Acoustic attenuation panel for low frequency waves

Abstract

The invention relates to an acoustic attenuation panel (40) comprising: - a perforated acoustic wall (42), - a first honeycomb structure (44) connected to the perforated acoustic wall (42) having a plurality of acoustic cells (440) delimited by perimeter partitions (445), - a second honeycomb structure (48) having a plurality of acoustic cells (480) delimited by perimeter partitions (485), - a septum (46) having a plurality of large perforations (460), the septum being interposed between the first honeycomb structure (44) and the second honeycomb structure (48), each acoustic cell (440) of the first honeycomb structure and each acoustic cell (480) of the at least one second honeycomb structure (48) being arranged opposite a distinct perforation (460) of the septum (46).

Description

TECHNICAL FIELD

[0001] The present invention relates to an acoustic attenuation panel for treating low frequency waves.

[0002] The acoustic attenuation panel according to the invention has particularly interesting applications in the aeronautical industry, in particular its applications in aircraft propulsion units. BACKGROUND

[0003] An aircraft is propelled by one or more propulsion units, each comprising a turbojet / turboprop engine housed inside a tubular nacelle. Each propulsion unit is connected to the aircraft by a mast, usually located under the wing or at the fuselage.

[0004] The nacelle generally has a structure comprising an upstream section forming an inlet duct upstream of the turbojet engine, an intermediate section for surrounding the fan of the turbojet engine, and a downstream section able to house a thrust reverser and intended to surround the combustion chamber of the turbojet engine. Also, the nacelle generally ends with an exhaust nozzle whose outlet is located downstream of the turbojet engine.

[0005] The inlet duct is intended to optimize the intake of air required to feed the fan of the turbojet engine and to direct the air towards the fan throughout the flight envelope.

[0006] The inlet duct mainly comprises an inlet duct lip forming a leading edge, the inlet duct lip being connected to a ring structure.

[0007] The ring structure comprises an outer fairing ensuring the continuity of the external aerodynamics of the nacelle and an inner fairing ensuring the continuity of the internal aerodynamics of the nacelle, in particular in the case where the outer fan case is at the intermediate section. The inlet duct lip ensures the upstream junction between the two fairings.

[0008] The inner fairing of the inlet duct is exposed to high air flows and is located in the vicinity of the blades of the fan. It thus contributes to the transmission of the noise originating from the turbojet engine towards the outside of the aircraft.

[0009] Furthermore, it is known from the prior art to equip the inner fairing of the inlet duct of the nacelle with acoustic panels in order to attenuate the transmission of the noise generated by the turbojet engine.

[0010] Generally, the acoustic panels comprise a perforated acoustic skin and a honeycomb core assembled on the acoustic skin.

[0011] The honeycomb core comprises a plurality of acoustic cells forming Helmholtz resonators, the acoustic cells being separated from each other by peripheral partitions.

[0012] The perforated skin is directed towards the noise emission area so that sound waves can pass through the openings of the perforated skin into the acoustic cells. The acoustic energy is dissipated by viscous-thermal effects within the acoustic cells.

[0013] In particular, the honeycomb core of the known acoustic panel comprises two layers of acoustic cells. The two layers of cells are separated from each other by a micro-perforated septum.

[0014] The presence of better acoustic cells allows to improve the acoustic performance of the panel.

[0015] In recent years, the development of propulsion systems pursues the reduction of the energy consumption of the aircraft. To solve this need, the overall development of the aircraft tends to provide propulsion units with greater turbojet engine size and with fans of greater diameter, in particular for commercial aircraft. It is also sought to provide shorter and lighter nacelles to reduce the drag generated in flight phase. The overall effect is to reduce fuel consumption.

[0016] The greater the size of the fan, the lower the rotational speed thereof, and then the lower the frequency loudness generated thereby. However, the acoustic panels of the prior art, which have optimal acoustic treatment results in medium or high frequencies, are not suitable for the acoustic treatment of low frequencies.

[0017] Therefore, there is a need to provide acoustic panels suitable for these new propulsion units, to provide sufficient acoustic treatment.

[0018] It is known in the public domain that the height of the cells allows to adjust the frequency at which the acoustic treatment is effective.

[0019] The increase in the size of the cells also allows the acoustic treatment of low frequencies.

[0020] The change to the panels of the prior art also consists in increasing the height of the cells of these panels. Moreover, the target overall height of the panels has been evaluated to be at least 50 mm to allow a sufficient acoustic treatment of low frequencies.

[0021] However, the available space in future propulsion units with short nacelles does not allow to integrate acoustic panels with excessive size.

[0022] Therefore, there is a need to provide an acoustic panel which is able to treat the acoustic emissions generated by these new propulsion units, while having a minimum volume. In particular, there is a need to provide an acoustic attenuation panel with a minimum volume which is able to provide an acoustic treatment comparable to that of a panel with an overall height in the range of 50 mm.

[0023] We know from the prior art, in particular from the applications WO2015023389 and GB2300081, that the low-frequency treatment with small volume consists in adding to each cell a conical obstacle with holes on the top.

[0024] However, this type of geometry is difficult to implement on an industrial scale.

[0025] There is therefore a need to provide an acoustic panel adapted to low frequency acoustic treatment, easy to manufacture on an industrial scale. SUMMARY

[0026] The object of the application is notably to overcome at least one of these drawbacks, and according to a first aspect, relates to an acoustic attenuation panel comprising:

[0027] - a perforated acoustic wall,

[0028] - a first honeycomb structure connected to the perforated wall, the first honeycomb structure having a plurality of acoustic cells delimited by perimeter partitions,

[0029] - a second honeycomb structure having a plurality of acoustic cells delimited by perimeter partitions,

[0030] - a septum having a plurality of large perforations, the septum being interposed between the first honeycomb structure and the second honeycomb structure,

[0031] Each acoustic cell of the first honeycomb structure and each acoustic cell of the at least one second honeycomb structure are arranged opposite a unique perforation of the septum.

[0032] By large perforation, it is understood a perforation having a diameter greater than or equal to 1 mm.

[0033] According to other features of the application, the acoustic attenuation panel of the application comprises one or more of the following optional features, taken separately or in all possible combinations.

[0034] According to one variant embodiment, each perforation of the septum has a diameter comprised between 1 mm and 2 mm.

[0035] According to one variant embodiment, each acoustic cell of the first honeycomb structure has a height comprised between 5 mm and 10 mm.

[0036] According to one variant embodiment, each acoustic cell of the second honeycomb structure has a height comprised between 10 mm and 20 mm.

[0037] According to one variant embodiment, the height of the acoustic cells of the first honeycomb structure is lower than the height of the acoustic cells of the second honeycomb structure.

[0038] According to one variant embodiment, the height of the acoustic cells of the first honeycomb structure is equal to the height of the acoustic cells of the second honeycomb structure.

[0039] According to one variant embodiment, the total height of the acoustic panel is less than 30 mm.

[0040] According to a variant embodiment, the large perforations are uniformly distributed in the septum so that three adjacent large perforations form an equilateral triangle, one side of which is equal to the diameter of the acoustic cell of the honeycomb structure and one height of which is equal to 0.86 times + / - 20% of the diameter of the acoustic cell of the honeycomb structure.

[0041] According to a variant embodiment, the diameter of the acoustic cell of the second honeycomb structure is greater than the diameter of the acoustic cell of the first honeycomb structure.

[0042] According to this variant embodiment, the large perforations are uniformly distributed in the septum so that three adjacent large perforations form an equilateral triangle, one side of which is equal to the diameter of the acoustic cell of the second honeycomb structure and one height of which is equal to 0.86 times + / - 20% of the diameter of the acoustic cell of the second honeycomb structure.

[0043] According to a variant embodiment, the first honeycomb structure is superimposed on the second honeycomb structure so that the peripheral septa of the cells of the first honeycomb structure are arranged in geometric continuity with the peripheral septa of the cells of the second honeycomb structure.

[0044] According to a variant embodiment, each cell of the first honeycomb structure and each cell of the second honeycomb structure is centered with respect to a single perforation of the septum.

[0045] According to a second aspect, the application relates to a nacelle in which a fan is arranged, the nacelle comprising an air intake comprising an inner surface directed opposite the fan, said inner surface housing at least one acoustic attenuation panel as previously described.

[0046] According to a variant, the nacelle comprises an air intake, a thrust reverser and an exhaust nozzle, wherein at least one component comprising the air intake, the thrust reverser and the exhaust nozzle receives an acoustic attenuation panel as previously described.

[0047] Other characteristics and advantages of the application will become apparent on reading the following non-limiting description and on examining the drawings which illustrate schematically an acoustic attenuation panel according to the application. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a schematic view of an acoustic attenuation panel of the prior art

[0049] Figure 2 is a schematic view in cross-section of an acoustic panel according to an embodiment of the application.

[0050] Figure 3 is a zoom on a superimposed cell of an acoustic panel according to an embodiment of the application.

[0051] Figure 4 is a top view of an acoustic panel according to an embodiment of the application.

[0052] Figure 5 is a diagram illustrating the difference in the range of treated frequencies between an acoustic panel according to the application and a prior art acoustic panel.

[0053] Figure 6 is a diagram showing the equivalence of the acoustic attenuation results obtained with an acoustic panel according to the application and an acoustic panel having a total height of 50 mm.

[0054] Figure 7 is a table listing the results obtained by varying the various parameters of the acoustic panel according to the application.

[0055] Figure 8 is a diagram of a propulsion unit comprising a nacelle according to the application, the intake of which houses an acoustic attenuation panel. DETAILED DESCRIPTION

[0056] For simplicity, in all the figures, identical elements have the same reference numerals.

[0057] Figure 1 is a partial view of an acoustic attenuation panel known in the prior art, in particular from the application FR2841031.

[0058] In this example, the acoustic panel 40' has a double resonator, that is to say, it comprises acoustic cells of two thicknesses and has a laminated structure comprising, in order along the Y axis direction, a perforated acoustic wall 42', a first honeycomb structure 44', a micro-perforated septum 46', a second honeycomb structure 48' and a solid skin 49' arranged on the second honeycomb structure 48'.

[0059] The perforated acoustic wall 42' is intended to be in contact with the circulating air and is crossed by a plurality of large perforations (not visible) through which the acoustic waves can penetrate.

[0060] Each of the first honeycomb structure 44' and the second honeycomb structure 48' comprises acoustic cells separated from each other by peripheral partitions.

[0061] The septum 46' is arranged between the first honeycomb structure 44' and the second honeycomb structure 46'. The micro-perforated septum of the prior panel is crossed by a plurality of micro-perforations, the diameter of which is generally in the range of 0.3 mm and the perforation density is in the range of 400,000 to 800,000 holes / m 2

[0062] The acoustic panel having a double resonator and a micro-perforated septum allows to achieve the attenuation of acoustic waves in the medium-high frequencies, that is to say, above 1,500 Hz.

[0063] Figure 2 is a diagram showing the equivalence of the acoustic attenuation results obtained with an acoustic panel according to the application and an acoustic panel having a total height of 50 mm.

[0064] ​The acoustic attenuation panel 40 is a double-resonator type acoustic panel comprising, in the Y axis direction, in succession, a perforated acoustic wall 42, a first honeycomb structure 44, a large perforated septum 46, a second honeycomb structure 48 and a solid wall 49 without perforation, called reflective wall.

[0065] Indeed, using an acoustic panel with a unique honeycomb structure does not allow to obtain a sufficient open surface ratio to achieve an optimal acoustic treatment.

[0066] The perforated acoustic wall 42 has a plurality of large perforations 420 uniformly distributed in the acoustic wall 42. Each perforation 420 has a diameter in the range of 1.5 mm, the perforation density being in the range of 40,000 to 100,000 holes / m 2

[0067] The open ratio of the perforated acoustic wall relative to the total surface of the wall is between 8% and 20%.

[0068] The first honeycomb structure 44 has a plurality of acoustic cells 440 delimited by peripheral partitions 445. The second honeycomb structure 48 has a plurality of acoustic cells 480 delimited by peripheral partitions 485.

[0069] The first honeycomb structure 44 is superimposed on the second honeycomb structure 48. The first honeycomb structure 44 is misaligned relative to the second honeycomb structure 48, that is to say, the peripheral partitions 445 of the first honeycomb structure 44 are not geometrically continuous with the peripheral partitions 485 of the second honeycomb structure 48.

[0070] The first honeycomb structure is directly connected to the perforated acoustic wall 42, for example by gluing.

[0071] The second honeycomb structure is connected to the solid wall 49, for example by gluing.

[0072] The acoustic cells 440 of the first honeycomb structure 44 extend in the Y axis and have a height H1 between 5 and 10 mm. The height of the acoustic cells 440 of the first honeycomb structure 44 must be at least equal to 5 mm to avoid coupling phenomena.

[0073] By height, it is understood the side of the face delimiting the honeycomb structure.

[0074] The acoustic cells 480 of the second honeycomb structure 48 extend in the Y axis and have a height H2 between 10 and 20 mm.

[0075] The principle according to the invention is that the height H1 of the acoustic cells 440 of the first honeycomb structure 44, in direct contact with the perforated acoustic wall 42, is less than the height H2 of the acoustic cells 480 of the second honeycomb structure 48. The aim is to make the volume of the acoustic cells 480 of the second honeycomb structure 48 resonate.

[0076] ​In an embodiment not shown, the height of the acoustic cells 440 of the first cellular structure 44 is equal to the height of the acoustic cells 480 of the second cellular structure 48.

[0077] In order to reduce the bulk, the total height HT of the acoustic attenuation panel 40 is less than or equal to 30 mm, preferably the total height HT is less than 25 mm.

[0078] The total height of the panel takes into account all the elements constituting said acoustic panel.

[0079] The first cellular structure 44 and the second cellular structure 44 are separated from each other by a septum 46. The septum 46 is interposed between the first cellular structure 44 and the second cellular structure 48. The septum 46 extends in a direction perpendicular to the Y axis over the entire surface between the two cellular structures.

[0080] The septum 46 differs from the septa of the prior art in that it is not micro-perforated but macro-perforated, that is to say, it has a plurality of perforations 460 with a diameter greater than or equal to 1 mm. Preferably, the macro-perforations 460 of the septum 46 have a diameter of between 1 mm and 2 mm.

[0081] Each acoustic cell 440 of the first cellular structure and each acoustic cell 480 of the at least one second cellular structure 48 are arranged opposite a unique perforation 460 of the septum 46. That is to say, each acoustic cell is arranged opposite a single macro-perforation of the septum 46.

[0082] The principle is to define the drilling pattern of the septum, that is to say, to define the distance between the macro-perforations so that generally the majority of the surface of the septum has a unique perforation for each acoustic cell in each cellular structure. Figure 4 It should be noted that, in the industrial production process, edge effects can occur at the edges, according to which the peripheral septa of the cellular structures can be located at the perforations. In these edge cases, there will be a half-perforation for each cell, or a full perforation and a half-perforation at one end of the cell. This does not call into question the effectiveness conferred on the panel, since the majority of the macro-perforations are arranged in a unique way, opposite each acoustic cell of the first and second cellular structures.

[0083] The inventors have surprisingly noted that the macro-perforations 460 of the septum 46 allow the acoustic attenuation behavior to be obtained at low frequencies without increasing the height of the acoustic cells. The macro-perforations 460 of the septum 46 allow the entire volume of the second cellular structure 48 to be forced to resonate on its height H2, thus allowing optimal acoustic treatment of the low-frequency waves.

[0084] The septum 46 can for example be made of an organic composite comprising one to three layers of glass fabric embedded in an epoxy resin which is hardened by polymerization.

[0085] Figure 3 is a magnified view of a partial section of an acoustic panel according to an embodiment of the application. Each acoustic cell has a hexagonal shape.

[0086] In this embodiment, the first honeycomb structure 44 is superimposed on the second honeycomb structure 48, so that the peripheral partitions 445 of the cells of the first honeycomb structure are arranged in geometric continuity with the peripheral partitions 45 of the cells of the second honeycomb structure 48.

[0087] Each acoustic cell 440 of the first honeycomb structure 44 is arranged opposite an acoustic cell 480 of the second honeycomb structure 48.

[0088] The septum 46 is located between the acoustic cells 440 of the first honeycomb structure 44 and the acoustic cells 480 of the second honeycomb structure 48.

[0089] The septum has a unique circular large perforation 46, which is arranged opposite the acoustic cells 440 of the first honeycomb structure 44 and the acoustic cells 480 of the second honeycomb structure 48. Thus, each acoustic cell 440 of the first honeycomb structure 44 and each cell 480 of the second honeycomb structure are centered with respect to the unique perforation 460 of the septum 46.

[0090] In this example, the height H1 of the acoustic cells 440 of the first honeycomb structure 44 is less than the height H2 of the acoustic cells 480 of the second honeycomb structure 48.

[0091] Figure 4 is a partial view of a longitudinal section of an acoustic attenuation panel according to an embodiment of the application.

[0092] Figure 4 The definition of the drilling pattern of the septum 46 is illustrated, that is to say, the definition of the distance between each large perforation 460 in the septum 46.

[0093] The definition of the drilling pattern is defined to obtain a unique large perforation 460 opposite each acoustic cell of each honeycomb structure.

[0094] This figure shows in the foreground the first honeycomb structure 44 comprising a plurality of acoustic cells 440, in the background the second honeycomb structure 48 comprising a plurality of acoustic cells 480, and the large perforation 460 of the septum arranged between the first honeycomb structure 44 and the second honeycomb structure 48.

[0095] In this example, the diameter D1 of the acoustic cells 440 of the first honeycomb structure 44 is equal to the diameter D2 of the acoustic cells 480 of the second honeycomb structure 48.

[0096] To obtain a unique large perforation 460 opposite each of the acoustic cells 440, 480, the drilling step in the spacer 46 must be in the range of the diameter D1, D2 of the acoustic cells 440, 480, while taking into account the tolerances. The diameter D1, D2 is defined as the diameter of the circle inscribed in the hexagon of the acoustic cell.

[0097] The drilling step P1 in the length of the plate of the spacer 46 is defined by the general rule: D + / - 20%. Where D corresponds to the diameter D1, D2 of the acoustic cells 440, 480.

[0098] The drilling step P2 in the width of the plate of the spacer 46 is defined by the general rule: (D x 0.86) + / - 20%. Where D corresponds to the diameter D1, D2 of the acoustic cells 440, 480.

[0099] Thus, the large perforations 460 are uniformly distributed in the spacer, so that three adjacent large perforations 360 form an equilateral triangle 5, one side P1 equal to the diameter D1, D2 of the acoustic cells 440, 480 of the first and second honeycomb structures 44, 48, and one height P2 equal to 0.86 times + / - 20% of the diameter D1, D2 of the acoustic cells 440, 480 of the first and second honeycomb structures 44, 48.

[0100] Thus, for example, by using a honeycomb structure with a size of 3 / 8" known to those skilled in the art, in which the acoustic cells have a diameter D1, D2 equal to 9.52 mm, the drilling step P1 on the length of the acoustic plate 46 should be set to 9.5 mm, and the drilling step P2 on the width of the acoustic plate should be set to 8.3 mm.

[0101] This allows to obtain a unique perforation 460 of the spacer 46 opposite each acoustic cell 440, 480 of the honeycomb structure, so that the acoustic attenuation is improved and optimized.

[0102] The application is not limited to these types of honeycomb structures, a person skilled in the art can use honeycomb structures with larger or smaller sizes.

[0103] In an embodiment not shown, the diameter D2 of the acoustic cells 480 of the second honeycomb structure 48 can be greater than the diameter D1 of the acoustic cells 44 of the first honeycomb structure 44. This embodiment also allows to benefit from the volume effect and to obtain equivalent acoustic attenuation results by reducing the height H2 of the acoustic cells of the second honeycomb structure 48, and thus to obtain a greater size gain.

[0104] In this embodiment, the drilling step will be defined according to the diameter D2 of the acoustic holes 480 of the second honeycomb structure 48, that is to say according to the largest diameter of the holes.

[0105] Thus, the large perforations 460 are evenly distributed in the septum, so that three adjacent large perforations 460 form an equilateral triangle 5, one side PI of which is equal to the diameter of the acoustic cell 480 of the second cellular structure 48, and one height P2 of which is equal to 0.86 times + / - 20% of the diameter of the acoustic cell 480 of the second cellular structure 48.

[0106] Figure 5 is a comparison graph of the results of the acoustic attenuation obtained between the acoustic panel according to the application and the acoustic panels of the prior art.

[0107] Curve A shows the results of the acoustic attenuation obtained with the acoustic panel according to the application, in which the height HI of the acoustic cell 440 of the first cellular structure 44 is equal to 5 mm, the height H2 of the acoustic cell 480 of the second cellular structure 48 is equal to 15 mm, and the diameter of the large perforations 460 of the septum 46 is equal to 1 mm.

[0108] Curve B shows the results of the acoustic attenuation obtained with an acoustic panel of the prior art, having a unique cellular structure called "single degree of freedom" and having a total height of 20 mm.

[0109] Curve C shows the results of the acoustic attenuation obtained with an acoustic panel of the prior art, having two cellular structures separated by a micro-perforated septum called "double degree of freedom" and having a total height of 20 mm.

[0110] As this graph shows, the acoustic panels of the prior art allow to provide satisfactory acoustic treatment in high frequency media. Furthermore, the so-called "double degree of freedom" panel with a micro-perforated septum allows to extend the acoustic attenuation to high frequencies compared to the so-called "single degree of freedom" panel having the same total height.

[0111] In turn, for the same volume, the acoustic panel according to the application allows to achieve the acoustic attenuation of the characteristic low frequency waves of the new propulsion units.

[0112] Figure 6 is a graph illustrating the results of the acoustic attenuation obtained with the acoustic panel according to the application and with an acoustic panel having a total height of 50 mm.

[0113] Curve A represents the acoustic attenuation obtained with the acoustic panel according to the application, in which the acoustic cell 440 of the first cellular structure 44 has a height HI equal to 5 mm, the acoustic cell 480 of the second cellular structure 48 has a height H2 equal to 15 mm, and the large perforations 460 of the septum 46 have a diameter of 1 mm.

[0114] Curve D represents the acoustic attenuation obtained with an acoustic panel having a unique cellular structure called "single degree of freedom" and having a total height of 50 mm.

[0115] Curve D illustrates a theory according to which the increase in the size of the cells allows to provide a treatment of the low frequency waves.

[0116] Curve A shows that the acoustic panel according to the application allows to obtain an optimal acoustic attenuation of the low frequency waves compared to an acoustic panel with a total height equal to 50 mm, with a size gain of 60%.

[0117] The acoustic attenuation panel according to the application allows to obtain an optimal acoustic treatment of the low frequency waves on a more selective frequency range.

[0118] Figure 7 is a table showing the acoustic length equivalences obtained with the acoustic panels according to the application, in which the different parameters of the acoustic panel have been changed.

[0119] The test parameters are the height H1 of the acoustic cells 440 of the first cellular structure, the height H2 of the acoustic cells 480 of the second cellular structure and the diameter of the large perforations 460 of the spacers 46.

[0120] The columns present from left to right: the height H1 of the acoustic cells 440 of the first cellular structure 44, the height H2 of the acoustic cells 480 of the second cellular structure 48, the total height of the acoustic attenuation panel according to the application, the diameter of the large perforations of the spacers 46 and the equivalences of the wavelengths.

[0121] The parameters of the height H1 of the acoustic cells of the first cellular structure are tested between 5 and 10 mm. The parameters of the height H2 of the acoustic cells of the second cellular structure are tested between 15 and 20 mm, so that the total height of the acoustic panel of the application is always less than 30 mm. The diameter of the large perforations of the spacers 46 is tested between 1 and 1.5 mm.

[0122] The results obtained show that the acoustic lengths obtained are equivalent to the acoustic lengths obtained with acoustic panels with a total height in the range of 50 mm. The acoustic panel according to the application allows to obtain acoustic lengths significantly greater than its total height.

[0123] In order to reduce the volume as much as possible, the principle is to select the height H2 of the acoustic cells 480 of the second cellular structure 48 and the diameter of the large perforations according to the target frequency, then to reduce the height H1 of the acoustic cells 440 of the first cellular structure 44 as much as possible.

[0124] Figure 8 is a schematic view of a propulsion unit 1 extending according to a longitudinal axis X, the propulsion unit 1 comprising a short nacelle 2 and a turbojet engine 3. The short nacelle 2 has a structure comprising an upstream section forming an air intake 200, an intermediate section 210, a downstream section 220 and a nozzle 230. The intermediate section 210 comprises a fan casing for the fan 30 of the turbojet engine 3, the downstream section 220 comprises a thrust reverser and is intended to surround the combustion chamber of the turbojet engine.

[0125] The intake duct 200 comprises an inner surface 205 opposite the fan 30, which houses at least one acoustic attenuation panel according to the present application.

[0126] As Figure 8 indicated, other components of the nacelle, such as the thrust reverser, can house acoustic attenuation panels according to the present application.

[0127] Therefore, thanks to the acoustic panels according to the present application, it is possible to provide a low-frequency acoustic treatment, the acoustic performance of which is comparable to that obtained with acoustic panels having a total height in the range of 50 mm, with a size gain in the range of 60% compared to these.

[0128] Of course, the present application is not limited to the examples just described and many arrangements can be made to these, for example the acoustic attenuation panels can have more than two honeycomb structures arranged on top of each other, without departing from the scope of the present application. Furthermore, the different features, shapes, variants and embodiments of the present application can be associated with each other in various combinations, to the extent that they are not incompatible or exclusive of each other. In particular, all the variants and embodiments described above can be combined together.

Claims

1. A sound attenuation panel (40), comprising: - a perforated acoustic wall (42), - a first honeycomb structure (44) connected to the perforated acoustic wall (42), the first honeycomb structure (44) having a plurality of acoustic cells (440) delimited by peripheral partitions (445), - a second honeycomb structure (48) having a plurality of acoustic cells (480) delimited by peripheral partitions (485), a spacer (46) having a plurality of large perforations (460), said spacer being interposed between said first honeycomb structure (44) and said second honeycomb structure (48), Each acoustic unit (440) of the first honeycomb structure and each acoustic unit (480) of at least one second honeycomb structure (48) are arranged opposite to the unique large perforation (460) of the septum (46), characterized in that: The large perforations (460) are evenly distributed in the spacer (46) so that three adjacent large perforations (460) form an equilateral triangle (5), one side (P1) of the equilateral triangle is equal to the diameter (D1, D2) of the acoustic unit (440, 480) of the honeycomb structure (44, 48), and a height (P2) of the equilateral triangle is equal to 0.86 times ±20% of the diameter of the acoustic unit (440, 480) of the first and second honeycomb structures (44, 48), the diameter (D1) of the acoustic unit (440) of the first honeycomb structure (44) is equal to the diameter (D2) of the acoustic unit (480) of the second honeycomb structure (48), The first honeycomb structure (44) is superimposed on the second honeycomb structure (48) so that the peripheral partition (445) of the acoustic unit (440) of the first honeycomb structure (44) is arranged to be not geometrically continuous with the peripheral partition (485) of the acoustic unit (480) of the second honeycomb structure (48), wherein each large perforation (460) of the spacer (46) has a diameter comprised between 1 mm and 2 mm, and the sound attenuation panel has an overall height (HT) less than 30 mm.

2. The sound attenuation panel (40) according to claim 1, characterized in that Each acoustic cell (440) of the first honeycomb structure (44) has a height (H1) comprised between 5 mm and 10 mm.

3. The sound attenuation panel (40) according to claim 1, characterized in that Each acoustic cell (480) of the second honeycomb structure (48) has a height (H2) comprised between 10 mm and 20 mm.

4. The sound attenuation panel (40) according to claim 1, characterized in that A height (H1) of the acoustic unit (440) of the first honeycomb structure (44) is smaller than a height (H2) of the acoustic unit (480) of the second honeycomb structure (48).

5. The sound attenuation panel (40) according to claim 1, characterized in that A height (H1) of the acoustic unit (440) of the first honeycomb structure (44) is equal to a height (H2) of the acoustic unit (480) of the second honeycomb structure (48).

6. A nacelle (2) in which a fan (30) is arranged, the nacelle comprising an air inlet (200) comprising an inner surface directed opposite to the fan (30), the inner surface accommodating at least one sound attenuation panel (40) according to any one of claims 1 to 5.

7. The nacelle according to claim 6, wherein: The nacelle comprises an air inlet (200), a thrust reverser (220) and a tail nozzle (230), wherein at least one of the components comprising the air inlet (200), the thrust reverser (220) and the tail nozzle (230) accommodates a sound attenuation panel (40) according to any one of claims 1 to 5.

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