Sound insulation panel containing balls and method for producing it

Background of the invention

0001The present invention relates to the general field of said sound absorbing "passive" operating on the basis of Helmholtz resonators.

0002It finds an advantageous application in the sector aviation, in particular in the conduits of a turbomachine subjected to elevated temperatures.

0003In the field of aeronautics, soundproofing panels are used to reduce noise emitted by aircraft turbine engines. These panels are generally willing directly in flow wall to reduce the acoustic energy generated by the various components of the turbomachine.

0004The present invention aims in particular to apply Such acoustical panels in hot duct of the turbomachine, for example at the output of turbines or at the level of the nozzle exhaust.

0005In this context, it is therefore necessary to carry out acoustical panels that resist high temperatures of these and conduits that reduce noise levels without penalizing mechanical strength, mass and bulk of the turbomachine.

0006One known method to achieve soundproofing panels is described in patent FR 2 775 216. This document proposes to make acoustic panels with a structure sandwiched between two skins and whose cavities are arranged in Helmholtz resonators. These Helmholtz resonators are formed as layers of hollow beads glued together and to the wall structure by a binder resin type.

0007Besides the difficulty of obtaining the soundproofing panel described in this patent still has some disadvantages for high temperature applications.

0008Indeed, it has poor soundproofing panel held heat when subjected to high temperatures and the use a binder between the balls created galvanic coupling problems resulting in one-time corrosion problems.

0009In addition, in the case of treatment with cavities of large, the disclosed method adapts trouble implementation of acoustic panels to changing acoustic characteristics. For example, the thickness of ball layers can hardly vary in the same cell so that the acoustic characteristics of this cell remain substantially constant.

Purpose and Summary of the Invention

0010The present invention thus seeks to mitigate such drawbacks by providing a soundproofing panel resistant to temperatures high, with improved acoustic performance and that can present scalable acoustic characteristics.

0011The invention also provides a method of making such a soundproofing panel.

0012The soundproofing panel includes a sandwiched soul between a solid wall and a porous wall, the core being linked to the walls and including partitions extending in the thickness direction between the two walls and forming cells, each comprising at least a sink layer of sound energy consists of spherical beads hollow with porous walls in mutual contact.

0013According to the invention, the balls are micro-perforated walls and are held in position in the thickness direction between the two walls by nets secured to the partitions.

0014Layers formed of hollow spherical ball walls porous and micro-perforated allow high energy dissipation sound by viscothermal effect of air. This can be understood by the that there remains between the balls of the same layer spaces component between the two threads a plurality of passages for the air molecules. These passages which form a pore network are causing the dissipation of sound energy by the air friction in movement in this network (viscothermal phenomenon).

0015The use of beads having porous and micro-perforated walls has benefit of increasing significantly the viscothermal effect of air relative to non micro-perforated beads. Indeed, the micro-perforation of the walls hollow beads allows to engage inside the ball passage of air molecules as well as the interstitial network.

0016The overall porosity of such energy dissipating layer sound can thus pass from 40% to 80% compared to a layer not micro-perforated beads.

0017In this way, the acoustic efficiency of the panel according to the invention is increased to the same size compared to a panel Conventional which results in more sound absorption band wide in terms of frequency. Similarly, acoustic performance Similar, the soundproofing panel according to the invention can enable get space gains.

0018The use of metallic nets for maintaining the ball instead of a binder also has many advantages. Especially, this simplifies the construction of the soundproofing panel and allows its acoustic characteristics vary locally (variable thickness the layer in the same cell, mixture of different types of spheres ...) and to adapt to demand, for example by sector radial or angular.

0019It is advantageous to use nets having the same composition that the walls on which they are attached, which avoids the problems of heterogeneous coupling responsible for corrosion. It is also possible to obtain in a single piece a soundproofing panel having a geometry of revolution.

0020The layers of sound energy dissipating panel Sound may be constituted by a mixture of beads homogeneous diameters (the manufacturing tolerance) or by a mixture of beads having diameters ranging from 1 to 3.5 mm.

0021In addition, these layers may sound energy dissipating have a constant or variable thickness in a single cell. As explained above, these advantageous characteristics allow varying the acoustic characteristics of the soundproofing panel.

0022The method of making a soundproofing panel according the invention is characterized in that: <ul><li>a soul is made comprising partitions forming cells;</li><li>for each cell of the soul, is positioned and fixed a subnet on the walls, is arranged on the lower net a desired thickness of hollow spherical beads with porous walls and micro-perforated, we sets a higher net by applying gentle pressure on beads and fixed the upper thread on the walls;</li><li>applies and fixed is one of the faces of the core to a porous wall; and</li><li>applies and fixed is the other face of the core to a solid wall.</li></ul>

Brief Description of Drawings

0023Other features and advantages of the present invention apparent from the description given below, with reference to the attached that show an embodiment without any limiting. In the figures:<ul><li>Figure 1 is a partial perspective view of a panel soundproofing according to one embodiment of the invention;</li><li>2A and 2B are sectional views in the direction of the thickness of acoustic panels in two other variants the invention;</li><li>Figure 3 is a graph showing the plot of the coefficients absorption for a soundproofing panel according to the invention in relation to a soundproofing panel using non micro-perforated beads;</li><li>Figure 4 is a graph showing the plot of coefficient sound absorption in a sample of micro-perforated beads and a sample of non-perforated micro beads; and</li><li>Figure 5 is a graph illustrating the acoustic impedance normalized to a sample of micro-perforated ball and a sample of non-micro-perforated beads.</li></ul>

Detailed description of an embodiment

0024Referring first to Figure 1 showing in perspective a soundproofing panel according to one embodiment of the invention.

0025The soundproofing panel 2 is a structure having a core 4 sandwiched between a wall (or skin) and a wall full 6 (Or skin) porous 8. The core 4 of the panel consists of a plurality of cells 10 separated by partitions 12 extending in the direction of the thickness of the web and perpendicular to walls 6, 8.

0026The cells may have a rectangular cross section (as shown in Figure 1), triangular or hexagonal, for example.

0027The partitions 12 are fixed to the walls 6, 8, for example by welding, to form the structure of the soundproofing panel and in ensure the rigidity and mechanical strength.

002810 each cell thus formed is divided in the direction of the thickness of the core 4 by at least one layer dissipating energy Sound 14 whose edges are attached to the bulkhead 12.

0029The dissipating layers 14 divide each cell into at least two cavities 16. In Figure 1 which has only one layer dissipating represented 14 per cell, the term cavity 16a background, ie that which is against the solid wall 6, and the cavity 16b input, ie that which is against the porous wall 8. The layer dissipating 14 is traversed right through by a plurality of passages 18 which connect the cavities 16a, 16b forming a interstitial network.

003016b the input cavity is in communication with the exterior of soundproofing panel via the porous wall 8. From so, the sound energy that comes from outside of the soundproofing panel 2 passes through the porous wall 8 and sets the resonant cavities 16 coupled together by the passages 18 formed in the dissipating layers 14.

0031The sound energy is mainly dissipated by effect air viscothermal through the dissipating layers, and in a lesser extent in the porous wall 8.

0032According to the invention, each layer of sound energy dissipating 14 consists of hollow spherical balls 20 with porous walls and micro-perforated.

0033Inside the dissipating layers, these balls 20 are mutual contact and are held in position in the thickness direction 6.8 between the walls by nets or mesh 22 against the rigid fixed partitions 12.

0034These nets 22, for example metallic, permit positioning and holding of the spherical beads 20 within the dissipating layer 14. They must therefore have a mesh size relatively small compared to the diameter of the beads (e.g. the order of 0.2 mm for the balls of diameters ranging from 1 to 3.5 mm). No binder is thus necessary to keep the ball in touch mutual.

0035The threads 22 are fixed by their edges 24 to 12 partitions per welding, brazing, riveting or any other equivalent technique. On the 1, the welding spots 26 are shown diagrammatically.

0036In order to prevent the balls 20 from moving within the dissipating layer, the threads 22 are assembled with light pressure (Of the order of 0.1 to 0.2 mbar). Although no significant displacement Ball is allowed, however, it remains a freedom of "micro-displacement "For damping vibrations suffered by the panel soundproofing.

0037Preferably, the threads 22 have the same composition as the 12 partitions on which they are fixed to avoid problems heterogeneous couplings that are responsible for corrosion. For example, the threads 22 and the partitions 12 are made of the same metal base nickel or stainless steel.

0038According to an advantageous characteristic of the invention, the wall porous 8 has perforations 28 in order to optimize the effect viscothermal air therethrough.

0039For example, the perforations 28 may be of the orifices circular having a diameter ranging from 0.5 to 2 mm and the rate of perforation of the porous wall 8 may be between 10 and 30%.

0040In Figure 1, only one sink layer 14 by cell 10 is shown. One can also imagine that each cell has several dissipating layers over the entire thickness of the core 4 according acoustic characteristics required for the soundproofing panel.

0041Thus, the soundproofing panel shown in Figure 2A comprises two dissipating layers 14a and 14b arranged sound energy in a cell 10.

0042Likewise, always in order to vary the acoustic characteristics of the soundproofing panel, the layers dissipating 14 may be constituted by a mixture of beads 20 homogeneous diameters or different diameters.

0043According to another advantageous characteristic of the invention, it is also possible to vary the acoustic characteristics of soundproofing panel by adjusting the thickness of the dissipating layers 14 in the same cell 10. This possibility is offered by varying the spacing between the threads 22.

0044In addition, the layout of the dissipating layers 14 permits advantageously to achieve in one piece panels acoustic revolution.

00452B illustrates these two advantages. The panel Sound shown in section in this figure is a part 30 about axis XX produced in a single piece and having a layer dissipating 14c of varying thickness around the axis XX.

0046We now describe the nature of the component balls 20 sound energy dissipating layers 14.

0047Each sink layer 14 has an impedance sound that is directly related to the acoustic characteristics of the layer. This impedance depends mainly on the type of beads used and their assembly.

0048The balls 20 of dissipating layers 14 are substantially spherical, hollow and porous walls. Their external diameter varies preferably between 0.5 and 5 mm with a layer thickness of 0.3 mm about.

0049They are preferably made of ceramic (type alumina) but may also be metal (sintered powder) or zirconia.

0050According to the invention, the beads 20 of the dissipating layers 14 which are substantially spherical, hollow and porous walls are also micro-perforated.

0051The microperforation is a form of porosity in addition to the intrinsic porosity of the walls of the beads. It is performed by drilling at least two holes which pass through from one side to the walls of the beads and is characterized by dimensions which are expressed in several hundred microns (on the order of 200 to 400 .mu.m) with respect to the pores whose dimensions are typically expressed in tens of microns ( the order of 10 microns).

0052For example, micro-perforations dimensions made in the walls of the balls 20 are greater than those of the pores of these walls of at least an order of about 20.

0053The shape and micro-perforations of the size varies depending on the production method. Thus, they may be substantially cylindrical (with a diameter of about 20 microns for example) or parallelepiped (with for example a width of about 200 .mu.m and a length of the order of 500 .mu.m).

0054Micro-perforation of the walls of balls 20 improves much the sound absorption capability of dissipating layers 14. Indeed, this micro-perforation allows to involve inside hollow beads to the passage of air molecules as well as the interstitial network formed between the balls.

0055The table below and the plot of the absorption coefficients (Figure 3) illustrate this improvement over non-micro-perforated beads.

0056Tests were made with a thick layer constant of 10 mm and composed of a loose assembly of balls of a same diameter of 1.5 mm (manufacturing tolerance). The results are as follows:<tables><table><tgroup cols="4"><tbody><row><entry align="center">Characteristics</entry><entry align="center">Flow resistance (in Pa.s)</entry><entry align="center">crookedness</entry><entry align="center">Porosity (in%)</entry></row><row><entry align="center">not micro-perforated beads</entry><entry align="center">13600</entry><entry align="center">3.36</entry><entry align="center">40</entry></row><row><entry align="center">micro-perforated beads</entry><entry align="center">15800</entry><entry align="center">3.91</entry><entry align="center">80</entry></row></tbody></tgroup></table></tables>

0057The flow resistance, expressed as the ratio between the pressure drop and the air flow rate through the dissipating layer represents the acoustic resistance. this resistance acoustic depends on the porosity and tortuosity and its value must be optimized to achieve maximum sound dissipation.

0058Porosity in the zone occupied by the ball represents the ratio of open volume and the total volume of the sink layer. As tortuosity, porosity must therefore be as high as possible to use the maximum available volume for the dissipation of sound energy, which secures the wedge in frequencies.

0059It should however ensure that the damping representing the acoustic resistance remains sufficient to allow dissipation sound energy.

0060In the plot of the absorption coefficients shown in Figure 3, we note that the value of having micro-perforated beads (curve 100) provides, in identical footprint to achieve a dunnage frequencies much lower in frequency than non beads micro-perforated (curve 102).

0061Another trial has highlighted the performance dissipation of sound energy produced by micro-perforated beads according to the invention compared to non-micro-perforated beads. this test comparison is performed under the following conditions:

0062<u>1^er sample:</u><ul><li>porous wall having a perforation rate of 22% with orifices perforation of 1.5 mm diameter and a thickness of 0.7 mm.</li><li>Bulk assembly to a thickness of 28, 5 mm spherical beads hollow ceramic pore walls and micro-perforated with diameters of 2 mm.</li></ul>

0063<u>2^nd sample</u> :<ul><li>porous wall having a perforation rate of 22% with orifices perforation of 1.5 mm diameter and a thickness of 0.7 mm.</li><li>Bulk assembly to a thickness of 28, 5 mm spherical beads hollow ceramic porous walls and not micro-perforated with diameters of 2 mm.</li></ul>

0064Both samples are subjected to an excitation level Sound of the order of 140.5 dB. The results of this test are shown in Figures 4 and 5 which respectively illustrate the layout of the coefficient sound absorption and the route of normalized acoustic impedance each of these two samples.

0065In Figure 4, curve 104 corresponds to the coefficient absorption first sample, while the curve 106 corresponds in the second sample absorption coefficient.

0066Similarly, in Figure 5, the curves 108a and 108b represent the normalized acoustic impedance of the first sample, respectively in terms of resistance and reactance, and curves 110a and 110b show the normalized acoustic impedance of the second sample, respectively in terms of resistance and reactance.

0067In Figure 4, we see that the characteristics of the layer dissipating micro-perforated beads of the first sample (curve 104) possible to obtain an absorption frequency configuration broad band.

0068Indeed, for the first sample, the absorption coefficient remains greater than 0.6 between 1000 Hz and 6500 Hz. against the characteristics of non dissipating layer micro-perforated beads of second sample (curve 106) shows a sound attenuation band narrower with further an absorption at low frequencies deficit.

0069The normalized impedance curves in terms of strength and reactance of figure 5 confirms that the timing frequency is in larger for the first sample (micro-perforated beads) for the second sample (not micro-perforated beads) and that the dissipation of sound energy is higher for the first sample.

0070We will now describe the method of producing a panel soundproofing panel according to the invention with reference again to Figure 1.

0071This process essentially consists in making a core 4, to applying and securing one of the faces of the core to a porous wall 8 and applying and securing the other face of the core to a solid wall 6.

0072According to the invention, prior to the application and fixing of the faces of the core on the walls 6, 8, is provided to cut two fillets 22 to the desired shape.

0073Is positioned and fixed a subnet (hereinafter referred Reference 22a) on the partitions 12 of the soul to a height <u>H</u> compared to the solid wall 6. The edges 24 of the subnet 22a are fixed on the partitions 12 for example by welding.

0074The next step is to settle on a subnet amount of balls 20 defining a desired thickness (<u>I</u>-<u>H</u>) For the layer dissipating 14, this thickness can be constant or variable.

0075An upper thread (denoted by the reference 22b) is then positioned on the "bed" of balls 20 with a slight pressure (e.g. of the order of 0.1 to 0.2 mbar) to avoid any movement of the beads in sink layer.

0076The edges 24 of the top net 22b are fixed to the partitions 12 (For example by welding). These operations are repeated for each cell 10 of the core 4.

0077The core 4 thus obtained is then placed between the walls 6,8 and then fixed thereto.

0078The method of making soundproofing panels shown by Figures 2A and 2B is identical thereto.

0079The method of making a soundproofing panel as described above has many advantages compared to a method using a binder to maintain the beads in the layers dissipative.

0080Indeed, the implementation of this process is greatly simplified. Any type of binder resin is also required, which avoids corrosion problems. The equipment used is also reduced.

0081This method thus allows for soundproofing panels with dissipating layers of variable thickness.