EP1213703B1 - Sandwich acoustic panel - Google Patents

Abstract

The acoustic panel comprises a resistive layer (14), a honeycomb structure (16) formed from at least two superposed honeycombed layers (18) and a rear reflector (17). A porous separator (24) placed between each pair of adjacent honeycombed layers carries on each of its surfaces tubular guides (26) which penetrate into certain honeycomb cells (20).

Description

Technical area

The invention relates to an acoustic panel sandwich, i.e. a reducing sandwich panel noise, designed to attenuate an incident sound wave directed towards an external face of the panel.

An acoustic panel according to the invention can in particular be used in nacelle walls or in turbojet engine housings, in conduits to be soundproofed, etc.

State of the art

Existing acoustic panels include generally one or more quarter wave resonators, superimposed on a total reflector. Each resonator itself consists of a resistive layer, more or less breathable, and of a honeycomb structure, most often of the honeycomb type. Layer resistive covers the face of the honeycomb structure facing outward, i.e. towards the wave incident sound. On the contrary, the total reflector covers the face of the resonator opposite to this wave incident. By convention, we call "front face" the face of the panel carrying the resistive layer and "face back "the opposite side of the panel, covered by the reflector.

In this classic arrangement of panels acoustic, the resistive layer has a dissipative role. When a sound wave passes through it, viscous effects that transform acoustic energy in heat.

The height of the honeycomb structure allows to tune the panel on the characteristic frequency noise to mitigate. Noise dissipation in the resistive layer is maximum when the height of cells of the dimpled core is equal to a quarter of the wavelength of the frequency of the noise to be attenuated. The cells of the honeycomb structure behave then like waveguides perpendicular to the surface of the panel, which gives them a response of "localized reaction" type. The cells form a assembly of quarter-wave resonators in parallel.

Rear reflector produces conditions of total reflection, essential for behavior described above of the honeycomb core.

In general, an acoustic panel should meet acoustic requirements.

A first of these requirements concerns the acoustic uniformity of the panel. In others terms, the acoustic treatment is all the more effective that it conforms to its specification on its entire surface. Compliance with this requirement depends the nature of the elements making up the panel, their relative arrangement and adhesives used for their assembly.

Another acoustic requirement is the requirement called "localized reaction". If this requirement is not not respected, it occurs inside the panel a transverse propagation of sound waves, called "lateral energy leakage", which opposes the "quarter wave" type of structure operation dimpled.

When the panel equips an aircraft engine, in addition to these acoustic requirements, requirements environmental resistance, requirements structural and aerodynamic requirements.

Thus, an acoustic panel integrated into a aircraft engine must be able to withstand conditions severe use. In particular, it should not delamination, even in the presence of strong depressions, the panel must be able to withstand corrosion and erosion due, for example, to sand, must have good electrical conductivity so including resisting sweeping lightning and it must ability to contribute to mechanical shock absorption following a loss of dawn.

An acoustic panel integrated into a motor aircraft must also have structural strength allowing him to support the weight of a man as well that the transfer of aerodynamic forces and inertials from the air inlet to the engine housing.

Finally, the surface condition of a panel acoustics built into an aircraft engine must be consistent with aerodynamic lines and continuity requirements of surfaces in contact with flowing air.

Known acoustic panels can be classified into three categories: single panels nonlinear degree of freedom or SDOF ("simple degree of freedom ") non-linear, single-degree panels linear freedom or linear SDOF, and the panels two degrees of freedom or DDOF ("double degree of freedom ").

In single degree of freedom panels nonlinear the resistive layer consists of a metallic or composite perforated layer.

A panel of this type has the advantages of allow good control of the area percentage open, to have good structural strength and be easy to perform.

On the other hand, it has the disadvantages of have strong acoustic non-linearity as well that a strong dependence on speed resistance of tangential flow at the surface. Moreover, since the frequency damped by each cell depends of its depth and like all the cells of the panel have the same depth, the frequency range damped by such a panel is restricted. In addition, when the resistive layer is made of composite material, the structure has a low resistance to erosion.

In single-degree acoustic panels of linear freedom, the resistive layer is a layer micro porous made for example of a fabric metallic, of a perforated sheet associated with a fabric acoustic or a metallic fabric associated with a fabric acoustic.

The use of a panel of this type allows to adjust the acoustic resistance by modifying the components of the micro porous layer. Its beach frequency is reasonable. Such panel also has the advantages of having a non-linearity low to moderate as well as low dependence of the acoustic resistance on the speed of tangential flow at the surface.

However, the realization of a panel single linear freedom sandwich is more complicated than that of a single degree panel of nonlinear freedom, because the resistive layer has two components. If the components or assembly processes are not under control, the structure may have areas of non-homogeneity acoustic as well as risks of delamination of the resistive layer. In addition, the risk of corrosion of the resistive layer impose a constraint additional regarding the choice of materials used. In addition, the method of assembling such a panel is long and expensive.

Finally, a double degree acoustic panel of freedom as described in patent EP 0 352 993 includes, in addition to a resistive layer perforated and a rear reflector, two cores superimposed dimples, separated by a layer resistive intermediate called "septum", generally micro porous.

Compared to other types of panels acoustic, double degree of freedom panels have a wider damped frequency range, possibility of resistance adjustment acoustics using two resistive layers and one weak to moderate acoustic non-linearity.

Double degree acoustic panels of freedom, however, has the disadvantage that areas of acoustic non-homogeneity appear due to the poor alignment of the cells of the two souls alveolate, which inevitably occurs during the panel shaping. There are also parasitic phenomena of transverse propagation in areas where the cells of the two dimpled souls are not not aligned. Finally, the process of assembling a panel of this type is long and expensive because it takes assemble one by one the various elements of the structure.

Statement of the invention

The subject of the invention is precisely a acoustic panel whose original design allows you to benefit from the advantages of several degrees of freedom, while removing the disadvantages due to misalignment of cells alveolar structures, such as risks acoustic inhomogeneity and transverse propagation acoustic waves.

According to the invention, this result is obtained by means of a sandwich acoustic panel comprising a resistive layer forming a front face of the panel, a honeycomb structure formed of at least two superimposed honeycomb layers each comprising a network of cells, a porous separator interposed between adjacent honeycomb layers and a reflector forming a rear face of the panel, characterized in that that the porous separator carries, on each of its faces, guides penetrating at least some of the cells of the honeycomb layers adjacent to the separator, distributed over the entire surface of the separator.

The presence of guides on each side porous separator ensures continuity of partitions and therefore cells of the honeycomb structure, between the inner surface of the resistive layer and reflector. The problems of local cell misalignments, which appear necessarily on panels with several degrees of freedom of the prior art, consisting of several superimposed honeycomb structures, are therefore removed. Consequently, the risks of non-uniformity do not exist more.

According to a preferred embodiment of the invention, the resistive layer, the layers dimpled, the porous separator and the reflector are joined together by gluing.

Advantageously, the resistive layer, the honeycomb layers, the porous separator and the reflector are all made of materials identical or compatible with the adhesive which ensures their assembly.

These materials are preferably chosen in the group comprising metallic materials, composites and thermoplastics.

Depending on the case, the guides include either aligned elements, reported on both sides of the porous separator, i.e. elements crossing the porous separator.

In the preferred embodiments of the invention, the guides are tubular or shaped solid rod, of circular section. This section can be uniform over the entire length of the guide, or otherwise have tapered ends to facilitate their installation. They can also present a different section, such as a section star-shaped with at least three branches, without depart from the scope of the invention. In addition, the rods can be made of any material porous or not.

Brief description of the drawings

• la figure 1 est une vue en coupe qui représente schématiquement un panneau acoustique sandwich conforme à l'invention ; et
• les figures 2a à 2c sont des vues en coupe, à plus grande échelle, qui représentent des variantes de réalisation des guides portés par le séparateur poreux.

A preferred embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended drawings, in which:

• Figure 1 is a sectional view which schematically shows a sandwich acoustic panel according to the invention; and
• Figures 2a to 2c are sectional views, on a larger scale, which show alternative embodiments of the guides carried by the porous separator.

Detailed description of a preferred embodiment of invention

As shown schematically on the Figure 1, a sandwich acoustic panel conforming to the invention consists of a stack of several constituents, joined together. For facilitate understanding, these constituents are shown slightly spaced from each other. In practice they are in close contact throughout the panel surface.

The acoustic panel according to the invention can be flat, as shown by way of example. However, it can also take any other form, and in particular a curved shape, as is the case when integrated into the nacelle or crankcase of a turbojet engine.

The panel structure will now be described from the outside face 10 thereof, called "front face", towards its inner face 12, called "back side". In Figure 1, the faces front 10 and rear 12 are oriented respectively down and up.

Thus, starting from the front face 10, the acoustic panel according to the invention comprises successively a resistive layer 14, a structure dimpled 16 and a rear reflector 17.

The resistive layer 14 is porous or perforated. It is in contact with the outside air and struck in first by the acoustic wave that we want to dampen. As in existing double acoustic panels degree of freedom, the resistive layer 14 is designed to transform the incident acoustic energy into heat.

When the panel is integrated in the nacelle of a turbojet engine, the resistive layer 14 also allows receive and transfer to links structural engine nacelle aerodynamic forces and inertials, as well as those related to the maintenance of the nacelle.

The honeycomb structure 16 comprises at least two superimposed honeycomb layers 18. The number of layers 18 forming the honeycomb structure 16 is equal to number of degrees of freedom desired for the panel acoustic. In the embodiment shown on Figure 1, the acoustic panel is two degrees from freedom and the honeycomb structure 16 therefore includes two acoustic layers 18. This number can however be greater than two without departing from the scope of the invention.

Each of the honeycomb layers 18 of the structure 16 comprises a network of cells 20, the cells of each network being delimited by partitions 22. The cell networks 20 of the different layers 18 are identical so that cells 20 and their partitions 22 can be aligned, as illustrated in figure 1. To this end, the shapes, the dimensions and distribution of cells 20 of each layers 18 are the same.

In a preferred embodiment of the invention, the honeycomb layers 18 have the form of honeycombs. The cells 20 then have a hexagonal section. However, honeycomb layers with cells 20 of different sections (circular, triangular, square, trapezoidal, etc.) can be used without departing from the scope of the invention.

The honeycomb structure 16 comprising the honeycomb layers 18 has the same function as in the acoustic panels with several degrees of freedom from prior art. This function is well known to skilled in the art, so she won't developed here.

A separator 24 is interposed between each pair of adjacent honeycomb layers 18 of the honeycomb structure 16. In the case of a panel with two degrees of freedom as illustrated in figure 1, a only separator is placed between the two layers dimpled 18. More generally, the number of separators 24 is one unit less than that of honeycomb layers 16.

Each separator 24 is made in one porous material. This material is chosen for its qualities of acoustic resistance, for its resistance to corrosion and for its low mass, because the level of structural stress to which it is subject is weak.

The porous material of the separator 24 can be metallic, synthetic or fiber-based fabric variety. It can also be a material thermoplastic or porous plastic. It ensures the same function that the porous separators interposed between the honeycomb layers of art acoustic panels prior to several degrees of freedom. This function is well known to those skilled in the art, so that it no description will be given here.

According to the invention, the separator porous 24 carries, on each of its faces, guides 26. These guides 26 are regularly distributed throughout the surface of the separator 24, according to a network superimposable on that of the cells 20 of the layers dimpled 18. In addition, the shape and sizing guides 26 are such that each of them can enter one of the 20 cells with a clearance too limited as possible.

The expression "stackable network" means that each of the guides 26 is located opposite a cell 20 when the honeycomb layers 18 and the or the separators 24 are superimposed. This result can be obtained either by providing on each side of the separator 24 as many guides 26 as there are cells 20 on the honeycomb layer 18 adjacent either, preferably by equipping the separator 24 with a number guides 26 lower than that of cells 20, as illustrated in Figure 1. In the latter case, the number guides 26 simply need to be sufficient for a correct alignment of cells 20 and partitions 22 is provided on the entire panel (we can provide, for example, a guide 26 for three to five cells 20 aligned). In order to satisfy this condition, the the number of guides 26 is greater the greater the curvature of the panel is important.

The shape presented by the guides 26 can be any, provided that the positioning effect mechanical sought is obtained. In the mode of embodiment shown in FIG. 1, the guides 26 are tubular. However, they may also present a different shape such as a shaped section star with three or four branches, without leaving the part of the invention.

When the guides 26 are tubular, they may in particular have a cross section circular or polygonal. This section can be uniform, as shown in Figure 1, or at otherwise variable, for example reduced and rounded in going towards the ends, to facilitate assembly, as shown in Figure 2a.

In another alternative embodiment, illustrated in FIGS. 2b and 2c, the guides 26 are made up of solid rods. In the case of Figure 2b, the rod is terminated by one end conical. In the case of FIG. 2c, the rod has, in section along its longitudinal axis, a shape rounded, for example substantially oval or elliptical.

The guides 26 can be produced in any materials, which mainly depend on the material chosen for the separator that carries them. according to the materials, the guides 26 can be fixed to the separator by welding, gluing, interlocking, etc.

In the embodiment illustrated in the Figure 1, the guides 26 include pairs of tubes 2 aligned, reported separately on either side of the separator 24. The alignment of the tubes 28 is ensured by the use of appropriate tools when the tubes are fixed to the separator, for example by bonding.

In an alternative embodiment, the guides 26 include elements 28 (in the form of tubes on FIG. 1) which cross the separator 24. The alignment is then ensured by construction, without that it is necessary to use a tool particular. However, in the case of guides tubular, they are then devoid of separator, unless you use tubular guides that you internally equipped with individual separators, before or after their attachment to the separator.

The rear reflector 17 is made of the same way as in art acoustic panels previous, according to the usual knowledge of the skilled person. It will therefore not be special description.

• 1)en plaçant la couche résistive 14 sur un moule ;
• 2)en collant une première couche alvéolée 18 sur la couche résistive 14, au moyen d'un adhésif ;
• 3)en collant le séparateur 24 muni de ses guides 26 sur la première couche alvéolée 18, en prenant soin de faire pénétrer dans les cellules de celle-ci les guides 26 montés sur la face du séparateur tournée vers la première couche alvéolée ;
• 4)en collant une deuxième couche alvéolée 18 sur le séparateur 24, en prenant soin de faire pénétrer dans les cellules de la deuxième couche alvéolée les guides 26 montés sur la face du séparateur tournée vers cette dernière ; et
• 5)en collant le réflecteur arrière 17 sur la deuxième couche alvéolée 18, au moyen d'un adhésif.

The different components of the acoustic panel according to the invention, that is to say the resistive layer 14, the honeycomb layers 18, the separator (s) 24 and the rear reflector 17, are assembled together by bonding. The assembly is carried out:

• 1) by placing the resistive layer 14 on a mold;
• 2) by bonding a first honeycomb layer 18 on the resistive layer 14, by means of an adhesive;
• 3) by sticking the separator 24 provided with its guides 26 on the first honeycomb layer 18, taking care to make the guides 26 mounted on the face of the separator facing the first honeycomb layer penetrate into the cells thereof;
• 4) by bonding a second honeycomb layer 18 on the separator 24, taking care to make the guides 26 mounted on the face of the separator facing the latter penetrate into the cells of the second honeycomb layer; and
• 5) by gluing the rear reflector 17 to the second dimpled layer 18, using an adhesive.

This description relates to the manufacture of a panel with two degrees of freedom, as illustrated on Figure 1. When the number of degrees of freedom is more important, steps 3) and 4) are repeated as many times as necessary.

The adhesive used to bond the different components of the panel can occur in the form of a film or be projected or sprayed on at least one of the components to be assembled.

In general, the different components of the panel can be made of various materials metallic, composite, thermoplastic, etc.

The use of the separator 24 in accordance with the invention enables the panel to be produced in materials identical or compatible with the adhesive used, i.e. in a single family of materials (for example, any composite). We thus avoid corrosion and galvanic torque problems. In addition, quality bonding is guaranteed between the different components.

In addition, and most importantly, the use of a separator 24 equipped with guides 26 ensures continuity of cells and partitions of the honeycomb layers 18, between the layer resistive front 14 and rear reflector 17. The cells 20 are thus automatically aligned whatever whatever the shape of the panel, and especially when it is a complex aerodynamic form or not developable. This arrangement also makes it possible to eliminate lateral energy leaks and therefore keep a localized acoustic reaction.

Claims (10)

1. Sandwich acoustic panel comprising a resistive layer (14) forming a front face of the panel, a compartmentalized structure (16) formed from at least two superposed compartmentalized layers (18) each comprising a network of cells (20), a porous separator (24) inserted between adjacent compartmentalized layers (18) and a reflector (17) forming a back face of the panel, characterized in that the porous separator (24) is fitted with guides (26) on each of its faces that penetrate into at least some of the cells (20) of the compartmentalized layers (18) adjacent to the separator, distributed over the entire surface of the separator.
2. Sandwich acoustic panel according to claim 1, in which the resistive layer (14), compartmentalized layers (18), the porous separator (24) and the reflector (17) are assembled to each other by bonding.
3. Sandwich acoustic panel according to either of claims 1 and 2, in which the resistive layer (14), the compartmentalized layers (18), the porous separator (24) and the reflector (17) are all made of identical or compatible materials using an adhesive to assemble them.
4. Sandwich acoustic panel according to claim 3, in which the said materials are chosen from the group comprising metallic, composite and thermoplastic materials.
5. Sandwich acoustic panel according to any one of claims 1 to 4, in which the guides (26) comprise aligned elements (28) added on each side of the porous separator (24).
6. Sandwich acoustic panel according to any one of claims 1 to 4, in which the guides (26) comprise elements passing through the porous separator (24).
7. Sandwich acoustic panel according to any one of claims 1 to 6, in which the guides (26) are tubular.
8. Sandwich acoustic panel according to any one of claims 1 to 6, in which the guides (26) are solid rods.
9. Sandwich acoustic panel according to any one of claims 1 to 8, in which the guides (26) are tapered at their ends.
10. Sandwich acoustic panel according to any one of claims 1 to 8, in which the cross-sections of the guides (26) are uniform over their entire length.

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