Method for manufacturing an acoustic attenuation structure with control of the positioning of an acoustic skin

Abstract

Method for manufacturing an acoustic attenuation structure with control of the positioning of an acoustic skin. A method for manufacturing an acoustic attenuation structure (100) comprising the following steps: - making a complex multi-element acoustic panel (120) comprising a plurality of hollow complex acoustic elements (121) and a plurality of partitions (131) forming acoustic cavities (132), each complex acoustic element being housed in an acoustic cavity so as to form an acoustic cell (150), - making a first skin, - drilling a plurality of perforations (111) at determined locations on the first skin to form a multi-perforated acoustic skin (110), - assembling the complex multi-element acoustic panel (120) with the multi-perforated acoustic skin (110).The multi-perforated acoustic skin (110) includes on its assembly face (112) a plurality of positioning studs (160) configured to cooperate with a hollow complex acoustic element (121) or an acoustic cavity when assembling the complex multi-element acoustic panel (120) with the multi-perforated acoustic skin (110). Figure for the abbreviation: Fig. 2.

Description

Title of the invention: Method for manufacturing an acoustic attenuation structure with control of the positioning of an acoustic skin technical field

[0001] The present invention relates to the general field of acoustic attenuation structures or panels. It relates more particularly to acoustic attenuation structures used to reduce noise produced in aircraft engines such as gas turbines or exhausts thereof. Previous technique

[0002] Acoustic attenuation structures typically consist of a plate or skin with an acoustic surface permeable to the acoustic waves to be attenuated and a solid, reflective plate or skin, known as a "closing plate," with a cellular body, such as a honeycomb or a porous structure, arranged between these two surfaces. As is well known, such panels form Helmholtz resonators that attenuate acoustic waves over a certain frequency range. Acoustic attenuation structures of this type are described, in particular, in US patent 5,912,442 and GB patent 2,314,526.

[0003] These acoustic attenuation structures are limited to simple cell shapes such as the alveoli of a classic NID A® type structure. Consequently, the acoustic performance obtained is limited to the absorption of a very narrow frequency range.

[0004] One solution for increasing the acoustic attenuation frequency range is to superimpose two cell bodies having different cell shapes and dimensions. This solution has the disadvantage of significantly increasing the size of the acoustic attenuation structure.

[0005] Another known solution consists of placing open truncated cones in cells as described in document FR 3 082 987. While this solution reduces the size of the acoustic attenuation structure, its manufacture proves difficult when using a multi-perforated acoustic skin. In this case, a plurality of perforations are made in the skin to allow the sound waves to be attenuated to penetrate the cells of the structure. The perforations can be made after assembling the acoustic skin with a simple honeycomb-type cell body. On the other hand, when using an acoustic attenuation panel comprising complex acoustic cells, such as truncated cones housed in cells, the perforation of the acoustic skin is more complex. This cannot be done after assembly without risking damage to the cone truncates or other elements housed in the cells. Indeed, it is not feasible to interrupt the drilling tool's (e.g., drill bit) stroke immediately upon exiting the skin, as the holes would not be clean at the surface, thus compromising the structure's acoustic performance.

[0006] Therefore, when manufacturing a sound-attenuating structure comprising complex acoustic cells, it is preferable to perforate the acoustic skin before assembling it with the sound-attenuating panel. However, in this case, a significant challenge remains regarding the precise positioning of the multi-perforated acoustic skin with the sound-attenuating panel. It is crucial that the perforations in the acoustic skin open into the cavities defined by the complex acoustic cells and not onto solid portions of the panel, such as the cell edges, in order to avoid compromising the acoustic performance of the structure. In large-scale sound-attenuating structures, even a slight local misalignment of the acoustic skin can lead to significant misalignments between the perforations and the cavities of the acoustic cells at the end of the component.

[0007] Furthermore, when perforations open onto solid portions of the panel due to misalignment, these perforations become blind holes which are both acoustically useless and aerodynamically detrimental because they generate an increase in acoustic skin drag. Description of the invention

[0008] The main purpose of the present invention is therefore to propose an acoustic attenuation structure which does not present the aforementioned disadvantages.

[0009] In accordance with the invention, this objective is achieved through a method for manufacturing an acoustic attenuation structure comprising the following steps: - creation of a complex multi-element acoustic panel comprising a plurality of hollow complex acoustic elements, each having a shape that gradually narrows between a base and a top, and a plurality of partitions forming acoustic cavities, each complex acoustic element being housed in an acoustic cavity so as to form an acoustic cell, - creation of a first layer, - drilling a plurality of perforations at specific locations on the first layer so as to form a multi-perforated acoustic layer, - assembly of one face of the complex multi-element acoustic panel with an assembly face of the multi-perforated acoustic skin, characterized in that the multi-perforated acoustic skin comprises on its face assembly a plurality of positioning studs present at determined locations, each positioning stud being configured to cooperate with a hollow complex acoustic element or acoustic cavity when assembling the complex multi-element acoustic panel with the multi-perforated acoustic skin.

[0010] Thanks to the process of the invention, it is possible to make the perforations in the acoustic skin before its assembly with the other elements of the structure, thus allowing a complete and homogeneous perforation of the skin, while ensuring a precise positioning of the perforations in relation to the acoustic cells of the complex multi-element acoustic panel when assembling the acoustic skin with said panel.

[0011] According to a first particular aspect of the method of the invention, one or more positioning pads have a shape complementary to the internal shape of the hollow complex acoustic elements.

[0012] According to a second particular aspect of the method of the invention, one or more positioning pads have a shape complementary to the shape of the acoustic cavities.

[0013] According to a third particular aspect of the method of the invention, the acoustic attenuation structure further comprises the assembly of the face of the complex multi-element acoustic panel opposite the face covered by the multi-perforated acoustic skin with an assembly face of a closing skin.

[0014] According to a fourth particular aspect of the method of the invention, at least one positioning block comprises a fastening element. Advantageously, the material reserve constituted by the positioning blocks is used as a base for fastening elements that can, in particular, contribute to securing the acoustic attenuation structure.

[0015] According to a fifth particular aspect of the method of the invention, the complex acoustic elements have a pyramidal, conical or spiral shape.

[0016] According to a sixth particular aspect of the method of the invention, at least the complex multi-element acoustic panel is made of a thermoplastic or thermosetting material, whether filled or not. This makes it possible to control the overall mass of the structure because the complex acoustic elements can be formed by injection molding and have very thin thicknesses.

[0017] According to a seventh particular aspect of the process of the invention, the complex multi-element acoustic panel is produced by injection-compression of a thermoplastic or thermosetting material, whether filled or not. Injection-compression makes it possible to further reduce the wall thickness of the complex acoustic elements.

[0018] According to an eighth particular aspect of the process of the invention, each positioning pad is manufactured with the acoustic skin by thermo- stamping-overmolding plastic or stamping with metal inserts.

[0019] According to a ninth particular aspect of the process of the invention, the positioning pads and the acoustic skin are made of a thermoplastic material, each positioning pad being attached by welding or by gluing to said skin.

[0020] According to a tenth particular aspect of the process of the invention, each positioning pad is produced by additive manufacturing.

[0021] According to an eleventh particular aspect of the process of the invention, the complex multi-element acoustic panel and the plurality of partitions are made in a single piece by injection of a thermoplastic or thermosetting material, whether filled or not. The manufacture of the acoustic attenuation structure is greatly simplified here because the complex acoustic elements are already positioned relative to the partitions.

[0022] The invention also relates to an acoustic attenuation structure comprising: - a complex multi-element acoustic panel comprising a plurality of hollow complex acoustic elements, each having a shape that gradually narrows between a base and a top, and a plurality of partitions forming acoustic cavities, each complex acoustic element being housed in an acoustic cavity so as to form an acoustic cell, - a multi-perforated acoustic skin comprising a plurality of perforations, said acoustic skin having an assembly face fixed to a face of the complex multi-element acoustic panel, characterized in that the multi-perforated acoustic skin comprises on its assembly face a plurality of positioning studs present at determined locations, each positioning stud cooperating with a hollow complex acoustic element or an acoustic cavity of the complex multi-element acoustic panel. Brief description of the drawings

[0023] [Fig-1] Fig. 1 is a schematic exploded perspective view of a structure acoustic attenuation according to one embodiment of the invention,

[0024] [Fig.2] Fig.2 is a schematic cross-sectional view of the attenuation structure acoustics of the [Fig. 1] once assembled,

[0025] [Fig.3] Fig.3 is a schematic exploded perspective view of an acoustic attenuation structure according to another embodiment of the invention,

[0026] [Fig.4] Fig.4 is a schematic cross-sectional view of the attenuation structure acoustics of the [Fig.3] once assembled. Description of the implementation methods

[0027] Figures 1 and 2 represent a 100 acoustic attenuation structure in accordance with an embodiment of the invention. The acoustic attenuation structure 100 here comprises an acoustic skin or plate 110, a complex multi-element acoustic panel 120 and a closing skin or plate 140.

[0028] The closing skin 140 corresponds to a solid surface designed to reflect sound waves entering the acoustic attenuation structure. The closing skin may be a constituent element of the acoustic attenuation structure, as in the example described here, or it may correspond to a structure of an object, for example, an aircraft engine. In the latter case, the acoustic attenuation structure of the invention does not include a closing skin and is mounted directly onto the structure of the object.

[0029] In the embodiment described here, the complex acoustic multi-element panel 120 is formed as a single piece extending lengthwise and widthwise along a horizontal direction DH and heightwise along a vertical direction Dv. The complex acoustic multi-element panel comprises a plurality of hollow complex acoustic elements 121, each having a shape that gradually narrows between a base 122 and an apex 123. In the example described here, the hollow complex acoustic elements 121 have a pyramidal shape. The base 122 of each hollow complex acoustic element 121 is in continuous contact with the base of the adjacent complex acoustic elements so as to form a continuous network of edges 124.

[0030] The complex acoustic multi-element panel 120 further comprises a plurality of partitions 131 formed by a network of ribs 130 that form a plurality of acoustic cavities 132. Each hollow complex acoustic element 121 is housed in an acoustic cavity 132. The upper edge 131a of the partitions 131 extends from the lower portion 122b of the bases 122 of the hollow complex acoustic elements 121 along the vertical direction Dv ([Fig. 2]). In this embodiment, the complex acoustic multi-element panel is formed in a single piece with the plurality of partitions, thus eliminating potential positioning problems between the hollow complex acoustic elements and the plurality of partitions. According to an alternative embodiment, the back skin can be integrated into the acoustic multi-element panel-partition assembly by polymer or composite additive manufacturing.

[0031] The complex acoustic multi-element panel 120 comprises a plurality of acoustic cells 150 each formed by a hollow complex acoustic element 121 and the partitions 131 surrounding it.

[0032] The complex acoustic multi-element panel 120 comprises a first assembly face constituted here by the upper portion 122a of the bases 122 of the hollow complex acoustic elements 121 corresponding to the exposed surface of the edges 124, this first assembly face is intended to be assembled with the acoustic skin 110. The complex multi-element acoustic panel 120 also includes a second assembly face consisting here of the lower edge 131b of the partitions 131.

[0033] The acoustic skin 110 has the function of allowing the sound waves to be attenuated to pass through the acoustic attenuation structure 100. For this purpose, the acoustic skin 110 comprises a plurality of perforations 111. The acoustic skin 110 has an assembly face 112 opposed to an external face 113 and which is intended to be placed opposite the complex multi-element acoustic panel 120.

[0034] According to the invention, the perforations 111 are made by drilling into the skin 110 before its assembly with the complex acoustic multi-element panel 120. This allows the acoustic skin to be drilled through without risk of damaging the complex acoustic elements. The perforations 111 are made at specific locations on the skin 110 so as to open into the free internal volume of the acoustic cells 150 and not on the edges 124 of the complex acoustic multi-element panel 120 once the acoustic skin 110 has been assembled with said complex acoustic multi-element panel. One or more perforations 111 can be aligned with each acoustic cavity 150. The perforations 111 can represent between 5% and 10% of the total surface area of ​​the acoustic skin HO. Also according to the invention, the acoustic skin comprises on its assembly face 112 a plurality of positioning studs 160, here four in number.The minimum number of positioning studs is two to allow for precise positioning of the acoustic skin with the complex multi-element acoustic panel. The positioning studs can be manufactured with the acoustic skin, for example, by thermoplastic overmolding or stamping with metal inserts. The studs can also be attached by welding or bonding in the case of studs and a skin made of thermoplastic material. The studs can also be formed by additive manufacturing. The studs can be metallic, made of thermoplastic matrix composite material, or thermosetting, filled or unfilled. The studs do not necessarily have to be solid.

[0035] The positioning studs are preferably positioned on the acoustic skin after the perforations have been made by drilling, the studs then serving as a reference for determining the drilling locations. This further increases the accuracy of the perforation positioning. However, it does not depart from the scope of the invention when the positioning studs are placed or formed on the acoustic skin after the perforations have been drilled.

[0036] In the example described here, the positioning studs 160 have a shape complementary to the internal shape of the hollow complex acoustic elements 121. The The pads can also have a shape that is not complementary to the internal shape of the complex hollow acoustic elements but is suitable for cooperating with these elements to allow for proper positioning. For example, spherical pads can be easily centered in hollow elements with a conical or pyramidal shape.

[0037] During the assembly of the assembly face 112 of the acoustic skin 110 with the first assembly face formed by the upper portion 122a of the bases 122 of the complex acoustic elements 121, the positioning studs 160 cooperate with the hollow complex acoustic elements 121 in order to facilitate the positioning of the acoustic skin 110 in relation to the complex acoustic multi-element panel 120 and thus ensure perfect alignment of the perforations 111 with the free internal volume of the acoustic cells 150 ([Fig.2]).

[0038] The acoustic skin 110 is fixed, by gluing or welding, to the upper portion 122a of the bases 122 of the complex acoustic elements 121 corresponding to the exposed surface of the edges 124 while the closing skin 140 is fixed, for example by gluing or welding, to the lower edge 131b of the partitions 131.

[0039] The height Hm of the complex acoustic elements 121 is less than the height H no of the acoustic cavities 132. More precisely, the height of the complex acoustic elements is between 10% and 99% of the height Hi32 of the acoustic cavities along the vertical direction.

[0040] Figures 3 and 4 represent another embodiment of an acoustic attenuation structure of the invention which differs from the structure illustrated in Figures 1 and 2 in that the positioning pads cooperate with acoustic cavities as explained in detail below.

[0041] The acoustic attenuation structure 200 here comprises an acoustic skin or plate 210, a complex acoustic multi-element panel 220 and a closing skin or plate 240.

[0042] The closing skin 240 corresponds to a solid surface designed to reflect sound waves entering the acoustic attenuation structure. The closing skin may be a constituent element of the acoustic attenuation structure, as in the example described here, or it may correspond to a structure of an object, for example, an aircraft engine. In the latter case, the acoustic attenuation structure of the invention does not include a closing skin and is mounted directly onto the structure of the object.

[0043] In the embodiment described here, the complex acoustic multi-element panel 220 is formed as a single piece extending lengthwise and widthwise along a horizontal direction DH and heightwise along a vertical direction Dv. The complex acoustic multi-element panel comprises a plurality of elements complex hollow acoustic elements 221, each having a shape that gradually narrows between a base 222 and an apex 223. In the example described here, the complex hollow acoustic elements 221 have a pyramidal shape. The base 222 of each complex hollow acoustic element 221 is in continuous contact with the base of the adjacent complex acoustic elements so as to form a continuous network of edges 224.

[0044] Still in the example described here, the complex acoustic multi-element panel 220 is devoid of hollow complex acoustic elements 221 at determined locations on the panel in order to allow the cooperation of acoustic cavities with positioning studs as explained below.

[0045] The complex acoustic multi-element panel 220 further comprises a plurality of partitions 231 formed by a network of ribs 230 that form a plurality of acoustic cavities 232. Each hollow complex acoustic element 221 is housed in an acoustic cavity 232. The upper edge 231a of the partitions 131 extends from the lower portion 222b of the bases 222 of the hollow complex acoustic elements 221 along the vertical direction Dv ([Fig. 4]). In this embodiment, the complex acoustic multi-element panel is formed in a single piece with the plurality of partitions, thus eliminating potential positioning problems between the hollow complex acoustic elements and the plurality of partitions.

[0046] The complex acoustic multi-element panel 220 comprises a plurality of acoustic cells 250 each formed by a hollow complex acoustic element 221 and the partitions 231 surrounding it.

[0047] The complex acoustic multi-element panel 220 has a first assembly face formed here by the upper portion 222a of the bases 222 of the hollow complex acoustic elements 221 corresponding to the exposed surface of the edges 124, this first assembly face being intended to be assembled with the acoustic skin 210. The complex acoustic multi-element panel 220 further has a second assembly face formed here by the lower edge 231b of the partitions 231.

[0048] The acoustic skin 210 has the function of allowing the sound waves to be attenuated to pass into the acoustic attenuation structure 200. For this purpose, the acoustic skin 210 comprises a plurality of perforations 211. The acoustic skin 210 has an assembly face 212 opposed to an external face 213 and which is intended to be placed opposite the complex multi-element acoustic panel 220.

[0049] According to the invention, the perforations 211 are made by drilling into the skin 210 before its assembly with the complex acoustic multi-element panel 220. This makes it possible to drill through the acoustic skin without risk of damaging the complex acoustic elements. The perforations 211 are The perforations are made at specific locations on the skin 210 so as to open into the free internal volume of the acoustic cells 250 and not on the edges 224 of the complex multi-element acoustic panel 220 once the acoustic skin 210 is assembled with said complex multi-element acoustic panel. One or more perforations 211 may be aligned with each acoustic cavity 250. The perforations 211 may represent between 5% and 10% of the total surface area of ​​the acoustic skin 210.

[0050] In accordance with the invention, the acoustic skin comprises on its assembly face 212 a plurality of positioning studs 260, here four in number. The number of positioning studs is at least two in order to allow precise positioning of the acoustic skin with the complex multi-element acoustic panel.

[0051] In the example described here, the positioning studs 260 have a shape complementary to the internal shape of the acoustic cavities 232. The complex acoustic multi-element panel 220 is devoid of hollow complex acoustic elements 221 in the places where the positioning studs 260 are intended to cooperate with the acoustic cavities 232.

[0052] During the assembly of the assembly face 212 of the acoustic skin 210 with the first assembly face formed by the upper portion 222a of the bases 222 of the complex acoustic elements 221, the positioning studs 260 cooperate with the acoustic cavities 232 in order to facilitate the positioning of the acoustic skin 210 in relation to the complex acoustic multi-element panel 220 and thus ensure perfect alignment of the perforations 211 with the free internal volume of the acoustic cells 250 ([Fig.4]).

[0053] The acoustic skin 210 is fixed, by gluing or welding, to the upper portion 222a of the bases 222 of the complex acoustic elements 221 corresponding to the exposed surface of the edges 224 while the closing skin 240 is fixed, for example by gluing or welding, to the lower edge 231b of the partitions 231.

[0054] The height H22i of the complex acoustic elements 221 is less than the height H 232 of the acoustic cavities 232. More precisely, the height H22i of the complex acoustic elements is between 10% and 99% of the height H232 of the acoustic cavities along the vertical direction.

[0055] According to one embodiment, the acoustic skin can be provided with both positioning studs having a shape complementary to the internal shape of the hollow complex acoustic elements such as studs 160 and positioning studs having a shape complementary to the internal shape of the acoustic cavities such as studs 260.

[0056] According to yet another embodiment, the complex acoustic elements hollows, on the one hand, and the partitions of the complex multi-element acoustic panel, on the other hand, can be made separately and joined together when assembling the acoustic attenuation structure.

[0057] The acoustic cavities formed by the plurality of partitions and in which the complex hollow acoustic elements are housed can have a square shape as illustrated in Figures 1 and 3 or any other shape suitable for acoustic treatment such as for example a honeycomb-type (hexahedral) shape.

[0058] The acoustic attenuation structure may have a flat shape as illustrated in Figures 1 to 4 or a curved shape exhibiting a single or multiple curvature.

[0059] One or more positioning studs can also be advantageously used as a support for a fixing device allowing the acoustic attenuation structure to be fixed to a part of an engine such as, for example, a blower housing of a gas turbine engine.

[0060] The complex acoustic multi-element panel is produced by injection molding a thermoplastic or thermosetting material, filled or unfilled. Injection molding allows for control of the overall mass of the structure because the complex acoustic elements can be formed with very thin thicknesses, for example, on the order of 1 mm.

[0061] According to a particular feature of the process of the invention, the complex acoustic multi-element panel is produced by injection-compression of a thermoplastic or thermosetting material, whether filled or not. Injection-compression consists of injecting the material into a partially open mold. Thus, even if the material solidifies, the channels become less clogged. Once the material is distributed throughout the mold, the mold is completely closed (by a closing force) to return to the correct dimensions. This makes it possible to obtain thinner wall thicknesses for the complex acoustic multi-element panel than with a conventional injection process. Injection-compression makes it possible to obtain wall thicknesses in complex acoustic multi-element panels of between 0.1 mm and 0.5 mm.

[0062] Thermoplastic materials that can be used for the injections described above include polyaryletherketones (PAEK) such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK).

[0063] Thermosetting materials that can be used for the injections described above include epoxy or polybismaleimides (BMI).

[0064] The plurality of partitions, the closing skin, as well as the assemblies combining in a single piece the plurality of partitions and the complex multi-element acoustic panel or the plurality of partitions and one of the skins, can also be made by injection of a thermoplastic or thermosetting material, filled or unfilled. A plurality of partitions can also be obtained by using a honeycomb structure, for example in aluminium or Nomex®.

Claims

Claims

1. A method of manufacturing an acoustic attenuation structure (100) comprising the following steps: - producing a complex acoustic multi-element panel (120) comprising a plurality of hollow complex acoustic elements (121) each having a shape gradually narrowing between a base (122) and a top (123) and a plurality of partitions (131) forming acoustic cavities (132), each complex acoustic element being housed in an acoustic cavity so as to form an acoustic cell (150), - producing a first skin, - drilling a plurality of perforations (111) at determined locations on the first skin so as to form a multi-perforated acoustic skin (110), - assembling a face (122a) of the complex acoustic multi-element panel (120) with an assembly face (112) of the multi-perforated acoustic skin (110),characterized in that the multi-perforated acoustic skin (110) comprises on its assembly face (112) a plurality of positioning studs (160) present at determined locations, each positioning stud (160) being configured to cooperate with a hollow complex acoustic element (121) or an acoustic cavity (150) during the assembly of the complex acoustic multi-element panel (120) with the multi-perforated acoustic skin (110) and in that one or more positioning studs have a shape complementary to the internal shape of the hollow complex acoustic elements.,

2. The method of claim 1, wherein one or more positioning pads have a shape complementary to the shape of the acoustic cavities.

3. Method according to claim 1 or 2, in which the acoustic attenuation structure (200) further comprises assembling the face of the complex acoustic multi-element panel (220) opposite the face covered by the multi-perforated acoustic skin (210) with an assembly face of a closing skin (240).

4. A method according to any one of claims 1 to 3, wherein at least one positioning pad comprises a fixing member.

5. A method according to any one of claims 1 to 4, wherein the complex acoustic elements (121) have a pyramidal, conical or spiral shape.

6. Method according to any one of claims 1 to 5, in which at least the complex acoustic multi-element panel (120) is made of a filled or unfilled thermoplastic or thermosetting material.

7. Method according to claim 6, in which at least the complex acoustic multi-element panel (120) is produced by injection of a thermoplastic or thermosetting material, loaded or not.

8. Method according to claim 7, in which the complex acoustic multi-element panel (120) and the plurality of partitions (131) are made in a single piece by injection of a filled or unfilled thermoplastic or thermosetting material.

9. A method according to any one of claims 1 to 8, wherein each positioning pad is manufactured with the acoustic skin by thermoplastic stamping-overmolding or stamping with metal inserts.

10. Method according to any one of claims 1 to 8, in which the positioning pads and the acoustic skin are made of a thermoplastic material, each positioning pad being attached by welding or by gluing to said skin.

11. A method according to any one of claims 1 to 8, wherein each positioning pad is produced by additive manufacturing.

12. Acoustic attenuation structure (100) comprising: - a complex acoustic multi-element panel (120) comprising a plurality of hollow complex acoustic elements (121) each having a shape gradually narrowing between a base (122) and a top (123) and a plurality of partitions (131) forming acoustic cavities (132), each complex acoustic element being housed in an acoustic cavity so as to form an acoustic cell (150), - a multi-perforated acoustic skin (110) comprising a plurality of perforations (111), said acoustic skin having an assembly face (112) fixed to a face (122a) of the complex acoustic multi-element panel (120), characterized in that the multi-perforated acoustic skin (110) comprises on its assembly face (112) a plurality of positioning studs (160) present at determined locations, each positioning stud (160) cooperating with a hollow complex acoustic element (121) or an acoustic cavity (150) of the complex acoustic multi-element panel (120) and in that one or more positioning pads have a shape complementary to the internal shape of the hollow complex acoustic elements.

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