Reduced-output sound-treatment panel for a turbojet engine

Description

Technical Field

[0001] The invention relates to an acoustic treatment panel for the absorption of sound waves, and more particularly to an acoustic absorption cell of an acoustic treatment panel for the absorption of low-frequency sound waves that can be integrated into a small footprint. Previous technique

[0002] Acoustic absorption cells are designed to reduce unwanted noise. This reduction is achieved for various reasons, such as protecting the human ear from damage or reducing the impact of noise. In equipment and technical components, acoustic protection may be necessary to ensure compliance with current noise emission standards, as well as to protect them from damage (noise fatigue). One of the challenges for high bypass ratio turbofan engines is certifying noise levels during takeoff and landing. Indeed, aircraft noise levels are subject to increasingly stringent international regulations to limit the noise footprint around airports.Furthermore, noise fatigue can occur, for example, on aircraft landing flaps near engines, or on payload components of space launch systems, due to the significant noise generated during launch.

[0003] Traditionally, reducing the noise of a turbojet engine, and more specifically the noise radiated by the interaction between the rotors and their surroundings, is achieved using absorbent panels placed on the wetted surfaces of the ducts through which sound waves propagate. Wetted surfaces are defined as surfaces in contact with a fluid flow. These panels are sandwich-type composite materials enclosing a honeycomb structure, whose absorbent properties are partly achieved through the principle of Helmholtz resonators, where the honeycomb cells act as acoustic absorption cells.

[0004] A Helmholtz resonator consists of a resonant cavity and one or more necks extending inside the cavity, each from an opening in a wall. These necks allow the resonant cavity to communicate with the surrounding medium through which the waves to be attenuated propagate. The necks thus ensure communication between the ambient environment and the internal air cavity. Once the device is optimized, the necks provide a visco-thermal dissipation effect, which corresponds to a rapid and alternating movement of sound waves through the necks, causing dissipation by friction.

[0005] In conventional processing technologies, the throat length is small compared to the cavity height. More precisely, in conventional technologies, the throat length is equal to the thickness of a composite sheet (carbon + resin) wall that constitutes the wetted surface of the treatment, since the throat is created by simply perforating this wall. The operation of the Helmholtz resonator is optimized by sizing the air cavity to obtain the maximum acoustic velocity at the throat. This optimization requires cavity heights on the order of a quarter of the wavelength of the principal frequency to be treated. For a sound wave at a frequency of 100 Hz in ambient air, which therefore has a wavelength of 3.4 m, this represents a necessary cavity height of: λ 4 = 3 , 4 4 = 0 , 85 m

[0006] It also offers very interesting properties in terms of the large frequency bandwidth covered.

[0007] However, current trends in propulsion system optimization are geared towards reducing the number of blades and the rotational speed of rotating assemblies such as the fan in ducted architectures (e.g., a turbofan) and / or the propellers in unducted architectures (e.g., an open-fan engine, particularly a twin-fan, counter-rotating open rotor). This results in a lowering of acoustic radiation frequencies.

[0008] Optimizing the processing panels then requires increasing their thickness to increase the cavity height and thus decrease the tuning frequency of the panels' resonant cavities. This makes the panels incompatible with the mass and size constraints associated with new UHBR (Ultra High Bypass Ratio) architectures, i.e., those with very high dilution ratios.

[0009] Indeed, the installation of conventional acoustic treatments without cones and tuned to these frequencies requires cavities 20 to 25 cm thick. And, with so-called slim nacelle sizes and an acoustic signature starting at particularly low frequencies, turbofan engines with very high bypass ratios cannot use such acoustic treatment panels and therefore require the use of specific acoustic coatings. More precisely, in addition to conventional absorbing structures that treat mid and high frequencies, there is a need for acoustic treatment capable of effectively attenuating low frequencies in the range of 150 to 800 Hz with compact acoustic treatment panels.

[0010] It is always possible to size the Helmholtz resonator so that it is effective at lower frequencies, for a reduced radial footprint, for example, by playing on both the height of the neck and the volume of the resonant cavity.

[0011] The drawback of this type of design, constrained by a limited footprint, is that the frequency band over which the treatment acts optimally is drastically reduced as the frequency decreases. Indeed, inserting a cone into the honeycomb resonant cavities increases the neck height, thus reducing the cell height to 7 or 8 cm rather than the 20 to 25 cm required for a cell without an internal cone. However, this comes at the cost of virtually no attenuation on the second- and third-order harmonic frequencies of the blower.

[0012] This restriction on the attenuation frequency bandwidth is very limiting, because the variation in fan speed according to flight phases and aircraft mass leads to significant changes in its sound emission frequency. Consequently, treatment designed in this way will only be effective over an extremely narrow speed range, particularly when the cone inlet is close to the perforated sheet metal.

[0013] A known solution for attenuating very low frequencies with relatively thin resonators is to insert a large conical or hyperbolic horn into a cavity fitted to the dimensions of this horn, the objective here being to increase the distance traveled by the sound wave in the resonant cavity.

[0014] An aeronautical application of acoustic absorption cells requires such a horn to be constructed with a very thin wall thickness of approximately 0.1 to 0.2 mm. However, the significant acoustic excitation levels, on the order of 140 to 150 dB, and the potential mechanical vibrations generated by the engine, prevent a horn with walls of this thickness from remaining sufficiently rigid to maintain its geometry and function optimally.

[0015] To enable its operation and increase the rigidity of the horn, a known solution from application FR 1 858 101 is to add tabs to attach the neck of the cone to the side walls or the bottom wall of the cell in order to minimize its movements.

[0016] Another known solution proposes to introduce a conical waveguide duct into the cavities of a honeycomb-type resonator that can be placed at several heights, possibly with a septum in the form of porous layers.

[0017] There is also a known solution proposing a septum, in the form of a porous surface or fabric, exhibiting variable acoustic attenuation and in a conical shape in acoustic resonators in the form of honeycomb cavities.

[0018] Another known solution proposes a manufacturing process for acoustic treatment with honeycomb cavities and a conical separating septum in each cavity, the septum being made using a porous surface or fabric.

[0019] Other known solutions propose to tilt the cavities of the resonators in order to increase the path traveled by the acoustic waves without increasing the thickness of the treatment.

[0020] It is also known from document FR 3 070 529 an acoustic panel with resonators in which obstacles have been added in the resonant cavities of the acoustic treatments in order to increase the distance traveled by the acoustic waves without increasing the thickness of the acoustic treatment, which makes it possible to improve the acoustic attenuation in low frequency.

[0021] Another known solution is to use folded cavities to increase the distance traveled by acoustic waves.

[0022] Another known solution involves using cavities with an elongated, cylindrical neck to improve low-frequency acoustic performance. US 2013 / 186707 A1 discloses a sound absorber. Description of the invention

[0023] The invention aims to provide an acoustic absorption cell for the absorption of low-frequency sound waves with a reduced cavity height and whose processing frequency range can be modulated as needed.

[0024] An object of the invention proposes an acoustic absorption cell comprising an acoustically opaque back wall, an acoustically porous entrance wall, such as a perforated sheet, an enclosure extending along an axial direction between a first axial end attached to the back wall and a second axial end attached to the entrance wall, and a first acoustic horn extending inside the enclosure between a first orifice of the first horn and a second orifice of the first horn smaller than said first orifice of the first horn, the first orifice being opposite said entrance wall.

[0025] According to a general characteristic of the object, the acoustic absorption cell comprises a second horn extending inside the enclosure between a first orifice of the second horn and a second orifice of the second horn smaller than said first orifice of the second horn.

[0026] Preferably, the first pavilion extends inside the enclosure in a direction parallel to or coinciding with the axial direction of the enclosure, and the second pavilion extends inside the enclosure in a direction parallel to or coinciding with said axial direction of the enclosure.

[0027] Preferably, the first and second pavilions are arranged one below the other, aligned or not, that is to say one after the other in a general direction parallel or coincident with their axial direction along which they extend, in other words a direction parallel or coincident with the axial direction of the cell.

[0028] An acoustically porous wall is defined as one with openings that allow some sound waves to pass through without significantly altering them in terms of intensity or frequency. An acoustically opaque wall is defined as one that does not allow any sound waves to pass through without altering them, at least partially, particularly in intensity. An acoustically opaque wall will reflect most of the incident sound waves.

[0029] Adding a second horn inside the acoustic absorption cell's enclosure allows for the duplication of low-frequency attenuation capabilities, thus providing a second low-frequency attenuation peak centered on a different frequency. This therefore increases the acoustic absorption cell's processing frequency range.

[0030] According to a first aspect of the acoustic absorption cell, the first orifice of the second pavilion can be arranged opposite the second orifice of the first pavilion and the second orifice of the second pavilion can be arranged opposite said background wall.

[0031] In one variant, the two pavilions can be coaxial and at least partially nested within each other.

[0032] In another variation, the first and second pavilions can be placed side by side, extending along two parallel axes. In yet another variation, the acoustic absorption cell can comprise a plurality of pavilions placed side by side, extending along parallel axes.

[0033] In another variant, the acoustic absorption cell may include a third horn extending in a direction parallel or coinciding with the first axial direction between a first orifice and a second orifice, the first and second horns being juxtaposed next to each other and both opening via their second orifice into the third horn.

[0034] According to a second aspect of the acoustic absorption cell, the enclosure can extend between the back wall and the entrance wall by forming a non-zero angle of inclination, β, which is not orthogonal to the entrance wall, on the one hand, and to the back wall, on the other.

[0035] Using cells inclined relative to the surface in contact with the external fluid flow increases the acoustic length of the cavity defined by the enclosure without altering the height of the cell separating the back wall and the entrance wall, measured along a direction orthogonal to the back and entrance walls. This increase in the acoustic length of the cavity allows the tuning frequencies of the acoustic absorption cell to be lowered, meaning that the acoustic absorption frequency(ies) of the cell are centered on a lower frequency than in a non-inclined configuration.

[0036] According to a third aspect of the acoustic absorption cell, the angle of inclination β is preferably between 0° and 60°, 0° excluding.

[0037] According to a fourth aspect of the acoustic absorption cell, the enclosure can define a cylinder whose generatrices extend along the first axial direction and whose base has a circular or polygonal shape.

[0038] According to a fifth aspect of the acoustic absorption cell, at least one of the first and second horns may include surfaces with non-zero roughness, in order to increase the viscous dissipation effects within the horn. The roughness may take the form of grooves or bumps on the internal and / or external surfaces of the horn walls.

[0039] According to a sixth aspect of the acoustic absorption cell, the cross-section of the second orifice of the first horn can be equal to the cross-section of the second orifice of the second horn, and the ratio between the cross-section of the enclosure at its second end and the cross-section of the second orifice of the first horn can be between 2 and 100.

[0040] According to a seventh aspect of the acoustic absorption cell, the distance between the first orifice and the second orifice of the first horn along the axial direction can be equal to the distance between the first orifice and the second orifice of the second horn along the axial direction, and the ratio between the distance between the first orifice and the second orifice of the first horn along the axial direction and the height of the enclosure measured along the axial direction between its first end and its second end can be between 0.2 and 0.6.

[0041] According to a eighth aspect of the acoustic absorption cell, at its first opening the first horn can form a first opening angle, α 1 , with a plane parallel to the entrance wall of the cell, and at its first opening the second horn can form a second opening angle, α 2 , with a plane parallel to the entrance wall of the cell.

[0042] According to a ninth aspect of the acoustic absorption cell, the second opening angle α 2 can be identical to the first opening angle α 1. In this configuration, the two opening angles are preferably between 60° and 90°.

[0043] According to a tenth aspect of the acoustic absorption cell, the first horn and the second horn can be of identical shape and dimensions, the first horn and the second horn each comprising a cylindrical portion with a cross-section equal to the cross-section of their second orifice, the ratio between the length of the cylindrical portion along the axial direction and the length of the horn along the axial direction being less than or equal to 0.8.

[0044] Designing acoustic absorption cells with a first and second pavilion of identical shape and dimensions reduces manufacturing costs.

[0045] In one variant, the shapes and dimensions of the first and second pavilions may be different.

[0046] According to an eleventh aspect of the acoustic absorption cell, the cell may further comprise a porous inner wall, or septum, extending inside the enclosure and over the first opening of the second horn. In other words, the porous inner wall is located inside the enclosure, and the second horn is in contact with the porous inner wall at the end bearing its first opening.

[0047] According to a twelfth aspect of the sound-absorbing cell, the inner wall, or septum, may be parallel to said entrance wall.

[0048] According to a thirteenth aspect of the acoustic absorption cell, the first pavilion may include a portion having a continuous non-straight slope extending from its first orifice, and the second pavilion may include a portion having a continuous non-straight slope extending from its first orifice.

[0049] In another object of the invention, an acoustic treatment panel is proposed for placement on at least one wall of an aircraft in contact with a fluidic flow, the panel comprising a plurality of acoustic absorption cells as defined above.

[0050] In another aspect of the invention, a turbojet engine for mounting on an aircraft is proposed, the turbojet engine comprising at least one acoustic treatment panel as defined above. The turbojet engine may be a ducted or unducted turbojet engine. In the case of an unducted turbojet engine, the acoustic panel may be installed on the structure surrounding the gas generator, and which is therefore in contact with the external airflow and / or the airflow generated by the fan.

[0051] In another object of the invention, an aircraft comprising at least one turbojet engine as defined above is proposed. Brief description of the drawings

[0052] The invention will be better understood upon reading the following, by way of example but not limitation, with reference to the attached drawings in which: [ Fig. 1 ] There figure 1presents a cross-sectional view of a turbojet engine according to an embodiment of the invention, in a longitudinal plane of the turbojet engine. Fig. 2 ] There figure 2 illustrates a partial perspective view of an acoustic treatment panel according to an embodiment of the invention. Fig. 3 ] There figure 3 schematically presents a perspective view of a sound-absorbing cell according to a first embodiment of the invention. Fig. 4 ] There figure 4 schematically presents a cross-sectional view of a plurality of acoustic absorption cells according to a first embodiment of the invention. Fig. 5 ] There figure 5 graphically represents the evolution of the absorption coefficient as a function of the sound wave frequency in Hertz according to two different configurations of acoustic absorption cells. Fig. 6 ] There figure 6schematically illustrates a perspective view of a sound-absorbing cell according to a second embodiment of the invention. Fig. 7 ] There figure 7 illustrates a cross-sectional view of a plurality of acoustic absorption cells according to a third embodiment of the invention. Fig. 8 ] There figure 8 illustrates a cross-sectional view of a plurality of acoustic absorption cells according to a fourth embodiment of the invention. Fig. 9 ] There figure 9 illustrates a cross-sectional view of a plurality of acoustic absorption cells according to a fifth embodiment of the invention. Description of the implementation methods

[0053] On the figure 1 is represented a cross-sectional view of a turbojet 1 according to a non-limiting embodiment of the invention, in a longitudinal plane of the turbojet 1.

[0054] The turbojet engine 1 comprises a nacelle 2, an intermediate casing 3, and an inner casing 4. The nacelle 2 and the two casings 3 and 4 are coaxial. The nacelle 2 defines, at one end, an inlet channel 5 for a fluid flow and, at the opposite end, an exhaust channel 6 for a fluid flow. The nacelle 2 and the intermediate casing 3 define a primary fluid flow path 7. The intermediate casing 3 and the inner casing 4 define a secondary fluid flow path 8. The primary and secondary fluid flows 7 and 8 are arranged along the axial direction of the turbojet engine between the inlet channel 5 and the exhaust channel 6.

[0055] The turbojet 1 further includes a fan 9 configured to deliver an airflow F as a fluidic flow, the airflow F being divided at the fan outlet into a primary flow FP circulating in the primary channel 7 and a secondary flow FS circulating in the secondary channel 8.

[0056] The turbojet 1 further includes at least one acoustic treatment panel 10 configured to attenuate the acoustic waves emitted by the turbojet 1 before these waves escape radially outside the nacelle 2 of the turbojet 1.

[0057] In the case of an unfaired turbomachine, the acoustic treatment would be configured to attenuate or limit the refractions of acoustic waves radiated by the propellers.

[0058] Each acoustic treatment panel 10 is configured to attenuate acoustic waves whose frequency belongs to a predetermined frequency range. In the embodiment illustrated in the figure 1 , the acoustic treatment panels 10 are integrated into the nacelle 2, the intermediate casing 3 and the internal casing 4. On the internal casing 4, the acoustic treatment panels 10 are integrated, on the one hand, on the portion upstream of the intermediate casing 3 in the axial direction and in particular on the portion carrying the blower 9, and, on the other hand, on a portion downstream of the intermediate casing 3.

[0059] On the figure 2 is shown a partial perspective view of an acoustic treatment panel 10 according to an embodiment of the invention.

[0060] In reference to the figure 2, the acoustic treatment panel 10 comprises a core 12, a reflective layer or wall 14 and an entrance layer or wall 16.

[0061] The core 12 has a honeycomb structure. More precisely, the core 12 comprises a plurality of acoustic cells 18, or alveoli, arranged in a known honeycomb structure.

[0062] Each alveolus 18 opens onto a first face 121 of the heart 12 and onto a second face 122 of the heart 18 located opposite the first face 121.

[0063] The first face 121 of the core 12 is intended to be oriented towards the airflow duct, primary 7 or secondary 8 depending on the location of the acoustic treatment panel 10. The second face 122 of the core 12 is intended to be oriented in the opposite direction to the airflow duct.

[0064] Depending on the embodiment, the core 12 can be made of metal, or of a composite material, such as a composite material formed of carbon fibers embedded in a hardened resin matrix.

[0065] The reflective layer 14 is adapted to reflect acoustic waves with a frequency belonging to the predetermined frequency range.

[0066] The reflective layer 14 extends opposite the second face 122 of the core 12, being in contact with the second face 122. More precisely, the reflective layer 14 is integral with the second face 122 of the core 12, for example glued to the second face 122 of the core 12.

[0067] Depending on the embodiment, the reflective layer 14 can be made of metal or a composite material, such as a composite material formed of carbon fibers embedded in a hardened resin matrix. In one variant, the layer forming the bottom of the cells 18 can be non-reflective.

[0068] The inlet layer 16 extends opposite the first face 121 of the core 12, being in contact with the first face 121. More precisely, the inlet layer 16 is integral with the first face 121 of the core 12, for example glued to the first face 121 of the core 12.

[0069] The inlet layer 16 is a perforated one-piece plate comprising a plurality of orifices 20 traversing the inlet layer 16 from the first face 161 to the second face 162 of the inlet layer 16. Each orifice 20 opens onto an alveolus 18 of the core 12, several orifices 20 being able to open onto the same alveolus 18.

[0070] On the figure 3is schematically illustrated a perspective view of an acoustic absorption cell 18 according to a first embodiment of the invention.

[0071] The cell 18 comprises an acoustically opaque back wall 180, an acoustically porous entrance wall 181, and a cylindrical enclosure 185 with a hexagonal base extending between the back wall 180 and the entrance wall 181.

[0072] The background wall 180 is formed by the reflective layer 14 of the acoustic treatment panel 10, while the entrance wall 181 is formed by the entrance layer 16 of the acoustic treatment panel 10.

[0073] The cylinder formed by the enclosure 185 defines an axial direction DA corresponding to the direction of the cylinder's generatrices. Furthermore, depending on the embodiment, the base of the cylinder formed by the enclosure 185 may have a shape other than hexagonal. The base may be triangular, quadrilateral, circular, or polygonal, for example.

[0074] According to the axial direction DA, the enclosure 185 comprises a first axial end 1850 attached to the bottom wall 180, and a second axial end 1855 attached to the entrance wall 181.

[0075] Each cell 18 of the acoustic treatment panel 10 further comprises two acoustic pavilions 30 and 40 within the enclosure 185. The first acoustic pavilion 30 extends along the axial direction DA, between a first opening 31 and a second opening 32, the second opening 32 being smaller than the first opening 31. Similarly, the second acoustic pavilion 40 extends along the axial direction DA between a first opening 41 and a second opening 42, the second opening 42 being smaller than the first opening 41.

[0076] In the embodiment illustrated in the figure 3The first pavilion 30 and the second pavilion 40 are aligned along the axial direction DA. More specifically, the first orifice 31 of the first pavilion 30 is opposite the entrance wall 181, the second orifice 32 of the first pavilion 30 and the first orifice 41 of the second pavilion 40 are opposite each other, or even in the same plane, and the second orifice 42 of the second pavilion 40 is opposite the back wall 180.

[0077] On the figure 4 is schematically represented a cross-sectional view of a plurality of acoustic absorption cells 18 according to the first embodiment of the invention.

[0078] As illustrated on the figure 4, each cell comprises a section having a first equivalent diameter D and height H, the height H being measured along the axial direction DA , and the first equivalent diameter D being measured in the plane in which the entrance wall 181 extends, and corresponding to the diameter of the circle circumscribed at the base of the cylinder formed by the enclosure 185.

[0079] The first orifice 31 of the first auricle 30 and the first orifice 41 of the second auricle 40 each have an equivalent diameter equal to the first equivalent diameter D of the alveolus 18. The second orifice 32 of the first auricle 30 and the second orifice 42 of the second auricle 40 have the same equivalent diameter which is equal to a second equivalent diameter d.

[0080] The first pavilion 30 has a length equal to the length of the second pavilion 40. The length L of each of the two pavilions 30 and 40 is measured along the same direction as the height H of the alveolus 18, in this case the axial direction DA.

[0081] The first opening 31 of the first horn 30 extends in a plane parallel to the first opening 41 of the second horn 40. From its first opening 31, the first horn 30 extends in a direction forming a first opening angle α1. Similarly, from its first opening 41, the second horn 40 extends in a direction forming a second opening angle α2. The first opening angle α1 and the second opening angle α2 are equal and are between 60° and 90°.

[0082] The ratio between the first equivalent diameter D and the second equivalent diameter d varies between 2 and 100. The ratio between the length of a pavilion 30 or 40 and the height H of the alveolus 18 varies between 0.2 and 0.6.

[0083] For example, an acoustic treatment panel 10 comprising cells 18 according to the first embodiment illustrated in the Figures 3 and 4 With a height H of 80 mm, a first equivalent diameter D of 19 mm, a horn length L of 35 mm, and a second equivalent diameter d of 6 mm, it is possible to obtain two attenuation peaks centered approximately around 300 Hz and 750 Hz, as illustrated on the figure 5 .

[0084] There figure 5is a graphical representation of the absorption coefficient as a function of the frequency of the sound wave in Hertz in a first case of an acoustic treatment cell with a single horn inside the enclosure, and in a second case of an acoustic absorption cell comprising two horns inside the enclosure according to the first embodiment of the invention.

[0085] Furthermore, as appears on the figure 4 , the first pavilion 30 includes a portion having a continuous non-straight slope extending from its first orifice 31, and the second pavilion 40 includes a portion having a continuous non-straight slope extending from its first orifice 41.

[0086] Furthermore, as depicted on the figure 4, the acoustic treatment panel 10 can include for each cell an internal septum formed by an internal porous wall 183 extending inside the enclosure 185 and over the first orifice 41 of the second horn 40.

[0087] On the figure 6 is schematically illustrated a perspective view of an acoustic absorption cell 18 according to a second embodiment of the invention.

[0088] The second embodiment illustrated on the figure 6 differs from the first embodiment illustrated on the Figures 3 and 4 in that the axial direction DA along which the generatrices of the cylinder formed by the enclosure 185 extend, is not orthogonal to the planes in which the bottom wall 180 and the entrance wall 181 extend respectively.

[0089] The axial direction DA along which the walls 185 extend forms an angle of inclination β with the entrance walls 181 and the bottom wall 180, the entrance wall 181 and the bottom wall 180 extending parallel to each other.

[0090] The angle of inclination β varies between 0° and 60°. This angle allows the volume of the cavity inside the enclosure to be increased without increasing the distance separating the entrance wall 181 and the bottom wall 180, this distance being measured along a direction orthogonal to the planes in which the bottom and entrance walls extend.

[0091] Furthermore, the second embodiment differs from the first embodiment in that the first orifices 31 and 41 and the second orifices 32 and 42 of the first and second pavilions 30 and 40 extend in planes not parallel to the planes in which the entrance wall 181 and the bottom wall 180 extend.

[0092] In one variant, the first pavilion 30 and the second pavilion 40 may include surfaces with non-zero roughness to increase viscoelastic friction and thus increase sound absorption.

[0093] On the figure 7 is schematically represented a cross-sectional view of a plurality of acoustic absorption cells 18 according to the third embodiment of the invention.

[0094] The third embodiment illustrated on the figure 7 differs from the first embodiment illustrated on the figure 4in that its acoustic cells 18 do not include an internal porous wall (or septum) 183 between the two bells, and in that, for each cell 18, the first bell 30 is partially inserted into the second bell 40. The second orifice 32 of the first bell 30 is thus arranged, along the axial direction DA, between the first orifice 41 of the second bell 40 and the second orifice 42 of the second bell 40.

[0095] This third embodiment is illustrated on the figure 7 allows for a more compact acoustic treatment panel than the panel according to the first embodiment illustrated in the figure 4 .

[0096] On the figure 8 is schematically represented a cross-sectional view of a plurality of acoustic absorption cells 18 according to the fourth embodiment of the invention.

[0097] The fourth embodiment illustrated on the figure 8differs from the first embodiment illustrated on the figure 4 in that each acoustic cell 18 comprises at least one third horn 50 extending along the axial direction DA between a first orifice 51 and a second orifice 52, and in that the first horn 30 and the second horn 40 of each cell 18 are juxtaposed next to each other along two directions parallel to the axial direction DA and open via their respective second orifices 32 and 42 onto the first orifice 51 of the third horn 50, the second orifice 52 of the third horn 50 being arranged opposite the bottom wall 180.

[0098] In one variant, more than two pavilions, juxtaposed to each other or in the same direction, may open into a single pavilion arranged between the back wall 180 and said three or more pavilions.

[0099] On the figure 9is schematically represented a cross-sectional view of a plurality of acoustic absorption cells 18 according to the fifth embodiment of the invention.

[0100] The acoustic cells 18 of the fifth embodiment illustrated on the figure 9 differ from the acoustic cells illustrated on the figure 6 in that the first and second orifices 31 and 32, and 41 and 42, of each pavilion 30 and 40 extend parallel to the entrance wall 181 and the bottom wall 180.

[0101] The invention thus makes it possible to provide an acoustic absorption cell for the absorption of low-frequency sound waves with a reduced cavity height and whose frequency range of processing can be modulated as needed.

Claims

1. An acoustic absorption cell (18) comprising an acoustically opaque bottom wall (180) an acoustically porous inlet wall (181), an enclosure (185) extending in an axial direction (DA) between a first axial end (1850) integral with the bottom wall (180) and a second axial end (1855) integral with the inlet wall (181), and a first acoustic horn (30) extending inside the enclosure (185) between a first opening (31) of the first horn (30) and a second opening (32) of the first horn (30) that is smaller than said first opening (31) of the first horn (30), the first opening (31) facing said inlet wall (181), characterized in that it comprises at least one second horn (40, 50) extending inside the enclosure (185) between a first opening (41, 51) of the second horn (40, 50) and a second opening (42, 52) of the second horn (40, 50) that is smaller than said first opening (41, 51) of the second horn (40, 50), the second opening (42, 52) of the second horn (40, 50) being arranged facing said bottom wall (180), and either the first opening (41, 51) of the second horn (40, 50) is arranged facing the second opening (32) of the first horn (30), either the first horn (30) and the second horn (40) are coaxial and the first horn (30) is inserted at least partially in the second horn (40).

2. The acoustic absorption cell (18) according to claim 1, wherein the enclosure (185) extends between the bottom wall (180) and the inlet wall (181) while forming a nonzero inclination angle (β) not orthogonal with the inlet wall (181), on the one hand, and with the bottom wall (180), on the other hand.

3. The acoustic absorption cell (18) according to one of claims 1 or 2, wherein the cross section of the second opening (32) of the first horn (30) is equal to the cross section of the second opening (42) of the second horn (40), and the ratio between the cross section of the enclosure (185) at its second end (1855) and the cross section of the second opening of the first horn (30) is comprised between 2 and 100.

4. The acoustic absorption cell (18) according to one of claims 1 to 3, wherein the distance between the first opening (31) and the second opening (32) of the first horn (30) in the axial direction (DA) is equal to the distance between the first opening (41) and the second opening (42) of the second horn (40) in the axial direction (DA), and the ratio between the distance between the first opening (31) and the second opening (32) of the first horn (30) in the axial direction (DA) and the height of the enclosure (185) measured in the axial direction (DA) between its first end (1850) and its second end (1855) is comprised between 0.2 and 0.6.

5. The acoustic absorption cell (18) according to one of claims 1 to 4, wherein at its first opening (31) the first horn (30) forms a first opening angle (α1) with a plane parallel with the inlet wall (181) of the cell (18) and, at its first opening (41) the second horn (40) forms a second opening angle (α2) with a plane parallel with the inlet wall (181) of the cell (18).

6. The acoustic absorption cell (18) according to claim 5, wherein the second opening angle (α2) is identical to the first opening angle (α1).

7. The acoustic absorption cell (18) according to one of claims 1 to 6, wherein the first horn (30) and the second horn (40) have identical shapes and dimensions, the first horn (30) and the second horn (40) each comprising a cylindrical portion with a cross section equal to the cross section of their second opening (32, 42), the ratio between the length of the cylindrical portion in the axial direction (DA) and the length of the horn in the axial direction (DA) being less than or equal to 0.8.

8. The acoustic absorption cell (18) according to one of claims 1 to 7, comprising at least one porous internal wall (183) extending inside the enclosure (185) and over the first opening (31, 41, 51) of one of the horns (30, 40, 50).

9. The acoustic absorption cell (18) according to claim 8, wherein the internal wall (183) is parallel to said inlet wall (181).

10. The acoustic absorption cell (18) according to one of claims 1 to 9, wherein the first horn (30) comprises a portion having a continuous non-straight slope extending from its first opening (31), and the second horn (40) comprises a portion having a continuous non-straight slope extending from its first opening (41).

11. An acoustic treatment panel (10) intended to be arranged over at least one wall of an aircraft in contact with a fluid flow (F, FP, FS), the panel (10) comprising a plurality of acoustic absorption cells (18) according to one of claims 1 to 10.

12. A turbomachine (1) intended to be mounted on an aircraft, the turbomachine (1) comprising at least one acoustic treatment panel (10) according to claim 11.

13. An aircraft comprising at least one turbomachine (1) according to claim 12.