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The present invention relates to an acoustic panel of the air sheet type in which a spacer (or a plurality of spacers) hold(s) two plates (or a plurality of plates) apart.
It is known that the effectiveness of an absorptive material such as a porous material is increased if a sheet of air is created, in particular between the absorptive material and the wall that receives acoustic excitation.
The problem posed is to have a spacer of low weight and sufficient mechanical strength to ensure that the walls are well spaced apart while presenting a small area of contact with the plates so as to minimize transfers of sound and possibly also of heat between the plates.
The invention thus provides an acoustic panel presenting at least two plates, together with at least one spacer for holding said two plates apart and for forming at least one sheet of air, wherein said spacer is constituted by a woven or knitted element that presents embossing.
The embossing may be inclined, e.g. at 45°, relative to two opposite edges of the spacer.
The embossing may advantageously be performed in two dimensions.
The knitted or woven element may be in the form of a sleeve. It may be flattened so as to present two superposed embossed thicknesses. Alternatively, the sleeve which constitutes the woven or knitted element forms a net in which a said plate is located, for example a central plate disposed between two outer plates, thereby defining two spacers and two sheets of air.
The woven or knitted element is advantageously a embossed wire fabric, in particular made of aluminum.
When the acoustic panel has a central plate and two outer plates separated by said spacer, it is advantageous for the central plate to be constituted by a foam, preferably a foam of melamine and/or by wool of low density in an aramid fiber cover.
The two outer plates may be made of a glass fiber and epoxy composite.
Other characteristics and advantages of the invention appear better on reading the following description with reference to the accompanying drawing, in which:
FIGS. 1 and 2 are respectively a section view of an acoustic panel in the embodiment of the invention and a view of a spacer in an embodiment of the invention, and
FIG. 3 shows the advantage provided by the spacer device relative to a panel without a sheet of air.
The acoustic panel shown in FIG. 1 comprises a central panel 1 of melamine foam, e.g. having a thickness of 5 millimeters (mm), and two outer plates 2 and 3 of five-ply glass fiber and epoxy composite. The spacers 4 and 5 are constituted by a embossed knit of aluminum wire. In this case a rolled wire is used having a diameter of 0.37 mm, which wire is subsequently knitted and then embossed so as to give it a thickness of 2 mm to 4 mm, for example.
The acoustic panel as made in this way presents a total thickness of about 15 mm.
FIG. 2 is a detail view of the embossed knit. The particular stitch used in unimportant: it is the embossing that serves to minimize contact between the outer plate 2 or 3 and the central plate 1 that provides sound insulation. The undulations constituted by the embossing have the effect of providing almost point contact between the wire and the plates.
The material constituting the knit or the fabric is selected as a function of constraints, in particular environmental constraints, for the intended application, however the material must be of a kind that enables it to be deformed mechanically when cold or when hot. For example, it is possible to select a metal or indeed a plastics material, in particular a thermoplastic material.
A knit made in this way provides good spacing between the walls, while the spacer nevertheless remains sufficiently flexible to follow any rounded outline of the acoustic panel.
Aluminum also presents the combined advantage of good stiffness for relatively low weight, and of fireproofing.
The spacer of the invention avoids contact between the inside faces of the plates 2 and 3 and the faces of the central panel.
Using a metal, and in particular using a fine loop size (e.g. of the order of a few mm), provides resistance against a non-grazing incident flame.
The embossed fabric or knit may be stuck to one of the two plates it separates or to both of them. It is also possible to omit any adhesive, in which case the sandwich can be held together via the outsides of the outer plates 2 and 3.
In a variant, the central plate 1 may be a block of acoustic foam (e.g. polyurethane foam). The central plate 1 may also be made using glass wool, in particular low density glass wool, possibly contained in a fine cover of aramid fibers (“Mylar”).
FIG. 3 shows a damping test TL expressed in dB plotted up the ordinate as a function of frequency expressed in hertz along the abscissa. Curve A applies to the panel of FIG. 1 (outer plates 2 and 3 of glass fiber and epoxy composite, central plate 1 of melamine foam, and spacers 4 and 5 of embossed aluminum wire knit), while curve B applies to the panel of FIG. 1 but in the absence of any spacer (panels 1, 2 and 3 coming face to face).
It can be seen that damping is improved above a certain frequency (in this case 800 Hz) and in particular that the region in which damping decreases down to a low point situated around 2000 Hz is eliminated.
The acoustic panel shown in FIG. 1 (outer plates 2 and 3 of glass fiber and epoxy composite, central plate 1 of melamine foam, and spacers 4 and 5 of embossed aluminum wire knit), is suitable for passing the FA 25-863 test (applying a flame to the edge of the panel for 12 seconds (s)). The melamine foam presents very good resistance to fire, as do the glass fiber and epoxy composite and the embossed aluminum wire knit, and the sheets of air on either side improve cooling.