FR2778780A1 - Sound absorbing lining, such as sound absorbent panels for vehicles making method - Google Patents

Abstract

The method involves defining an efficient absorption frequency range for the material and forming it using at least one layer (Ca) of balls to form a specific resistance dependent on the selected frequency range.

Description

The present invention relates to the technical field aimed at reducing the

  noise, in the general sense, and it aims, more specifically, at the production of a material

  ensuring noise attenuation while having a low specific weight.

  The object of the invention finds a particularly advantageous application in the aeronautics, automobile or space sectors. In the above technical fields, it is known to use light composite materials called "honeycombs" which are formed by corrugated walls of semi-polygonal section joined together by welding, gluing or brazing, in order to constitute a three-dimensional material with a polygonal section. Such a material has practical limits of use coming essentially from the anisotropy of these mechanical properties and from its non-formability character without

  destruction of its mechanical properties.

  It is known, moreover, from patent FR 2 585 445, a material having a reduced specific weight compatible in particular, with use in the aerospace sector while offering excellent mechanical characteristics making it possible to use it as constituent material of 'a working structure. The material described by this patent comprises a plurality of balls geometrically arranged in a distribution of the compact crystallographic type and assembled

  to each other exclusively by their contact area.

  If this material has a low specific gravity and has good mechanical characteristics, even at high temperatures, such a material does not offer

  interesting properties of acoustic attenuation.

  It thus appears the need to have a material having a low density and good mechanical characteristics and acoustic attenuation. The Applicant expressed more generally, the need for a low density material designed to allow a modification of its acoustic properties as a function of the frequency range to be attenuated. The object of the present invention therefore aims to satisfy this need by proposing a method for producing, from spheres assembled together, a

  sound absorption material for noise.

  According to the invention, the method consists in - defining an effective absorption frequency range for the material, - and in constituting a material comprising at least one layer of spheres assembled together and having a specific normal impedance adapted to allow absorption of the noise in the beach

frequency determined.

  Another object of the invention is to propose a sound absorption material for noise, comprising spheres assembled together. According to the invention, the sound absorption material consists of at least one layer of spheres each having a specific specific normal impedance,

  so as to allow the absorption of noise in a determined frequency range.

  Various other characteristics will emerge from the description given below.

  below with reference to the accompanying drawings which show, by way of examples not

  limiting, embodiments and implementation of the subject of the invention.

  Fig. 1 is a sectional view schematically showing a

  embodiment of a material according to the invention.

  Fig. 2 illustrates the variation of the absorption coefficient ca as a function of

  the frequency for two types of materials according to the invention.

  Figs. 3 and 4 illustrate two other exemplary embodiments of a

material according to the invention.

  As shown more precisely in FIG. 1, the material 1 according to the invention is designed to ensure an acoustic attenuation or absorption of a sound wave coming from a noise source 2 illustrated diagrammatically and which can be of any nature known per se. In accordance with the invention, the material 1 comprises at least one and, in the example illustrated, three layers C1, C2, C3 each made from spheres S, S2, S3 respectively. Each layer CI, C2, C3 or generally, C, consists of an agglomeration of spheres or hollow balls S1, S2, S3, or generally S., which can be of the metallic type, by example, nickel, tin or silver or of the organic type, such as ceramic or epoxy. The hollow spheres have external diameters less than 5 mm and skin thicknesses controlled by the manufacturing process, greater than 5 μm to ensure the mechanical strength of the spheres. It should be noted that the material according to the invention is not limited to the use of hollow spheres

  and can be made from solid spheres, for example, made of glass.

  Preferably, each layer is formed from spheres identical to each other but different from one layer to another from the point of view of their size, their nature and their method of assembly. It should be noted that provision may be made for one or more layers each to be formed from non-identical spheres,

  for example, having different diameters.

  In order to assemble the spheres with a view to constituting a layer, provision may be made for assembling with a binder of the epoxy resin type. In another form of assembly, a metallic type binder is deposited by heat treatment during a brazing operation. In another form of assembly, the connection of the hollow spheres can be carried out from a deposition operation

  electrolytic. Other methods can be envisaged.

  The binder for assembling the spheres is determined according to the mechanical characteristics required for the material but also according to its environment of use. The environmental conditions in terms of sound pressure, vibration levels, temperature, humidity, chemical attack and corrosive atmosphere define the type of binder to be used for assembling the spheres. In use in a harsh environment, the assembly of the spheres by brazing makes it possible to obtain a completely metallic material which preserves its structural integrity and, consequently, its acoustic properties. In general, the assembly binder must not obstruct the

  pores of the material so that it retains its acoustic properties.

  The spheres thus assembled constitute a layer of a material with a rigid and porous structure which can be qualified as resistive. It should be noted that the method of assembling the spheres, of the random type or organized according to a crystallographic distribution, is determined for each layer according to the characteristics

  desired and defined in the following description. For each layer, the spheres

  create by their cohesion, air circulation ducts which define an interstitial network. It is thus possible to obtain a dissipation of the acoustic wave in heat

  by friction of the moving air in this interstitial network.

  Each resistive layer Ca has a specific specific normal impedance P, such that '= Z. / Z0, with: Z. = P / V, where P corresponds to the acoustic pressure at the surface of the layer and V to the acoustic speed normal to the surface of the layer, and Z0 = p. C, corresponding to the impedance of the surrounding or external medium with density p and speed C. The impedance of each layer depends on the following elementary physical quantities: - the porosity which represents the ratio of the open volume to the total volume of the resistive layer; - the flow resistance which links the speed of air circulation in the resistive layer to the pressure variation; - the tortuosity which characterizes the shape of the air circulation duct of the

resistive layer.

  These three elementary physical quantities are defined for each

  resistive layer and are related to the type of spheres used and their assembly.

  It should be considered that the increase in the value of the flow resistance results in a reduction of the sound absorption. Too much resistance to flow (> 100,000 Pa.s) implies limited circulation

  air in the interstitial network and therefore reduced sound absorption.

  However, too low a flow resistance (<1000 Pa.s) concentrates

  the absorption efficiency of the resistive layer in a narrow frequency zone.

  The porosity which is linked, among other things, to the method of assembling the spheres, makes it possible to control the width of the frequency efficiency band of the resistive layer. Thus, an increase in the value of the porosity leads to an increase in the width of the frequency efficiency range of the resistive layer. According to a preferred embodiment characteristic, the porosity of each layer must be maximum while retaining a high resistance to flow. However, since porosity is linked to flow resistance, it is necessary to obtain the best porosity-flow resistance couple. The porosity of a layer is linked to the organization of the assembly of the spheres in the following way for the same type of spheres:

TYPE OF POROSITY ASSEMBLY

  - Bulk assembly ........... 36% - Cubic assembly ........... 47% - Hexagonal assembly ......... 40% - Tetrahedral assembly ........ 26% The tortuosity of the resistive layer increases the low frequency efficiency of the layer by lengthening the circulation circuit of the acoustic wave in the interstitial network. Thus, the increase in the value of the tortuosity allows

  increase the absorption of low frequencies of the sound wave.

  According to a preferred embodiment characteristic, provision is made to use at least two layers of resistive C. According to this advantageous variant of the invention, the different layers of resistive C. are assembled in a superimposed manner, being organized so to allow, in combination, the absorption of noise within a determined frequency range. The different layers of resistive C, are stacked in a given direction, so as to produce a succession of layers with progressive impedances. This arrangement of the layers allows the incident wave to gradually enter the porous medium, in order to dissipate there.

  acoustic energy by viscothermal effect.

  Thus, provision may be made to arrange the resistive layers C ,, so that the values of their resistance to flow follow a progression in a given direction of stacking of the layers. In a first embodiment, the resistive layers C, are arranged so that their resistance to flow has values which are lower from one layer to the other in a direction of stacking of the layers considered to be analogous to the direction of progression of the incident wave through the complex material 1. It can thus be obtained

  progressive reduction of the speed of the acoustic wave inside the material.

  According to this example, each layer C, to C3 respectively has a flow resistance equal to 10,000, 25,000 and 30,000 Pa.s. Thus, it can be provided to stack the layers C1, C2, C3 according to this order by placing the layer C, closest to the noise source 2, so that the porosity values increase according to the direction of propagation of the acoustic wave. This kind of stacking makes it possible to obtain a given type of acoustic absorption, as illustrated by curve A of FIG. 2 giving the variation of the absorption coefficient ex as a function of the frequency f of the acoustic wave. It thus appears that this kind of stacking makes it possible to obtain an accentuated attenuation centered around a frequency (of the order of

  2300 Hz in the example illustrated) and on a relatively limited frequency band.

  It should be noted that provision may be made to reverse the order of the stacking of the layers C1 to C3 as described above, so as to obtain other

  sound absorption properties, as illustrated by curve B in fig. 2.

  According to this example for which the porosity values decrease according to the direction of propagation of the acoustic wave, the material allows attenuation at low

  lower frequency but offers a wider attenuation spectrum.

  According to another characteristic of the invention, provision may be made to stack the layers of resistive C. so that the values of their porosity follow a progression in a given direction of stacking of the layers. In a first embodiment, the resistive layers CD are arranged so that their porosity has values which are greater from one layer to the other, in a direction of stacking of the layers, considered to correspond to the direction of progression. of the incident wave through the complex material 1. For example, each layer CI to C3 has a porosity of 45%, 36% and% respectively. According to the example considered, the layers CI to C3 are stacked in this order starting from the noise source 2, so that the values of the porosity decrease according to the direction of progression of the incident wave. Of course, it can be envisaged to reverse the stacking order of the layers so that the values of the

  porosity increases according to the direction of progression of the incident wave.

  As is clearer from the above description,

  the object of the invention makes it possible to define a material 1 capable of treating certain frequency ranges as a function of the resistive layers, their stacking and the spheres used. To this end, the manufacturing process consists in defining an effective absorption frequency range for the material and in producing at least one layer from spheres assembled together and having a specific normal impedance adapted to allow noise absorption in the frequency range determined. According to a preferred alternative embodiment, provision is made to produce at least two layers of spheres each having a specific specific normal impedance. These layers are then mounted in a superimposed manner, being organized so as to allow, in combination, the absorption of the wave.

  sound in the frequency range determined.

  It should be considered that the material according to the invention can be in different possible forms. According to an elementary form of presentation, the complex material 1 may consist solely of a layer of resistive or by the assembly of the different layers of resistive chosen. For example, provision may be made for assembly between the different layers by bonding by depositing a resin at the interfaces between the layers to ensure their fixing. In the case of a thermal assembly of the layers, a solder powder is deposited at the interface between the different layers to be assembled. This powder is chosen to have a melting point lower than the melting point of the brazing of the resistive layers. The heating of the complex material is thus carried out up to the melting point of the brazing powder placed at the interfaces. According to another presentation characteristic, the different layer or layers of resistive may be associated with a supporting structure 3. As shown more precisely in FIG. 3, this support structure 3 can be constituted by a honeycomb or "honeycomb" structure of all types known per se. Integration of resistive layers inside a "honeycomb" structure increases performance

  acoustic of the material thus formed.

  According to another alternative embodiment illustrated more particularly in FIG. 4, provision may be made for the supporting structure to be constituted by a plate 6 and a counter plate 7 between which the layer or layers of spheres are arranged. Preferably, the plate 6 located on the side of the noise source is perforated or perforated to constitute an acoustic resonator, for example, of the type

of Helmholtz.

  The layers can be produced one after the other in the supporting structure or be produced simultaneously. In the latter case, separators must be provided between the different layers to avoid the interpenetration of

  spheres of a layer with the spheres of an adjacent layer.

  It should be noted that provision may be made to leave the interior of the support structure, as illustrated in FIG. 4, an air gap 8 making it possible to obtain an equivalent density reduced at constant thickness. This air gap 8 which is in the form of an air layer of shape similar to the resistive layers is arranged in the stack of layers at a determined location for

  ensure the desired sound absorption.

  The sound absorption material 1 according to the invention has a low density while having modifiable sound absorption capacities as a function of the frequency range to be attenuated. It should be noted that the sound absorption characteristics of the material change as a function of frequency. The absorption coefficient oe can, depending on the resistive layers used,

  be between 0.7 and 1.0 over several thirds of consecutive octaves.

  An advantage of this material in the configuration of hollow spheres and metallic binder is its use as an acoustic treatment material in an environment where the temperature ranges vary between -100 C to + 1100 C. This single or multilayer material based on hollow spheres is characterized by a low density. Depending on the constituent material used, the skin thickness and the manufacturing technologies, this material has

  a density varying between 200 kg / m3 to several hundred kg / m3.

  Such a material in metallic configuration has the following specific properties: rot-proof, structural rigidity, hygrometric stability

and mechanical isotropy.

  The hollow spheres used in this material have microscopic porosity properties which allow them to undergo a variation in atmospheric pressure without degradation. This porosity allows the material to be applied in humid areas, because it gradually evacuates water or any other

  liquid likely to be present in its environment.

  The material according to the invention is particularly suitable for ensuring acoustic treatment in the aeronautical sector (engine treatment, nacelle, fan, aircraft nozzle), in the space sector (treatment of components of launcher engine or satellite equipment, ...) or in the automotive sector

  (exhaust silencer, engine encapsulation).

  The invention is not limited to the examples described and shown, because

  various modifications can be made without departing from its scope.

Claims (12)

CLAIMS:   1 - Method for producing, from spheres assembled together, an acoustic absorption material for noise, characterized in that it consists: - of defining an effective absorption frequency range for the material, - and of constituting a material comprising at least one layer (Cj) of spheres assembled together and having a specific normal impedance adapted to allow absorption of noise in the range frequency determined.   2 - Method according to claim 1, characterized in that it consists - of constituting at least two layers (CO) from spheres assembled together and different from one layer to another, each having a specific specific normal impedance determined , - and ensuring the superimposed mounting of at least two layers (C) of spheres to constitute the sound absorption material, the layers of spheres being organized so as to allow, in combination, the absorption of noise in the frequency range determined.   3 - Method according to claim I or 2, characterized in that it consists, for each layer of spheres (Ce), in determining their normal impedance by defining the porosity, the resistance to flow and the tortuosity for each between them.   4 - Method according to claim 3, characterized in that it consists, for each layer of spheres (Ca), in determining their porosity whose increase in value results in an increase in the width of the frequency range sound absorption.   - Method according to claim 3, characterized in that it consists, for each layer of spheres (Cn), in determining their resistance to flow,   increasing the value results in a reduction in sound absorption.   6 - Process according to claim 3, characterized in that it consists, for each layer of spheres (C,), in determining their tortuosity whose increase in   value increases the absorption of low frequencies.   7- Method according to claims 1 and 3, characterized in that it consists of   arrange the layers of spheres (CO), so that the values of their porosity follow a progression in a given direction of stacking of the layers.   8- Method according to claims 1 and 3, characterized in that it consists of   arrange the layers of spheres (CO), so that the values of their resistance to flow follow a progression in a given direction stacking layers.   9 - Method according to one of claims 1 to 8, characterized in that it consists of   mount the layers of spheres in a supporting structure (3).   - Sound absorption material for noise, comprising spheres assembled together, characterized in that it consists of at least one layer (C) of spheres (S) each having a specific specific normal impedance determined, so as to allow noise absorption in a frequency range determined.   11i - Sound absorption material according to claim 10, characterized in that it comprises at least two layers (C) of spheres (S.) assembled together and different from one layer to another, the layers of spheres (C,) each having a specific normal impedance determined by being organized so as to allow, in combination, the absorption of noise in   a specific frequency range.   12 - Sound absorption material according to claim 11, characterized in that it comprises a support structure (3) in which the layers of spheres are arranged.   13 - Sound absorption material according to claim 12, characterized in that the support structure (3) is a honeycomb structure in which the or layers of spheres are arranged.   14 - Sound absorption material according to claim 12 or 13, characterized in that the supporting structure (3) consists of a plate (6) and a counterplate (7) between which the layer or layers of spheres (CO ) are arranged. - Sound absorption material according to claim 14, characterized in that the plate (6) is an openwork structure to constitute an acoustic resonator.   16 - Sound absorption material according to claims 10 to 15, characterized   in that it comprises an air space (8).