Method for manufacturing acoustic panels by welding

Abstract

The invention relates to a method for manufacturing an acoustic panel (120) by welding at least two acoustic components (121, 122) of thermoplastic material, wherein a thermal gradient is applied to the acoustic components (121, 122) during the welding operation in order to reduce the temperature above the glass transition temperature or melting temperature (T f ) temperature (T j ) is applied to the area to be welded and in order to reduce the temperature to a value lower than or equal to the geometric stability temperature of the material (T s ) temperature (T e ) is applied to the portion farthest from the area to be welded.

Description

Technical Field

[0001] The present invention relates to the general field of sound attenuation structures. The present invention relates more particularly to sound attenuation structures for reducing the noise generated in aircraft engines and gas turbines or their exhaust gases. Background Art

[0002] The sound attenuation structure typically consists of an acoustic panel or skin permeable to the sound waves to be attenuated and a fully reflective panel or skin, known as a "closing plate", between which a porous body is arranged. The porous body is usually made of a set of partitions, for example in the shape of a honeycomb, and of an acoustic panel comprising a composite hollow acoustic element, such as a cone. This structure forms a Helmholtz-type resonator in a well-known manner, which makes it possible to attenuate sound waves within a specific frequency range. This type of sound attenuation structure is particularly described in documents US 5 912 442 and GB 2 314 526.

[0003] In order to produce a large-sized sound attenuation structure, a large-sized acoustic panel must be manufactured.

[0004] Acoustic panels are conventionally manufactured by injection and / or stamping. However, the size of the achievable structure is limited by the manufacturing apparatus, for example by the size of the injection tool.

[0005] Thus, to produce a large-sized sound attenuation structure, it may be necessary to assemble several smaller acoustic subpanels, also called acoustic components.

[0006] The assembly of the acoustic components can be carried out using fixing systems, including holes for example. However, such fixing systems reduce the acoustic performance and increase the bulk. In fact, the functional surface of the acoustic component intended for sound attenuation is reduced to allow the placement of these fixing systems.

[0007] Acoustic components can also be assembled by welding, by significantly heating the joints between the acoustic components. However, by heat diffusion, welding also causes heating of the parts of the acoustic components away from the joints, which leads to deformation of the components and thus to uncontrolled changes in their geometry.

[0008] Therefore, despite the high temperatures, large-sized holding tools must be used to maintain the geometry of the acoustic component. The size of the achievable acoustic structure is therefore limited by the size of the holding tool. Summary of the invention

[0009] Therefore, the main object of the present invention is to manufacture acoustic structures of large dimensions while overcoming the above mentioned drawbacks.

[0010] According to the invention, this object is achieved by means of a method for manufacturing an acoustic panel, the method comprising manufacturing at least a first acoustic component and a second acoustic component made of thermoplastic material, the first acoustic component and the second acoustic component each comprising a plurality of hollow acoustic elements, the first component comprising a first joining edge and the second component comprising a second joining edge, the first component and the second component being welded to each other via the first and second joining edges to form at least a part of the acoustic panel,

[0011] The method is characterized in that, during the welding operation, the tool covers the portion of the first and second components including the first and second joining edges so as to match the geometry of the covered hollow acoustic element, and in that a thermal gradient is applied to the portion of the first and second components covered by the tool so as to apply a temperature above the glass transition temperature or melting temperature of the thermoplastic material to the first and second joining edges and so as to apply a temperature below or equal to the geometric stability temperature of the thermoplastic material to that portion of the hollow acoustic element covered by the tool which is farthest from the first and second joining edges.

[0012] Therefore, in order to manufacture acoustic panels of large dimensions, it is of particular interest to assemble these acoustic components by welding them to one another so as to maintain a large functional surface from an acoustic point of view without increasing the volume. The installation costs are also reduced, since one large acoustic component can be handled instead of several small ones. Finally, the overall mass of the acoustic panel is reduced by strongly limiting the number and size of the necessary fixing elements or even eliminating them completely.

[0013] The welding of the joint edges of the components requires high temperatures, which may deform or change the geometry of the acoustic component by heat diffusion, including in the parts far from the weld. Therefore, in order to maintain the geometry of the acoustic component despite the high temperature without having to use large-sized heating or holding tools, the present invention proposes the use of a thermal gradient.

[0014] Thus, only the parts of the acoustic component situated close to the weld are subjected to high temperatures, while at the tool exit the temperature of the acoustic components is low enough to avoid any deformation or modification of their geometry. Thus, the dimensions of the welding tool used are significantly smaller than the dimensions of the acoustic panel to be manufactured, thus making it possible to manufacture acoustic panels of large dimensions. The welding tool used is also easier to handle and can be reused very quickly. Thus, it makes it possible to carry out several welds in succession between different acoustic components.

[0015] According to a particular feature of the invention, a portion of the hollow acoustic element of the first and second parts is located outside the tool.

[0016] According to another particular feature of the invention, the second joining edge partially covers the first joining edge.

[0017] According to another particular feature of the invention, there is an additional thickness of material along the first and second joining edges.

[0018] According to another particular feature of the invention, the thermal gradient is created by pulsed air by carrying out control of each tool temperature zone.

[0019] According to another particular feature of the invention, the thermoplastic material is polyetherimide.

[0020] According to another particular feature of the invention, the temperature applied to the first and second joining edges is greater than 420° C., and the temperature applied to the hollow acoustic element covered by the tool farthest from the first and second joining edges is less than 215° C. Preferably, the temperature applied to the hollow acoustic element covered by the tool farthest from the first and second joining edges is less than 200° C.

[0021] In the case of acoustic components made of polyetherimide (PEI), the temperatures described herein are particularly relevant.

[0022] The invention further relates to a method for manufacturing a sound attenuation structure, the method comprising the following steps:

[0023] - manufacturing an acoustic panel according to the method of the invention,

[0024] - manufacturing a porous body comprising an acoustic panel and a plurality of partitions forming an acoustic cavity, each hollow acoustic element of the acoustic panel being housed in the acoustic cavity to form an acoustic chamber,

[0025] - Assemble the first surface of the porous body with the assembly surface of the acoustic surface.

[0026] According to a particular feature of the invention, the method further comprises assembling a second face opposite to the first face covered by the acoustic skin and an assembled face of the closing skin.

[0027] The invention further relates to an aerospace engine casing or an aerospace propulsion assembly casing comprising at least one sound attenuation structure manufactured according to the method of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a perspective view of two acoustic parts to be welded to obtain an acoustic panel.

[0029] Figure 2 yes Figure 1 Perspective view of the positioning of the two acoustic components.

[0030] Figure 3 yes Figure 1 A perspective view of the positioning of two acoustic components, with the joining edge including additional thickness of material.

[0031] Figure 4 yes Figure 1 A perspective view of the positioning of two acoustic components with the joining edges overlapping at the junction of the two components.

[0032] Figure 5 is a cross-sectional view of a welding tool in which an acoustic component to be welded is positioned.

[0033] Figure 6 yes Figure 5 Representation of the temperature of the acoustic component inside the welding tool as a function of the distance of the acoustic component from the joint.

[0034] Figure 7 is an exploded perspective view of an acoustic structure including an acoustic panel manufactured according to the method of the present invention.

[0035] Figure 8 yes Figure 7 Cross-sectional view of the acoustic structure represented in . DETAILED DESCRIPTION

[0036] Figures 1 to 6 A method for manufacturing an acoustic panel comprising a composite hollow acoustic element according to the invention is shown. The acoustic panel is intended to be assembled with an acoustic skin, a plurality of partitions and possibly a closing skin to form a complete sound attenuation structure.

[0037] Figure 1 A first acoustic component 121 and a second acoustic component 122 are shown.

[0038] The acoustic members 121 and 122 extend in width along the X direction and extend in length along the Y direction.

[0039] The first acoustic member 121 includes an upper surface 121c and a lower surface 121d opposite to the first surface 121c. The surfaces 121c and 121d extend along the X direction and the Y direction. The upper surface 121c of the first acoustic member 121 includes a plurality of hollow composite acoustic elements 121b.

[0040] The second acoustic member 122 includes an upper surface 122c and a lower surface 122d opposite to the upper surface 122c. The surfaces 122c and 122d extend along the X direction and the Y direction. The upper surface 122c of the second acoustic member 122 includes a plurality of hollow composite acoustic elements 122b.

[0041] exist Figures 1 to 8In the embodiment shown, the acoustic components 121 and 122 are straight. Of course, if the acoustic components are curved or have angles, there is no deviation from the framework of the invention. In this case, the welding tool described below must be adapted to the curvature or angle of the acoustic components.

[0042] The first acoustic component 121 and the second acoustic component 122 are intended to be welded to each other to form an acoustic panel. Thus, the upper face 121c of the first acoustic component 121 comprises a first joining edge 121a, and the upper face 122c of the second acoustic component 122 comprises a second joining edge 122a. Thus, the first joining edge 121a of the first acoustic component 121 is intended to be welded to the second joining edge 122a of the second acoustic component 122.

[0043] The acoustic components 121 and 122 are formed as a single piece. They can be manufactured in a known manner for thermoplastic materials, by injection or stamping. The thermoplastic material can be loaded with short fibers or continuous fibers. The thermoplastic material can be unloaded.

[0044] The acoustic components 121 and 122 can also be manufactured in a well-known manner by injection compression of loaded or unloaded thermoplastic material. Injection compression consists in injecting the material into a half-open mold. Thus, even if the material freezes, the passage becomes less blocked. When the material is distributed throughout the mold, it is completely closed by the closing force to return to the correct size. This makes it possible to obtain thinner wall thicknesses for acoustic components than with conventional injection methods.

[0045] Acoustic components 121 and 122 can also be manufactured in a known manner by temperature control of a tool loaded or unloaded with thermoplastic material. Injection with tool temperature control consists in monitoring the temperature of the tool or mold by means of a system for servo-controlling the tool temperature, for example with a heat transfer fluid or with air.

[0046] Thermoplastic materials that can be used to produce the acoustic components 121 and 122 are in particular polyaryletherketones (PAEK), such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), polyetherimide (PEI), polyphenylene sulfide (PPS), polyethersulfone (PESU) and polysulfone (PSU).

[0047] exist Figures 1 to 8 In the embodiment presented in the embodiment, the hollow composite acoustic element has a pyramidal shape. However, it does not depart from the framework of the invention if the hollow composite acoustic element has, for example, a cone, a spiral or a funnel shape. Preferably, the hollow composite acoustic element has a shape that tapers gradually from the upper side of the component towards the lower side of the acoustic component.

[0048] The acoustic components 121 and 122 are then put into position to allow welding, such as Figure 2As shown. Thus, the first joining edge 121a of the first acoustic component 121 is placed near or in contact with the second joining edge 122a of the second acoustic component 122 to define a joint j between the acoustic components 121 and 122. The positioning of the acoustic components 121 and 122 relative to each other makes it possible to obtain the shape of the acoustic panel to be manufactured before welding.

[0049] according to Figure 3 In a variation shown in , the additional thickness 123 may be present on the upper face 121c of the first acoustic component 121 along the first joint edge 121a. Similarly, the additional thickness 123 may be present on the upper face 122c of the second acoustic component 122 along the second joint edge 122a. The additional thickness of the material 123 rises perpendicularly to the directions X and Y along the joint j. These additional thicknesses of the material 123 are conventionally made of thermoplastic material, preferably the same material as that used to make the acoustic component 121 or 122. Preferably, these additional thicknesses 123 are made simultaneously with the rest of the acoustic component to form a single-piece whole. These additional thicknesses 123 facilitate welding at the joint j of the acoustic components 121 and 122 by providing excess thermoplastic material, so that the gap between the two components can be filled more easily during the welding process. Vertical pressure can also be applied to the entire tool or only to the joint j so as to force the additional thickness 123 into the gap present between the two components.

[0050] according to Figure 4 In a variation shown in FIG. 1 , an overlap of the first acoustic component 121 and the second acoustic component 122 can be made at the joint j between the two components 121 and 122. In particular, the overlap can be made by reducing the thickness of the first joint edge 121a on a portion and / or by reducing the thickness of the second joint edge 122a on a portion.

[0051] Figure 5 There is shown a welding tool 500 comprising a die 510 and a counter-die 520. The die 510 comprises teeth 510c and the counter-die 520 comprises a molding cavity 520d intended to cooperate with the teeth 510c to match the shape of the acoustic components 121 and 122 near their joints j.

[0052] The mold and / or the counter-mold of the welding tool may have several different parts in order to facilitate the application of different pressures or temperatures in specific areas of the acoustic component, for example by differentiating the joining area, in other words the welding area, from other areas.

[0053] like Figure 5As shown, acoustic components 121 and 122 are positioned in welding tool 500 such that first and second joining edges 121a and 122a exist entirely within tool 500 , while portions of acoustic components 121 and 122 farthest from joining edges 121a and 121b are preferably located outside tool 500 .

[0054] In particular, the upper surface 121c of the first acoustic component 121 and the upper surface 122c of the second acoustic component 122 are placed in contact with the mold 510 so that the teeth 510c of the mold 510 match the shapes of the hollow composite acoustic elements 121b and 122b of the acoustic components 121 and 122. In addition, the lower surface 121d of the first acoustic component 121 and the lower surface 122d of the second acoustic component 122 are placed in contact with the counter mold 520 so that the molding cavity 520d of the counter mold 520 matches the shapes of the hollow composite acoustic elements 121b and 122b of the acoustic components 121 and 122.

[0055] The welding tool 500 has a temperature regulation system so that a temperature gradient can be applied to the acoustic components 121 and 121 which are partially arranged in the welding tool 500 .

[0056] When the acoustic components 121 and 122 are placed in the welding tool 500, a thermal gradient is applied in the mold 510 and / or in the counter mold 520. Figure 6 As shown, the thermal gradient thus achieved makes it possible to apply a temperature T at the joint of the acoustic components 121 and 122. j , the temperature T j Higher than the melting temperature T of thermoplastic materials f In practice, in the case of non-amorphous thermoplastics, preferably a temperature higher than the melting temperature T of the material is applied at the junction of the acoustic components 121 and 122 . f Temperature T j However, there is no departure from the framework of the invention if the temperature applied to the joint of the acoustic component is just above the glass transition temperature of the material. In the case of amorphous thermoplastics, a temperature above the glass transition temperature of the material can be applied to the joint of the acoustic component.

[0057] A temperature higher than the melting temperature T is applied to the joint of the acoustic components 121 and 122. f Temperature T j The thermal gradient also makes it possible to apply a gradually decreasing temperature to the portion e of the acoustic components 121 and 122 that is farthest from the joint j. At the end of the welding tool 500, the thermal gradient applies a temperature lower than the geometric stability temperature T of the thermoplastic material in the portion e of the acoustic components 121 and 122. s Temperature Te Therefore, the temperature that will diffuse into the portion of the acoustic components 121 and 122 located outside the mold will be lower than the temperature T existing at the end of the tool 500. e , and thus below the geometric stability temperature T s .

[0058] Figure 6 The distribution diagram of the thermal gradient presented in is given only as an example and is not limiting. In fact, other distribution diagrams are also possible, such as a Gaussian curve.

[0059] For example, in the case where the acoustic component is made of polyetherimide (PEI), the temperature T applied at the joint j of the two acoustic components is j The temperature T applied to the portion e covered by the mold and farthest from the joint j is preferably greater than 420°C. e It will preferably be below 215°C, and ideally below 180°C, to maintain a stable material.

[0060] Therefore, the portions of the acoustic components 121 and 122 that are located outside the welding tool 500 will not deform. s The teeth 510 of the mold 510 and the molding cavity 520 c of the counter mold 520 make it possible to maintain the geometric shapes of the acoustic components 121 and 122 during welding and during cooling.

[0061] By means of a heat transfer fluid, a thermal gradient can be made in the welding tool 500. For example, this technology is described in document US 2012267828A1. By induction, in a well-known manner, a thermal gradient can also be made. Finally, according to a preferred embodiment of the present invention, a thermal gradient can be made by pulsed air using the "PtFS" (Production to Functional Specifications) technology from the company "Surface Generation". This technology makes it possible to apply a very significant and local temperature increase or decrease ramp, typically from 30°C to 50°C per minute. In addition, the surface temperature can be monitored to the nearest degree Celsius.

[0062] A support system or tool may be used to support the portion of the acoustic component that is outside of the mold while positioned in the welding tool or during a welding operation (not shown).

[0063] During welding, pressure may be applied at the joint between the two acoustic components. In fact, when thermoplastic materials are brought to very high temperatures, pores may be created in the thermoplastic material. Therefore, the application of pressure on the acoustic components during welding helps to reduce these pores. In addition, this application of pressure may promote the mutual penetration of the thermoplastic chains at the joint between the two acoustic components. Finally, as mentioned above, in the case of an extra thickness at the joint between the two acoustic components, the application of pressure makes it possible to move the material of the extra thickness into the gap to be filled between the two acoustic components.

[0064] exist Figure 5 and Figure 6 In the embodiment shown, the welding tool 500 covers three rows of hollow composite acoustic elements 121b of the first acoustic component 121 from the joint j, and covers three rows of hollow composite acoustic elements 122b of the second acoustic component 122 from the joint j. Of course, if the welding tool covers more than three rows of acoustic elements of the acoustic component from the joint, it does not deviate from the framework of the present invention. Preferably, the welding tool covers at least 6 rows of hollow acoustic elements of the first component from the joint, and covers at least 6 rows of hollow acoustic elements of the second component from the joint. Preferably, the welding tool covers a maximum of 8 rows of hollow acoustic elements of the first component from the joint, and covers a maximum of 8 rows of hollow acoustic elements of the second component from the joint.

[0065] According to a preferred embodiment of the invention, in order to facilitate the application of the temperature gradient, the welding tool covers the same number of rows of acoustic elements on each side of the joint j, that is to say, the same number of rows of acoustic elements for each acoustic component. In fact, this configuration makes it possible to apply the thermal gradient symmetrically on either side of the joint j. If the tool covers a different number of rows of acoustic elements for each acoustic component, it does not deviate from the framework of the invention.

[0066] After the welding operation described above, an acoustic panel 120 is obtained by welding two acoustic components 121 and 122 comprising a set of composite hollow acoustic elements 121b and 122b. By manufacturing an acoustic panel by welding two or more acoustic components, it is possible to obtain an acoustic panel of very large dimensions. According to a particular embodiment of the invention, the acoustic panel thus obtained may comprise a side portion having a length greater than or equal to 1,000 mm.

[0067] exist Figures 1 to 6 In the embodiment described in , the acoustic panel 120 is manufactured by welding two acoustic components 121 and 122. Of course, if the acoustic panel is manufactured by welding more than two acoustic components, it does not deviate from the framework of the present invention.

[0068] exist Figures 1 to 6In the embodiment presented in , the first acoustic component is identical to the second acoustic component. It does not depart from the framework of the invention if one or more acoustic components have dimensions or geometries that differ from the other components.

[0069] exist Figures 1 to 6 In the described embodiment, the acoustic panel 120 is manufactured to have a planar shape. If the acoustic panel is manufactured to be curved, there is no departure from the framework of the invention, in which case the acoustic components, welding means and any supporting means will be adapted.

[0070] Figure 7 and 8 A method for manufacturing a sound attenuation structure according to one embodiment of the present invention will be described. Here, the sound attenuation structure 100 includes an acoustic skin or plate 110 , an acoustic panel 120 manufactured as described above, a plurality of partitions 131 , and a closing skin or plate 140 .

[0071] The closed surface 140 corresponds to a solid surface intended to reflect the sound waves entering the sound attenuation structure. The closed surface can be a constituent element of the sound attenuation structure as in the embodiments described herein, or it can correspond to the structure of an object, such as an aircraft engine wall. In the latter case, the sound attenuation structure of the invention does not include a closed surface and is mounted directly on the structure of the object.

[0072] The acoustic skin 110 has the function of allowing sound waves to be attenuated to pass through the interior of the acoustic attenuation structure 100. For this purpose, and in the embodiment described here, the acoustic skin 110 comprises a plurality of perforations 111.

[0073] As described above, the acoustic panel 120 is formed by welding at least two acoustic members 121 and 122 , and extends in length along the X direction and in width along the Y direction.

[0074] In the exemplary embodiment described herein, the plurality of dividers 131 are made as a single piece, namely a grid of ribs 130 that, once assembled with the acoustic panel 120, form dividers around the composite hollow acoustic elements 121b and 122b.

[0075] Still in the embodiment described here, the sound attenuation structure 100 is made by assembling the acoustic panel 120 with a plurality of partitions 131, the upper edges 131a of the partitions 131 being fixed, for example by gluing or welding, based on the composite hollow acoustic elements 121b and 122b. The self-positioning of the hollow composite acoustic elements and the partitions greatly facilitates the assembly between these two elements.

[0076] The closing skin 140 is fixed, for example by gluing or welding, to the lower edge 131b of the partition 131, while the acoustic skin 110 is fixed, by gluing or welding, to the upper part of the base of the composite hollow acoustic elements 121b and 122b corresponding to the exposed surface of the edge. Thus, the acoustic skin and the closing skin are each fixed to a perfectly planar support, which ensures a very good seal between the skins and the assembly of the acoustic panel with a plurality of partitions.

[0077] Once assembled, the attenuation structure 100 includes a plurality of acoustic chambers 150, each formed by a composite hollow acoustic element 121b or 122b and a partition 131 surrounding it. The height H120b of the composite hollow acoustic elements 121b and 122b is less than the height H150 of the acoustic unit 150.

[0078] Multiple partitions can be made by injection compression of loaded or unloaded thermoplastic or thermosetting materials, by injection of loaded or unloaded thermoplastic or thermosetting materials, or by injection under temperature control of a tool of loaded or unloaded thermoplastic or thermosetting materials. Acoustic skins and closing skins can be made by covering or by stamping materials with continuous fibers, by RTM injection.

[0079] For example, the sound attenuation structure thus produced may be used in an aircraft engine casing, in particular in a casing of an aircraft propulsion assembly.

Claims

1. A method for manufacturing an acoustic panel (120), comprising manufacturing at least a first acoustic component (121) and a second acoustic component (122) made of thermoplastic material, the first acoustic component and the second acoustic component each comprising a plurality of hollow acoustic elements (121b, 122b), the first acoustic component (121) comprising a first joining edge (121a), and the second acoustic component (122) comprising a second joining edge (122a), the first acoustic component (121) and the second acoustic component (122) being welded to each other via the first joining edge (121a) and the second joining edge (122a) to form at least a portion of the acoustic panel (120), The method is characterized in that during the welding operation, the tool (500) covers a portion of the first acoustic component (121) including a first joint edge (121a) and a portion of the second acoustic component (122) including a second joint edge (122a) so as to match the geometry of the covered hollow acoustic element, The invention further comprises applying a thermal gradient to the portions of the first acoustic component (121) and the second acoustic component (122) covered by the tool (500) so as to increase the temperature above the glass transition temperature or melting temperature (T f ) temperature (T j ) is applied to the first joint edge (121a) and the second joint edge (122a) so as to be lower than or equal to the geometric stability temperature (T s ) temperature (T e ) is applied to the hollow acoustic element (121b, 122b) covered by the tool (500) which is farthest from the first joining edge (121a) and the second joining edge (122a).

2. The method according to claim 1, wherein: A portion of the hollow acoustic elements (121b, 122b) of the first acoustic component (121) and the second acoustic component (122) is located outside the tool (500).

3. The method according to claim 1 or 2, wherein: The second joining edge (122a) partially covers the first joining edge (121a).

4. The method according to claim 1 or 2, wherein: There is an additional thickness of material (123) along the first joining edge (121a) and the second joining edge (122a).

5. The method according to claim 1 or 2, wherein: The thermal gradient is created by pulsing air.

6. The method according to claim 1 or 2, wherein: The thermoplastic material is polyetherimide.

7. The method according to claim 6, wherein: The temperature (T j ) is greater than 420° C., and wherein the temperature (T) applied to the hollow acoustic element (121b, 122b) covered by the tool (500) which is farthest from the first joining edge (121a) and the second joining edge (122a) is greater than 420° C. e ) is less than 215℃.

8. A method for manufacturing a sound attenuation structure (100), comprising the steps of: - manufacturing an acoustic panel (120) according to the method of any one of claims 1 to 7, - manufacturing a porous body comprising an acoustic panel (120) and a plurality of partitions (131) forming an acoustic cavity, each hollow acoustic element (121b, 122b) of the acoustic panel (120) being accommodated in the acoustic cavity to form an acoustic chamber (150), - Assembling the first surface of the porous body with the assembly surface of the acoustic surface layer (110).

9. The method according to claim 8, further comprising assembling a second surface of the porous body opposite to the first surface covered by the acoustic surface layer (110) with an assembling surface of the closed surface layer (140).

10. An aircraft engine casing comprising at least one sound attenuation structure (100) manufactured according to the method of claim 8 or 9.

Images