absorbing structure according to Application Example 1 or Application Example 2, In a longitudinal section of the cylindrical member, The cross-sectional shape of the reduced portion is rectangular, A sound-absorbing structure in which the length of the upper edge is 2.0 mm or less and the length of the upper edge is less than the length of the bottom edge, when the portion of the inner surface of the tubular member where the reduced portion is not formed overlaps with an extension line of the line extending toward the side connected to the cavity, and the edge of the reduced portion closer to the central axis is defined as the upper edge. [Explanation of symbols]

[0069] 1, 1r, 1A-1C, 2, 3...Sound absorbing structure 10...Housing 10f…Top surface 20, 20r, 20A to 20C, 22, 23... Cylindrical members B, Br, BA, BC...bottom C...Top CV…Cavity EL…Extension line F...flow path H1…Aperture H2…Aperture IN…part OL…center axis R, Rr, RA, RC…reduction part T…Tip h,hr,hA,hC…distance

Claims

1. A sound absorbing structure, a housing having an internal cavity; a cylindrical member having a flow path formed therein that connects the cavity with the outside of the housing, A sound-absorbing structure, wherein a reducing portion that reduces the cross-sectional area of ​​the flow path toward the central axis of the cylindrical member is formed at the end of the cylindrical member that is connected to the cavity.

2. The sound absorbing structure according to claim 1, In a longitudinal section of the cylindrical member, A sound-absorbing structure in which the distance from an extension line obtained by extending a line passing over a portion of the inner surface of the tubular member where a reduction portion is not formed toward the side connecting to the cavity to the tip of the reduction portion is smaller than the value obtained by multiplying the distance from that portion to the central axis of the tubular member by 0.

8.

3. The sound absorbing structure according to claim 2, In a longitudinal section of the cylindrical member, The cross-sectional shape of the reduced portion is an isosceles triangle, When the overlapping portion of the extension line and the reduction portion is defined as the base of the reduction portion, the length of the base is not more than five times the distance from the extension line to the tip of the reduction portion.

4. The sound absorbing structure according to claim 2, In a longitudinal section of the cylindrical member, A sound-absorbing structure, wherein the cross-sectional shape of the reduced portion is a right-angled triangle.

5. The sound absorbing structure according to claim 4, In a longitudinal section of the cylindrical member, When the overlapping portion of the extension line and the reduction portion is defined as the base of the reduction portion, the length of the base is not more than 2.5 times the distance from the extension line to the tip of the reduction portion.

6. The sound absorbing structure according to claim 4, The sound-absorbing structure, wherein the reduced portion is formed in a range from a position on the cylindrical member that is connected to the outside of the housing to an end portion on the side that is connected to the cavity.

7. The sound absorbing structure according to claim 1 or 2, In a longitudinal section of the cylindrical member, The cross-sectional shape of the reduced portion is rectangular, A sound-absorbing structure in which the bottom edge of the reduction portion is defined as the portion where an extension line passing over the portion of the inner surface of the tubular member where a reduction portion is not formed, extended toward the side connected to the cavity, overlaps with the reduction portion, and the edge of the reduction portion closer to the central axis is defined as the top edge, and the length of the top edge is 2.0 mm or less and is less than the length of the bottom edge.

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