ACOUSTIC ATTENUATION PANEL WITH IMPROVED PERFORMANCES IN LOW AND MEDIUM FREQUENCIES

Abstract

An acoustic attenuation panel including: a main layer made of a porous material and having two opposite faces; a coating layer arranged on one of the two faces of the main layer; an adhesive film, arranged at the interface between the main layer and the coating layer to assemble same. The main layer is a body made of melamine resin foam having a density between 6 and 6.8 kg/m.sup.3 and a porosity rate between 0.978 and 0.984; the coating layer is a fabric with a density between 484 and 526 kg/m.sup.3 and a porosity rate between 0.771 and 0.817; the adhesive film is a film made of a thermoplastic material provided with holes passing through and having a surface density between 40 g/m.sup.2 and 56 g/m.sup.2

Claims

1. Acoustic attenuation panel comprising: a first layer, so-called main layer, made of a porous material and having two opposite faces; and a second layer, so-called coating layer, disposed on one of the two faces of the main layer; wherein the main layer is a body made of melamine resin foam having a density between 6 and 6.8 kg/m.sup.3 and a porosity rate between 0.978 and 0.984, wherein the coating layer is a fabric having a density between 484 and 526 kg/m.sup.3 and a porosity rate between 0.771 and 0.817, and wherein the main layer and the coating layer are assembled one with the other by an adhesive film that is disposed at the interface between the main layer and the coating layer, the adhesive film being a film made of a thermoplastic material provided with through holes and having a surface density between 40 g/m.sup.2 and 56 g/m.sup.2.

2. Panel according to claim 1, wherein the main layer has a thickness between 9.5 cm and 11 cm.

3. Panel according to claim 1, wherein the coating layer has a resistivity between 2,021,988 and 2,471,130 N.Math.s/m.sup.4 and an airflow resistance (AFR) between 707 and 865 N.Math.s/m.sup.3.

4. Panel according to claim 3, wherein the coating layer has a thickness less than 1 mm.

5. Panel according to claim 1, wherein the coating layer is a fabric produced from a mixed yarn consisting of polyester fibres and of cotton fibres, the mixture ratio of polyester fibres and of cotton fibres being preferably of 50/50.

6. Panel according to claim 1, wherein the melamine resin foam forming the main layer is a formaldehyde, melamine and sodium bisulphite copolymer.

7. Panel according to claim 6, wherein the melamine resin foam has a resistivity between 9,752 and 11,196 N.Math.s/m.sup.4 and an airflow resistance (AFR) between 990.7 and 1,137.3 N.Math.s/m.sup.3.

8. Panel according to claim 1, wherein the adhesive film has a thickness between 0.16 and 0.20 mm.

9. Panel according to claim 8, wherein the adhesive film has a melting temperature between 180° C. and 190° C.

10. Panel according to claim 1, wherein: the main layer is a body made of melamine resin foam having a surface density between 600 g/m.sup.2 and 680 g/m.sup.2 preferably equal to 650 g/m.sup.2, and a thickness between 9.5 cm and 11 cm; the coating layer is a fabric having a surface density between 110 and 130 g/cm.sup.2, preferably equal to 120.75 g/cm.sup.2, and a thickness between 0.3 and 0.4 mm; the adhesive film has a surface density between 40 g/m.sup.2 and 56 g/m.sup.2, preferably equal to 52 g/m.sup.2.

11. Part having a wall whereon a panel is attached such as defined according to claim 1, said part being chosen from a space launcher nose cone, a space launcher inter-stage structure, a fuselage of an aircraft, a building, a motorised vehicle and a train.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Other advantages and features of the invention will become clear upon reading the following detailed description of preferred embodiments of it, given by way of non-limiting example, and made with reference to the appended drawings wherein:

[0046] FIG. 1 shows an acoustic attenuation panel according to the invention, according to a sectional view, disposed against a wall;

[0047] FIG. 2 shows the sound absorption coefficient depending on the frequencies, of the sound absorption panel according to the prior art used in ARIANE 5;

[0048] FIG. 3 shows the sound absorption coefficient depending on the frequency of the sound absorption panel produced according to a preferred embodiment of the invention;

[0049] FIG. 4 is a schematic representation, according to a top view, of a web-shaped adhesive film.

[0050] It is specified that the various elements in FIG. 1 are not shown to scale.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0051] In FIG. 1 an acoustic attenuation panel 1 is shown according to the invention. It includes a main layer 2 made of a porous material, a coating layer 3 and an adhesive film 4.

[0052] The panel 1 is shown positioned against a wall 5 of a structure. It may for example concern the inner wall of a space launcher nose cone.

[0053] The panel may be integrated into the wall during the design of the structure or be added subsequently.

[0054] The inventors have sought to optimise the features of the panel of the prior art, used in the payload cavity of the ARIANE 5 space launcher, having an absorption profile such as illustrated in FIG. 2. More specifically, the inventors have sought to design a panel having a sound absorption coefficient of at least 0.3 at 63 Hz and of at least 0.55 at 125 Hz. Obtaining a coefficient of 0.3 at 63 Hz would indeed make it possible to reduce the pressure in the octave band centre frequency of 63 Hz inside the payload cavity by a factor of 1.25 (i.e. a reduction of 2 dB in relation to the reference solution of the Ariane 5 launcher).

[0055] For this purpose, the inventors have in particular used, as porous material for the main sound absorption layer, a melamine resin foam having the features indicated in Table 1 below.

TABLE-US-00001 TABLE 1 features of the melamine resin foam and of the laminated fabric used in a preferred embodiment of the invention Fabric 11689C with ULb melamine resin foam lamination effect Standard Standard Parameters Value Deviation Value Deviation Porosity 0.981 0.003 0.794 0.023 rate Resistivity 10,474 722 2,246,559 224,571 (N.s.m.sup.−4) AFR 1,064 73.3 786 79 (N.s.m.sup.−3) Tortuosity 1 0 1 0 Viscous 99.1 12.3 1 0 characteristic length (μm) Thermal 149.7 10.9 70 0.2 characteristic length (μm) Density 6.4 0.4 505 21 (kg/m.sup.3) Young's 117.4 11.7 — — modulus (Pa) Poisson's 0.41 0.07 — — ratio Sound 0.053 0.008 — — insulation

[0056] The choice amongst the existing melamine resin foams is made depending on the temperature constraints to which the panel must resist, as well as to the mass constraints and, obviously to the acoustic attenuations to be achieved.

[0057] The same applies for the fabric and the adhesive film, the choice being made depending on the temperature, mass and acoustic attenuation constraints. Regarding this subject matter, it is specified that the features of the “fabric 11689C with lamination effect” indicated in Table 1 above correspond to the features of said fabric after it has undergone a lamination step with an adhesive film.

[0058] By way of example, for a use of the panel in a space launcher nose cone, the various elements of the panel must pass the space environmental qualification tests, namely the ESA (European cooperation for Space Standardization) specifications.

[0059] Table 2 below groups a few ESA references and the subject matter to which each reference relates.

TABLE-US-00002 TABLE 2 a few ESA references Reference Subject matter ECSS-E-ST-10C System engineering general requirements ESSS-Q-ST-70-71C Materials, processes and their selection ECSS-E-HB-31-01 Part 5A Thermal design handbook ECSS-E-HB-32-20 Part 1A Structural materials handbook ECSS-E-HB-32-21A Adhesive design handbook ECSS-E-ST-10-06C Technical requirements specification ECSS-E-ST-32C Structural general requirements ECSS-E-ST-08C Materials

[0060] The melamine resin foam manufactured by BASF under the name Basotect™ is suitable for use in a space launcher nose cone, an aircraft cabin, a train or a building, because it has an M1 fire-resistance rating (non-flammable product). Said particular melamine resin foam is a formaldehyde, melamine and sodium bisulphite copolymer.

[0061] The panel according to the preferred embodiment of the invention includes a Basotect™ melamine layer bearing the reference “UltraLight b” (or ULb), having a thickness of 4 inches (i.e. 10.16 cm) and the features indicated in Table 1 above.

[0062] The so-called “UltraLight b” melamine is a foam derived from the melamine currently used in the space industry, but it is 30% lighter than the conventional Basotect™ According to a preferred embodiment, and in order to limit the weight of the panel, the inventors have chosen to use, as coating layer, the fabric having features indicated in Table 1 and a thickness less than 1 mm.

[0063] The inventors have chosen a woven textile (or fabric) for the coating layer, because the unwoven textiles, mostly, have the fibres thereof that are interlinked by a thermoplastic resin that does not withstand a temperature of 160° C.

[0064] Namely, the woven textile used in the preferred embodiment is that manufactured by Victor Textile, located in Canada, under the reference 11689C. It includes the features indicated in Table 1 and a thickness of 0.35 mm, and it is produced from a textured filament yarn of a mixture of polyester/cotton (50/50) fibres.

[0065] The woven textile of reference 11689C from Victor Textile has a sound absorption coefficient of at least 0.3 at 63 Hz and of at least 0.55 at 125 Hz. Furthermore, it is light (average surface density of 120.75 g/cm.sup.2) and it has a substantially smooth surface (more specifically, it has little surface relief and there are no free fibres on the surface), which has the advantage of ensuring a better adhesion with the foam. Furthermore, as there are no free fibres on the surface of the coating layer, this prevents, in the particular case of space launcher nose cones, the emission of particles during the forced ventilation under the nose cone of the launcher and during the phase of separating/moving away of the launcher from the half-nose cones.

[0066] In order to ensure a good holding of the coating layer (fabric) on the main layer (foam) and to increase the airflow resistance of the acoustic attenuation panel, said two elements are bonded on one another by a lamination method by placing an adhesive film made of thermoplastic material at the interface thereof.

[0067] The adhesive film will therefore make it possible to bond said two elements, but it also contributes to the acoustic attenuation. It is chosen so that it includes through holes both to enable the air to escape and to contribute to the sound absorption performance of the panel. Preferably, the adhesive film has a web shape, whereof an example is illustrated in FIG. 4. Namely, we have used a hot-melt adhesive from Protechnic.

[0068] Protechnic proposes hot-melt adhesive films bearing the name Thermoplast™ and available in the form of solid films or having cutouts according to various patterns (in the shape of strips, hexagons, etc.) forming for example nets or webs.

[0069] In the preferred embodiment, we have used a thermoplastic film bearing the reference BB8, which has a web pattern (filaments superimposed giving the impression of a web with cutouts in the form of irregularly placed holes of various diameters) and that meets the requirements of the ESA space thermo-mechanical environment qualification tests (whereof the references are indicated in Table 2 above), by being able to withstand a temperature of 185° C.

[0070] Tests have been performed by varying the density of the thermoplastic film.

[0071] A first panel has been produced using a film having a surface density of 40 g/m.sup.2 and a second panel has been produced by using a film having a surface density of 56 g/m.sup.2. The sound absorption results obtained for a panel produced with the films of 40 g/m.sup.2 (curve 2) and 56 g/m2 (curve 3) are shown in FIG. 3 (the curve 1 showing the acoustic attenuation objective to be reached; it is noted that the acoustic attenuation is greater than that of the panel of the prior art used in the Ariane 5 launcher and presented in FIG. 2). The optimum absorption is obtained by interpolation with a surface density of 52 g/m.sup.2.

[0072] By comparing the acoustic performances of the panel of the prior art used in ARIANE 5 and those of the panel produced according to the preferred embodiment of the invention, it is noted that the sound absorption is improved in the low frequency bands with an acceptable loss at high frequencies. More particularly, it is noted that the panel according to the invention satisfies the targeted sound absorption conditions of 0.3 and 0.55 respectively at 63 Hz and 125 Hz, since it provides a sound absorption of 0.35 at 63 Hz and of 0.62 at 125 Hz. Said sound absorptions are greater than those provided by the reference panel of the prior art (see the curve in FIG. 2), and are in particular substantially greater than a factor 2 in the frequency band of the central frequency 63 Hz, which is a critical frequency band for the space launchers. Said factor 2 leads to a reduction of the noise inside the fairing of a launcher of 3 dB.

[0073] The panel thus produced satisfies the difficult requirements in terms of sound absorption in the low and medium frequency bands, without the acoustic performances thereof being degraded at higher frequencies.

[0074] Furthermore, the panel according to the invention, for the same thickness of sound absorption material made of melamine resin foam, is 30% lighter compared to that of the prior art. Having decreased the weight of the panel has the advantage of improving the payload mass carrying performances of the launcher whereon it is intended to be applied.

[0075] We have just described a panel adapted to a use in a space launcher nose cone. But it is clearly understood that the panel according to the invention may be adapted to a use in all kinds of structures, such as for example in an aircraft fuselage, in a carriage of a high-speed train, in a building, etc.