Acoustic attenuation structure provided with s-shaped cells having drained barriers and method for manufacturing same
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-07
- Publication Date
- 2026-03-18
AI Technical Summary
Current acoustic attenuation structures in aircraft engines are limited in their ability to effectively handle low-frequency noise, requiring thick cell bodies that increase bulk and mass, while also facing issues with liquid retention due to their design.
The proposed acoustic panel structure features S-shaped cells with partial obstacles and drainage orifices to increase sound wave path length and prevent liquid accumulation, utilizing a cellular body with drainage orifices adjacent to the obstacles to enhance drainage efficiency.
This design effectively attenuates a broader range of frequencies, including low frequencies, while maintaining compactness and preventing liquid retention, thus reducing the size and mass of the sound attenuation structure.
Smart Images
Figure FR2024050603_21112024_PF_FP_ABST
Abstract
Description
[0001] Description
[0002] Title of the invention: Acoustic attenuation structure with drained barrier S-shaped cells and method of manufacturing the same
[0003] Technical Field
[0004] The present invention relates to the general field of acoustic attenuation structures and to their manufacturing method. It relates more particularly to acoustic attenuation structures used in reduced volumes, in particular to reduce the noise produced in aircraft engines such as in gas turbines or their exhausts.
[0005] Prior art
[0006] The challenges of commercial aviation regarding the reduction of fuel consumption and the ecological impact lead to a rethinking of aircraft engine architectures. Two characteristics are particularly studied for their influence on the performance of the propulsion system: the bypass ratio (ratio of the air flow passing through the secondary flow to the air flow passing through the primary flow of the engine) and the drag induced by the propulsion system.
[0007] It has been proven and shared by the entire industry that an increase in the bypass ratio improves engine efficiency (reduces its consumption and reduces its harmful emissions). This increase in the bypass ratio is achieved in practice by increasing the diameter of the turbomachinery fan. However, this increase in the diameter of the secondary flow induces an increase in the same proportions of the diameter of the casing and the nacelle. If the architecture of the fan module is not redesigned, it is easy to understand the negative impact of the increase in drag induced by the propulsion unit having larger external surfaces in contact with the air. One of the paths studied therefore leads to shortening and thinning the external casing and the nacelle of the engine to reduce its mass and drag.
[0008] On current engines, sound attenuation is partly achieved using acoustic panels placed in the casing and nacelle. To gain compactness on the external casing, the acoustic treatments themselves will have to become more compact. Acoustic panels are mechanical elements similar to honeycomb boxes that reduce engine noise. The shape of the honeycombs and the thickness of these panels are designed to minimize engine noise pollution, particularly during certain operating phases such as takeoff and landing. Noise reduction is an even more important issue around airports and neighboring towns.
[0009] In future engine architectures, two new difficulties arise: the large diameter fan will rotate more slowly and therefore generate lower frequency sound waves and the structural casing will no longer completely cover the engine and therefore no longer effectively confine noise.
[0010] Acoustic attenuation structures typically consist of an acoustic surface plate or skin permeable to the acoustic waves that are to be attenuated and a reflective solid plate or skin called a "closing plate", a cellular body being arranged between these two skins. The cellular body is generally constituted by a set of partitions, for example in the form of a honeycomb. As is well known, such structures form Helmholtz-type resonators that make it possible to attenuate acoustic waves in a certain frequency range. Acoustic attenuation structures of this type are described in particular in documents US 5,912,442 and GB 2,314,526. However, the acoustic attenuation structures previously described can only absorb a very limited frequency range.
[0011] With such a structure, the absorbed frequency f is of the order of c / 4e with e the thickness of the cell body and c the speed of the sound. Or conversely, the processing thickness necessary to process a frequency f is of the order of c / 4f.
[0012] Thus, if the frequency of the noise to be treated decreases, the required treatment thickness increases. Typically, in the absence of grazing flow effects, a 30 mm thick cell body is suitable for attenuating frequencies close to 2000 Hz, and a 70 mm thick cell body is suitable for attenuating frequencies close to 880 Hz. The thickness of the cell body corresponds to the distance between the permeable acoustic skin and the impermeable acoustic skin. In other words, the acoustic length of the cells is approximately equal to the height of the alveolar core.
[0013] This attenuation constraint works against reducing the thickness of the structural casing and its surface area to satisfy mass and drag reduction.
[0014] However, it is desirable to produce sound attenuation structures that largely address low frequencies, while exhibiting satisfactory performance in the medium and high frequencies, for example in the case of a slow-moving engine fan that produces low frequencies and harmonics. In addition, the size and mass of the sound attenuation structure should preferably be limited, for example when it is mounted on an aircraft.
[0015] To expand the absorbed frequency range, it is known to superimpose two cellular bodies, preferably each having a honeycomb structure of different sizes to handle different frequencies. We speak of DDOF structures in English for "Double Degree Of Freedom" (or also 2DOF) for acoustic panels having two distinct superimposed cellular bodies, and SDOF structures in English for "Single Degree Of Freedom" for acoustic panels with a single cellular body.
[0016] However, this solution with two stacked cellular bodies has limitations for the treatment of low frequencies. Indeed, to reduce the lowest frequencies, it is necessary to use very thick cellular bodies. Thus, the acoustic attenuation structure comprising two stacked cellular bodies to treat both low and high frequencies will be relatively bulky.
[0017] Different solutions exist to treat low frequencies while limiting the increase in thickness: the use of a cone introduced into the cells, the inclination of a classic honeycomb or the manufacture of labyrinth or S-shaped cells.
[0018] EP 3 676 825 proposes an acoustic panel structure with a cell body having cells having an S-shaped structure inside to increase the distance traveled by sound inside the cell. The acoustic panels described in this document comprise cells having inside at least one partial obstacle extending from the inner wall of the cell and preventing the wave from propagating directly along the direction in which the cell extends, i.e. the direction of the thickness of the cell body.
[0019] However, acoustic panels must generally also meet a requirement for non-liquid retention. However, depending on the gravity orientation of acoustic panels whose honeycomb cores have additional partial partitions to form an internal S-shaped structure, excessive liquid retention may occur, which may be detrimental or even unacceptable for the proper functioning and reliability of the system.
[0020] Statement of the invention
[0021] The main aim of the present invention is therefore to propose an acoustic panel structure with a cellular body having cells having an S-shaped structure inside and with a configuration which further makes it possible to meet the drainage requirement while maintaining the adaptation of attenuation to acoustic frequencies.
[0022] According to an object of the invention, there is provided an acoustic panel with resonators for an aircraft propulsion unit nacelle, the acoustic panel comprising acoustic cells joined to each other in a plane orthogonal to a direction of acoustic propagation of the sound waves to form a honeycomb core, each acoustic cell comprising an enclosure extending along a main axis parallel to the axis of acoustic propagation of the sound waves and, inside the enclosure, at least one partial obstacle which extends from a wall of the enclosure in a transverse direction which is orthogonal to the main axis, said at least one partial obstacle comprising at least one edge secured to a wall of the enclosure and an internal passage off-centered relative to a central axis of the enclosure parallel to or coincident with the main axis of the enclosure to increase the length of the path traveled by the sound waves through the acoustic cell.According to a general characteristic of the invention, said at least one partial obstacle further comprises at least one drainage orifice adjacent to an edge secured to a wall of the enclosure.
[0023] Said at least one drainage orifice provided in a partial obstacle thus makes it possible to evacuate any residual liquid which could accumulate in the cell at the junctions between the enclosure and a partial obstacle.
[0024] By adjacent, we mean "which is in the immediate vicinity" as defined in a dictionary. Thus, by "a drainage orifice adjacent to an edge integral with a wall of the enclosure", we mean an orifice located in the direct vicinity of the edge of the partial obstacle, the orifice being able to be attached to said edge or close to the edge of the obstacle but without being attached to this obstacle edge, the edge considered being an integral edge and therefore attached to a wall of the enclosure.
[0025] In a first embodiment of the acoustic panel according to the invention, said at least one partial obstacle may comprise a cutout off-center relative to the central axis of the enclosure, the cutout forming the off-center internal passage.
[0026] In a second embodiment of the acoustic panel according to the invention, said at least one partial obstacle may comprise a free edge distant from a wall of the enclosure, the off-center internal passage being formed between the free edge and the wall of the enclosure opposite said free edge.
[0027] In this second embodiment, said free edge of the partial obstacle is opposite an edge secured to the wall of the enclosure.
[0028] In a third embodiment of the acoustic panel according to the invention, said at least one partial obstacle may comprise several edges integral with a wall of the enclosure over the entire length of the corresponding wall, said length extending in a direction orthogonal to the acoustic propagation axis, and said at least one partial obstacle comprising at least two drainage orifices.
[0029] Providing two drainage holes instead improves drainage efficiency in terms of drainage speed but also by increasing the number of possible sites for draining residual liquid. In a fourth embodiment of the acoustic panel according to the invention, each of said edges secured to a wall of the enclosure over the entire length of the corresponding wall comprises at least one drainage hole.
[0030] Providing a drainage hole per edge integral with the acoustic cell enclosure improves the chances of draining any residual liquid regardless of the orientation of the acoustic cell relative to gravity.
[0031] Said at least one partial obstacle may comprise two drainage orifices adjacent to the same edge secured to a wall of the enclosure.
[0032] Preferably, said at least one drainage orifice comprises a width greater than or equal to 0.1 mm and a length greater than or equal to 0.2 mm, and a section less than 1 mm. 2 and greater than 0.05 mm 2 .
[0033] If the drainage holes are too small, they do not allow water to pass through efficiently. If they are too large, there is a loss of acoustic efficiency.
[0034] Preferably, said at least one orifice comprises a section less than 0.5 mm 2 .
[0035] More preferably, said at least one orifice comprises a section less than 0.2 mm. 2 .
[0036] In a fifth embodiment of the acoustic panel according to the invention, said at least one drainage orifice may comprise an elongated shape extending to the edge of the partial obstacle with which it is associated, the drainage orifice being laterally open on said wall of the enclosure integral with said edge with which the drainage orifice is associated.
[0037] In a sixth embodiment of the acoustic panel according to the invention, the acoustic panel may further comprise an acoustically porous skin and an acoustically opaque skin, the alveolar core being interposed, along the main axis, between the acoustically porous skin and the acoustically opaque skin, and each enclosure of the alveolar core comprises at least two drainage notches, each drainage notch being arranged in a separate wall of the enclosure of the cell on a portion in contact with the acoustically opaque skin.In a seventh embodiment of the acoustic panel according to the invention, at least one acoustic cell comprises a first partial obstacle and a second partial obstacle, the first partial obstacle and the second partial obstacle being offset along the main axis, the passage delimited by the free end edge of the first partial obstacle and the passage delimited by the free end edge of the second partial obstacle being offset radially to form a baffle intended to increase the length of the path traveled by the sound waves through the associated acoustic cell.
[0038] In another subject matter, there is provided an aircraft propulsion assembly nacelle, which comprises at least one acoustic panel as defined above.
[0039] In another object, a method of manufacturing an acoustic panel according to the invention is proposed, said at least one partial obstacle being made from at least one ribbon.This method comprises: a step of perforating the ribbon to form at least one drainage orifice, a fixing step in which said at least one ribbon is fixed at least to a first plate, and in which a second plate is fixed to the first plate by respective nodal parts, a forming step in which non-nodal parts of the first and second plates are shaped so that each non-nodal part of the first plate constitutes, with a respective non-nodal part of the second plate, said peripheral wall delimiting a corresponding acoustic cell of the alveolar core, and so that a part of said at least one ribbon forms said at least one partial obstacle in this acoustic cell, said at least one partial obstacle comprising at least one drainage orifice.
[0040] A method of manufacturing an acoustic panel according to the invention is also proposed, said at least one partial obstacle being made from at least one ribbon.This method comprises: a step of perforating the ribbon to form at least one drainage orifice, a fixing step in which said at least one ribbon is fixed at least to a first plate, and in which a second plate is fixed to the first plate by respective nodal parts, a forming step in which non-nodal parts of the first and second plates are shaped so that each non-nodal part of the first plate constitutes, with a respective non-nodal part of the second plate, said peripheral wall delimiting a corresponding acoustic cell of the cellular core, and so that a part of said at least one ribbon forms said at least one partial obstacle in this acoustic cell, said at least one partial obstacle comprising at least one drainage orifice.
[0041] The method may further comprise a step of cutting the ribbon to form at least one cutout defining said at least one offset internal passage, the cutting step being able to be combined with the perforation step.
[0042] Brief description of the drawings
[0043] [Fig. 1] Figure 1 is a schematic sectional view illustrating a nacelle equipped with a plurality of acoustic panels according to the invention;
[0044] [Fig. 2] Figure 2 is a schematic cross-sectional view illustrating cells of one of the acoustic panels of Figure 1 equipped with two obstacles forming baffles;
[0045] [Fig. 3] Figure 3 is a schematic cross-sectional view illustrating cells of one of the acoustic panels of Figure 1 equipped with three obstacles forming baffles;
[0046] [Fig. 4] Figure 4 is a schematic perspective view with cutaways illustrating a hexagonal cell of one of the acoustic panels of Figure 1 equipped with two obstacles forming baffles;
[0047] [Fig. 5] Figure 5 illustrates a top view of a honeycomb core of an acoustic panel of Figure 2 according to a first embodiment of the manufacturing method of the invention.
[0048] [Fig. 6] Figure 6 illustrates a top view of a honeycomb core of an acoustic panel of Figure 2 according to a second embodiment of the manufacturing method of the invention.
[0049] [Fig. 7] Figure 7 shows a flowchart of a manufacturing method for one of the acoustic panels of Figure 2 according to an embodiment of the invention.
[0050] Description of embodiments Identical or similar elements are identified by identical reference signs throughout the figures.
[0051] In the description, the terminology longitudinal, vertical and transverse will be adopted without limitation with reference to the trihedron L, V, T indicated in the figures.
[0052] The expressions “front” and “rear” will also be used without limitation in reference to the lower part and the upper part respectively of figures 2 to 5.
[0053] Figure 1 shows a nacelle 10 equipped with a plurality of acoustic panels 12 with acoustic attenuation resonators shown schematically in strong lines. Some or all of these, or other acoustic panels may be totally or partially equipped with honeycomb cores according to the invention.
[0054] The acoustic panels 12 are designed to attenuate the noise emitted by the components which are housed in the nacelle 10, such as an engine or a fan (not shown). According to exemplary embodiments of the invention described here, the acoustic panels 12 are integrated into an air inlet shroud 14, into the secondary vein 16 and into an ejection nozzle 20.
[0055] With reference to Figure 2, which illustrates a first example of an embodiment of an acoustic panel 12, the acoustic panel 12 comprises successively, from front to back along the longitudinal axis L, a front acoustic skin 22 which is acoustically porous to sound waves, a honeycomb core 24, and a rear skin 26 which is solid and therefore acoustically reflective.
[0056] The front acoustic skin 22 and the rear skin 26 extend parallel to each other and transversely, that is to say in the transverse direction T which is orthogonal to the longitudinal direction L.
[0057] The front acoustic skin 22 has a plurality of perforations 27, or a permeability formed by a mesh, the perforations 27 being adapted to allow sound waves to penetrate into the alveolar core 24.
[0058] The alveolar core 24 comprises a plurality of acoustic cells 28, or alveoli 28, which are joined to each other in the transverse direction T and the vertical direction V to form a hollow structure such as, for example, a “honeycomb”. The direction of the longitudinal axis L corresponds to the direction of acoustic propagation of a sound wave entering an acoustic cell 28 via the acoustic skin 22.
[0059] Each cell 28 is delimited by a peripheral enclosure 30 extending substantially parallel to the longitudinal direction L from the acoustic skin 22 to the rear skin 26. The shape of the cell 28 may be of hexagonal cross-section, as can be seen in FIG. 4, or of rectangular, or square, or circular shape, or any other polygonal, curvilinear or mixed geometric shape.
[0060] Also, each cell 28 extends along a main longitudinal axis L, corresponding overall to an axis of propagation of the sound waves, from a front end 32 of the cell 28 resting on the acoustic skin 22, to a rear end 34 resting on the rear skin 26.
[0061] It will be noted that the cells 28 are acoustically independent. The term “acoustically independent cells” refers to cells whose enclosure 30 does not significantly propagate acoustic waves from one cell 28 to another. These terms refer to cells separated by sealed walls or walls perforated with one or a few small and limited number of orifices whose function is mainly to facilitate the evacuation of liquids that may penetrate into the cells. These orifices are preferably two to four in number with a unitary section of the order of 1 to 4 mm. 2 , and located in the enclosure 30 of the cells, in the immediate vicinity of the rear end 34 of the acoustic core against the rear skin 26.
[0062] As can be seen in the exemplary embodiment illustrated in Figure 2, each cell 28 comprises at least a first partial obstacle 36 and even, in this example, a second partial obstacle 38 which extend generally in the transverse direction T from the enclosure 30 of the cell 28. In other words, the obstacles 36 and 38 extend in a plane perpendicular to the main axis of the associated cell 28, the main axis of the cell 28 being merged with the direction of the longitudinal axis L. In addition, each obstacle 36, 38 has a free edge 40 which delimits a passage 42 with the wall 30 opposite, to allow the passage of the sound waves which penetrate into the associated cell 28.
[0063] The obstacles 36, 38 are offset in depth along the main longitudinal axis L of the associated cell 28.
[0064] The obstacles 36, 38 are substantially opposite, that is to say that the first obstacle 36 extends from a first gripping edge 41a on the left side of the wall 30, according to FIGS. 2 and 4, and the second obstacle 38 extends from a second gripping edge 41b on the opposite right side of the wall 30, to form a baffle intended to increase the length of the path traveled by the sound waves through the associated cell 28.
[0065] In addition, the length of each obstacle 36, 38 is adapted so that the obstacles 36, 38 partially overlap in a longitudinal projection view on a surface perpendicular to the longitudinal direction.
[0066] Thus, the sound waves follow a sinuous path between the obstacles 36, 38, from the front end 32 to the rear end 34 of the associated cell 28. This sinuous path is therefore longer than the straight-line distance between the two end faces 32 and 34.
[0067] As illustrated by arrow F in Figure 2, sound waves follow a winding path that has an apparent length greater than the length of a straight path.
[0068] Furthermore, each obstacle 36, 38 comprises at least one drainage orifice 37 located close to or even attached to the wall of the enclosure 30 to which the obstacle 36, 38 is fixed. Each drainage orifice 37 thus makes it possible to evacuate any liquid retained by the obstacle 36, 38.
[0069] As illustrated in Figure 4, each acoustic cell 28 further comprises two drainage notches 31 each provided in a wall of the enclosure 30 at the junction with the rear skin 26.
[0070] According to another exemplary embodiment shown in Figure 3, which is similar to the example shown in Figure 2, each cell 28 comprises a third obstacle 44, the three obstacles 36, 38, 44 have dimensions such that two successive obstacles in the longitudinal direction have a cumulative surface area greater than the section of the cell and a projected surface area covering the entire section. In other words, the obstacles are arranged to impose a sinuous path on the sound waves which travel through the associated cell 28, as shown by arrow F.
[0071] The cells 28 may be made of thermoplastic composite material and the obstacles of metallic material. Also, the obstacles 36, 38 may be welded to the material forming the cells 28.
[0072] In Figure 5 is illustrated a top view of an obstacle 36, 38 according to a particular embodiment. In this embodiment, the obstacle 36, 38 comprises four drainage orifices 37 each placed at a vertex of the hexagon formed by the obstacle 36, 38. Each vertex comprising the hexagon comprising a drainage orifice 37 is formed by two edges of the obstacle 36, 38 which are integral with a wall of the enclosure 30.
[0073] In Figure 6 is illustrated a top view of an obstacle 36, 38 according to another particular embodiment different from that of Figure 5. This embodiment differs from that of Figure 5 in that the drainage orifices 37 have an elongated shape rather than circular as in Figure 5. The shape of the drainage orifices 37 is elongated up to the wall of the enclosure 30, the orifice therefore being in direct contact with the wall of the enclosure 30.
[0074] Such an acoustic panel can be manufactured according to a manufacturing method as described in document FR 3 070 530 by adding a preliminary step of perforating the ribbons to thus provide drainage holes in the obstacles 36, 38.
[0075] Figure 7 schematically shows another method of manufacturing an acoustic panel according to the invention with at least one partial obstacle made from at least one ribbon. The manufacturing method shown in Figure 7 comprises a step 700 of cutting the ribbon, a step 710 of perforating the ribbon to form at least one drainage orifice, a fixing step 720 and a forming step 730.
[0076] In step 700 of cutting the ribbon, the ribbon is cut to form at least one cutout defining at least one offset internal passage.
[0077] In step 720 of perforating the ribbon, the ribbon is attached to at least a first plate and a second plate is attached to the first plate by respective nodal portions.
[0078] In forming step 730, non-nodal portions of the first and second plates are shaped such that:
[0079] - each non-nodal part of the first plate constitutes, with a respective non-nodal part of the second plate, the peripheral wall delimiting a corresponding acoustic cell of the alveolar core, and so that
[0080] - a portion of the ribbon forms the partial obstacle in this acoustic cell, the partial obstacle comprising at least one drainage orifice.
[0081] The cutting step 700 can be combined with the perforation step 710.
[0082] The acoustic panel according to the invention thus offers an acoustic panel structure with a cellular body having cells having an S-shaped structure inside and with a configuration further making it possible to meet the drainage requirement while maintaining the adaptation of attenuation to the acoustic frequencies.
Claims
Claims
1. Acoustic panel (12) with resonators for an aircraft propulsion unit nacelle, the acoustic panel (12) comprising acoustic cells (28) joined to each other in a plane orthogonal to a direction of acoustic propagation of the sound waves to form a honeycomb core (24), each acoustic cell (28) comprising an enclosure (30) extending along a main axis (L) parallel to the axis of acoustic propagation of the sound waves and, inside the enclosure (30), at least one partial obstacle (36, 38) which extends from a wall (41a, 41b) of the enclosure (30) in a transverse direction (T) which is orthogonal to the main axis (L), said at least one partial obstacle (36, 38) comprising at least one edge (41a,41b) secured to a wall of the enclosure (30) and an internal passage (42) off-centered relative to a central axis of the enclosure (30) parallel to or coincident with the main axis (L) of the enclosure (30) to increase the length of the path traveled by the sound waves through the acoustic cell (28), said at least one partial obstacle (36, 38) further comprising at least one drainage orifice (37), characterized in that said at least one drainage orifice (37) is adjacent to an edge secured to a wall of the enclosure (30).,
2. The acoustic panel (12) of claim 1, wherein said at least one partial obstacle (36, 38) comprises a cutout offset from the central axis of the enclosure, the cutout forming the internal off-center passage (42).
3. Acoustic panel (12) according to claim 1, wherein said at least one partial obstacle (36, 38) comprises a free edge (40) distant from a wall of the enclosure (30), the off-center internal passage (42) being formed between the free edge (40) and the wall of the enclosure (30) opposite said free edge (40).
4. Acoustic panel (12) according to claim 3, wherein said free edge (40) of the partial obstacle (36, 38) is opposite an edge secured to the wall of the enclosure.
5. Acoustic panel (12) according to one of claims 1 to 4, wherein said at least one partial obstacle (36, 38) comprises several edges integral with a wall of the enclosure (30) over the entire length of the wall corresponding, said length extending in a direction orthogonal to the axis (L) of acoustic propagation, and said at least one partial obstacle (36, 38) comprising at least two drainage orifices (37).
6. Acoustic panel according to claim 5, wherein each of said edges integral with a wall of the enclosure (30) over the entire length of the corresponding wall comprises at least one drainage orifice (37).
7. Acoustic panel according to one of claims 1 to 6, in which said at least one partial obstacle (36, 38) comprises two drainage orifices (37) adjacent to the same edge secured to a wall of the enclosure (30).
8. Acoustic panel (12) according to one of claims 1 to 7, wherein said at least one drainage orifice (37) comprises a width greater than or equal to 0.1 mm and a length greater than or equal to 0.2 mm, and a section between 0.05 mm 2 and 1 mm 2 .
9. Acoustic panel (12) according to one of claims 1 to 8, wherein said at least one drainage orifice (37) comprises a section less than 0.5 mm2 .
10. Acoustic panel (12) according to one of claims 1 to 8, wherein said at least one drainage orifice (37) comprises a section less than 0.2 mm 2 .
11. Acoustic panel (12) according to one of claims 1 to 10, wherein said at least one drainage orifice (37) comprises an elongated shape extending to the edge of the partial obstacle with which it is associated, the drainage orifice (37) being laterally open on said wall of the enclosure (30) integral with said edge with which the drainage orifice (37) is associated.
12. Acoustic panel (12) according to one of claims 1 to 11, further comprising an acoustically porous skin (22) and an acoustically opaque skin (26), the alveolar core (24) being interposed, along the main axis (L), between the acoustically porous skin (22) and the acoustically opaque skin (26), and each enclosure (30) of the alveolar core (24) comprises at least two drainage notches (31), each drainage notch (31) being arranged in a separate wall of the enclosure (30) of the cell (28) on a portion in contact with the acoustically opaque skin (22).
13. Acoustic panel according to one of claims 1 to 12, in which at least one acoustic cell (28) comprises a first partial obstacle (36) and a second partial obstacle (38), the first partial obstacle (36) and the second partial obstacle (38) being offset along the main axis (L), the passage (42) delimited by the free edge (40) of the first partial obstacle (36) and the passage (42) delimited by the free edge (40) of the second partial obstacle (38) being offset radially to form a baffle intended to increase the length of the path traveled by the sound waves through the associated acoustic cell (28).
14. Nacelle (10) of an aircraft propulsion unit, characterized in that it comprises at least one acoustic panel (12) according to any one of claims 1 to 13,
15. A method of manufacturing an acoustic panel (12) according to any one of claims 1 to 13, wherein said at least one partial obstacle (36, 38) is made from at least one ribbon, characterized in that it comprises: - a step (710) of perforating the ribbon to form at least one drainage orifice (37), - a fixing step (720) in which the at least one ribbon is fixed to at least one first plate, and in which a second plate is fixed to the first plate by respective nodal parts, - a forming step (730) in which non-nodal portions of the first and second plates are shaped so that each non-nodal portion of the first plate constitutes, with a respective non-nodal portion of the second plate, said peripheral wall delimiting a corresponding acoustic cell (28) of the cellular core (24), and so that a portion of said at least one ribbon forms said at least one partial obstacle (36, 38) in this acoustic cell (28), said at least one partial obstacle (36, 38) comprising at least one drainage orifice (37).