Acoustic panel for aircraft turbomachine and related methods and apparatus

Abstract

The invention relates to a sound-absorbing panel (9) for an aircraft turbomachine (1), comprising:-a first shell layer (10) formed from a first composite material having a first polymeric matrix and reinforcing fibres embedded in the matrix,-a second shell layer (11) formed from a second composite material having a second polymeric matrix and reinforcing fibres embedded in the matrix, the invention relates to a composite material comprising:-a first shell layer (10) having a first thermoplastic polymer matrix and reinforcing fibers embedded therein,-a second composite material having a second thermoplastic polymer matrix and reinforcing fibers embedded therein, and-a core (12) having a honeycomb structure, the core (12) being arranged between the first shell layer (10) and the second shell layer (11), characterized in that the second shell layer (11) comprises pores (11a) having a diameter of less than 1 mm.

Description

Technical Field

[0001] The present invention relates to the field of sound absorbing panels for aircraft turbines, in particular for aircraft turbine nacelles.

[0002] The invention also relates to the field of manufacturing these sound absorbing panels. Background Art

[0003] The prior art is shown by documents US-B2-6 609 592, EP-A1-2 821 621, US-A1-2019 / 337632 and US-A1-2021 / 324794.

[0004] An aircraft turbomachine comprises, for example, from upstream to downstream in the direction of the air flow along the longitudinal axis, a fan, a low-pressure compressor and a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine and an exhaust nozzle.

[0005] The fan enables the suction of an air flow which is divided into a primary flow and a secondary flow. The primary flow passes through the primary duct of the turbine, while the secondary flow is directed towards a secondary duct surrounding the primary duct.

[0006] The main flow is compressed in the compressor. The compressed air is then mixed with fuel and burned in the combustion chamber. The gases produced by the combustion pass through the turbine and are then discharged through a nozzle whose cross section enables these gases to be accelerated to produce propulsion.

[0007] The fan generally comprises a disk rotatable about a longitudinal axis and blades mounted on the disk. The blades are surrounded by a fan housing, which is centered on the longitudinal axis and is configured to hold the blades in the event of, for example, blade damage.

[0008] The fan housing is usually surrounded by a nacelle that enables the fan to be protected. This type of fan is called a ducted fan, as opposed to a non-ducted fan where the blades are not surrounded by a housing.

[0009] Turbines, and in particular fans, are a major source of noise pollution and there is a strong desire to reduce this type of pollution. For this reason, it has been proposed that the nacelle be equipped with sound absorbing panels to reduce the noise generated by the turbine fan.

[0010] The sound absorbing panel generally comprises a sandwich structure including a first shell and a second shell, and a core is arranged between the first shell and the second shell, the core having a honeycomb structure configured to absorb sound waves. The first shell is attached to the housing of the fan, and the second shell is in contact with the secondary flow. The second shell generally has holes to enable sound waves to propagate into the core, through which the sound waves are absorbed. The diameter of the holes formed in the second shell is generally greater than 1 mm.

[0011] Although this type of sound absorbing panel significantly reduces the noise pollution generated by the turbine, this type of sound absorbing panel is not completely satisfactory. The second shell is the radially inner shell, that is, the shell closest to the longitudinal axis in the radial direction, and the first shell is the radially outer shell, that is, the shell farthest from the longitudinal axis in the radial direction. Therefore, the second shell is in contact with the secondary air flow, and the second shell is an element in the aerodynamic performance of the turbine. The holes provided in this second shell tend to increase the drag and reduce the aerodynamic performance of the turbine. However, these holes are necessary for the sound waves to pass through the sound absorbing panel and be absorbed. Therefore, noise reduction is currently achieved at the expense of the aerodynamic performance of the turbine.

[0012] Therefore, there is a need to provide a sound absorbing panel for an aircraft turbine that has good sound insulation performance while minimizing drag to maintain the aerodynamic performance of the turbine. Summary of the invention

[0013] To this end, the present invention proposes a sound absorbing panel for an aircraft turbine, the sound absorbing panel comprising:

[0014] a first shell layer formed from a first composite material having a first polymer matrix and reinforcing fibers embedded in the matrix,

[0015] a second shell layer formed from a second composite material having a second thermoplastic polymer matrix and reinforcing fibers embedded in the second matrix, and

[0016] - a core having a honeycomb structure, the core being arranged between a first shell and a second shell, the second shell comprising pores having a diameter of less than 1 mm.

[0017] The acoustic panel is notable in that it comprises a coating comprising a layer comprising a ceramic material and deposited by thermal spraying, the first skin layer being arranged between the coating and the core.

[0018] According to the invention, the sound absorbing panel comprises a second skin layer having holes with a diameter less than 1 mm.

[0019] It has been found that a second shell layer having holes of this diameter significantly reduces the aerodynamic drag of the turbine without affecting the soundproofing qualities of the acoustic panel.Thus, the invention can reduce noise pollution from the turbine while maintaining the aerodynamic performance of the turbine.

[0020] Furthermore, according to the invention, the second shell is made of a composite material with a thermoplastic polymer matrix. The polymer matrix enables holes with a diameter of less than 1 mm to be made without damaging the second shell. In fact, it has been found that certain drilling methods for making small diameter holes are not suitable for all materials. For example, laser drilling requires the use of a matrix that can withstand temperatures exceeding 300° C. Thermosetting polymer matrices cannot withstand such high temperatures, and drilled holes with a diameter of less than 1 mm in such matrices tend to degrade the second shell around the drilled hole area, thereby reducing the quality of the acoustic panel.

[0021] The invention thus makes it possible to provide a sound absorbing panel having good acoustic and aerodynamic properties.

[0022] The sound absorbing panel according to the present invention may include one or more of the following features, which may be used independently of each other or in combination with each other:

[0023] - the diameter of the holes is between 0.1 mm and 0.8 mm, preferably between 0.3 mm and 0.8 mm,

[0024] - the second shell has a thickness less than 1 mm, advantageously between 0.5 mm and 0.8 mm,

[0025] - the second polymer matrix has a melting temperature higher than 300° C., preferably higher than 400° C.,

[0026] - the coating further comprises an additional layer of glass fibers, the additional layer of glass fibers being arranged between the layer and the first shell layer,

[0027] - the additional layer comprises a first sub-layer of non-woven glass fibers and a second sub-layer of woven glass fibers,

[0028] - reinforcements,

[0029] The reinforcement comprises a frame comprising walls arranged on the edge of the second shell and / or studs arranged on the second shell.

[0030] The invention also relates to a method for manufacturing a sound absorbing panel according to any one of the preceding features, the method comprising the following steps:

[0031] - (100) forming a second shell by automated fiber placement,

[0032] -(102) holes are made in the second shell,

[0033] -(104) forming a core on the second shell by additive manufacturing, and

[0034] -(106) Forming a first shell layer on the core by additive manufacturing.

[0035] -(108) Depositing a layer of the coating onto the first shell layer by thermal spraying.

[0036] The method may include one or more of the following features, taken alone or in combination with each other:

[0037] -The holes are made by laser drilling.

[0038] The invention also relates to a device for manufacturing a sound absorbing panel according to any of the preceding features, the device comprising:

[0039] - a first station for producing the second shell, the first station comprising:

[0040] - an unwinder for a ribbon comprising fibers impregnated in a thermoplastic polymer matrix,

[0041] - rollers for compacting the strip,

[0042] - means for heating the strip, and

[0043] - means for drilling holes in the second shell,

[0044] - a second station for manufacturing the core and the first shell, the second station comprising an additive manufacturing device,

[0045] - a third station for producing the coating, comprising a thermal spraying device. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Other features and advantages will emerge from the following description of non-limiting embodiments of the invention and with reference to the accompanying drawings, in which:

[0047] Figure 1 is a schematic diagram of a longitudinal cross section of an aircraft turbine.

[0048] Figure 2 is a schematic perspective view of a sound absorbing panel according to the present invention,

[0049] Figure 3 is a schematic perspective view of an example of an embodiment of a core of a sound absorbing panel,

[0050] Figure 4 is a block diagram of a method for manufacturing a sound absorbing panel according to the present invention,

[0051] Figure 5 is a schematic diagram of an apparatus for manufacturing a sound absorbing panel according to the present invention,

[0052] Figure 6 is a partial schematic diagram of a manufacturing station for the second shell. DETAILED DESCRIPTION

[0053] Figure 1 An example of a turbomachine 1 for an aircraft is shown. The turbomachine 1 extends around and along a longitudinal axis X.

[0054] In the present application, the terms “upstream”, “downstream”, “axial” and “axially” are defined relative to the direction of the gas flow along the longitudinal axis X in the turbomachine.

[0055] The terms “radial”, “radially”, “inner”, “inner”, “outer”, “outer”, and “externally” are defined relative to a radial axis Z perpendicular to the longitudinal axis X and relative to the distance along the radial axis Z from the longitudinal axis X.

[0056] The turbomachine 1 includes, from upstream to downstream, a fan 2 , at least one compressor (eg, a low-pressure compressor 3 and a high-pressure compressor 4 ), a combustion chamber 5 , at least one turbine 6 (eg, a high-pressure turbine and a low-pressure turbine), and a nozzle.

[0057] The fan 2 enables suction of an air flow which is divided into a main flow F1 and a secondary flow F2. The main flow F1 passes through the engine of the turbomachine 1, while the secondary flow F2 is directed towards a secondary duct.

[0058] The main flow F1 is compressed in the low-pressure compressor 3 and then in the high-pressure compressor 4. The compressed air is then mixed with fuel and burned in the combustion chamber 5. The gases formed by the combustion pass through the high-pressure turbine and the low-pressure turbine. Finally, the gases are discharged through a nozzle whose cross section enables these gases to be accelerated to generate propulsion.

[0059] The fan 2 comprises a disk rotatably movable about a longitudinal axis X and blades 7 mounted on the disk. The blades 7 extend radially from the disk. The blades 7 are surrounded by a housing 8. The housing 8 is centered on the longitudinal axis X. In addition, a nacelle (not shown) is arranged around the housing 8 and the nacelle is coaxially attached to the housing.

[0060] In order to reduce the noise pollution generated by the turbine 1, the turbine 1 further comprises at least one sound absorbing panel 9, which is attached to the interior of the shell 8. Typically, a plurality of sound absorbing panels 9 are attached to the interior of the shell 8. The sound absorbing panels 9 have a complex shape that enables them to adapt to the shape of the shell 8. Each sound absorbing panel 9 extends, for example, over an angular sector.

[0061] Each sound absorbing panel 9 is capable of absorbing sound energy in the frequency range between 100 Hz and 1500 Hz.

[0062] Advantageously, each sound absorbing panel 9 is formed in one piece.

[0063] Reference Figure 2, each sound absorbing panel 9 has a thickness e between 10 mm and 50 mm. This thickness provides good sound insulation without increasing resistance. Each sound absorbing panel 9 has a sandwich structure. Each sound absorbing panel 9 includes a first shell 10, a second shell 11 and a core 12 arranged between the first shell 10 and the second shell 11. Advantageously, the second shell 11 and the first shell 10 are substantially parallel.

[0064] When the acoustic panel 9 is mounted in the housing 8, the first shell 10 is located outside the second shell 11. Advantageously, the second shell 11 forms the innermost shell.

[0065] The first shell 10 is made of a first composite material having a first polymer matrix and reinforcing fibers embedded in the matrix. The polymer material of the first matrix is ​​for example selected from thermoplastics such as polyolefins, for example polyethylene or polypropylene, or thermosetting plastics such as epoxy resins.

[0066] The reinforcing fibers are, for example, selected from carbon fibers, glass fibers and polyamide fibers.

[0067] Carbon fibers are, for example, polyaryletherketone (PAEK) fibers such as polyetherketone (PEK), polyetheretherketone (PEEK) or polyetherketoneketone (or PEKK) or polyacrylonitrile (PAN) fibers such as those sold by Hexcel Corporation. AS4, AS7 or IM7 fibers).

[0068] Advantageously, the fibres are continuous fibres, ie rovings consisting of an assembly of parallel or twisted continuous filaments.

[0069] Advantageously, the fibers are high strength fibers and / or have an intermediate modulus, for example a modulus between 200 GPa and 250 GPa.

[0070] For example, the first shell layer 10 has a thickness between 0.5 mm and 2 mm, preferably between 0.8 mm and 2 mm.

[0071] Advantageously, the first shell 10 is produced by additive manufacturing.

[0072] The second shell 11 is made of a second composite material. The second composite material comprises a second polymer matrix selected from thermoplastics and reinforcing fibers embedded in the second matrix.

[0073] The thermoplastic material of the second matrix may be the same or different from the thermoplastic material of the first polymer matrix. The thermoplastic of the second matrix is ​​for example selected from polyolefins such as polyethylene, polypropylene or fluorinated polymers or polyamides or polyaryletherketones such as polyetheretherketone (PEEK).

[0074] Advantageously, the melting temperature of the second matrix is ​​higher than 300°C, preferably higher than 400°C, even more preferably between 400 and 500°C, preferably between 420 and 480°C.

[0075] The reinforcing fibers are, for example, selected from carbon fibers, glass fibers and polyamide fibers.

[0076] Carbon fibers are, for example, polyaryletherketone (PAEK) fibers (such as polyetherketone (PEK) fibers, polyetheretherketone (PEEK) fibers or polyetherketoneketone (PEKK) fibers) or polyacrylonitrile (PAN) fibers (such as those sold by Hexcel Corporation). AS4, AS7 or IM7 fibers).

[0077] Advantageously, the fibres are continuous fibres, ie rovings consisting of an assembly of parallel or twisted continuous filaments.

[0078] Advantageously, the fibers are high strength fibers and / or have an intermediate modulus, for example a modulus between 200 GPa and 250 GPa.

[0079] The reinforcing fibers may be the same as or different from the reinforcing fibers in the first shell layer 10 .

[0080] The second shell 11 is formed by an automated fiber placement method, which enables the second shell 11 to be manufactured automatically.

[0081] The second shell 11 has holes 11a. The holes 11a are through holes. The holes pass through the entire thickness of the second shell 11. According to the invention, the holes 11a have a diameter less than 1 mm, advantageously between 0.1 mm and 0.8 mm, even more advantageously between 0.3 mm and 0.8 mm. The diameter of the holes 11a makes it possible to reduce the aerodynamic drag of the turbine 1. The second shell 11 is in contact with the secondary flow F2, and the presence of the second shell causes an aerodynamic drag that reduces the performance of the turbine 1. It has been found that holes 11a of such size significantly reduce this drag.

[0082] The hole 11 a has, for example, a circular cross section.

[0083] The holes 11a are made by laser drilling or mechanical drilling. The advantage of laser drilling is that holes with a diameter of less than 1 mm can be automatically made in the formed second shell 11, thereby significantly reducing the time and cost of manufacturing the sound absorbing panel 9. In addition, since the second substrate is made of thermoplastic, laser drilling will not damage the second shell 11, which is different from thermosetting materials, which may not be able to withstand the temperature used during laser drilling.

[0084] Advantageously, the thickness of the second shell layer 11 is less than 1 mm, preferably between 0.5 mm and 0.8 mm.

[0085] The core 12 is arranged between the first shell 10 and the second shell 11. The core 12 has a honeycomb structure. The core 12 includes honeycomb cells 12a. Each honeycomb cell 12a has, for example, a hexagonal or triangular cross section. The cross sections of the honeycomb cells 12a may be different from each other. Each honeycomb cell 12a is hollow and has an internal cavity connected to at least one hole 11a. This enables each honeycomb cell 12a to absorb some of the sound energy emitted by the fan 2.

[0086] Advantageously, the core 12 comprises a superposition of honeycomb cells 12a. The core 12 comprises, for example, a first circle of honeycomb cells 12a and a second circle of honeycomb cells 12a (not shown). The first circle and the second circle are separated by a diaphragm. In the art, a diaphragm is a perforated membrane that is permeable to certain frequency ranges and impermeable to other frequency ranges. This configuration of the core 12 enables the sound absorbing panel 9 to absorb sound energy within a wider frequency range. The first circle enables the absorption of sound energy within a first frequency range, while the second circle enables the absorption of sound energy within another frequency range. In addition, due to the diameter of the holes 11a, the height of the sound absorbing panel 9 can be increased to absorb sound energy within a wide frequency range without increasing resistance. In general, the greater the height of the panel, the greater the aerodynamic resistance.

[0087] The core 12 is made of, for example, a metal material such as aluminum or a polymer material or a composite material. The core 12 is made of, for example, the same material as the first shell 10 .

[0088] Advantageously, the core 12 is made by additive manufacturing directly on the second skin 11. In this way, the manufacture of the acoustic panel 9 according to the invention can be fully automated without requiring any manual assembly steps.

[0089] Reference Figure 3 The acoustic panel 9 advantageously comprises a reinforcement, preferably a plurality of reinforcements. The reinforcements make it possible to reinforce the acoustic panel 9. This makes it possible to reduce the risk of the acoustic panel 9 collapsing.

[0090] The reinforcement includes, for example, a frame 13 and / or studs 14 .

[0091] Advantageously, the frame 13 is arranged on the second shell 11. The frame 13 is, for example, square or rectangular. The frame 13 includes walls 13a arranged on the edges (advantageously, the four edges of the second shell 11) and spacers 13b arranged inside the walls 13a. The height of the frame 13 is, for example, approximately equal to the height of the honeycomb unit 12a.

[0092] The studs 14 are arranged on the second shell 11 and between the honeycomb units 12a. The studs 14 have, for example, a circular cross section. The height of the studs 14 is advantageously equal to the height of the honeycomb units 12a. The studs 14 have, for example, an inner hole. Therefore, in addition to giving the sound absorbing panel 9 rigidity, the studs 14 also enable the sound absorbing panel 9 to be attached to the housing 8 by fastening screws (not shown) that pass through the studs 14 and are attached to the housing 8.

[0093] Preferably, the acoustic panel 9 further comprises a coating 15 arranged on the first shell 10. Thus, the first shell is arranged between the core 12 and the coating 15. The coating 15 comprises a layer 15a comprising a ceramic material, preferably comprising a majority of ceramic material, and even more preferably consisting of a ceramic material. Ceramic materials include, for example, yttrium zirconium oxide, silicon dioxide and aluminum oxide. The layer 15a makes it possible to improve the heat resistance and heat insulation of the acoustic panel 9.

[0094] The layer 15a is deposited, for example, by thermal spraying.

[0095] In order to further improve the insulating properties of the coating 15 , the coating also comprises an additional layer of glass fibers. The additional layer is arranged between the layer of ceramic material 15 a and the first shell 10 .

[0096] The additional layer comprises, for example, a first sublayer 15b, for example made of non-woven fiberglass, and a second sublayer 15c, for example made of woven fiberglass or thermoplastic, to promote adhesion to the first shell 10. The second sublayer 15c is arranged between the first sublayer 15b and the first shell 10. The second sublayer 15c is applied, for example, by wet surface treatment such as dipping or spraying.

[0097] For example, the coating 15 has a thickness between 0.2 mm and 5 mm.

[0098] Now refer to Figure 4 A method for manufacturing an acoustic panel 9 is described. The method comprises a first step 100 of forming a second shell 11 by automated fiber placement. During this first step 100, a plurality of ribbons 210 of pre-impregnated fibers are deposited on a support 220. The ribbons 210 are then compacted and heated to facilitate bonding of the ribbons 210 to each other. The heating temperature of the ribbons 210 is between 400° C. and 500° C., preferably between 400° C. and 450° C., even more preferably at 420° C.

[0099] Then, in a second step 102, a hole 11a is made in the second shell 11. During this second step 102, the second shell 11 is drilled by mechanical drilling or laser drilling.

[0100] Laser drilling is particularly advantageous because it enables small diameter holes to be made in the shaped second shell 11 , the matrix of which is made of a thermoplastic polymer material, without damaging the second shell 11 .

[0101] Laser drilling is performed, for example, using a laser. The laser is, for example, a short pulse power laser or an ultrashort pulse power laser. Advantageously, the laser generates radiation with a wavelength between 1 nm and 1 mm. Thus, the generated radiation is in the infrared, visible or ultraviolet range.

[0102] Then, in a third step 104, the core 12 is formed on the second shell 11 by additive manufacturing. Thus, the core 12 is formed directly on the second shell 11. No additional assembly steps are required. The core 12 is formed, for example, by fused filament fabrication (FFF).

[0103] Then, in a fourth step 106, the first shell 10 is formed on the core 12 by additive manufacturing. Thus, the first shell 10 is formed directly on the core 12. No additional assembly steps are required. The first shell 10 is formed, for example, using a fused filament fabrication (FFF) technique.

[0104] Then, in an optional fifth step 108, a coating 15 is deposited on the first shell layer 10. In a first sub-step, an additional layer is deposited on the first shell layer 10 and then a ceramic layer 15a is deposited by thermal spraying.

[0105] Finally, in an optional sixth finishing step 110 , the acoustic panel 9 is machined.

[0106] According to the present invention, the manufacturing method for manufacturing the sound absorbing panel 9 is fully automated, which makes it possible to reduce the manufacturing time of the sound absorbing panel 9 and significantly reduce the cost. In addition, due to the laser drilling method, the quality of the holes 11a in terms of shape and size makes it possible to provide a sound absorbing panel 9 with significant sound absorption performance and low resistance.

[0107] Now refer to Figure 5 and Figure 6 An apparatus for manufacturing an acoustic panel 9 is described.

[0108] The apparatus comprises a first station 200 for manufacturing the second shell 11 and a second station 300 for manufacturing the core 12 and the first shell 10 .

[0109] For example, Figure 6A first station 200 is shown. The first station 200 comprises an unwinder 230 for a ribbon 210 of fibers pre-impregnated in a second thermoplastic polymer matrix. The unwinder 230 enables the ribbon 210 to be deposited on a support 220.

[0110] The first station 200 also includes a compacting roller 240. The compacting roller 240 enables pressure to be applied to the strips 210 arranged on the support 220 to promote adhesion between the strips 210. The compacting roller 240 can be moved in translation in the direction D1 along the strips 210. This enables pressure to be applied along the entire length of the strips 210 to facilitate adhesion of the strips 210 to each other along the entire length of the strips.

[0111] The first station 200 also comprises means 250 for heating the ribbon 210. The heating means 250 make it possible to heat the ribbon 210 when it is deposited on the support 220. This makes it possible to facilitate the bonding of the ribbons 210 to each other by melting the thermoplastic matrix. Thus, the combination of pressure and temperature facilitates the bonding of the ribbons to each other. The heating temperature is between 400° C. and 500° C., preferably between 400° C. and 450° C., even more preferably 420° C. The heating means 250 is, for example, a laser such as an infrared laser.

[0112] The first station 200 further comprises a member 260 for drilling the second shell 11. The drilling member 260 is, for example, a laser or a mechanical drill. The laser is, for example, a short pulse power laser or an ultrashort pulse power laser. Advantageously, the laser generates radiation with a wavelength between 1 nm and 1 mm. Thus, the radiation generated is in the infrared, visible or ultraviolet range.

[0113] The second station 300 for manufacturing the core 12 and the first shell 11 comprises an additive manufacturing device (not shown). The additive manufacturing device comprises, for example, a first filament dispenser, a first extruder and a molten filament manufacturing plate. According to another example of embodiment, in particular when the core 12 and the first shell 10 are made of different materials, the additive manufacturing device further comprises a second filament dispenser and a second extruder.

[0114] The apparatus may further comprise a third station 400 for producing the coating 15. The third station 400 comprises a thermal spraying device. The thermal spraying device comprises, for example, a torch for spraying powder via a carrier gas, the powder comprising, preferably mainly, ceramic material.

[0115] The device described in the present invention thus enables the online manufacture of the acoustic panels 9 without human intervention.

Claims

1. A sound absorbing panel (9) for an aircraft turbine (1), the sound absorbing panel (9) comprising: a first shell layer (10) formed from a first composite material having a first polymer matrix and reinforcing fibers embedded in the matrix, - a second shell layer (11), said second shell layer being formed from a second composite material having a second thermoplastic polymer matrix and reinforcing fibers embedded in said second matrix, and - a core (12) having a honeycomb structure, said core (12) being arranged between said first shell (10) and said second shell (11), The second shell (11) comprises holes (11a) with a diameter less than 1 mm, Characterized in that the sound absorbing panel further comprises a coating (15), the coating comprising a layer (15a), the layer comprising a ceramic material and deposited by thermal spraying, the first shell layer (10) being arranged between the coating (15) and the core (12).

2. The sound absorbing panel according to the above claim, characterized in that The diameter of the hole (11a) is between 0.1 mm and 0.8 mm, preferably between 0.3 mm and 0.8 mm.

3. A sound absorbing panel according to any one of the preceding claims, characterised in that The second shell (11) has a thickness less than 1 mm, advantageously between 0.5 mm and 0.8 mm.

4. A sound absorbing panel according to any one of the preceding claims, characterised in that The second polymer matrix has a melting temperature above 300°C, preferably above 400°C.

5. A sound absorbing panel according to any one of the preceding claims, characterised in that The coating (15) further comprises an additional layer of glass fibers, which is arranged between the layer (15a) and the first shell layer (10).

6. Sound absorbing panel according to the preceding claim, characterized in that The additional layer comprises a first sub-layer (15b) of non-woven glass fibers and a second sub-layer (15c) of woven glass fibers.

7. A sound absorbing panel according to any one of the preceding claims, characterised in that The sound absorbing panel includes a reinforcement member.

8. Sound absorbing panel according to the preceding claim, characterized in that The reinforcement comprises a frame (13) comprising walls (13a) arranged on the edge of the second shell (11) and / or studs (14) arranged on the second shell (11).

9. A method for manufacturing an acoustic panel (9) according to any one of the preceding claims, the method comprising the steps of: - (100) forming the second shell (11) by automated fiber placement, - (102) forming the hole (11a) in the second shell layer (11), - (104) forming the core (12) on the second shell (11) by additive manufacturing, - (106) forming the first shell (10) on the core (12) by additive manufacturing, and - (108) depositing said layer (15a) of said coating (15) onto said first shell layer (10) by thermal spraying.

10. The method according to the preceding claim, characterized in that The holes (11a) are made by laser drilling.

11. A device for manufacturing a sound absorbing panel (9) according to any one of claims 1 to 8, the device comprising: - a first work station (200) for manufacturing the second shell layer (11), the first work station comprising: an unwinder (230) for a ribbon (210) comprising fibers impregnated in a matrix of said second thermoplastic polymer, a roller (240) for compacting the strip (210), means (250) for heating the ribbon (210), and a member (260) for drilling holes in the second shell layer (11), - a second station (300) for manufacturing the core (12) and the first shell (10), the second station comprising an additive manufacturing device, and - a third station (400) for producing said coating (15), said third station (400) comprising a thermal spraying device.

Images