Sound absorbing structure and manufacturing method thereof

Abstract

To provide a sound absorption structure body capable of reducing sound in a broad frequency band.SOLUTION: A sound absorption structure body comprises a main body which has a porous part which is opened on a surface side, and a resonator which is built in the main body, and has a cavity part and a neck part serving as a conduit reaching the cavity part from the surface side of the main body. The main body and resonator are formed, for example, in one body. The resonator, for example, has a plurality of cavity parts and neck parts, respectively. A cavity part is, for example, at least partially formed of a solid wall surface (shell). At least one of the neck parts may be a conduit bypassing an outer peripheral side of the cavity part. The resonator further may have a communication pipe linking two or more of the cavity parts. A sound absorption structure body like this is manufactured by, for example, a powder lamination method.SELECTED DRAWING: Figure 1A

Description

[Technical field]

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

[0002] Generally, the quieter the equipment (machines, devices, equipment, etc.), the better. Noise reduction measures include reviewing the structure and operating conditions to suppress the generation of noise itself, as well as taking sound absorption (deadening) measures to reduce or eliminate generated or transmitted sound. The following document contains information related to the latter: [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 4-225398 [Patent Document 2] Patent Publication No. 11-259076 [Patent Document 3] Patent Publication No. 2000-8950 [Non-patent literature]

[0004] [Non-Patent Document 1] Zhengqing Liu et al., “Acoustic properties of a porous polycarbonate material produced by additive manufacturing”, Materials Letters ,Vol.181(2016),pp.296-299. Summary of the Invention [Problem to be solved by the invention]

[0005] Patent documents 1 to 3 contain descriptions of Helmholtz resonators. Non-patent document 1 describes the sound absorption characteristics of a structure with about 100 0.8 mm round holes at a pitch of about 2 to 3 mm, formed by stereolithography (3D printer). In all cases, the range of sound frequencies that are absorbed (called the "sound absorption frequency band" or simply the "frequency band") is limited, and sounds over a wide frequency band cannot be reduced, blocked, or eliminated.

[0006] Patent Document 2 proposes forming a Helmholtz-type resonator by press-molding and drilling a sound-absorbing material (coarse felt) that has been attached to sound-insulating panels (engine covers, etc.). A skin material (aluminum foil, etc.) is attached to the surface of the sound-absorbing material of Patent Document 2 to prevent scattering and ensure shape retention, and a film material that blocks ventilation is also attached to the inner wall surface of the Helmholtz-type resonator (see

[0012] ,

[0013] ,

[0019] , Figure 3, etc. of Patent Document 2). In this case, since the skin material blocks ventilation, air does not enter (penetrate) into gaps (for example, between fibers) in the sound-absorbing material, and sound absorption due to the viscous resistance of air is not expected. At a glance, the graph shown in Figure 4 of Patent Document 2 appears to have achieved a wide sound absorption frequency band. However, there is no description of actual measurements of the sound absorption frequency band in Patent Document 2, and the graph shown in Patent Document 2 is not considered to be compatible with the sound absorbing material described in the examples. If the sound absorbing material reduces the high frequency sound, this is considered to be due to resonance caused by membrane vibration of the skin material, or the sound absorbing material functioning as a vibration damping material that suppresses vibration of the sound insulation panel.

[0007] The present invention has been made in consideration of the above circumstances, and has an object to provide a new sound-absorbing structure etc. that is capable of reducing sound over a wide frequency band, unlike conventional structures. [Means for solving the problem]

[0008] As a result of intensive research into solving this problem, the inventor came up with the idea of ​​incorporating a resonator into a porous body, and confirmed that this would enable sound absorption over a wide frequency range. By expanding on this result, the inventor has completed the present invention, which will be described below.

[0009] Sound-absorbing structure (1) The present invention is a sound-absorbing structure comprising a main body having a porous portion opening on its surface side, and a resonator built into the main body and having a hollow portion and a neck portion which is a conduit leading from the surface side of the main body to the hollow portion.

[0010] (2) The sound-absorbing structure (also simply referred to as the "structure") of the present invention can reduce sound over a wide frequency range, enabling effective sound absorption (deadening, silencing) in various structures (machines, equipment, devices, buildings, etc.).

[0011] The mechanism by which the structure of the present invention absorbs sound is believed to be as follows. First, sound waves (air vibrations) that penetrate (enter) the porous portion from the outside have their acoustic energy (simply referred to as "energy") reduced due to viscous loss of the air and heat exchange loss. As a result, for example, sound is absorbed mainly on the high frequency side.

[0012] Next, the sound waves (air vibrations) that enter the cavity from the outside are reduced in energy by the Helmholtz resonance effect. This allows, for example, sound absorption mainly on the low frequency side. Thus, according to the structure of the present invention, the synergistic effect of the porous part provided in the main body and the resonator built into the main body makes it possible to absorb sound over a wide frequency range.

[0013] Here, the porous portion needs to be at least on the surface side (surface layer side) of the main body. In other words, the main body may be porous (porous portion) as a whole, or the portion other than the porous portion may be solid (solid portion), or may be hollow for the purpose of weight reduction, etc. Furthermore, even if the porous portion is thin (layered), it can still exhibit a sufficient sound absorbing effect. For this reason, there is a large degree of freedom in the shape of the main body (and thus the structure), and it can itself be a component of various members.

[0014] In addition, by changing the shape of the porous portion, the hollow portion, and the neck portion, it is possible to adjust the frequency band to be absorbed or the sound absorption coefficient in a specific frequency range, etc. Therefore, the structure of the present invention is not necessarily limited to the case where the porous portion absorbs high-frequency sound and the resonator absorbs low-frequency sound.

[0015] <<Method for manufacturing sound-absorbing structure>> (1) The present invention can be understood as a method for manufacturing a sound absorbing structure. There are various methods for the manufacturing method, but it is preferable to use a powder lamination method, for example.

[0016] According to the powder lamination method, it is possible to form a resonator consisting of a porous portion consisting of a minute gap (hole), a minute cavity portion, or a neck portion. Specifically, it is possible to manufacture a porous portion, a cavity portion, or a neck portion whose size is adjusted in units of 1 mm, 0.5 mm, 0.1 mm, or even 0.05 mm.

[0017] Furthermore, with the powder lamination method, it is possible, for example, to integrally form a resonator within a porous body, freely adjust its form (shape, size, length, number, etc.), and form a separate sound-absorbing structure (e.g., a lattice structure) or vibration-damping structure within the cavity of the resonator.

[0018] The powder remaining in the structure (main body) may be discharged to the outside through the porous portion or the neck portion, or may be discharged from a separately provided powder removal hole. Furthermore, a part of the powder may remain inside.

[0019] (2) Incidentally, the powder additive manufacturing is a type of additive manufacturing (AM) method or additive manufacturing method using powder, and may be either powder sintering additive manufacturing or powder bonding additive manufacturing. In the powder sintering additive manufacturing method, a high-energy beam, which is a heat source, is irradiated onto the raw powder of each layer, and the raw powder is successively sintered (including melting and solidifying) to obtain a molded object (e.g., the selective laser melting (SLM) method). In the powder bonding additive manufacturing method, an adhesive (ink) is sprayed onto the raw powder of each layer in succession, and the raw powder is successively bonded to obtain a molded object (e.g., the binder jet method).

[0020] The powder sintering additive manufacturing method may be powder bed fusion (PBF) or directed energy deposition (DED). PBF is a method in which a high-energy beam (laser, electron beam, etc.) is scanned along a predetermined path to melt and solidify the raw material powder every time a thin layer of raw material powder is laid, and a desired object (bulk body) is obtained. DED is a method in which raw material powder projected near the focus of the high-energy beam is melted and solidified, and the melting and solidification position is scanned (moved) to obtain a desired object. Note that powders of various materials (metal, resin, compound (ceramics), etc.) can be used as raw material powder, but the most typical raw material powder is metal powder.

[0021] "others" Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit value or upper limit value to create a new range such as "a to b". Furthermore, unless otherwise specified, "x to y μm" in this specification means x μm to y μm. The same applies to other unit systems. [Brief description of the drawings]

[0022] [Figure 1A] 1 is a perspective view showing the appearance, the inside, and a cross section of a sample 1. [Figure 1B] 2 is a perspective view showing the appearance, the inside, and a cross section of a sample 2. FIG. [Diagram 2] 1 is a graph showing the relationship between frequency and sound absorption coefficient for each sample. [Figure 3A] FIG. 13 is a perspective view showing a modified example of sample 2. [Figure 3B] FIG. 13 is a perspective view showing another modified example of the sample 2. [Figure 4] 1A to 1C are explanatory diagrams showing application examples of the sound absorbing structure. [Diagram 5] FIG. 1 is an explanatory diagram of a resonator (Helmholtz resonator). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] One or more components selected from the present specification may be added to the components of the present invention. The contents described in this specification apply not only to the sound absorbing structure of the present invention but also to the manufacturing method thereof as appropriate. A method-related component may also be a component related to an object.

[0024] "Sound absorption principle" (1) Resonator The principle of sound absorption by a resonator (Helmholtz resonator) with a cavity (resonance chamber) and a neck (connecting tube) can be considered as follows. As shown in Figure 5, a resonator has a resonance frequency (natural frequency) f that is determined mainly by the shape of the cavity and neck. Of the sound (air vibrations) that enter (propagate) from the neck into the cavity, components near the resonance frequency resonate (reflect) within the cavity, and the vibrations are attenuated due to interference. In this way, sound near the resonant frequency of the resonator is reduced.

[0025] The resonance frequency may also be affected by air vibrations near the neck opening. Figure 5 shows an example of a formula for calculating the resonance frequency (f) with open-end correction. The open-end correction shown is for a case where the neck opening (on the surface side of the body) is flange-shaped. If the neck opening is not flange-shaped (for example, if the neck opening is located at a position that protrudes from the surface of the body), L'=L+1.7(d / 2) may be replaced with L'=L+1.5(d / 2).

[0026] Incidentally, sound that enters the cavity can also be reduced within the neck as energy is wasted through friction (viscous resistance of air). However, since the former type of resonance is usually the main source of sound absorption, the frequency band of sound absorbed by the resonator can be narrow. However, if a resonator is provided that consists of multiple cavities and / or necks with different specifications, the multiplexing or distribution of resonance frequencies can expand or shift the frequency band of sound absorbed by the resonator. The specifications of the resonator include the internal volume (V) of the cavity, and the cross-sectional area (S) and length (l) of the neck.

[0027] (2) Porous section The principle of sound absorption by porous parts can be considered as follows. Sound that enters the numerous gaps (holes) opened on the surface side loses energy due to friction (viscous resistance of air) between the air and the walls and pillars of the porous part. Generally, such sound absorption depends on the wavelength, and a thickness (guideline) of about 1 / 4 of the wavelength of the frequency to be absorbed is required. In reality, due to spatial constraints, the thickness of the sound-absorbing material often has to be thin. For this reason, the frequency band that is absorbed (deadened) by the porous part is usually on the high frequency side. However, it is also possible to adjust the frequency band of sound absorption by the porous part by changing the form (shape, size, etc.) of each gap (hole).

[0028] Resonator (1) The resonator has a cavity and a neck. The shapes of the cavity and the neck may be adjusted according to the required specifications of the structure. The cavity may be, for example, substantially cylindrical, substantially square prism, substantially triangular prism, or a distorted irregular shape. The neck may be, for example, circular tubular, angular tubular, or a distorted irregular tubular shape. The path of the neck may be, for example, linear, curved, or broken line.

[0029] There should be at least one cavity and one neck. If there are multiple cavities, there may be one or more necks for each cavity, or conversely, there may be one neck for some cavities. In the latter case, it is preferable that two or more cavities are in communication with each other.

[0030] When there are multiple cavities or necks, each cavity or neck may have the same or different shapes. By providing cavities or necks with different shapes, the sound absorption frequency band of the resonator can be expanded. Also, when there are multiple necks, each opening may be arranged in the same direction or face side, or in different directions or faces. The former can effectively absorb directional sound, while the latter can absorb sound from multiple directions.

[0031] (2) The walls of the cavity and neck may be solid or porous (mesh-like). Even in the case of porous walls, if the size of the gaps is small (for example, the maximum length (diameter) is 200 μm or less, or even 100 μm or less), the air permeability (sound transmission) is suppressed and the function as a resonator can be ensured. Of course, if at least a part of the cavity is a solid wall (shell), the sound absorption rate of the resonator can be increased.

[0032] (3) The neck portion is made of a conduit that communicates (penetrates) between the wall surface of the cavity and the outer surface of the body. When there are multiple neck portions, the shapes of the conduits may be the same or different. Also, at least one of the neck portions may be a conduit (a bypass pipe) that bypasses the outer periphery of the cavity. This lengthens the neck portion, and the sound absorption frequency band (resonance frequency band) can be shifted to the lower frequency side (see Figure 5).

[0033] 《Main body》 (1) The porous portion may be present on the entire outer surface of the main body, or on only a part of it. The opening of the neck of the resonator may be present either inside or outside the porous portion.

[0034] The shape of each hole constituting the porous portion may be various. For example, each hole may be a through hole or a bottomed cylindrical shape. Also, each hole may be substantially angular (for example, rectangular or lattice), substantially circular (including elliptical, oval, etc.), or irregular. The average hole size may be, for example, 0.05 to 2 mm, 0.1 to 1 mm, or 0.15 to 0.5 mm.

[0035] The pore size is indicated by the length (□: circle) of one side (√si) of a square with an area equal to the area (si) of each closed pore (gap). The average pore size is the square root (√sm) of the average area (sm=S / n) obtained by dividing the total area (S=Σsi) of the closed pores (gaps) in the field of view of the observation image obtained by enlarging the surface of the porous part with a microscope or the like by the number of counts of the pores (n=Σi). The average pore size is obtained, for example, by image processing the observation image. The image processing may be performed using an attached function of the microscope or using other software (such as free software ImageJ). The average pore size may be calculated using 5 to 100 or even 10 to 50 pores randomly extracted, excluding extremely small or excessively large pores.

[0036] (2) The main body may be entirely porous, or may be solid (solid portion) except for the porous portion and the resonator built in. The porous portion may be present, for example, only near the surface of the main body.

[0037] The main body may be all or part of the case (case, housing, etc.) of an equipment (machine, device, etc.), the wall of a component, the skeleton of a structural component, etc. This allows for space saving and noise reduction measures without the need to provide separate sound-absorbing materials, etc.

[0038] The main body may be made of metal, resin, ceramics, etc. A metal main body (structure) is superior to general porous sound-absorbing materials in strength, rigidity, thermal conductivity, heat resistance, moisture resistance, non-flammability, etc., and can therefore provide a stable sound-absorbing (noise-reducing) effect even in high-temperature environments (e.g., engine rooms).

[0039] Sound-absorbing structure The structure can be of any shape, size, or purpose. The shape, size, installation position (area), installation direction, characteristics, etc. of the main body (porous portion) and resonator are appropriately selected according to the specifications. The sound absorbing structure may be a general-purpose product or a dedicated product. If it is a dedicated product, as described above, the main body may be shaped to suit a specific device, etc.

[0040] The structure can be formed by additive processing such as the powder lamination method described above, as well as by removal processing such as laser processing, cutting processing, punching, and even plastic processing. An appropriate manufacturing method should be selected depending on the specifications and applications of the structure. The structure may be one in which the main body and the resonator are integrally formed, or one in which multiple members are combined or joined (glued, welded, fastened, etc.) to assemble the structure. EXAMPLES

[0041] A number of samples (sound absorbing structures) were produced and their sound absorbing properties were evaluated. Based on these specific examples, the present invention will be described in more detail below.

[0042] [First Example] <Sample Preparation> (1) Form Samples 1 and 2 shown in Figures 1A and 1B (collectively referred to as "Figure 1") were manufactured by powder sintering additive manufacturing. For the sake of convenience, the directions indicated by the arrows in the figures are taken as the up-down direction and the left-right direction.

[0043] Samples 1 and 2 consist of a short cylindrical body and a resonator formed inside the body. The body is entirely porous except for the resonator part.

[0044] The resonator is composed of multiple cavities and necks. One neck is provided for each cavity. Each cavity is composed of a shell having an approximately triangular prism shape. The shell walls are solid walls unless otherwise specified. A shell composed of a solid wall is appropriately referred to as a "solid shell." Each neck is composed of a straight tube (conduit) extending from the upper surface of each cavity to the upper surface of the main body. The open end of each neck is flush with the upper surface of the main body. Sample 1 has four pairs of cavities and necks, and sample 2 has eight pairs of cavities and necks.

[0045] Body size: φ29mm x 10mm, hollow volume (V): 246.5mm 3 The wall thickness of the hollow portion (shell thickness) was 0.5 mm, the inner diameter of the neck portion (d) was φ1 mm, and the length of the neck portion (l) was 2 mm.

[0046] The porous part on the upper surface of the main body is a roughly square lattice shape (two-dimensional extrusion shape in the vertical direction), with a lattice wall thickness of 100 μm (target value) and a lattice wall spacing (gap width) of 200 μm (target value). The porous part of the sample actually produced had an average pore size of 154 μm.

[0047] (2) Manufacturing method The specimen was fabricated by the powder sintering additive manufacturing method using a so-called 3D printer (SLM280 manufactured by SLM Solutions). Commercially available AlSi10Mg powder (composition: Al-10wt%Si-0.35wt%Mg, particle size: 20~63μm) was used as the raw powder. The remaining powder in the cavity was discharged from the neck.

[0048] (3) Comparison sample As a comparative sample, sample C made of polyurethane foam (BS2F manufactured by Misumi Corporation), a typical porous sound absorbing material, was prepared. The appearance of sample C was cylindrical, the same as samples 1 and 2 shown in FIG.

[0049] "measurement" The sound absorption characteristics (sound absorption coefficient) of each sample were measured according to the standard (measurement of sound absorption coefficient and impedance using an acoustic tube / ISO 10534-2:1998, JIS A 1405-2:2007). A Brüel & Kjær Type 4206 was used as the acoustic tube. The frequency range that could be evaluated was 500 to 6300 Hz. Sound was incident vertically from above on the porous part and neck opening of each sample. The sound absorption characteristics (sound absorption coefficient for each frequency) measured for each sample are summarized in Figure 2.

[0050] The resonance frequency of the resonators of Samples 1 and 2 was calculated to be 1809 Hz based on the formula shown in Figure 5. The frequency at which the sound absorption coefficients of Sample 2 and Sample C overlap was 4768 Hz.

[0051] "evaluation" As is clear from Figure 2, Samples 1 and 2 had significantly higher sound absorption coefficients over a wider frequency range than Sample C. It was also found that in Samples 1 and 2, the sound absorption peak due to the resonator appears on the low frequency side (e.g., 1500 to 1600 Hz), while the sound absorption peak due to the porous portion appears on the high frequency side (e.g., 4000 to 6000 Hz).

[0052] The frequency at which the sound absorption peak due to the resonators appears is near the above-mentioned resonance frequency (1809 Hz), and the two are almost the same. The magnitude of the sound absorption peak on the high frequency side is different between sample 1 and sample 2. The sound absorption peak at 4000 to 6000 Hz is due to the effect of the 10 mm thick porous part. It is thought that the sound absorption coefficient is smaller in sample 2, which has a large number of resonators, because the area for the incident sound waves is smaller.

[0053] [Second Example] Modified examples of the structure of sample 2 shown in FIG. 1B are shown in FIG. 3A and FIG. 3B. As shown in each figure, the neck portion may be a conduit (bypass pipe) extending along the outer periphery of the hollow portion (shell). For example, as shown in FIG. 3A, the neck portion may be a conduit extending from the lower surface side of the hollow portion to the upper surface side of the main body. Also, as shown in FIG. 3B, the neck portion may be a conduit extending from the upper surface side of the hollow portion, winding around the outer periphery side of the hollow portion, and then reaching the upper surface side of the main body. By changing the shape of the neck portion in this way (particularly the length l), it is also possible to adjust the frequency band in which the sound absorption peak of the resonator appears. For example, by increasing the length of the neck portion, it is possible to absorb (deaden) sound at lower frequencies.

[0054] Two or more of the cavities may be connected by a communication pipe provided separately from the neck portion. The communication pipe can be used to adjust the sound absorption frequency band, to discharge the powder remaining in the cavities produced by the powder sintering additive manufacturing method, etc.

[0055] [Third Example] As an application example of a structure, Fig. 4 shows a case (housing wall) of an electric motor disposed in the engine room of an automobile constructed from a sound-absorbing structure. By using such a case, not only the sound generated by the electric motor itself but also various sounds generated or reflected from other devices (A to E in the figure, etc.) in the engine room are absorbed (deadened). Therefore, measures to silence the vehicle can be taken without the need to separately add (retrofit) sound-absorbing materials, etc.

[0056] In addition, the structure (such as the case) may be configured such that, for example, the area where a large stress acts (such as around the bolts of the flange) is solid, while porous areas or resonators are provided in areas where the acting stress is small. Such a structure is preferably made of metal, which can ensure rigidity, strength, etc. In particular, structures used in high temperature or high humidity environments are preferably made of metal.

[0057] In this way, by using the sound absorbing structure of the present invention as a part of a component, noise reduction (reduction in sound pressure level, etc.) can be achieved over a wide frequency range without the need to add a new component.

Claims

1. A main body having a lattice-shaped porous portion opening on a surface side thereof; a resonator that is built into the main body and has a hollow portion and a neck portion that is a conduit that extends from a surface side of the main body to the hollow portion; The sound absorbing structure is formed by integrally forming a porous portion on the surface side of the main body and a hollow portion inside the main body.

2. 2. The sound absorbing structure according to claim 1, wherein the porous portion has an average pore size of 0.05 to 2 mm.

3. The sound absorbing structure according to claim 1 or 2, wherein the main body further comprises a solid portion.

4. 4. The sound absorbing structure according to claim 1, wherein the resonator has a plurality of cavities and a plurality of necks.

5. The sound absorbing structure according to claim 4 , wherein the resonator has a plurality of resonant frequencies.

6. 6. The sound absorbing structure according to claim 4, further comprising a communication pipe connecting two or more of the cavities.

7. 7. The sound absorbing structure according to claim 1, wherein at least a part of the wall surface of the cavity portion is a solid surface.

8. 8. The sound absorbing structure according to claim 1, wherein at least one of the neck portions is a conduit that bypasses the outer periphery of the hollow portion.

9. 9. The sound absorbing structure according to claim 1, wherein the main body is entirely made of metal.

10. The sound absorbing structure according to any one of claims 1 to 9, which constitutes at least a part of a housing.

11. A method for producing a sound absorbing structure according to any one of claims 1 to 10, comprising the steps of: producing the sound absorbing structure according to any one of claims 1 to 10 by a powder lamination method.

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