Acoustics – Sound-modifying means – Sound absorbing panels
Reexamination Certificate
2001-10-17
2003-03-25
Hsieh, Shih-yung (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
C181S284000, C181S286000, C181S291000
Reexamination Certificate
active
06536556
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 00 13977 filed in France on Oct. 31, 2000; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The invention relates to a noise reduction sandwich panel of the type including a waffle core mounted between a porous resistive layer and a reflector.
Such a sandwich panel may advantageously be used in an aircraft turbojet engine, for example in order to form the internal wall of the air intake and of the thrust reverser.
STATE OF THE ART
In aircraft turbojet engines developed during the last couple of years, noise reduction is one of the priority goals. Indeed, present regulations relative to noise level around airports impose a threshold not to be exceeded. Beyond this threshold, airlines are under obligation to pay financial penalties to the airport authorities.
For this purpose and as
FIG. 1
of the appended drawings illustrates this in a sectional view and very schematically, it is common practice to produce the major portion of the inner wall
1
of the air intake of a turbojet engine, as well as the walls
2
of the thrust reverser as noise reduction sandwich panels.
These noise reduction sandwich panels comprise, from the surface of the panel turned towards the outside, a more or less air permeable resistive layer, a waffle core most frequently having a structure of the honeycomb type, and a rear total reflector.
In this conventional layout of noise reduction sandwich panels, the resistive layer has a dissipate role. When a sound wave crosses it, viscous effects are produced which transform acoustical energy into heat.
The height of the waffle core enables the panel to be tuned to the characteristic frequency of the noise to be damped. Dissipation of noise in the resistive layer is maximum when the height of the cells of the waffle core is equal to a quarter of the wavelength of the noise frequency to be damped. The cells of the waffle structure then behave as waveguides perpendicular to the surface of the panel, which give them a response of the “localized reaction” type. The cells form an assembly of quarter wave resonators in parallel.
The rear reflector produces total reflection conditions absolutely required for obtaining the behavior described above of the waffle core.
A noise reduction sandwich panel implanted in an aircraft turbojet engine should meet various requirements of an acoustical, mechanical, structural and aerodynamically nature. For fulfilling the noise reduction function, there are presently different types of sandwich panels.
In the so-called “non-linear one-degree-of-freedom” panels, the resistive layer consists of a metal or composite perforated layer. This structure has the advantage of providing good control of the percentage of open surface, of exhibiting good structural strength and of being easy to produce. However, it exhibits a strong acoustical non-linearity as well as a strong dependence of the strength on the surface tangential flow velocity. Further, as the frequency damped by each cell depends on its depth and as all the cells of the panel have the same depth, the range of frequencies damped by such a structure is limited. In addition, as the resistive layer is in a composite material, the structure has low erosion resistance.
So-called “linear one-degree-of-freedom” noise reduction sandwich panels are also known. In this case, the resistive layer is a microporous layer for example consisting of metal fabric, of perforated sheet metal associated with an acoustical fabric or of a metal fabric associated with an acoustical fabric. Such a structure enables the acoustical resistance to be adjusted by changing the components of the microporous layer. Its effective frequency range is reasonable. It has a low to moderate non-linearity as well as a low dependency of the acoustical resistance on the surface tangential flow velocity.
However, the production of a linear one-degree-of-freedom sandwich panel is more complicated than that of non-linear one-degree-of-freedom panel, because the resistive layer comprises two constituents. If the components or the assembly methods are not under control, the structure may have areas of acoustical non-homogeneity and there are also risks of delamination of the resistive layer. Further, risks of corrosion of the resistive layer impose an additional constraint as to the choice of the materials used. Finally, the assembly method of such a panel is lengthy and costly.
“Two-degrees-of-freedom” noise reduction sandwich panels are also known. Such a panel comprises, in addition to a perforated resistive layer and a rear reflector, two superimposed waffle cores, separated by an intermediate resistive layer called a “septum”, which is generally microporous.
As compared with other types of sandwich panels, the panels with two degrees of freedom have a larger range of damped frequencies, a possibility of adjusting the acoustical resistance by means of both resistive layers, and a low to moderate acoustical non-linearity. However, areas of acoustical non-homogeneity appear because of misalignment of the cells from both waffle cores which inevitably occurs upon forming the panel. There are also parasitic transverse propagation phenomena in the areas where the cells of both waffle cores are not aligned. Finally, the assembly method for a panel of this type is lengthy and costly, as the different components of the structure must be assembled one by one.
Various solutions have been suggested in order to overcome the drawbacks of the two-degrees-of-freedom panels resulting from misalignments of cells from both waffle cores.
Thus, in document GB-A-2,252,076, a sandwich panel with two degrees of freedom is obtained from a waffle core produced as a single piece. The intermediate resistive layer is obtained by positioning a separation sheet on one face of this core and by pressing down on the sheet in such a way that it is cut out into pieces having the dimensions of the cells, by the edges of the walls of the latter. These pieces are then pushed in and then stuck in the cells in a predetermined position. However, the problem of accurately placing the different pieces, repeatedly and reliably, is not solved in this document.
Document GB-A-2,098,926 describes a method for integrating a separation sheet into a sandwich panel comparable to the one described in document GB-A-2,252,076. More specifically, this document suggests the use of a press for cutting out the separation sheet to the dimension of the cells by means of the waffle core. As soon as this operation is completed, the waffle core incorporating the pieces cut out of the separation sheet is placed in a bath of dense liquid, the depth of which is carefully monitored in order to push these pieces into their predetermined definitive position. With this technique a two-degrees-of-freedom sandwich panel may be obtained, wherein the cells are aligned properly. However, this method is relatively lengthy and delicate to implement and may prove to be dangerous because of the use of a liquid such as mercury. Further, it is completely unsuitable in the case of a non-planar sandwich panel.
In document GB-A-1,463,918, a two-degrees-of-freedom sandwich panel is obtained by using a single waffle core, the cells of which are divided, in the direction of height, into two subcells by separative components mounted in each cell. In all the described solutions, the separative components have the same hexagonal shape and the same dimensions as the cells in which they are received.
Further, among the various solutions proposed in document GB-A-1,463,918, certain propose joining up several separative components received in aligned cells. These solutions seem advantageous from an industrial point of view, as they lead to a reduction of the setup time for the separative components, which is all the more significant as the number of cells of a sandwich panel is generally very large. More specifically, according to document GB-A-1,463,918, the adjacent separative components are joined up either by a
Batard Hervé
Porte Alain
Airbus France
Hsieh Shih-yung
Krebs Robert E.
Thelen Reid & Priest LLP
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