Surface trim for acoustic absorption

Abstract

A surface liner member includes an external wall which is provided with multiple holes, a spacer structure, and tubes which are respectively associated with the holes in the external wall. The surface liner member is intended to be applied to a base surface in order to reduce, by acoustic absorption, an acoustic wave reflected on this base surface. The acoustic absorption efficiency is improved, particularly for frequencies below 500 Hz, by varying the shape of the holes in the external wall such that the ratio of the hole perimeter to the hole area varies. Such a surface liner member may be adapted to form a surface portion of an aircraft engine nacelle, or of a leading edge of an aircraft wing.

Claims

1. A surface liner member to be applied to a base surface to reduce, by acoustic absorption, an acoustic wave reflected on the base surface through the surface liner member, said surface liner member comprising: an external wall provided with multiple holes, each hole of the multiple holes individually forming an open passage through said external wall; a spacer structure connected to an inner face of the external wall and adapted to set a gap between said external wall and the base surface so as to form at least one acoustic cavity between the inner face of the external wall and the base surface; and tubes connected to the external wall, and which extend into the acoustic cavity from the inner face of said external wall towards the base surface, two opposite ends of each tube of said tubes being open and being dedicated to one of the holes in the external wall with a hole section that is contained in an inner section of the tube at the external wall, wherein a length of each tube is smaller than the gap between the external wall and the base surface, said tube length being measured in a direction perpendicular to the inner face of said external wall, so that at least 10% of the gap between the external wall and the base surface, against said base surface, is devoid of tubes, and wherein a subset of the holes in the external wall have different cross-sectional shapes, such that a quotient of a hole cross-sectional perimeter divided by a hole cross-sectional area varies between at least some of said holes so that an apparent total acoustic absorption peak of the surface liner member is broadened for acoustic wave frequencies that are less than 500 Hz.

2. The surface liner member according to claim 1, wherein at least some of the tubes have respective lengths which are different, each measured in the direction perpendicular to the inner face of the external wall.

3. The surface liner member according to claim 1, wherein each tube extends perpendicularly to the inner face of the external wall.

4. The surface liner member according to claim 1, wherein the length of each tube is less than the gap between the external wall and the base surface, by a distance of between 2 mm and 40 mm, in the direction perpendicular to the inner face of said external wall.

5. The surface liner member according to claim 1, wherein the internal cross-section of at least one of the tubes varies as a function of the distance measured from the inner face of the external wall along the direction perpendicular to said inner face of the external wall.

6. The surface liner member according to claim 1, wherein a peripheral edge of at least one of the holes is a polygon with more than four sides.

7. The surface liner member according to claim 6, wherein the peripheral edge of at least one of the holes has a fractal pattern of second order or higher than second order.

8. The surface liner member according to claim 1, having a shape adapted to form a surface portion of one of an aircraft engine nacelle, a portion of a leading edge of an aircraft wing, a portion of an acoustic absorption device for an air conditioning system, and a portion of a blower inner wall.

9. The surface liner member according to claim 1, wherein one end of the spacer structure which is opposite to the external wall is adapted to be bonded to the base surface.

10. A method for manufacturing a surface liner member which is in accordance with claim 1, wherein the tubes are formed by a method of three-dimensional printing or injection molding from the inner face of the external wall.

11. The surface liner member according to claim 2, wherein each tube extends perpendicularly to the inner face of the external wall.

12. The surface liner member according to claim 2, wherein the length of each tube is less than the gap between the external wall and the base surface, by a distance of between 2 mm and 40 mm, in the direction perpendicular to the inner face of said external wall.

13. The surface liner member according to claim 3, wherein the length of each tube is less than the gap between the external wall and the base surface, by a distance of between 2 mm and 40 mm, in the direction perpendicular to the inner face of said external wall.

14. The surface liner member according to claim 2, wherein the internal cross-section of at least one of the tubes varies as a function of the distance measured from the inner face of the external wall along the direction perpendicular to said inner face of the external wall.

15. The surface liner member according to claim 3, wherein the internal cross-section of at least one of the tubes varies as a function of the distance measured from the inner face of the external wall along the direction perpendicular to said inner face of the external wall.

16. The surface liner member according to claim 4, wherein the internal cross-section of at least one of the tubes varies as a function of the distance measured from the inner face of the external wall along the direction perpendicular to said inner face of the external wall.

17. The surface liner member according to claim 2, wherein a peripheral edge of at least one of the holes is a polygon with more than four sides.

18. The surface liner member according to claim 3, wherein a peripheral edge of at least one of the holes is a polygon with more than four sides.

19. The surface liner member according to claim 4, wherein a peripheral edge of at least one of the holes is a polygon with more than four sides.

20. The surface liner member according to claim 5, wherein a peripheral edge of at least one of the holes is a polygon with more than four sides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will be apparent from the following description of some non-limiting exemplary embodiments, with reference to the appended drawings, in which: FIG. 1 is a sectional view of a surface liner member according to the invention; FIGS. 2a to 2g show cross-sectional shapes of holes that can be used in a surface liner member according to the invention; FIGS. 3a, 3b, 3c, and 3d are diagrams of acoustic absorption as a function of acoustic wave frequency; FIGS. 4a and 4b are perspective views, in two opposite directions, of a surface liner member according to the invention; and FIGS. 5a and 5b are cross-sectional views of tubes which can be used in various embodiments of the invention, having varying tube lengths, or cross-sectional sizes which vary along each tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) For the sake of clarity, the dimensions of the members represented in these figures do not correspond to actual dimensions or actual dimension ratios. In addition, identical references indicated in different figures denote members which are identical or have identical functions.

(3) In the figures, the indicated references have the following meanings: 100 structural member for receiving the surface liner member SB surface of the structural member 100, onto which the surface liner member is applied; called the base surface 10 surface liner member 1 external wall of the surface liner member 10 FI inner face of the external wall 1 FE outer face of the external wall 1 2 open holes through the external wall 1 3 tube associated with each hole 2, extending from the inner face FI of the external wall 1 4 partition, extending from the inner face FI of the external wall 1 4a distal end of each partition 4, opposite to the external wall 1 V intermediate volume between the inner face FI of the external wall 1 and the base surface SB of the structural member 100 OA incident acoustic wave on the outer face FE of the external wall 1 A-A central axis of each of the tubes 3 E gap between the inner face FI of the external wall 1 and the base surface SB of the structural member 100 l length of each tube 3, measured along the axis A-A of the tube from the inner face FI of the external wall 1 d.sub.r residual distance between the distal end of each tube 3, opposite to the external wall 1, and the base surface SB of the structural member 100

(4) The structural member 100 may be a portion of an aircraft engine nacelle, for example. However, for clarity in the figures but in a non-limiting manner, the base surface SB of the structural member 100 is shown as being flat.

(5) The surface liner member 10 constitutes a covering of the structural member 100. For this purpose, the surface liner member 10 is fixed on the surface SB of the structural member 100, for example by bonding the distal ends 4a of the partitions 4 to the base surface SB.

(6) The structural member 10 comprises the external wall 1, the tubes 3, and the partitions 4. The tubes 3 and the partitions 4, which are rigid, may be attached to the inner face FI of the external wall 1, or may be of continuous material with the external wall 1, for example by being formed starting from the inner face FI. They may extend perpendicularly to the inner face FI of the external wall 1, but not necessarily. According to a preferred method for manufacturing the structural member 10, at least the tubes 3, but possibly also the partitions 4, may be produced by three-dimensional printing or by using an injection molding process.

(7) The length of the partitions 4 determines the gap E (FIG. 1) when the surface liner member 10 is fixed on the surface SB of the structural member 100. Preferably, the partitions 4 have identical lengths so that the inner face FI and the base surface SB are parallel. Without the tubes 3, the spacing E would be substantially equal to a quarter of the value of the wavelength of the acoustic wave OA for which a maximum acoustic absorption is desired. However, in the presence of the tubes 3, the gap E can be from 1/20 to 1/30 for example of the acoustic wavelength for which the maximum acoustic absorption is desired. The maximum absorption frequency for the acoustic wave OA can therefore be reduced by the presence of the tubes 3 by a factor of 5 to 7 in comparison to a resonator without tubes 3, for a given gap E. In addition, the distance between two partitions 4 which are facing each other is preferably less than half of the smallest acoustic wavelength recommended for the surface liner member 10. In these conditions, the wave travels through the external wall 1 primarily through the holes 2, propagates along the tubes 3, then in the volume V towards the structural member 100, is reflected by the base surface SB, propagates again in the volume V but in the direction of the external wall 1, then again travels through the tubes 3 and the outer wall 1 to the outside. In a known manner, the absorption of the acoustic wave OA essentially occurs at each passage through the tubes 3 and external wall 1.

(8) Each partition 4 may be of any geometry. In particular, each partition 4 may have one or more openings while fulfilling its function of rigid spacer between the external wall 1 and the structural member 100.

(9) Each hole 2 extends from the outer face FE of the external wall 1 to its inner face FI, forming an open passage between the outside and the intermediate volume V. It thus allows a portion of the acoustic wave OA to pass through the external wall 1. The acoustic absorption spectrum that affects the wave OA during such passage is shifted to lower frequency values when the quotient of the perimeter of the hole 2 divided by its cross-sectional area is higher. Tables 1 and 2 below show the quotient values for the hole perimeter divided by its cross-sectional area for different shapes and for two values of hole cross-sectional area:

(10) TABLE-US-00001 TABLE 1 for a cross-sectional area of each hole 2 of about 1.27 mm.sup.2 Shape of round slit Regular Vicsek cross- six-pointed fractal section star with pattern point angles of first of 60° order (cross) Figure 2a 2b 2c 2d Dimension(s) diameter width: side of arm arm thickness (mm) d = 1.27 d = 0.80 d = 0.86 d = 0.5 length: L = 1.60 Quotient ~3.14 ~4.3 ~4.7 ~4.8 perimeter/area (mm.sup.−1)

(11) TABLE-US-00002 TABLE 2 for a cross-sectional area of each hole 2 of about 3.14 mm.sup.2 Gosper Regular Island six-pointed Vicsek Vicsek fractal star with fractal fractal pattern, point pattern of pattern of Shape of cross- Hexa- second angles of first order second section round gonal order 60° (cross) order FIG. 2a 2e 2f 2c 2d 2g Dimension diameter side side of side of arm arm arm (mm) d = 2.0 d = 1.1 island d = 1.35 thickness thickness d = 0.42 d = 0.79 d = 0.355 Quotient ~2.0 ~2.1 ~2.4 ~3.0 ~3.0 ~5.9 Perimeter/area (mm.sup.−1)

(12) In general, the smallest distance between two portions of the edge of a hole that are parallel and opposite, is preferably greater than twice an acoustic boundary layer thickness δ.sub.ac, calculated according to the formula: δ.sub.ac=(ρ.Math.τ.Math.f/μ).sup.−1/2, where ρ is the density of the air at 25° C. (degrees Celsius) and 10.sup.5 Pa (Pascal), μ is the viscosity of the air under the same conditions, and f is the frequency of the acoustic wave OA, preferably comprised between 100 Hz and 500 Hz, for example equal to 200 Hz. This condition ensures that a significant portion of the acoustic wave OA enters the intermediate volume V through the hole 2.

(13) Each hole 2 is provided with a tube 3 which extends from the inner face FI of the external wall 1 in the direction of the structural member 100. In the embodiments described, but without this being essential for the invention, each tube 3 has a central longitudinal axis A-A which is straight and perpendicular to the inner face FI at the location of the hole. In addition, the internal cross-section of each tube 3 at the inner face FI is identical and coincident with the cross-section of the corresponding hole 2. Furthermore, each tube 3 has a length l which is less than 90% of the gap E between the inner face FI of the external wall 1 and the base surface SB of the structural member 100.

(14) Under these conditions, the diagrams of FIGS. 3a, 3b, 3c, and 3d each show the effect of the shape of the holes 2 on the absorption spectrum of the acoustic wave OA. The horizontal axis of each diagram indicates the frequency f of the acoustic wave OA, expressed in hertz (Hz), and the vertical axis indicates the spectral absorption denoted Abs(f), normalized with respect to its maximum value of 1.

(15) The diagram of FIG. 3a corresponds to the value of 10 mm (millimeters) for the gap E between the inner face FI of the external wall 1 and the base surface SB of the structural member 100. The diagram of FIG. 3b corresponds to the value of 20 mm for this gap E. All the tubes 3 have a length l which is equal to 5 mm (millimeters) and an internal cross-sectional area which is about 1.27 mm.sup.2 for the two diagrams of FIGS. 3a and 3b. For each curve in these diagrams, all the holes 2 are identical, of the shape indicated in the inset legend of the corresponding diagram and with the hole dimension(s) indicated in Table 1 above for that shape of hole. A comparison of these two diagrams shows that the main absorption peak at low frequency shifts towards the lower values of the frequency f when the gap E increases, and that at constant value of the gap E, this peak is also shifted towards the lower values of the frequency f when the quotient of the hole perimeter divided by the hole cross-sectional area increases. In addition, FIG. 3b shows a broadening of approximately 7% of the lower main absorption frequency band as the quotient of the perimeter divided by the hole cross-sectional area increases.

(16) To move the absorption peak to around or below 500 Hz, the length l of the tubes 3 can be set at 15 mm for a gap E of 20 mm (FIG. 3c) or 30 mm (FIG. 3d) between the inner face FI of the external wall 1 and the base surface SB of the structural member 100. All the holes 2 related to each of the curves of FIG. 3c or 3d still have the same shape, which is indicated in the inset legend of the corresponding diagram, with a hole cross-sectional area of about 3.14 mm.sup.2 (see Table 2 above for the dimensions of the holes that correspond to the curves of FIGS. 3c and 3d). The same behaviors are then observed as when the length l of the tubes 3 is equal to 5 mm and the hole cross-sectional area is about 1.27 mm.sup.2 (FIGS. 3a and 3b), in other words a maximum absorption that is governed by the quotient of the hole perimeter divided by the hole cross-sectional area.

(17) A surface liner member 10 according to the invention is illustrated in FIGS. 4a and 4b. For clarity in FIGS. 4a and 4b, the partitions 4 have not been represented in these two figures but are present in the actual surface liner member. In this liner member 10, the shape varies between several of the holes 2, such that the value of the quotient of the cross-sectional perimeter of a hole divided by its cross-sectional area varies within the same liner member. Thus, the same surface liner member 10 simultaneously has holes 2 of at least two different patterns among a round, cross-shaped, slit-shaped, hexagonal, or six-pointed star pattern, or fractals of various patterns, etc., and whose orientations in the outer face FE may also vary. The absorption spectrum of such a surface liner member 10 is therefore a combination of the spectra that correspond to each of the hole cross-sectional shapes. As a consequence, the resulting absorption spectrum exhibits a main absorption peak which is broadened at low acoustic frequency, with a constant value for the gap E. This broadened peak produces sound absorption values which are greater when the acoustic frequency f is less than 500 Hz. The gain in absorption efficiency that is achieved in this manner at 500 Hz can be 7% or more.

(18) The tube 3 which is associated with each hole 2 may have an internal cross-sectional shape which is identical to the cross-section of the corresponding hole 2. In the embodiment of the invention which is illustrated in FIGS. 4a and 4b, the internal cross-section of each tube 3 does not vary along each tube, and all the tubes 3 have the same length.

(19) FIG. 5a illustrates first variants of the invention, in which some of the tubes 3 of the liner member 10 of FIGS. 4a and 4b may have lengths that vary from tube to tube. The residual open distance, denoted d.sub.r, which exists between the distal end of one of the tubes 3, opposite to the external wall 1, and the structural member 100, is equal to E-I. For surface liner members in accordance with the invention, this residual open distance d.sub.r is greater than 10% of the gap E. This condition is maintained when all the tubes 3 have identical lengths. In general, it guarantees an optimum combination between the resonance effect produced by the gap E and the increased sound absorption produced by the various shapes of the holes 2. Preferably, the residual free distance d.sub.r can be greater than 2 mm for all tubes 3 of the surface liner member 10.

(20) FIG. 5b illustrates second variants of the invention, in which some of the tubes 3 of the liner member 10 may have internal cross-sections that vary as a function of the distance x from the inner face FI of the external wall 1. For example, the internal cross-section of a tube 3 may vary between two planes which are perpendicular to the axis A-A of this tube and which correspond to different values for the distance x, homothetically, with a ratio of homothety which depends on the distance x. For x=0, the internal cross-section of the tube may be identical to that of the corresponding hole 2. Then, the distance r between the inner surface of a tube 3 and the central axis A-A of that tube can vary according to the distance x in a manner which is identical in all meridional planes which contain the axis A-A. r(x) is thus the ratio of homothety introduced above. In different embodiments of the invention, r(x) can be an affine, increasing, or decreasing function segment, or an exponential function segment which may be increasing or decreasing, or a parabolic segment, a hyperbolic segment, etc. It is possible for r(x) to be a linear combination of several of these functions and, in general, r(x) may be any function as long as the distal end of the tube 3 is open. Thus, each tube 3 of variable cross-section along its central axis A-A may constitute a horn that is conical, exponential, parabolic, hyperbolic, etc., which links the intermediate space V to the open passage formed by the corresponding hole 2 through the external wall 1. In a known manner, such a horn which widens with distance from the inner face FI of the external wall 1 encourages the transfer of acoustic energy through the hole 2. In addition, such a horn has a cut-off frequency F.sub.c, below which the acoustic wave OA is no longer transmitted through the horn (corresponding to f<F.sub.c). This cutoff frequency is F.sub.c=C/(4τ.Math.α), where α is a characteristic length of variation of the function r(x).

(21) The features of the first and second variants of the invention, illustrated by FIGS. 5a and 5b, may also be combined in improved embodiments.

(22) The number of holes 2 per unit surface area of the external wall 1 may be comprised between 1 hole/cm.sup.2 (hole per square centimeter) and 8 holes/cm.sup.2. Given the cross-sectional area of each hole, which may be comprised between 1 mm.sup.2 and 4 mm.sup.2 for example, all the holes may occupy a surface area fraction of the external wall 1 of 4% to 8% for example.

(23) Finally, the partitions 4 may form a set of separate cells which are next to one another on the inner face FI of the external wall 1. A single hole 2, with its associated tube 3, may be contained in each cell, but it is also possible to have several holes 2, with the tubes 3 associated with them, inside a single cell.

(24) It is understood that the invention may be reproduced while modifying secondary aspects thereof relative to the examples detailed above. In particular, the partitions 4 may be replaced by a spacer structure which has a different geometry, and any distribution with respect to the holes 2. In addition, the numerical values which have been mentioned were provided as examples only.