Sound insulation panel containing balls and method for producing it

Background of the invention

The present invention relates to the general field of acoustic panels called "passive" operating on the basis of Helmholtz resonators.

It finds an advantageous application in the aviation sector, notably in the ducts of a turbine engine subjected to high temperatures.

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

The present invention aims in particular to apply such acoustic panels in hot ducts of the turbomachine, for example at the output of turbines or at the exhaust nozzle.

In this context, it is necessary to perform acoustical panels that resist high temperatures and these ducts that reduce noise levels without penalizing the mechanical strength, weight and size of the engine.

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

In addition to its difficulty in obtaining, the soundproofing panel described in this patent still has certain disadvantages for high temperature applications.

Indeed, this sound absorbing panel has poor thermal stability when subjected to high temperatures and the use of a binder between the balls created galvanic coupling problems resulting in point of corrosion problems.

Moreover, in the case of a treatment with large-size cavities, the disclosed method adapts badly to the implementation of sound-absorbing panels to changing acoustic characteristics. For example, the thickness of ball layers can hardly be varied within the same cell so that the acoustic characteristics of the cell remain substantially constant.

Purpose and Summary of the Invention

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

The invention also provides a process for producing such a soundproofing panel.

The objectives are achieved by the panel as claimed in claim 1 and the method for producing as claimed in claim 8.

The soundproofing panel comprises a socket core sandwiched between a solid wall and a porous wall, the core being bonded to the walls and having partitions extending in the thickness direction between the two walls and forming cells, each comprising at least one sound energy dissipating layer constituted by hollow spherical beads with porous walls in mutual contact.

According 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.

Layers formed of hollow spherical beads having porous and micro-perforated walls allow a high dissipation of the sound energy by viscothermal effect of the air. This can be understood by the fact that there remains between the balls of the same layer forming spaces between the threads a multitude of passages to the air molecules. These passages which form an interstitial network are responsible for the dissipation of sound energy by friction of the air moving in the network (viscothermal phenomenon).

The use of beads having porous and micro-perforated walls has the advantage of significantly increasing the viscothermal effect of the air with respect to non-micro-perforated beads. Indeed, micro-perforation of the walls of hollow spheres allow to involve inside ball to the passage of air molecules as well as the interstitial network.

The overall porosity of such sink layer of sound energy can then rise from 40% to 80% compared to a layer of non-perforated micro beads.

In this way, the acoustic efficiency of the panel of the invention is increased at the same size compared to a conventional panel which is reflected by a larger sound absorption band in terms of frequency. Likewise, similar acoustic performance, soundproofing panel according to the invention can allow to obtain in space gains.

The use of metallic nets for maintaining the balls in place of a binder also has many advantages. In particular, this simplifies the construction of the soundproofing panel and allows for locally varying its acoustic characteristics (variable thickness of the layer in the same cell, mixture of different types of spheres ...) and adapted to request, for example by radial or angular sector.

It is advantageous to use nets having the same composition as the partitions on which they are fixed, 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.

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

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

The method of making a soundproofing panel according to the invention is characterized in that: <ul><li>a soul is made comprising partitions forming cells;</li><li>for each cell of the core, is positioned and fixed on a subnet partitions, is arranged on the lower net a desired thickness of hollow spherical beads having porous and micro-perforated walls, positioning an upper net by applying slight pressure on the ball and sets the upper net 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

Other features and advantages of the present invention will become apparent from the description given below, with reference to the accompanying drawings that show an embodiment having no limiting character. In the figures:<ul><li>Figure 1 is a partial perspective view of a soundproofing panel according to one embodiment of the invention;</li><li>2A and 2B are sectional views in the direction of the thickness of sound-absorbing panels according to two other embodiments of the invention;</li><li>Figure 3 is a graph showing the plot of the absorption coefficients for a soundproofing panel according to the invention with respect to a sound absorbing panel using non-micro-perforated beads;</li><li>Figure 4 is a graph showing the plot of the sound absorption coefficient for a sample of micro-perforated ball and a sample of non-perforated micro beads; and</li><li>Figure 5 is a graph showing the normalized acoustic impedance for a sample of micro-perforated ball and a sample of non-micro-perforated beads.</li></ul>

Detailed description of an embodiment

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

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

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

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

Each 10 thus formed cell is divided in the direction of the thickness of the core 4 by at least one sound energy dissipating layer 14 whose edges are attached to the bulkhead 12.

The dissipating layers 14 divide each cell into at least two recesses 16. In Figure 1 which has only one sink layer 14 represented by cell, 16a denotes the bottom of the cavity, ie that which is against the solid wall 6, and 16b by the input cavity, ie that which is against the porous wall 8. the dissipative layer 14 is traversed right through by a plurality of passages 18 which bring the cavities in communication 16a, 16b forming an interstitial network.

16b the input cavity is in communication with the outside of the soundproofing panel via the porous wall 8. In this way, 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.

The sound energy is dissipated primarily by viscothermal effect of the air passing through the dissipating layers, and to a lesser extent in the porous wall 8.

According to the invention, each sound energy dissipating layer 14 is constituted of balls 20 hollow spheres with porous walls and micro-perforated.

Inside the dissipating layers, these balls 20 are in mutual contact and are held in position in the thickness direction between the walls 6,8 by netting or mesh 22 fixed against the rigid walls 12.

These nets 22, for example metal, allow the positioning and holding of spherical balls 20 inside the dissipating layer 14. They must therefore have a relatively small mesh size to the diameter of the balls (eg the about 0.2 mm for the balls of diameters ranging from 1 to 3.5 mm). No binder is thus necessary to keep the beads in mutual contact.

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

In order to prevent the balls 20 from moving inside the sink layer, the threads 22 are assembled with a slight pressure (of the order of 0.1 to 0.2 mbar). Although no significant displacement of the ball is allowed, however, it remains a freedom of "micro-displacement" for damping vibrations suffered by the soundproofing panel.

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

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

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

In Figure 1, only one sink layer 14 by cell 10 is shown. It is also conceivable that each cell comprises several dissipating layers over the entire thickness of the core 4 to the acoustic characteristics necessary for the acoustic panel.

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

Similarly, always with the aim of varying the acoustic characteristics of the acoustic panel, the dissipating layers 14 may be constituted by a mixture of beads 20 of uniform diameter or of different diameters.

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

In addition, the layout of the dissipating layers 14 advantageously allows for one piece of soundproofing panels revolution.

2B illustrates these two advantages. The soundproofing panel shown in section in this figure is a part 30 about the axis XX produced in a single piece and having a variable 14c dissipating layer thickness around the axis XX.

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

Each sink layer 14 has an acoustic impedance which is directly related to the acoustic properties of the layer. This impedance depends mainly on the type of balls used and their assembly.

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

They are advantageously made of ceramic (alumina type) but can also be metal (sintered powder) or zirconia.

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

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

Thus, the dimensions of micro-perforations in the walls of the balls 20 are greater than those of the pores of these same walls of at least an order of about 20.

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

The micro-perforation of the walls of balls 20 significantly improves the sound absorption capability of dissipating layers 14. In fact, the micro-perforation allows to engage inside the hollow ball for the passage of air molecules as well as interstitial network formed between the balls.

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

Tests were performed with a constant thickness layer of 10 mm and composed of an assembly of loose beads of the same diameter of 1.5 mm (manufacturing tolerance). The results are as follows:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="41mm" /><colspec colnum="2" colname="col2" colwidth="55mm" /><colspec colnum="3" colname="col3" colwidth="20mm" /><colspec colnum="4" colname="col4" colwidth="26mm" /><thead valign="top"><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></thead><tbody><row><entry align="center">not micro-perforated beads</entry><entry align="center">13600</entry><entry align="char" char="," charoff="32">3.36</entry><entry align="center">40</entry></row><row><entry align="center">micro-perforated beads</entry><entry align="center">15800</entry><entry align="char" char="," charoff="32">3.91</entry><entry align="center">80</entry></row></tbody></tgroup></table></tables>

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

The porosity in the beads by occupied area is the ratio of open volume to the total volume of the dissipating layer. As for the tortuosity, porosity should be as high as possible in order to use the maximum available volume for dissipation of sound energy, allowing to set the timing in frequency.

However, it must ensure that the amortization representing the acoustic resistance remains sufficient to allow the dissipation of the sound energy.

In the plot of the absorption coefficients shown in Figure 3, we see that the value of having micro-perforated beads (curve 100) provides, in identical footprint to perform a calibration in much lower frequencies frequencies that with non microperforated balls (curve 102).

Another trial has highlighted the dissipation of sound energy performance obtained by micro-perforated beads of the invention compared to non micro-perforated beads. This comparative test is performed under the following conditions:

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

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

These two samples are subjected to a level of sound excitation in the range of 140.5 dB. The results of this test are shown in Figures 4 and 5 which respectively illustrate the path of the sound absorption coefficient and the route of normalized acoustic impedance for each of these two samples.

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

Similarly, 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 the curves 110a and 110b show the normalized acoustic impedance of the second sample, respectively terms of resistance and reactance.

In Figure 4, we see that the characteristics of the dissipating layer micro-perforated beads of the first sample (curve 104) enable a configuration of frequency absorption broadband.

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

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

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

This process essentially consists in making a core 4 to be applied and fix 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.

According to the invention, prior to the application and fixing faces of the core on the walls 6, 8, there is provided two cutting threads 22 to the desired shape.

Is positioned and fixed a subnet (designated by the reference 22a) of the partitions 12 of the core at a height <u>H</u> with respect to the solid wall 6. The edges 24 of the subnet 22a are fixed to the partitions 12 for example by welding.

The next step consists of depositing on the subnet an amount of balls 20 defining a desired thickness (<u>I</u>-<u>H</u>) For the dissipative layer 14, this thickness can be constant or variable.

An 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 displacement of the balls in dissipating layer.

The 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.

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

The method of making the acoustical panels illustrated in FIGS 2A and 2B is identical thereto.

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

In fact, the implementation of this method is greatly simplified. No type binder resin is also necessary, which avoids corrosion problems. The equipment used is also reduced.

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