Sound absorber and method for producing same

Abstract

The invention relates to a sound absorber (100) extending between two opposite surfaces (100a), (100b) and combining one or more quarter-wave acoustic resonators (102), (102') with a microporous element (101). Each acoustic resonator (102), (102') has a length L, which is substantially greater than a thickness t of the sound absorber (100), between a first open end (102a) on a first surface (100a) of the two opposite surfaces (100a), (100b) of the sound absorber (100) and a second closed end (102b). The microporous element (101) is made up of a plurality of periodically repeating unit cells adjacent to the acoustic resonators (102), (102'). The invention also relates to a method for producing this sound absorber, which method comprises at least one additive manufacturing step.

Description

Description Title of the invention: Acoustic absorber and its production process Technical Field

[0001] The present invention relates to the field of acoustic absorbers, as well as to their production. Previous technique

[0002] Acoustic absorbers have a wide range of applications. These include aeronautics, where such elements are used to absorb at least some of the noise generated by aircraft engines and thus reduce its transmission to the external environment. Among the most common aircraft engines are turbofan engines. A turbofan engine comprises a fan and a gas generator incorporating at least one compressor, a combustion chamber, a turbine, and a nozzle. The total noise produced by such a turbofan engine can therefore include jet, combustion, fan, compressor, and turbine noise. However, the most dominant noise is usually that emitted by the fan, which can extend over a wide frequency band, as illustrated in Fig.13, with tonal components corresponding to the fan blade passage frequencies. In order to increase the energy efficiency of turbofan engines, the general trend is to increase their bypass ratio, that is, the proportion of the airflow supplied by the fan relative to that used for combustion in the gas generator, and therefore the fan diameter. Consequently, the fans of the latest generations of turbofan engines tend to rotate more slowly, and therefore emit noise at lower frequencies.

[0003] In order to reduce the noise emitted by aircraft engines, it is therefore common to cover certain areas, such as the nacelles containing these engines, with acoustic absorbers such as honeycomb sandwich panels. With this type of acoustic absorber, each cell of the honeycomb can function as a Helmholtz resonator to attenuate noise. However, the frequency range of acoustic attenuation of such absorbers is limited, and to be effective at low frequencies, they must be particularly large, which is all the more problematic given that the surface area to be covered can be very large for turbofan engines with high bypass ratios.

[0004] As an alternative to honeycomb sandwich panels, the use of porous materials, whose individual pores act as Helmholtz resonators, has been proposed. However, most available porous materials have insufficient mechanical strength, while the strongest, such as the metallic material disclosed in US 7,963,364 B2, are excessively heavy. Furthermore, these materials consist primarily of an interconnected pore structure, which, in the case of aircraft engine applications, can disrupt the airflow within the engine and thus degrade engine efficiency. Despite these drawbacks, the minimum frequency for perfect absorption of porous materials is typically achieved when their thickness is approximately one-quarter of the acoustic wavelength.As a result, to achieve high noise absorption at 1000 or 500Hz, for example, this thickness must be approximately 86 or 171 mm, respectively, resulting in elements that are far too bulky for the increasingly limited space in new generations of high or ultra-high bypass ratio motors.

[0005] The use of additive manufacturing was proposed by Z. Liu, J. Zhan, M. Fard, and JL Davy in "Acoustic properties of a porous polycarbonate material produced by additive manufacturing," Materials Letters, vol. 181, pp. 2960-299 (Oct. 2016) for producing acoustic absorbers with microchannels. However, these acoustic absorbers also have a rather narrow absorption frequency range.

[0006] It has also been proposed, for example by Qian, YJ, Kong, DY, Liu, SM, Sun, SM, & Zhao, Z., in "Investigation on micro-perforated panel absorber with ultra-micro perforations," Applied Acoustics, 74(7), pp; 931-935 (2013), to use micro-perforated panels as absorbers acoustics. To broaden the frequency range of sound absorption, Liu, Z., Zhan, J., Fard, M., & Davy, J., in "Acoustic properties of multilayer sound absorbers with a 3D printed micro-perforated panel," Applied Acoustics, 121, pp. 25-32 (2017), and Yang, W., Bai, X., Zhu, W., Kiran, R., An, J., Chua, CK, & Zhou, K., in "3D Printing of Polymeric Multi-Layer Micro-Perforated Panels for Tunable Wideband Sound Absorption," Polymers, 12(2), p. 360 (2020), also proposed layering several of these panels and producing them by additive manufacturing. However, these relatively fragile acoustic absorbers seem difficult to apply in environments where they would be subjected to abrasion or other mechanical stresses, such as aircraft engine nacelles.

[0007] Acoustic metamaterials with multiple superimposed layers in the thickness direction, produced by additive manufacturing, were proposed in French patent application FR 1 761 722, as well as by Guild, MD, Rohde, C., Rothko, MC, & Sieck, CF in "3D printed acoustic metamaterial sound absorbers using functionally-graded sonic crystals," Proceedings of Euronoise (2018). An acoustic metamaterial can be understood as a periodically structured medium whose periodically repeated constituent units collectively affect the passage of acoustic waves. In the case of the aforementioned metamaterials, each superimposed layer can exhibit a lattice with a different periodicity, thereby broadening its attenuation frequency range.However, their acoustic absorption is more reduced at the lowest frequencies, which are particularly important to absorb in the context of engines with very high bypass ratio blowers. Description of the invention

[0008] This disclosure aims to address these drawbacks by proposing a compact acoustic absorber with good sound absorption properties, including at low frequencies. According to one aspect, this acoustic absorber can extend between two opposing surfaces and include one or more first quarter-wave acoustic resonators. Each resonator has a first length, substantially greater than the thickness of the acoustic absorber between two opposing surfaces, between a first end, open to one of the two opposing surfaces of the acoustic absorber, and a second closed end, and a microporous element, consisting of a plurality of periodically repeated unit cells, adjacent to the first acoustic resonators. Thanks to the combination of acoustically different structures, the absorber is capable of providing high sound absorption over a wide frequency spectrum.

[0009] According to a second aspect, to obtain a length significantly greater than the thickness of the acoustic absorber, the first quarter-wave acoustic resonators can be inclined relative to a direction along the thickness of the acoustic absorber. In particular, the first quarter-wave acoustic resonators can be at least partially helical, so as to limit their size in the direction perpendicular to the thickness. Alternatively, or in addition to their inclination relative to the thickness, the first quarter-wave acoustic resonators can be bent.

[0010] According to a third aspect, the microporous element may comprise at least a first and a second layer superimposed in the thickness of the acoustic absorber, and the unit cells constituting the first layer may be different from the unit cells constituting the second layer, so as to adjust the acoustic properties of the microporous element.

[0011] According to a fourth aspect, each unit cell can comprise a channel and / or interwoven strands. The microporous element can thus take the form of a set of microchannels or a microlattice.

[0012] According to a fifth aspect, the first quarter-wave acoustic resonators can be arranged around the microporous element. Thus, their walls can confine the microporous element, so as to limit the propagation of acoustic waves beyond it, particularly when the microporous element takes the form of a microlattice with micropores interconnected in a direction perpendicular to the thickness of the acoustic absorber.

[0013] According to a sixth aspect, the acoustic absorber may comprise one or more second quarter-wave acoustic resonators, each having a second length substantially different from the first length, in order to absorb acoustic energy at two substantially different wavelengths. The first and second acoustic resonators may be arranged in adjacent rows, which may, in particular, be concentric.

[0014] A seventh and an eighth aspect relate, respectively, to a gas turbine engine and an aircraft incorporating an acoustic absorber according to any one of the preceding aspects.

[0015] A ninth aspect relates to a process for producing an acoustic absorber according to any one of the first to sixth aspects, comprising an additive manufacturing step of the microporous element and / or the quarter-wave acoustic resonators, in particular by molten material deposition, which may in particular be carried out along a zig-zag trajectory in order to limit fluid communication between adjacent unit cells of the microporous element, between adjacent quarter-wave acoustic resonators and / or between the microporous element and the quarter-wave acoustic resonators.

[0016] According to a tenth aspect, the process may include a subsequent step of assembling the microporous element with the quarter-wave acoustic resonators, in particular by shrink fitting. The microporous element and / or the quarter-wave acoustic resonators can thus be more easily manufactured separately, before their assembly. Brief description of the drawings

[0017] [Fig. 1] Figure 1 schematically illustrates a gas turbine engine for aircraft propulsion.

[0018] [Fig. 2A][Fig. 2B] Figures 2A and 2B represent detailed views of two different types of microporous element.

[0019] [Fig. 3A][Fig. 3B] Figures 3A and 3B are, respectively, top and side views of an acoustic absorber according to a first embodiment.

[0020] [Fig. 4] Figure 4 illustrates an acoustic absorber according to a second embodiment.

[0021] [Fig. 5] [ Fig. 6] [ Fig. 7] [ Fig. 8] [ Fig. 9] [ Fig. 10] Figures 5 to 10 are graphs comparatively illustrating the acoustic responses of different embodiments.

[0022] [Fig. 11][Fig. 12] Figures 11 and 12 illustrate an additive manufacturing step of a manufacturing process for an acoustic absorber according to an embodiment.

[0023] [Fig. 13] Figure 13 is a graph illustrating the frequency spectrum of acoustic emissions from an engine such as that in Figure 1. Description of the implementation methods

[0024] Figure 1 schematically illustrates a gas turbine engine 1. In the direction of fluid flow, this gas turbine engine 1 may comprise a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6, a low-pressure turbine 7, and a nozzle 8. The entire assembly may be enclosed by a nacelle 9. The compressors 3 and 4, the combustion chamber 5, and the turbines 6 and 7 together form the gas generator 10, which may itself be enclosed by a shroud 11 leading into the nozzle 8. Thus, an air stream 12 from the fan 2 can be defined between the shroud 11 of the gas generator 10 and an internal wall 13 of the nacelle 9.The high-pressure turbine 6 can be connected to the high-pressure compressor 4 by a first rotating shaft 14 for driving the latter, while the low-pressure turbine 7 can be connected to the fan 2 and the low-pressure compressor 3 by a second rotating shaft 15 coaxial with the first rotating shaft 14, in a similar manner. In the context of high and very high bypass ratio engines, a reduction gear 16 can be mechanically interposed between the second rotating shaft 15 and the fan 2, in order to reduce the rotational speed of the fan 2 and prevent the blade tips of the fan 2 from reaching excessive speeds.

[0025] Each of these components of the gas turbine engine 1 can generate noise, but the noise generated by the fan 2 is generally dominant. Furthermore, in high and very high bypass ratio engines, and particularly in those equipped with a reduction gear 16, a large portion of the fan 2 noise can be concentrated in low frequencies, as illustrated in Figure 10, which shows the sound pressure level (SPL) as a function of frequency f. In order to absorb at least some of the fan 2 noise, sound absorbers 17 can be integrated into the inner wall 13 of the nacelle 9, particularly upstream and downstream of the fan 2 blades. As illustrated, however, it is also possible to integrate sound absorbers 17 into the shroud 11 of the gas generator 10, or even into the casing of the latter.

[0026] Typically, the acoustic absorbers 17 are formed by honeycomb sandwich panels. However, in engines with high or even very high bypass ratios, these panels can represent a significant penalty in terms of mass and size. Furthermore, it can be difficult to position them directly opposite the fan blade tips, where noise emission may be most intense, since the inner wall 13 of the nacelle 9 typically includes an abradable material 18 in this area to absorb the occasional friction of the fan blade tips 2 due to their transient deformations.

[0027] An acoustic absorber 100 according to an embodiment may comprise a microporous element 101 and one or more acoustic resonators 102, as illustrated in Figures 3A, 3B and 4, so as to combine their acoustic absorption properties over a wide frequency spectrum, which may in particular include the frequencies corresponding to the emission peaks of a gas turbine engine.

[0028] The microporous element 101 and / or the acoustic resonators 102 may be made of thermoplastic polymer, for example polyetherimide (PEI) or polyetheretherketone (PEEK), or of thermosetting resin, for example an epoxy resin such as that forming the abradable material sold by 3M® under the name Scotch-Weld® EC-3524 B / A. In order to reinforce this material, particularly when the acoustic absorber 100 is intended to be placed near rotating parts, and especially the rotating blades of a blower 2. The material can be reinforced with solid particles embedded within it, such as fibers, particularly carbon fibers, microspheres (e.g., glass microbeads), or nanoparticles like silica powder. Depending on the material and reinforcements used in its manufacture, the acoustic absorber 100 can exhibit significant mechanical and thermal resistance as well as abradability.

[0029] The microporous element 101 can be composed of a plurality of periodically repeated unit cells 110, so as to form a periodic metamaterial. Each unit cell 110 can comprise a channel 111 and / or interwoven strands 112, as illustrated in Figures 2A and 2B respectively, so that the microporous element takes the form of a set of microchannels or a microlattice, respectively. Furthermore, the microporous element 101 can comprise several superimposed layers within the thickness of the acoustic absorber 100, and the different layers can be composed of different unit cells than the different unit cells 110. By superimposing several layers with different properties, it is possible, in particular, to absorb acoustic energy over a broader frequency spectrum.

[0030] As illustrated in Figures 3A, 3B, and 4, the acoustic resonators 102 can, in particular, take the form of tubular waveguides, with a first open end 102a and a second closed end 102b. The acoustic absorber 100 can be used as a sound-absorbing coating on a wall (not shown) that is substantially impermeable to sound. In this case, the second ends 102b of the acoustic resonators 102 can simply be closed by said wall. Thus, these tubular waveguides can function as quarter-wave acoustic resonators, absorbing acoustic waves with a wavelength equal to four times the length of the acoustic resonator 102 between its open end 102a and its closed end 102b. In order to absorb acoustic energy at low frequencies, this length can be substantially greater than the thickness t of the acoustic absorber 100 between its two opposite surfaces 100a, 100b.For this purpose, the acoustic resonators 102 can be inclined at an angle P with respect to the direction of the thickness of the acoustic absorber 100. More particularly, they can be at least partially helical, as illustrated in the figures. 3A, 3B and 4, so as to also limit their extension in each direction perpendicular to the direction of the thickness of the acoustic absorber 100. Alternatively or in addition to their inclination with respect to the direction of the thickness, the acoustic resonators 102 can also be bent in order to increase the ratio between their length and the thickness t of the acoustic absorber 100.

[0031] Furthermore, as illustrated in Figures 3A, 3B, and 4, the acoustic resonators 102 can be arranged in one or more rows 110, 110'. As shown in Figure 3, each row 110, 110' can follow a closed line, for example, circular, oval, or polygonal. Thus, the acoustic resonators 102 can be arranged around the microporous element 101, so as to confine it. The circulation of acoustic waves perpendicular to the thickness of the acoustic absorber 100 can be limited in this way, which may be particularly preferable when the microporous element 101 takes the form of a microtrellis with micropores interconnected in a direction perpendicular to the thickness of the acoustic absorber 100. In addition, as illustrated in Figure 4, when the rows 110,110' follow closed lines, they can notably be concentric.

[0032] Figure 5 comparatively illustrates the respective curves of the sound absorption coefficient α as a function of frequency for samples of acoustic absorbers 100 having a thickness t of 30 mm and comprising a single-layer microporous element 101 with pores having a diameter D of 290 pm, and for different numbers and lengths L of acoustic resonators 102 formed by tubes, each with a diameter D tbetween 3 and 5 mm, wound helically around the microporous element 101. Curve 501 corresponds to an acoustic absorber 100 with seven acoustic resonators 102, each forming 1.25 turns around the microporous element 101, to obtain a length L of each acoustic resonator 102 of 109 mm. Curve 502 corresponds to an acoustic absorber 100 with five acoustic resonators 102, each forming 1.75 turns around the microporous element 101, to obtain a length L of each acoustic resonator 102 of 150 mm. Curve 503 corresponds to an acoustic absorber 100 with three acoustic resonators 102, each forming three turns around the microporous element 101, to obtain a length L of each acoustic resonator 102 of 220 mm. For comparison, curve 504 corresponds to the acoustic response of the single-layer microporous element 101 alone, without the acoustic resonator sheath. As can be seen, the acoustic resonators 102 offer additional peaks in the sound absorption coefficient a. More specifically, the seven-acoustic resonator example 102 offers an additional peak in the sound absorption coefficient a with a value of 0.95 at a frequency f of 1200 Hz on curve 501, the five-acoustic resonator example 102 offers an additional peak in the sound absorption coefficient a with a value of 0.92 at a frequency f of 1060 Hz on curve 502 and the three-acoustic resonator example 102 offers an additional peak in the sound absorption coefficient a with a value of 0.89 at a frequency f of 668 Hz on curve 503.

[0033] Figure 6 comparatively illustrates the respective curves of the sound absorption coefficient a as a function of frequency for samples of acoustic absorbers 100 having a thickness t of 30 mm and comprising a two-layer microporous element 101 and the different numbers and lengths L of acoustic resonators 102 formed by tubes between 3 and 5 mm in diameter wound helically around the microporous element 101. More specifically, the microporous element 101 comprises a first layer 101a, on the side of the first surface 100a of the acoustic absorber 100, with a thickness ti of 2 mm and a pore diameter Di of 100pm, and a second layer 101b, on the side of the second surface 100b of the acoustic absorber 100, with a thickness t2 of 28 mm and a pore diameter D2 of 4.6 mm.Curve 601 corresponds to an acoustic absorber 100 with seven acoustic resonators 102, each forming 1.25 turns around the microporous element 101, resulting in a length L of each acoustic resonator 102 of 109 mm. Curve 602 corresponds to an acoustic absorber 100 with five acoustic resonators 102, each forming 1.75 turns around the microporous element 101, resulting in a length L of each acoustic resonator 102 of 150 mm. Curve 603 corresponds to an acoustic absorber 100 with three acoustic resonators 102, each forming three turns around the microporous element 101, resulting in a length L of... Each 220 mm acoustic resonator 102. For comparison, curve 604 corresponds to the acoustic response of the two-layer microporous element 101 alone, without the acoustic resonator sheath. As can be seen, the additional peaks of the sound absorption coefficient α at frequencies f of 1200 Hz on curve 601, 1060 Hz on curve 602, and 668 Hz on curve 603 are also present.

[0034] Figure 7 comparatively illustrates the respective curves of the sound absorption coefficient a as a function of frequency for samples of sound absorbers 100 having a thickness t of 30 mm and comprising a four-layer microporous element 101 and the different numbers and lengths L of acoustic resonators 102 formed by tubes of between 3 and 5 mm in diameter wound helically around the microporous element 101.More specifically, the microporous element 101 comprises a first layer 101a, on the side of the first surface 100a of the acoustic absorber 100, with a thickness ti of 1 mm and a diameter Di of the pores of 100 pm and, successively in the direction of the second surface 100b of the acoustic absorber 100, a second layer 101b with a thickness t2 of 13 mm and a diameter D2 of the pores of 4.6 mm, a third layer 101c with a thickness t3 of 2 mm and a diameter D3 of the pores of 100 pm, and a fourth layer 101d with a thickness t4 of 13 mm and a diameter D4 of the pores of 4.6 mm. Curve 701 corresponds to an acoustic absorber 100 with seven acoustic resonators 102, each forming 1.25 turns around the microporous element 101, to obtain a length L of each acoustic resonator 102 of 109 mm.Curve 702 corresponds to an acoustic absorber 100 with five acoustic resonators 102, each forming 1.75 turns around the microporous element 101, resulting in a length L of 150 mm for each acoustic resonator 102. For comparison, curve 703 corresponds to the acoustic response of the two-layered microporous element 101 alone, without the acoustic resonator sheath, and also includes curve 603, corresponding to an acoustic absorber 100 with three acoustic resonators 102 around the two-layered microporous element 101. As can be seen, the peaks are also present. additional to the sound absorption coefficient a at frequencies f of 1200 Hz on curve 701 and 1060 Hz on curve 702.

[0035] Figure 8 illustrates the influence of diameter D tacoustic resonators on the acoustic absorption coefficient a. The four curves 801 to 804 correspond to an acoustic absorber 100 with seven acoustic resonators 102 of length L, each forming 1.25 turns around a four-layered microporous element 101. The first layer 101a, on the side of the first surface 100a of the acoustic absorber 100, has a thickness ti of 1 mm and a pore diameter Di of 100 µm. Subsequently, towards the second surface 100b of the acoustic absorber 100, there is a second layer 101b with a thickness t2 of 13 mm and a pore diameter D2 of 4.6 mm, a third layer 101c with a thickness t3 of 2 mm and a pore diameter D3 of 100 µm, and a fourth layer 101d with a thickness t4 of 13 mm and a pore diameter D4. of 4.6 mm. However, curve 801 corresponds to a diameter D t of each 102 acoustic resonator of 4 mm, the 802 curve has a diameter D tof 3 mm, curve 803 has a diameter D t of 2 mm, and curve 804 has a diameter D t of 1 mm. Thus, depending on the desired absorption spectrum, it is possible to optimize not only the configuration of the microporous element 101, and the length and number of the acoustic resonators 102, but also their individual diameters.

[0036] Thus, the diameter D t Optimal for acoustic resonators 102 of an acoustic absorber 100 to seven acoustic resonators 102 of 109 mm length L, each forming 1.25 turns around the aforementioned four-layer microporous element 101, can be, for example, 3.2 mm, resulting in a sound absorption coefficient a as a function of frequency f following the curve 901 of Figure 9, while the diameter D toptimal for acoustic resonators 102 of an acoustic absorber 100 with the same microporous element but only three acoustic resonators 102 of 220 mm in length, each forming three turns around the microporous element 101, can be for example 4.8 mm, resulting in the curve 902.

[0037] Furthermore, as illustrated in Figure 4, the acoustic absorber 100 can comprise acoustic resonators 102, 102' of different lengths, in order to absorb acoustic energy at different wavelengths. Thus, as illustrated in Figure 4, a first set of helical acoustic resonators 102, each having a first angle P of inclination with respect to the direction of the thickness, and therefore a first length L, can be arranged in a first circular row 110, and a second set of helical acoustic resonators 102', each having a second angle P' of inclination with respect to the direction of the thickness, and therefore a second length L', can be arranged in a second circular row 110' concentric with respect to the first row 110, so as to confine together the microporous element 101.

[0038] Figure 10 thus compares the curve 901 of the acoustic absorber 100 with seven acoustic resonators 102 of 109 mm length L around the four-layer microporous element 101 with a curve 903 corresponding to an acoustic absorber with the same four-layer microporous element 101, but surrounded by two concentric sheaths, respectively formed by a first and a second set of acoustic resonators 102, 102', where the first set is formed by seven acoustic resonators 102 of 109 mm length L arranged circularly and wound around the microporous element 101, while the second set is formed by three acoustic resonators 102' of 220 mm length arranged circularly and also wound around the microporous element 101. As can be seen, this curve 903 combines the acoustic absorption peaks of curves 901 and 902.

[0039] The acoustic resonators 102, 102' and / or the microporous element 101 can be produced, together or separately, by an additive manufacturing process based on material extrusion, such as the fused deposition modeling (FDM) process used for thermoplastics. These processes, particularly well-suited to manufacturing complex shapes with thin walls, comprise several consecutive material deposition steps. In each of these steps, an extruder head 200 can move along a path 201 in a transverse plane XY, depositing the material 202, which then solidifies to form a layer 203. By moving this transverse plane XY along an orthogonal direction Z after the deposition of each layer 203, it is possible to stack these layers 203 to form the element microporous 101 and / or acoustic resonators 102, as illustrated in Figure 11. In order to form tubes and / or channels, each layer 203 can comprise a plurality of periodically repeated cells, separated by the walls formed by the deposition of the material 202, and the layers 203 deposited in consecutive material deposition steps can be stacked with their respective cells aligned.

[0040] In order to at least partially avoid the interlacing of the extruded material 202 during the deposition of a layer 203, which could cause the formation of pores between adjacent cells, the trace 201 can be zig-zag, as illustrated in Figure 12. To avoid an accumulation of material and the formation of pores at the intersections between the walls, a gap O can be maintained between the angles 205 of the trace 201 at these intersections.

[0041] When the acoustic resonators 102, 102' and the microporous element 101 are manufactured separately, they can then be assembled to form the acoustic absorber 100. This assembly can be achieved, in particular, by shrink-fitting. Thus, when the acoustic resonators 102, 102' are arranged to form one or more annular sheaths, as illustrated in Figures 3 or 4, the microporous element 101 can then be shrink-fitted inside these sheaths. A strong connection between these sheaths of acoustic resonators 102, 102' and the microporous element 101 inside can thus be obtained through radial pressure and the resulting friction between them.

[0042] Although the present invention has been described with reference to specific embodiments, it is evident that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered illustrative rather than restrictive.

Claims

Demands

1. Acoustic absorber extending between two opposite surfaces and comprising: one or more first quarter-wave acoustic resonators each having a first length substantially greater than a thickness of the acoustic absorber between the two opposite surfaces, between a first end, open on a first surface of the two opposite surfaces of the acoustic absorber, and a second closed end, and a microporous element, consisting of a plurality of periodically repeated unit cells, adjacent to the first acoustic resonators.

2. Acoustic absorber according to claim 1, wherein the first quarter-wave acoustic resonators are inclined with respect to a direction of the thickness of the acoustic absorber.

3. Acoustic absorber according to claim 2, wherein the first quarter-wave acoustic resonators are at least partially helical.

4. Acoustic absorber according to any one of claims 1 to 3, wherein the first quarter-wave acoustic resonators are bent.

5. Acoustic absorber according to any one of claims 1 to 4, wherein the microporous element comprises at least a first and a second layer superimposed in the thickness of the acoustic absorber, and the unit cells constituting the first layer are different from the unit cells constituting the second layer.

6. Acoustic absorber according to any one of claims 1 to 5, wherein each unit cell comprises a channel and / or interlaced strands.

7. Acoustic absorber according to any one of claims 1 to 6, wherein the first quarter-wave acoustic resonators are arranged around the microporous element.

8. Acoustic absorber according to any one of claims 1 to 7, comprising one or more second quarter-wave acoustic resonators each having a second length, substantially different from the first length.

9. Acoustic absorber according to claim 8, in which the first acoustic resonators and the second acoustic resonators are arranged in adjacent rows.

10. Acoustic absorber according to claim 9, wherein the adjacent rows are concentric.

11. Gas turbine engine incorporating a sound absorber according to any one of the preceding claims.

12. Aircraft incorporating an acoustic absorber according to any one of claims 1 to 10.

13. A method for producing an acoustic absorber according to any one of claims 1 to 10, comprising an additive manufacturing step of the microporous element and / or quarter-wave acoustic resonators.

14. A production method according to claim 13, wherein the additive manufacturing step is carried out by deposition of molten material.

15. A production method according to claim 14, wherein the molten material is deposited along a zig-zag trajectory.

16. A production method according to any one of claims 13 to 15, comprising a subsequent step of assembling the microporous element with the quarter-wave acoustic resonators.

17. A production method according to claim 14, wherein the assembly of the microporous element with the quarter-wave acoustic resonators is carried out by shrink fitting.

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