Sound absorber and production method thereof

Abstract

The invention relates to a sound absorber (100) extending between two opposing 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 between a first open end (102a) and a second closed end (102b) on a first surface (100b) of two opposing surfaces (100a), (100b) of the sound absorber (100), which length is substantially greater than a thickness t of the sound absorber (100). The microporous element (101) is composed of a plurality of periodically repeating unit cells adjacent to the acoustic resonators (102, 102 '). The invention also relates to a production method for such a sound absorber, comprising at least one additive manufacturing step.

Description

Technical Field

[0001] The invention relates to the field of sound absorbers and the field of their production. Background Art

[0002] Sound absorbers have a wide range of applications. These include in particular aeronautics, where these elements are used to at least partially absorb the noise generated by aircraft engines and thereby reduce its transmission to the external environment. The most common aircraft engines include turbofan engines. A turbofan engine comprises a fan and a gas generator, which gas generator includes at least a compressor, a combustion chamber, a turbine and a nozzle. The total noise generated by such a turbofan engine may therefore include: the noise of the jet, combustion, fan, compressor and turbine. However, the most dominant noise is usually the noise emitted by the fan, such as Fig.13 As exemplified in , this noise can extend over a large frequency band, with the tonal components corresponding to the passing frequencies of the fan blades. In order to improve the energy efficiency of turbofan engines, the general trend is to increase their bypass ratio, in other words, to increase the ratio of the airflow driven by the fan to the airflow used for combustion in the gas generator, thereby increasing the diameter of the fan. As a result, the fans of the latest generation of turbofan engines tend to rotate more slowly and therefore emit a lower frequency noise.

[0003] Therefore, in order to reduce the noise emitted by aircraft engines, certain areas such as the cabin containing these engines are usually covered with sound absorbers such as honeycomb sandwich panels. In this type of sound absorber, each cell of the honeycomb can act as a Helmholtz resonator in order to attenuate the noise. However, the frequency range of the sound attenuation of such absorbers is limited and, in order to be effective at low frequencies, they must be particularly bulky, which is even more disadvantageous because, for turbofan engines with very high bypass ratios, the surface to be covered can be very large.

[0004] Therefore, as an alternative to honeycomb sandwich panels, it has been proposed to use porous materials whose individual holes act as Helmholtz resonators. However, most of the available porous materials have too low mechanical strength, while the strongest ones (such as the metal materials disclosed in US 7,963,364 B2) are too heavy. In addition, these materials are mainly composed of structures with interconnected holes, which, when applied to aircraft engines, interfere with the airflow in the engine, thereby reducing the engine efficiency. In addition to these disadvantages, the minimum frequency for complete absorption of the porous material is usually reached when the thickness of the porous material is approximately equal to one-quarter of the wavelength of the sound wave. Therefore, in order to obtain a high noise absorption rate at 1000 Hz or 500 Hz, for example, the thickness must be about 86 mm or 171 mm, respectively, resulting in many components being too bulky for the increasingly limited space in the new generation engines with high or ultra-high bypass ratios.

[0005] The use of additive manufacturing for producing sound absorbers containing microchannels has been 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. 296-299, (Oct. 2016). However, these sound absorbers have only a very narrow absorption frequency range.

[0006] For example, Qian, YJ, Kong, DY, Liu, SM, Sun, SM and Zhao, Z. have also proposed the use of microperforated panels as sound absorbers in "Investigation on micro-perforated panel absorber with ultra-microperforations" in Applied Acoustics, 74(7), pp; 931-935 (2013). In order to broaden the range of sound absorption frequencies, Liu, Z., Zhan, J., Fard, M., & Davy, J., “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, C. K., & Zhou, K, “3D Printing of Polymeric Multi-Layer Micro-Perforated Panels for Tunable Wideband Sound,” Polymers, 12(2), p. 360 (2020). It has also been proposed in "3D Printing of Polymer Multilayer Microporous Panels for Tunable Broadband Sound Absorption" to stack multiple such panels and produce them by additive manufacturing. However, these relatively fragile absorbers seem difficult to use in environments where they will be subject to wear and tear or other mechanical stresses (for example, aircraft engine nacelles).

[0007] Acoustic metamaterials with multiple layers stacked in the thickness direction produced by additive manufacturing have been proposed in the French patent application with publication number FR 1 761 722 and in "3D printed acoustic metamaterial sound absorbers using functionally-graded sonic crystals" by Guild, MD, Rohde, C., Rothko, MC and Sieck, CF in Proceedings of Euronoise (2018). The term "acoustic metamaterial" can be understood as a medium with a periodic structure, whose periodically repeated constituent units jointly influence the passage of sound waves. In the case of the above-mentioned metamaterials, each stacked layer can have a lattice of different periodicity in order to broaden its attenuation frequency range. However, their sound absorption is reduced more at lower frequencies, which is particularly applicable to the sound absorption in fan engines with very high bypass ratios. Summary of the invention

[0008] The present disclosure aims to overcome these disadvantages by proposing a sound absorber of small size but with good sound absorption performance (including at low frequencies). According to a first aspect, the sound absorber can extend between two opposing surfaces and comprises: one or more first quarter-wave acoustic resonators, each of which has a first length between a first open end and a second closed end on a first surface of the two opposing surfaces of the sound absorber, the first length being substantially greater than the thickness of the sound absorber between the two opposing surfaces; and a microporous element consisting of a plurality of periodically repeated unit cells adjacent to the first acoustic resonator. From an acoustic point of view, through a combination of different structures, the sound absorber is able to provide higher sound absorption over a wider frequency range.

[0009] According to a second aspect, in order to obtain a length substantially greater than the thickness of the sound absorber, the first quarter wave acoustic resonators may be tilted relative to the direction of the thickness of the sound absorber. In particular, the first quarter wave acoustic resonators may be at least partially helical so as to limit their size in a direction perpendicular to the thickness. Alternatively or in addition to their tilt relative to the thickness direction, the first quarter wave acoustic resonators may be curved.

[0010] According to a third aspect, the microporous element may include at least a first stacked layer and a second stacked layer in the thickness of the sound absorber, and the unit cells constituting the first layer may be different from the unit cells constituting the second layer in order to adjust the acoustic characteristics of the microporous element.

[0011] According to the fourth aspect, each unit cell may include channels and / or cross-wires. The microporous element may be in the form of an assembly of microchannels or microlattices.

[0012] According to a fifth aspect, the first quarter-wave acoustic resonator can be arranged around the microporous element. Thus, their walls can delimit the microporous element, thereby limiting the circulation of sound waves outside thereof, in particular when the microporous element takes the form of a microlattice, wherein the micropores are interconnected in a direction perpendicular to the thickness of the sound absorber.

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

[0014] The seventh and eighth aspects relate to a gas turbine engine and an aircraft comprising the sound absorber according to any one of the preceding aspects, respectively.

[0015] A ninth aspect relates to a method for producing a sound absorber according to any one of the first to sixth aspects, the method comprising: a step of additive manufacturing of the microporous element and / or the quarter-wave acoustic resonator, in particular by depositing molten material, in particular, this can be performed along a zigzag path so as 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 resonator.

[0016] According to a tenth aspect, the method may comprise a subsequent step of assembling the microporous element with the quarter wave acoustic resonator, in particular by shrink fitting. Thus, the microporous element and / or the quarter wave acoustic resonator may be manufactured more easily separately before they are assembled. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A gas turbine engine for propelling an aircraft is schematically illustrated.

[0018] Figure 2A and Figure 2BDetailed views of two different types of microporous elements are shown.

[0019] Figure 3A and Figure 3B 1 and 2 are a top view and a side view, respectively, of the sound absorber according to the first embodiment.

[0020] Figure 4 A sound absorber according to a second embodiment is illustrated.

[0021] Figures 5 to 10 is a graph illustrating comparative acoustic responses of various embodiments.

[0022] Fig.11 and Fig.12 Additive manufacturing steps from a method for manufacturing a sound absorber according to an embodiment are illustrated.

[0023] Fig.13 is an example of Figure 1 Graph of the frequency spectrum of the engine's acoustic emission. DETAILED DESCRIPTION

[0024] Figure 1 A gas turbine engine 1 is schematically illustrated. In the direction of fluid flow, the gas turbine engine 1 may include a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6 and a low-pressure turbine 7, and a nozzle 8. The whole may be surrounded by a nacelle 9. The compressors 3 and 4, the combustion chamber 5 and the turbines 6 and 7 together form a gas generator 10, which itself may be surrounded by a fairing 11 terminating in the nozzle 8. Thus, the air flow 12 of the fan 2 may be confined between the fairing 11 of the gas generator 10 and the inner wall 13 of the nacelle 9. The high-pressure turbine 6 may be connected to the high-pressure compressor 4 by a first rotating shaft 14 for driving the high-pressure compressor 4, while the low-pressure turbine 7 may be connected to the fan 2 in a similar manner by a second rotating shaft 15 coaxial with the first rotating shaft 14, and to the low-pressure compressor 3. In the case of engines with a high and very high bypass ratio, a reduction gear 16 may be mechanically interposed between the second rotating shaft 15 and the fan 2 in order to reduce the rotation speed of the fan 2 and prevent the ends of the blades of the fan 2 from reaching excessively high speeds.

[0025] Each of these elements of the gas turbine engine 1 generates noise, but the noise generated by the fan 2 is generally dominant. In addition, in engines with high and very high bypass ratios, and especially in engines equipped with a reduction gear 16, most of the noise from the fan 2 may be concentrated at low frequencies, such as Fig.10, which shows the sound pressure level (SPL) as a function of the frequency f. In order to absorb at least part of the noise of the fan 2, a sound absorber 17 can be installed in the inner wall 13 of the nacelle 9 (in particular upstream and downstream of the fan blades 2). However, as illustrated in the figure, the sound absorber 17 can also be installed in the fairing 11 of the gas generator 10 (or even in its casing).

[0026] The sound absorbers 17 are usually formed of honeycomb sandwich panels. However, in engines with a high or very high bypass ratio, these panels may present a significant disadvantage in terms of mass and size. Furthermore, it may be difficult to arrange them directly opposite the ends of the blades of the fan, where, however, the noise emitted may be the strongest, since the inner wall 13 of the nacelle 9 usually comprises an abradable material 18 at this location in order to absorb the friction that occasionally occurs at the ends of the fan blades 2 due to their transient deformations.

[0027] like Figure 3A , Figure 3B and Figure 4 As illustrated in , a sound absorber 100 according to an embodiment may include a microporous element 101 and one or more acoustic resonators 102 to combine their sound absorption properties over a wider frequency spectrum, in particular, the spectrum may include frequencies corresponding to emission peaks of a gas turbine engine.

[0028] The microporous element 101 and / or the acoustic resonator 102 may be made of a thermoplastic polymer, such as polyetherimide (PEI) or polyetheretherketone (PEEK), or may be made of a thermosetting resin, such as a resin formed of Scotch Epoxy resin of a wearable material sold under the name EC-3524B / a. To reinforce this material, in particular when the sound absorber 100 is intended to be arranged opposite a rotating part (and in particular the rotating blades of the fan 2), the material can be reinforced by solid particles embedded in the substance, such as fibers (and in particular carbon fibers), microspheres (for example glass microspheres) or nanoparticles (such as silica powder). Depending on the materials and reinforcements used to manufacture the sound absorber 100, it can have significant mechanical strength and heat resistance and wear resistance.

[0029] The microporous element 101 may be composed of a plurality of periodically repeated unit cells 110 to form a periodic metamaterial. Figure 2A and Figure 2BAs respectively illustrated in , each unit cell 110 may include a channel 111 and / or a cross line 112, so that the microporous element may take the form of an assembly of microchannels or microlattices, respectively. Furthermore, the microporous element 101 may include a plurality of superimposed layers over the thickness of the sound absorber 100, and different layers may consist of different unit cells than different unit cells 110. By superimposing several layers with different characteristics in this way, in particular, it is possible to absorb sound energy over a wider spectrum.

[0030] like Figure 3A , Figure 3B and Figure 4 As illustrated in , in particular, the acoustic resonator 102 can take the form of a tubular waveguide, wherein the first end 102a is open and the second end 102b is closed. The sound absorber 100 can be used as a sound-absorbing coating on a wall (not illustrated) that is substantially impermeable to sound. In this case, the second end 102b of the acoustic resonator 102 can simply be closed by the wall. Therefore, these tubular waveguides can be used as quarter-wave acoustic resonators to absorb sound waves having a length equal to one-quarter of the length of the acoustic resonator 102 between its open end 102a and the closed end 102b. In order to absorb low-frequency sound energy, this length must be substantially greater than the thickness t of the sound absorber 100 between its two opposing surfaces 100a, 100b. To this end, the acoustic resonator 102 can be inclined at an angle β relative to the thickness direction of the sound absorber 100. More specifically, as Figure 3A , Figure 3B and Figure 4 As illustrated in , they may be at least partially helical so as to also limit the extension in every direction perpendicular to the thickness direction of the sound absorber 100. Alternatively, or in addition to their inclination relative to the thickness direction, the acoustic resonators 102 may also be curved so as to increase the ratio between their length and the thickness t of the sound absorber 100.

[0031] In addition, if Figure 3A , Figure 3B and Figure 4 As illustrated in , the acoustic resonators 102 may be arranged along a plurality of rows 110, 110'. As illustrated in FIG. 3, each row 110, 110' may follow a closed line (e.g., a circular, elliptical or polygonal line). Thus, the acoustic resonators 102 may be arranged around the microporous element 101 to delimit it. In this way, the circulation of sound waves perpendicular to the thickness of the sound absorber 100 may be limited, which may be particularly preferred when the microporous element 101 is in the form of a microlattice with micropores that are interconnected in a direction perpendicular to the thickness of the sound absorber 100. Furthermore, as Figure 4 As illustrated in , when the rows 110 , 110 ′ follow a closed line, in particular, they may be concentric.

[0032] Figure 5 The corresponding curves of the sound absorption coefficient α as a function of frequency for a sample of a sound absorber 100 having a thickness t of 30 mm and comprising: a single-layer microporous element 101 having pores with a diameter D of 290 μm, and acoustic resonators 102 of different numbers and lengths L formed by tubes, each tube having a diameter D between 3 mm and 5 mm are illustrated by way of comparison. t , spirally wound around the microporous element 101. Curve 501 corresponds to a sound absorber 100 having seven acoustic resonators 102, each of which forms 1.25 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 109 mm. Curve 502 corresponds to a sound absorber 100 having five acoustic resonators, each of which forms 1.75 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator of 150 mm. Curve 503 corresponds to a sound absorber 100 having three acoustic resonators 102, each of which forms 3 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator of 220 mm. By comparison, curve 504 corresponds to the acoustic response of a single-layer microporous element 101 alone, without the sheath of the acoustic resonator. As can be seen, the acoustic resonator 102 gives an additional peak in the sound absorption coefficient α. More specifically, on curve 501, the example with seven acoustic resonators 102 gives an additional peak value of the sound absorption coefficient α of 0.95 at a frequency f of 1200 Hz, on curve 502, the example with five acoustic resonators 102 gives an additional peak value of the sound absorption coefficient α of 0.92 at a frequency f of 1060 Hz, and on curve 503, the example with three acoustic resonators 102 gives an additional peak value of the sound absorption coefficient α of 0.89 at a frequency f of 668 Hz.

[0033] Figure 6 The corresponding curves of the sound absorption coefficient α for the sound absorber 100 samples with frequency are comparatively illustrated, the sound absorber having a thickness t of 30 mm and comprising: a microporous element 101 having two layers, and acoustic resonators 102 of different numbers and lengths L, the acoustic resonators being formed of tubes having a diameter between 3 mm and 5 mm and spirally wound around the microporous element 101. More specifically, the microporous element 101 comprises: a first layer 101a (whose thickness t is 30 mm) located on one side of the first surface 100a of the sound absorber 100; 1 2mm, aperture D 1 100 μm), and a second layer 101b (whose thickness is t 2 28mm, aperture D 24.6 mm). Curve 601 corresponds to a sound absorber 100 having seven acoustic resonators 102, each forming 1.25 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 109 mm. Curve 602 corresponds to a sound absorber 100 having five acoustic resonators 102, each forming 1.75 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 150 mm. Curve 603 corresponds to a sound absorber 100 having three acoustic resonators 102, each forming 3 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 220 mm. By comparison, curve 604 corresponds to the acoustic response of a microporous element 101 having only two layers, without a sheath of the acoustic resonators. As can be seen from the figure, additional peaks of the sound absorption coefficient are also found at a frequency f of 1200 Hz on the curve 601 , at a frequency f of 1060 Hz on the curve 602 , and at a frequency f of 668 Hz on the curve 603 .

[0034] Figure 7 The corresponding curves of the sound absorption coefficient α for the sound absorber 100 samples with frequency are comparatively illustrated, the sound absorber having a thickness t of 30 mm and comprising: a microporous element 101 having four layers, and acoustic resonators 102 of different numbers and lengths L, the acoustic resonators being formed of tubes having a diameter between 3 mm and 5 mm and spirally wound around the microporous element 101. More specifically, the microporous element 101 comprises: a first layer 101a having a thickness t of 1 mm and located on one side of the first surface 100a of the sound absorber 100; 1 and a pore size D of 100 μm 1 and a second layer 101b (having a thickness t of 13 mm) facing the second surface 100b of the sound absorber 100 in turn 2 and 4.6mm aperture D 2 ), the third layer 101c (which has a thickness of 2 mm) 3 and a pore size D of 100 μm 3 ), and the fourth layer 101d (which has a thickness of 13 mm) 4 and 4.6mm aperture D 4). Curve 701 corresponds to a sound absorber 100 having seven acoustic resonators 102, each forming 1.25 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 109 mm. Curve 702 corresponds to a sound absorber 100 having five acoustic resonators 102, each forming 1.75 turns around the microporous element 101, so as to obtain a length L of each acoustic resonator 102 of 150 mm. By comparison, curve 703 corresponds to the acoustic response of a microporous element 101 having only two layers, without a sheath of the acoustic resonators, and curve 603 is also considered, which corresponds to a sound absorber 100 having three acoustic resonators 102 surrounding only two layers of microporous elements 101. As can be seen, additional peaks of the sound absorption coefficient will also be found at a frequency f of 1200 Hz on curve 701 and at a frequency f of 1060 Hz on curve 702.

[0035] Figure 8 The diameter D of the acoustic resonator is shown as t The four curves 801 to 804 correspond to a sound absorber 100 having seven acoustic resonators 102 with a length L of 109 mm, each of which forms 1.25 turns around a microporous element 101 having four layers, the four layers comprising: a first layer 101a located on one side of a first surface 100a of the sound absorber 100, having a thickness t of 1 mm 1 and a pore size D of 100 μm 1 and a second layer 101b (having a thickness t of 13 mm) facing the second surface 100b of the sound absorber 100 in turn 2 and 4.6mm aperture D 2 ), the third layer 101c (which has a thickness of 2 mm) 3 and a pore size D of 100 μm 3 ), and the fourth layer 101d (which has a thickness of 13 mm) 4 and 4.6mm aperture D 4 ). However, the curve 801 is consistent with the diameter D of each acoustic resonator 102 of 4 mm. 1 Correspondingly, curve 802 and diameter D of 3 mm t Correspondingly, curve 803 and diameter D of 2 mm t Correspondingly, curve 804 is related to the diameter D of 1 mm. t Therefore, as a function of the desired absorption spectrum, not only the configuration of the microporous element 101 and the length and number of the acoustic resonators 102 can be optimized, but also their respective diameters.

[0036] Therefore, as described above, the optimum diameter D of the acoustic resonator 102 for the sound absorber 100 having seven acoustic resonators 102 (each acoustic resonator forming 1.25 turns around the microporous element 101 having four layers) with a length L of 109 mm is t It can be, for example, 3.2 mm, so that according to Fig. 9 The curve 901 shows the sound absorption coefficient α as a function of the frequency f, and the optimum diameter D of the acoustic resonator 102 for the sound absorber 100 having the same microporous element but only three acoustic resonators 102 of length 220 mm (each acoustic resonator is formed as three turns around the microporous element 101). t It may be, for example, 4.8 mm, resulting in curve 902 .

[0037] In addition, if Figure 4 As illustrated in FIG. 1 , the sound absorber 100 may include acoustic resonators 102, 102' of different lengths to absorb acoustic energy of various wavelengths. Figure 4 As illustrated in the figure, the first group of spiral acoustic resonators 102 can be arranged along a first annular row 110, each of the first group of spiral acoustic resonators having a first inclination angle β relative to the thickness direction, and therefore having a first length L, and the second group of spiral acoustic resonators 102' can be arranged along a second annular row 110', each of the second group of spiral acoustic resonators having a second inclination angle β' relative to the thickness direction, and therefore having a second length L', and the second annular row 110' is concentrically arranged relative to the first row 110 so as to define the microporous elements 101 together.

[0038] therefore, Fig.10 The curve 901 of the sound absorber 100 with seven acoustic resonators 102 with a length L of 109 mm around a microporous element 101 with four layers is compared with the curve 903 corresponding to the sound absorber with the same microporous element 101 with four layers but surrounded by two concentric sheaths, the two sheaths being formed by a first group of acoustic resonators 102 and a second group of acoustic resonators 102', respectively, wherein the first group is formed by seven acoustic resonators 102 with a length L of 109 mm arranged annularly and wound around the microporous element 101, and the second group is formed by three acoustic resonators 102' with a length of 220 mm arranged annularly and wound around the microporous element 101. As can be seen, the curve 903 combines the sound absorption peaks of the 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 method based on extrusion of materials (for example, a method for depositing molten yarns of thermoplastic materials). These methods are particularly suitable for manufacturing complex shapes with thin walls, and the method includes multiple consecutive material deposition steps. In each of these steps, the extruder head 200 can move along the path 201 in the transverse plane XY while depositing the material 202, and then the material is solidified to form a layer 203. By moving the transverse plane XY in the orthogonal direction Z after depositing each layer 203, these layers 203 can be stacked to form the microporous element 101 and the acoustic resonator 102, as shown in FIG. Fig.11 To form tubes and / or channels, each layer 203 may include a plurality of periodically repeating unit cells separated by walls formed by deposition of material 202, and in successive material deposition steps, the layers 203 may be stacked together by aligning their respective unit cells.

[0040] In order to at least partially avoid the crossing of the extruded material 202 during the deposition of the layer 203 (which may result in the formation of holes between adjacent cells), the path 201 may be zigzag-shaped, such as Fig.12 To avoid accumulation of material and formation of holes at intersections between walls, a gap O may be maintained between the angles 205 of the paths 201 at these intersections.

[0041] When the acoustic resonators 102, 102' and the microporous element 101 are manufactured separately, they can be assembled together to form the sound absorber 100. The assembly can be performed by shrink fitting. Figure 4 As illustrated in , when the acoustic resonators 102, 102' are deposited to form one or more annular sheaths, the microporous element 101 can then be shrink-fitted to the inside of these sheaths. Thus, due to the radial pressure and the friction generated between them, a firm connection to the inside can be obtained between these sheaths of the acoustic resonators 102, 102' and the microporous element 101.

[0042] Although the present invention has been described by reference to specific exemplary embodiments, it is apparent that various modifications and changes may be made to these examples without exceeding the general scope of the invention as defined in the claims. In addition, the various features of the different embodiments described may be combined in other embodiments. Therefore, the description and drawings should be regarded as illustrative rather than restrictive.

Claims

1. A sound absorber extending between two opposing surfaces, the sound absorber comprising: one or more first quarter wave acoustic resonators, each of the one or more first quarter wave acoustic resonators having a first length between a first open end and a second closed end on a first of the two opposing surfaces of the acoustic absorber, the first length being substantially greater than a thickness of the acoustic absorber between the two opposing surfaces; and A microporous element is formed of a plurality of periodically repeated unit cells adjacent to the first acoustic resonator.

2. The sound absorber according to claim 1, wherein: The first quarter-wave acoustic resonator is inclined with respect to a thickness direction of the sound absorber.

3. The sound absorber according to claim 2, wherein: The first quarter wave acoustic resonator is at least partially helical.

4. The sound absorber according to any one of claims 1 to 3, wherein: The first quarter-wave acoustic resonator is curved.

5. The sound absorber according to any one of claims 1 to 4, wherein: The microporous element includes at least a first stacked layer and a second stacked layer in the thickness of the sound absorber, and the unit cells constituting the first stacked layer are different from the unit cells constituting the second stacked layer.

6. The sound absorber according to any one of claims 1 to 5, wherein: Each unit cell includes channels and / or cross-wires.

7. The sound absorber according to any one of claims 1 to 6, wherein: The first quarter-wave acoustic resonator is arranged around the microporous element.

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

9. The sound absorber according to claim 8, wherein: The first acoustic resonators and the second acoustic resonators are arranged along adjacent rows.

10. The sound absorber according to claim 9, wherein The adjacent rows are concentric.

11. A gas turbine engine comprising a sound absorber according to any one of the preceding claims.

12. An aircraft comprising the sound absorber according to any one of claims 1 to 10.

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

14. The production method according to claim 13, wherein: The additive manufacturing step is performed by depositing molten material.

15. The production method according to claim 14, wherein: The molten material is deposited along a zigzag path.

16. The production method according to any one of claims 13 to 15, comprising: Subsequent assembly steps of the microporous element and the quarter-wave acoustic resonator.

17. The production method according to claim 14, wherein: The assembly of the microporous element and the quarter-wave acoustic resonator is performed by shrink fitting.

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