EP4297016A1 - Broadband acoustic absorbent panel based on a bilayer meta-material - Google Patents

Abstract

The disclosure relates to an elementary cell of absorbent acoustic metamaterial. The elementary cell comprises a first layer comprising a first resonator. The first resonator forms a conduit comprising a first inlet opening and an outlet opening. The elementary cell further comprises a second layer comprising one or more resonators. Each resonator of the second layer forms a cavity comprising a second input opening connected to the output opening of the conduit of the first resonator so that all or part of an incident sound wave received by the first input opening can penetrate in each resonator of the second layer via this second input opening. The elementary cell is used to improve acoustics and sound comfort in a simple and effective way.

Description

Technical field of the invention

The invention relates to an elementary cell of absorbent acoustic metamaterial and also to an absorbent acoustic panel comprising at least one elementary cell of absorbent acoustic metamaterial.

State of the prior art

An acoustic absorbent material allows part of the incident sound wave to be dissipated and only a fraction of it to be reflected. The performance of such a device is measured in terms of absorption rate α (also called absorption coefficient) and frequency bandwidth Δ f in which this absorption rate α is guaranteed. An effective absorbent panel will have an acoustic impedance very close to that of air over the frequency band considered. Inherently absorbent materials such as foams, wools and fibrous materials make up the vast majority of sound absorption solutions used in various areas of industry. However, the use of this type of solution is not always suitable and sometimes, simply not conceivable depending on the applications. Indeed, these materials are fragile and very often polluting so that they are always encased in a frame and covered with panels comprising perforations or any other openings which allow the incident wave to reach the absorbing material. This, with a degradation in acoustic performance compared to that of bare absorbent material. They are, therefore, excluded in applications and environments for which the rate of particulate pollution must be maintained at drastically low levels and/or the maintenance of performance must be sustainable in all circumstances (for example, humidity, flooding and /or extreme temperatures). In addition, the efficiency of these porous materials drops suddenly at low frequencies corresponding to the first five octaves of the sound band (<1 kHz). This is why a whole section of the needs for improving acoustic comfort and noise reduction is unaffordable with this type of absorbent materials.

The very limited low frequency performance of classic solutions based on porous materials (for example wool, foam, fibers, and extruded materials) are usually replaced by absorbers with Helmholtz type resonances. Such absorption panels are made with solid materials because their acoustic properties are due to the geometric dimensions of the Helmholtz resonators. The absorption characteristic of this kind of resonator has the form of a Lorentz resonance where the quality factor and the resonance frequency are tuned with these dimensional values. The ideal for an absorbing device having such an absorption characteristic is that the reduction of the central frequency can be done without a significant increase in the quality factor so as not to narrow the absorption frequency band. In this way, it would be possible to process low frequencies over a wide band. This is unfortunately not the case for Helmholtz type acoustic resonators for which a duality between the quality factor and the resonance frequency exists. Indeed, the width of the frequency band where absorption takes place is proportional to the central frequency to the power of 4. Thus, when a resizing of the Helmholtz resonator is carried out to reduce by half the resonance frequency of the latter , its bandwidth becomes 16 times smaller.

To remedy this, we use a resonator diversification technique which consists of constructing a panel composed of several of its resonators, each having a resonance frequency close to the other and circumscribed in the targeted frequency band. The incident acoustic wave has simultaneous access to all the conduits of these resonators. This is a transverse coupling (acoustic openings on the same surface parallel to that of the panel) of acoustic resonators. It is a technique used in particular for the sound-absorbing “liner” covering of aircraft engines.

The disadvantage of these resonators is dimensional since the manufacture of such panels requires machining precision of at least 1/20 of a millimeter for conduit diameters not exceeding 3.5 mm and an aspect ratio of up to 1 :15. The high cost of such manufacturing and the reduced lifespan of these panels due to obstruction of the conduits (or rather capillaries) constitute the main obstacle to the use of this technique.

We know another type of absorber panels with resonances, which are also made up of Fabry-Perot type resonators. These resonators are in the form of conduits whose effective length corresponds to 1/4 of the wavelength for which the absorption is the highest. It is also a type of resonator whose absorption characteristic has the form of a Lorentz resonance. But, unlike Helmholtz type resonators, it is possible to adjust the resonance frequency and the quality factor independently. Indeed, the effective length of the conduit is the only parameter which fixes the resonance frequency. The quality factor can then be adjusted via the conduit section. In this way, we can target very low frequencies by manufacturing very long conduits but folded enough times to be installed in a thin panel.

In this case again, uniform sizing of the resonators of an absorbing panel hinders the broadening of the frequency band. The resonator diversification technique is also used to broaden the absorption frequency band of these resonators.

The limitation of this method lies in the direct proportionality link that exists between the quality factor of each of the diversified resonators and the absorption rate α that they induce at their respective resonance frequency. Thus, to obtain a constant acoustic absorption over an entire frequency band, it is necessary to simultaneously optimize the air in the section of each duct to maximize its absorption rate α and also the duct length so that the resonance frequencies remain sufficiently close. This has the direct effect of increasing the number of elementary resonators necessary to guarantee maximum absorption on the targeted frequency band. As a result, the latter will have as a limit the maximum permitted thickness of the absorbent panel.

Another technique is the superposition of absorbent layers which aims in particular to combine in an additive manner the acoustic performances of the different layers. The latter are often of distinct natures. This is the case, for example, of structures which combine a layer of intrinsically absorbent material such as foams, wool and fibrous materials with another layer made up of acoustic resonators in the form of a body bottom or several cavities. . This technique is often presented as an artifice aimed at enhancing the effectiveness of the intrinsically absorbent material. We can also present things differently by stating that it is the affixing of a thin layer of intrinsic absorber (a foam, in this case) on a structure of judiciously sized resonators which improves the absorption of such composite structure. This technique is therefore an improvement on previous techniques since they consist of covering with a layer of porous material (most often a foam) a panel of Fabry-Perot or Helmholtz type acoustic resonators coupled transversely. This makes it possible to sufficiently widen the operational frequency band by smoothing out the ripples in the absorption coefficient. The latter having been altered due to a choice of sizing of the acoustic resonators which favors the extent of the absorption frequency band on the account of the frequency constancy of the absorption coefficient. This technique of mixed superposition of absorbent layers (intrinsically or not) is not possible in decontaminated and purified environments and in hostile environments (for example, extreme temperature and/or latent humidity).

In the category of acoustic absorbers using exclusively inert materials, there is a more recent technique which consists of manufacturing absorbent panels made of membrane resonators. This type of resonator consists of an air cavity obstructed by a membrane to which a massive part is glued to increase its inertia and thus target low frequencies.

This type of acoustic absorber presents significant limitations linked on the one hand to the fragility of the membranes and on the other hand to the relative narrowness of the operational frequency band. In fact, the membranes have a limited lifespan directly linked to the duration of exposure and the sound power absorbed. This is aggravated by the part stuck on it and whose mass increases with the lowering of the targeted frequencies.

There is therefore no absorbent acoustic metamaterial that can both make it possible to maintain maximum sound absorption efficiency over a significantly large bandwidth for a reduced footprint, and present robustness and a significant operational frequency bandwidth.

Summary of the invention

We propose an elementary cell of absorbent acoustic metamaterial. The elementary cell comprises a first layer comprising a first resonator. The first resonator forms a conduit comprising a first inlet opening and an outlet opening. The elementary cell further comprises a second layer comprising one or more resonators. Each resonator of the second layer forms a cavity comprising a second input opening connected to the output opening of the conduit of the first resonator so that all or part of an incident sound wave received by the first input opening can penetrate in each resonator of the second layer via this second input opening.

• le premier résonateur présente en outre un taux d'absorption non nul étendu sur une bande fréquentielle Δf qui est centrée autour d'une fréquence caractéristique f ₀ pour laquelle le coefficient d'absorption α₀(f=f ₀) est maximal sur cette bande fréquentielle Δf, la valeur maximale étant, par exemple, inférieur à 30% ;
• chacun des uns ou plusieurs résonateurs de la deuxième couche présente en outre un taux d'absorption non nul étendu respectivement sur une bande fréquentielle Δf _k, plus étroite que la bande fréquentielle Δf, qui est centrée respectivement autour d'une fréquence caractéristique f _k pour laquelle le coefficient d'absorption α_k(f=f _k) est maximal sur cette bande fréquentielle Δf _k, la valeur maximale étant, par exemple, inférieur ou égal à 30% ;
• chacun des uns ou plusieurs résonateurs de la deuxième couche est dimensionné de manière à positionner sa fréquence caractéristique respective f _k sur un intervalle de fréquence Δf±δ où Δf est la bande fréquentielle du premier résonateur et δ représente une différence de fréquence minime ;
• le conduit de la première couche et/ou la cavité de la deuxième couche comprend au moins un conduit du type sélectionné parmi :
  • un tube ;
  • une chambre ;

• de préférence le au moins un conduit du type tube comprend un ou plusieurs coudes ;
  • le conduit de la première couche et/ou la cavité de la deuxième couche comprend au moins deux conduits montés en cascade, chaque conduit étant du type sélectionné parmi :
    • un tube ;
    • une chambre ;
• de préférence au moins un des deux conduits est du type tube comprenant un ou plusieurs coudes ;
  • au moins un des deux conduits est du type tube se prolongeant à l'intérieur d'un conduit du type tube ou chambre qui le précède ou qui lui succède ;
  • tout ou une partie du conduit de la première couche a sensiblement la forme d'un prisme, de préférence le prisme étant un parallélépipède, un cube, un cylindre, un cône ou un prisme hexagonal ;
  • tout ou une partie de la cavité de la première couche a sensiblement la forme symétriques ou régulière d'un cuboïde, un cylindre, un cône ou un prisme hexagonal, un parallélépipède, un cube ou un cylindre ;
  • la deuxième ouverture d'entrée d'au moins une des cavités de la deuxième couche comprend en outre une extension qui s'étend dans le conduit de la première couche, de préférence l'extension étant droite ou repliée dans le conduit de la première couche ;
  • le matériau de la première couche et/ou le matériau de la deuxième couche est du type sélectionné parmi :
    • un matériau rigide ;
    • un matériau mou ;
    • un matériau plein, de préférence sélectionné parmi un métal, un alliage, une céramique, une terre cuite, l'argile, le béton, une plaque de pierre naturelle ou artificielle, le plastique, le verre et un textile aggloméré ;
    • un matériau structuré de préférence sélectionné parmi les essences naturelles de bois et les bois dérivés ;
    • un matériau expansé, de préférence sélectionné parmi une pierre, une argile, un liège une matière plastique, un caoutchouc naturel, un caoutchouc synthétique et une résine ;
    • un matériau extrudé de préférence sélectionné parmi un métal un alliage et un polymère ;
    • un matériau malléable, de préférence sélectionné parmi un métal, un plastique, un bois naturel, un caoutchouc naturel, un caoutchouc synthétique, du papier, du carton ;
    • un matériau sensiblement non-absorbant acoustiquement ; et/ou
    • un matériau inerte ; et/ou
  • la première couche est en contact contre la deuxième couche.

The elementary cell may include one or more of the following characteristics:

• the first resonator also has a non-zero absorption rate extended over a frequency band Δ f which is centered around a characteristic frequency f ₀ for which the absorption coefficient α ₀ ( f = f ₀ ) is maximum on this frequency band Δ f , the maximum value being, for example, less than 30%;
• each of one or more resonators of the second layer also has a non-zero absorption rate extended respectively over a frequency band Δ f _k , narrower than the frequency band Δ f , which is centered respectively around a characteristic frequency f _k for which the absorption coefficient α _k ( f = f _k ) is maximum on this frequency band Δ f _k , the maximum value being, for example, less than or equal to 30%;
• each of one or more resonators of the second layer is dimensioned so as to position its respective characteristic frequency f _k on a frequency interval Δ f ±δ where Δ f is the frequency band of the first resonator and δ represents a minimal frequency difference;
• the conduit of the first layer and/or the cavity of the second layer comprises at least one conduit of the type selected from:
  • a tube ;
  • room ;

• preferably the at least one tube-type conduit comprises one or more elbows;
  • the conduit of the first layer and/or the cavity of the second layer comprises at least two conduits mounted in cascade, each conduit being of the type selected from:
    • a tube ;
    • room ;
• preferably at least one of the two conduits is of the tube type comprising one or several elbows;
  • at least one of the two conduits is of the tube type extending inside a tube or chamber type conduit which precedes or succeeds it;
  • all or part of the conduit of the first layer has substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cylinder, a cone or a hexagonal prism;
  • all or part of the cavity of the first layer has substantially the symmetrical or regular shape of a cuboid, a cylinder, a cone or a hexagonal prism, a parallelepiped, a cube or a cylinder;
  • the second inlet opening of at least one of the cavities of the second layer further comprises an extension which extends into the conduit of the first layer, preferably the extension being straight or folded into the conduit of the first layer ;
  • the material of the first layer and/or the material of the second layer is of the type selected from:
    • a rigid material;
    • a soft material;
    • a solid material, preferably selected from a metal, an alloy, a ceramic, a terracotta, clay, concrete, a natural or artificial stone slab, plastic, glass and an agglomerated textile;
    • a structured material preferably selected from natural wood species and derived woods;
    • an expanded material, preferably selected from a stone, a clay, a cork, a plastic, a natural rubber, a synthetic rubber and a resin;
    • an extruded material preferably selected from a metal, an alloy and a polymer;
    • a malleable material, preferably selected from a metal, a plastic, a natural wood, a natural rubber, a synthetic rubber, paper, cardboard;
    • a substantially non-acoustically absorbent material; and or
    • an inert material; and or
  • the first layer is in contact against the second layer.

We also propose an acoustic panel comprising at least one elementary cell.

The acoustic panel can be flat or curved.

We also propose an acoustic plate comprising at least one acoustic panel, the plate being able to be fixed on a surface to be soundproofed, preferably a wall and/or a ceiling, preferably the plate is flat or curved.

Brief description of the designs

• FIG. 1 est un exemple, selon un plan [yz], d'une représentation générique de la constitution structurelle d'une cellule élémentaire selon l'invention ;
• FIG. 2 est un exemple d'une coupe de la deuxième couche (couche profonde) de l'exemple de FIG. 1 selon un plan [xz] opérée suivant l'axe β_k ;
• FIG. 3 est un exemple d'une coupe de la deuxième couche (couche profonde) de l'exemple de FIG. 1 selon un plan [xz] opérée suivant l'axe β_k ;
• FIG. 4 est un exemple d'une coupe de la première couche (couche superficielle) de l'exemple de FIG. 1 selon un plan [xz] opérée suivant l'axe γ ;
• FIG. 5 est un exemple d'une coupe de la première couche (couche superficielle) de l'exemple de FIG. 1 selon un plan [xz] opérée suivant l'axe γ ;
• FIG. 6 est un exemple d'une coupe de la première couche (couche superficielle) de l'exemple de FIG. 1 selon un plan [xz] opérée suivant l'axe γ ;
• FIG. 7 est un exemple d'une projection selon un plan [xz] d'un exemple d'une cellule élémentaire selon l'invention ;
• FIG. 8 est un exemple d'une vue de l'exemple de la FIG. 7 ;
• FIG. 9 est un exemple d'une vue d'un exemple d'un panneau acoustique comprenant des cellules élémentaires de la FIG. 7 ;
• FIG. 10 est un exemple de coupes du panneau acoustique de la FIG. 9 ;
• FIG. 11 est un graphe avec des exemples des valeurs calculées du coefficient d'absorption
• α de la première couche d'une cellule élémentaire (courbe 1102), de la deuxième couche d'une cellule élémentaire (courbe 1101), d'une cellule élémentaire (courbe 1103) et des valeurs mesurées d'une plaque comportant plus d'un panneau absorbant (courbe 1105) en fonction de la fréquence d'une onde sonore incidente, mesuré en Hz ;
• FIG. 12 est un exemple d'une projection selon un plan [xz] d'une cellule élémentaire selon l'invention ;
• FIG. 13 est un exemple d'une vue de l'exemple de la FIG. 12 ;
• FIG. 14 est un exemple d'une vue en perspective cavalière d'un panneau acoustique comprenant des cellules élémentaires de la FIG. 12 ;
• FIG. 15 est un exemple de coupes du panneau acoustique de la FIG. 14 ;
• FIG. 16 est un graphe avec des exemples des valeurs calculées du coefficient d'absorption
• α de la première couche d'une cellule élémentaire (courbe 1602), de la deuxième couche d'une cellule élémentaire (courbe 1601), d'une cellule élémentaire (courbe 1603) et des valeurs mesurées d'une plaque comportant plus d'un panneau absorbant (courbe 1605) en fonction de la fréquence d'une onde sonore incidente, mesuré en Hz ;
• FIG. 17 est une photo d'un exemple d'une représentation générique d'une plaque comprenant au moins un panneau correspondant au panneau représenté FIG. 14.
• FIG. 18 est un exemple d'une projection selon un plan [xz] d'une cellule élémentaire selon l'invention ;
• FIG. 19A, FIG. 19B et FIG. 19C sont des exemples de la cellule élémentaire de la FIG. 18 qui montrent une vue de dessus de la cellule, une vue en perspective transparente de la cellule et une vue en perspective de l'évidement de la cellule, respectivement ;
• FIG. 20 est un graphe avec des exemples des valeurs calculées du coefficient d'absorption α de la première couche d'une cellule élémentaire (courbe 2002), de la deuxième couche d'une cellule élémentaire (courbe 2001), d'une cellule élémentaire (courbe 2003) et des valeurs mesurées d'un panneau acoustique (courbe 2005) en fonction de la fréquence d'une onde sonore incidente, mesuré en Hz ;
• FIG. 21 est un exemple d'une projection selon un plan [xz] d'une cellule élémentaire selon l'invention ;
• FIG. 22A, FIG. 22B et FIG. 22C sont des exemples de la cellule élémentaire de la FIG. 21 qui montrent une vue de dessus de la cellule, une vue en perspective transparente de la cellule et une vue en perspective de l'évidement de la cellule respectivement ;
• FIG. 23 est un graphe avec des exemples des valeurs calculées du coefficient d'absorption α de la première couche d'une cellule élémentaire (courbe 2302), de la deuxième couche d'une cellule élémentaire (courbe 2301), d'une cellule élémentaire (courbe 2303) et des valeurs mesurées d'un panneau acoustique (courbe 2305) en fonction de la fréquence d'une onde sonore incidente, mesuré en Hz.

Embodiments of the invention will now be described by means of non-limiting examples of the invention, and with reference to the figures, where:

• FIG. 1 is an example, according to a plan [ yz ], of a generic representation of the structural constitution of an elementary cell according to the invention;
• FIG. 2 is an example of a section of the second layer (deep layer) of the example of FIG. 1 according to a plane [ xz ] operated along the β _k axis;
• FIG. 3 is an example of a section of the second layer (deep layer) of the example of FIG. 1 according to a plane [ xz ] operated along the β _k axis;
• FIG. 4 is an example of a section of the first layer (surface layer) of the example of FIG. 1 according to a plane [ xz ] operated along the γ axis;
• FIG. 5 is an example of a section of the first layer (surface layer) of the example of FIG. 1 according to a plane [ xz ] operated along the γ axis;
• FIG. 6 is an example of a section of the first layer (surface layer) of the example of FIG. 1 according to a plane [ xz ] operated along the γ axis;
• FIG. 7 is an example of a projection along a plane [ xz ] of an example of an elementary cell according to the invention;
• FIG. 8 is an example of a view of the example of the FIG. 7 ;
• FIG. 9 is an example of a view of an example of an acoustic panel comprising elementary cells of the FIG. 7 ;
• FIG. 10 is an example of sections of the acoustic panel of the FIG. 9 ;
• FIG. 11 is a graph with examples of calculated absorption coefficient values
• α of the first layer of an elementary cell (curve 1102), of the second layer of an elementary cell (curve 1101), of an elementary cell (curve 1103) and of the measured values of a plate comprising more than an absorbent panel (curve 1105) as a function of the frequency of an incident sound wave, measured in Hz;
• FIG. 12 is an example of a projection along a plane [ xz ] of an elementary cell according to the invention;
• FIG. 13 is an example of a view of the example of the FIG. 12 ;
• FIG. 14 is an example of a cavalier perspective view of an acoustic panel comprising elementary cells of the FIG. 12 ;
• FIG. 15 is an example of sections of the acoustic panel of the FIG. 14 ;
• FIG. 16 is a graph with examples of calculated absorption coefficient values
• α of the first layer of an elementary cell (curve 1602), of the second layer of an elementary cell (curve 1601), of an elementary cell (curve 1603) and of the measured values of a plate comprising more than an absorbent panel (curve 1605) as a function of the frequency of an incident sound wave, measured in Hz;
• FIG. 17 is a photo of an example of a generic representation of a plate comprising at least one panel corresponding to the panel represented FIG. 14 .
• FIG. 18 is an example of a projection along a plane [ xz ] of an elementary cell according to the invention;
• FIG. 19A, FIG. 19B and FIG. 19C are examples of the elementary cell of the FIG. 18 which show a top view of the cell, a transparent perspective view of the cell and a perspective view of the cell recess, respectively;
• FIG. 20 is a graph with examples of the calculated values of the absorption coefficient α of the first layer of an elementary cell (curve 2002), of the second layer of an elementary cell (curve 2001), of an elementary cell (curve 2003) and measured values of an acoustic panel (curve 2005) as a function of the frequency of an incident sound wave, measured in Hz;
• FIG. 21 is an example of a projection along a plane [xz] of an elementary cell according to the invention;
• FIG. 22A, FIG. 22B and FIG. 22C are examples of the elementary cell of the FIG. 21 which show a top view of the cell, a transparent perspective view of the cell and a perspective view of the recess of the cell respectively;
• FIG. 23 is a graph with examples of the calculated values of the absorption coefficient α of the first layer of an elementary cell (curve 2302), of the second layer of an elementary cell (curve 2301), of an elementary cell (curve 2303) and measured values of an acoustic panel (curve 2305) as a function of the frequency of an incident sound wave, measured in Hz.

Detailed description of the invention

The present invention relates to an elementary cell of absorbent acoustic metamaterial. The elementary cell comprises a first layer comprising a first resonator. The first resonator forms a conduit comprising a first inlet opening and an outlet opening. The elementary cell further comprises a second layer comprising one or more resonators. Each resonator of the second layer forms a cavity comprising a second input opening connected to the output opening of the conduit of the first resonator so that all or part of an incident sound wave received by the first input opening can penetrate in each resonator of the second layer via this second input opening.

The present invention therefore makes it possible to meet the needs mentioned above. In particular, the invention proposes a device for improving acoustics and sound comfort in a simple and effective manner. This is achieved by an elementary cell of absorbent acoustic metamaterial comprising a first layer and a second layer which are coupled such that an incident sound wave can enter the elementary cell via the first layer and finish its course after having penetrated into the second layer via their respective conduits. Having a first layer coupled with a second layer, and thus having the resonators of the second layer cascaded with the resonator of the first layer (i.e., the resonators can be represented by impedance quadrupoles), creates a longitudinal coupling which makes it possible to benefit from the width of the frequency band of the resonator of the first layer and at the same time from the enhancement of the absorption rate provided by the resonators of the second layer. This corresponds to a acoustic impedance adaptation whose effectiveness is due to the fact that the first layer behaves like an acoustic resonator with a low quality coefficient with a high value of acoustic inertance, while the second layer reacts as multiple resonators with a high value of compliance which are distributed over an expanded operational frequency band. Combined, these two elements behave as an effective acoustic resonator having a wide resonance range in which the effective impedance is very close to that of air. Consequently, the absorbent acoustic metamaterial makes it possible to obtain an optimal absorption rate α for a wide frequency band. In addition, the two layers which make up this absorbent acoustic metamaterial have, each taken separately, very limited performance requirements which thus allow a very small footprint. This results in a significant reduction in the bulk of such a low-frequency broadband acoustic absorption device.

An “ elementary cell ” designates a unitary element which can be positioned with other unitary elements, to form, for example, an acoustic panel, or possibly, an acoustic plate.

An “ absorbing acoustic metamaterial ” means a metamaterial developed to manipulate and dissipate an incident sound wave in a sub-wavelength thickness.

A “ resonator ” is subsequently synonymous with the expression: acoustic resonator.

A “ duct ” refers to an acoustic channel that the incident or reflected sound wave passes through. The conduit designates an empty volume that a sound wave (also called an “acoustic wave”) can access to propagate there, by entering through an entrance opening and leaving through an exit opening, i.e. the duct has two acoustic openings. Here “empty” means a space without material forming the resonator; we therefore understand that the conduit comprises air or any gas allowing an acoustic wave to propagate. The conduit of the first layer may comprise at least one conduit of the type selected from a tube and a chamber. Preferably, the at least one tube-type conduit may comprise one or more bends in the first layer. The conduit of the first layer may comprise at least two conduits mounted in cascade (that is to say, in successive stacks), each conduit being of the type selected from a tube, a chamber. Preferably, at least one of the two conduits may be of the tube type comprising one or more bends in the first layer. Additionally or alternatively, at least one of the two conduits may be of the tube type extending inside a tube or chamber type conduit which precedes or succeeds it. All or part of the conduit of the first layer may have substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cylinder, a cone or a hexagonal prism.

An “ input aperture ” means an acoustic aperture that allows an incident sound wave to enter a resonator.

An “ exit opening ” means an acoustic opening that allows a wave incident sound coming out of a resonator. Here an “acoustic opening” designates an access allowing an incident sound wave to enter a resonator or a reflected sound wave to exit the resonator. The surface S _{[ AB ]} ( λ ) of an acoustic opening designates the area of the section of the duct or cavity to which this opening provides access. This area can change and evolve along the conduit or cavity going from point A to point B, points A and B being arbitrary points along the length of the conduit or cavity. The variable λ represents a position between A and B, along the conduit or the cavity; this is the position along the path of a propagating sound wave (e.g. an incident sound wave). The shape and dimensions of the section of the conduit or the cavity can thus vary between the inlet opening of the conduit (or the cavity) and the outlet opening of the conduit (or the bottom of the cavity). The effective length Λ _{[ ABL ]} designates the length perceived by the sound wave when propagating from point A to point B along the path of length L. This length differs from L due to the thermo viscous effects of air and of the variation of the section area along the acoustic path.

A “ cavity ” designates an empty volume to which the wave has access to propagate and reflect when leaving through the same access. It is therefore a blocked duct. The cavity can consist of a succession of tubes and chambers with a single acoustic opening, that is to say a single entrance opening. Here “empty” means a space without material forming the resonator; we therefore understand that the cavity includes air or any gas allowing an acoustic wave to propagate. The cavity can have various symmetrical or regular shapes such as for example a cuboid, a cylinder, a cone, a prism, preferably the prism being a parallelepiped or a hexagonal prism, a cube or a cylinder. At least one cavity of the second layer may comprise at least one conduit of the type selected from a tube and a chamber. Preferably, the at least one tube-type conduit may comprise one or more bends in the second layer. As for the conduit, at least one cavity of the second layer may comprise at least two conduits mounted in cascade of the type selected from a tube, a chamber. Preferably, at least one of the two conduits may be of the tube type comprising one or more bends in the second layer, and more preferably at least one of the two conduits may be of the tube type extending inside a conduit of the tube or chamber type which precedes or succeeds it.

The volume defined by the cavity can be an effective volume equal to V _{[ ABL ]} which differs from the purely geometric calculation according to the product of form S _{[ AB ]} ( λ ) ×Λ _{[ ABL ]} . Likewise, the relationship between the sound wavelength considered and the dimensions of the cavity or conduit can be biased; all this in connection with the thermo viscous properties of the air or fluid considered.

A “ chamber ” designates a conduit with a high global aspect ratio S _{[ AB ]} ( λ )/Λ _{[ ABL ]} (the term “global” designating a calculation carried out by integration over the entire domain of definition of lambda, or from A to B). That is to say, a room is a conduit comprising a conduit section larger, and even considerably larger, than the length perceived by the sound wave propagating from point A to point B along the path of length L.

A “ tube ” designates a conduit with a low shape ratio S _{[ AB ]} ( λ )/Λ _{[ ABL ]} . That is to say, a tube is a conduit comprising a section of conduit equal to or smaller than, and even considerably smaller than, the length perceived by the sound wave in propagating from point A to point B along the path of length L.

A “ layer ” refers to a structure composed of at least one acoustic resonator to form an acoustic metamaterial. The acoustic resonator(s) composing the first and second layer may be of the same nature or different (for example, Helmholtz, Quarter-wave, or a conduit or cavity comprising at least two conduits mounted in cascade). A “ first layer ” designates the layer through which the sound wave enters the elementary cell. A “ second layer” designates the layer in which the sound wave ends its course after passing through the first layer.

There FIG. 1 shows an example of a section, along a plane [ yz ], of an elementary cell 109 of absorbent acoustic metamaterial comprising a first layer 101 (also called the surface layer) and a second layer 102 (also called the deep layer). The first layer 101 is the one which first receives the incident wave to be attenuated which enters the metamaterial through the first entrance opening 112. The first layer 101 comprises a first resonator 103. The first resonator 103 forms a conduit comprising a first inlet opening 112 and an outlet opening 124a, 124b, 124c, 124d, 124e. The first input opening 112 gives the incident sound wave access to this first resonator 103 and therefore to this first layer 101. The first input opening 112 can be flat on the surface of the first resonator (i.e. that is, without an outward extension of the resonator 103) to reduce the thickness of the absorbent metamaterial. The first resonator 103 forms a conduit which, in this example, comprises a composition of a tube 111 followed by a chamber 116, the first inlet opening 112 being able to receive an incident sound wave coming from the outside. This is an example where the conduit formed by the resonator 103 of the surface layer 101 is a succession of conduits (that is to say, conduits mounted in cascade). Such a succession of conduits can have a section shape which evolves between the tube 111 and the chamber 116. Indeed, the tube 111 of length " l " can access the chamber 116 with an extension in the chamber 116 of a length of “ ll _e ”, l _e being the thickness of the wall 128 separating the resonator 103 from the first layer 101 and the medium producing the acoustic wave to be absorbed. The first inlet opening 112 of the conduit can be substantially coincident with the exterior surface of the first layer 101. The tube 111 can open (or “extend”) into the chamber 116 of the conduit of the first layer 101. The tube 111 can be straight or folded in the first layer 101. That is to say, the conduit can include one or more bends (or is “folded”) in the first layer 101. The tube 111 can be folded into the chamber 116. The tube 111 can be straight in the chamber 116. The tube 111 can be confused (that is to say, the tube 111 can join the chamber in a regular way instead of opening into the chamber 116) with the chamber 116 so that the entire conduit is folded into the first layer 101.

The second layer 102 of the FIG. 1 comprises, in this example, five acoustic resonators 104a, 104b, 104c, 104d, 104e. Each resonator 104a, 104b, 104c, 104d, 104e of the second layer 102 forms a cavity. Each cavity comprises a second inlet opening 115a, 115b, 115c, 115d, 115e connected to the outlet opening 124a, 124b, 124c, 124d, 124e of the conduit of the first resonator 103 so that all or part of a wave incident sound received by the first input opening 112 can penetrate into each resonator 104a, 104b, 104c, 104d, 104e of the second layer via this second input opening 115a, 115b, 115c, 115d, 115e. Each cavity may include a conduit in the form of a tube 118a, 118b, 118c, 118d, 118e. Additionally or alternatively, the cavity may include a conduit in the form of a chamber. Additionally or alternatively, the cavity may comprise at least one conduit of the type selected from a tube, a chamber. It will be understood that the number of acoustic resonators of the second layer 102 can vary depending on the examples.

Still in the example of FIG. 1 , the first layer 101 is placed against the second layer 102, that is to say that the first layer 101 is in contact against the second layer 102. In examples, one or more layers of materials can be interposed between the first and the second layer 101, 102; in this case the first layer 101 is no longer in (direct) contact against the second layer 102. It will be understood that the first and/or the second layer may comprise one or more sub-layers. In other words, the first and/or the second layers may very well be made up of several strata; for example, due to the shape of the conduits which change over several layers but which are nevertheless in the same layer. It will also be understood that whatever the constitution of the first and second layers, the first layer presents a strong inertance and the second layer presents a strong compliance. Each resonator of the second layer 102 comprises a second input opening 115a, 115b, 115c, 115d, 115e opening into the chamber 116 of the first resonator 103 of the first layer 101. The second input opening 115a, 115b, 115c, 115d, 115e allows the propagation of an acoustic wave. Thus, the incident sound wave received by the first resonator 103 is propagated towards all of the resonators 104a, 104b, 104c, 104d, 104e of the second layer. Likewise, the sound wave reflected by the bottom of the cavities of the second layer 102 can be propagated through the first resonator 103 to the first input opening 112 of the elementary cell 109. In a manner similar to that which has been described for the resonator 103 of the first layer 101 forming a conduit, at least one among the one or more cavities formed by one or more resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 may comprise a succession of conduits of different nested shapes (i.e., “stacked” or “cascaded”) in a continuous or discontinuous manner. Two differences nevertheless remain in this similarity: (i) the resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 correspond to cavities and are therefore obstructed and (ii) as shown in the FIG. 1 , each cavity formed by each resonator 104a, 104b, 104c, 104d, 104e can have a second unique inlet opening 115a, 115b, 115c, 115d, 115e (and a single acoustic opening) which can open out one like the other in the conduit of the first layer 101 with an extension 117a, 117b, 117c, 117d, 117e of lengths “ p _k=1..n ”. Indeed, the second inlet opening 115a, 115b, 115c, 115d, 115e of at least one of the cavities of the second layer 102 may comprise an extension 117a, 117b, 117c, 117d, 117e which extends into the conduit of the first layer 101. Additionally, the extension 117a, 117b, 117c, 117d, 117e can be straight or folded (or may include “elbows”) in the conduit of the first layer 101. Such an extension 117a, 117b, 117c, 117d, 117e can be chosen for reasons of dimensional optimization and acoustic performance. Additionally or alternatively, at least one cavity of the second layer 102 may comprise at least two conduits mounted in cascade of the type selected from a tube, a chamber. At least one of the two conduits may be of the tube type, comprising one or more bends in the second layer 102. At least one of the two conduits may be of the tube type which extends inside a tube or chamber type conduit who precedes or succeeds him. At least one of the conduits can be folded over all or part of the resonator 103 of the first layer 101.

The second inlet opening 115a, 115b, 115c, 115d, 115e of each resonator 104a, 104b, 104c, 104d, 104e of the second layer 102 connected to the outlet opening 124a, 124b, 124c, 124d, 124e of the conduit of the first resonator 103 allows each resonator of the second layer 102 to comprise a cavity 118a, 118b, 118c, 118d, 118e connected to the outside (outside the cell) via the resonator 103. The acoustic openings 115a, 115b, 115c , 115d, 115e of the resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 are therefore an access to the second layer 102 for an acoustic wave which has passed through the first layer 101. In this way, we can combine the reactive effect of the resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 to the inertial effect of the acoustic resonator 103 of the first layer 101. All or part of the conduit of the first layer 101 can have substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cylinder, a cone or a hexagonal prism. For example, the chamber 116 may have substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cylinder or a hexagonal prism. The shape of all or part of the conduit of the first layer 101 can be chosen to occupy the maximum of the volume allocated to it on the elementary cell 109. The shape has intrinsically only a limited impact for the processing of frequencies low (e.g. <1KHz). For example, if the elementary cell 109 has a hexagonal shape, a hexagonal cavity can make it possible to take advantage of the largest possible space of the elementary cell 109. In this case, the second layer 102 could include six resonators and have a triangular shape. The shape of one or more conduits of each cavity of the second layer 102 can also be chosen as a function of the targeted frequency for each resonator 104a, 104b, 104c, 104d, 104e forming each cavity. Alternatively, instead of being limited to the frequency band targeted for the entire elementary cell 109, these individual resonance frequencies can have higher or lower frequency values.

In examples, one or more of the cavities of the second layer 102 may have at least one conduit which has, for example, without being limited to, substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cone, cylinder or hexagonal prism. The shape of said one or more conduits of the cavities of the second layer 102 can be chosen to occupy the maximum of the volume of the elementary cell 109, in particular of the second layer 102. This can be done thanks to the bends (or "folding") or a succession of duct bends of different shapes.

Whatever its shape, the conduit of the first layer 101 (that is to say the conduit formed by the first resonator of the first layer 101) gives access to n resonators 104a, 104b, 104c, 104d, 104e which are located in the second layer 102. The second layer 102 can have a thickness d and a characteristic surface S _{[ wz ]} . These resonators 104a, 104b, 104c, 104d, 104e the second layer 102 form cavities, each having the shape of an obstructed conduit. Each resonator n° k (with k = 1.. n ) can have an effective length l _k and an acoustic opening with a surface area equal to S _k ( λ ). The variation of the geometry of each resonator of the second layer 102, from one end to the other can bring the shape, for example, of a resonator 104a, 104b, 104c, 104d, 104e closer to that of a resonator of Helmholtz type or a Fabry-Perot type resonator. Indeed, the notation S _k ( λ ), designates a section of conduit which is a function of the position λ >0 along the conduit to the obstructed bottom ( λ = l _k ).

As already said above, the conduit of the first layer 101 may comprise at least two conduits mounted in cascade, each conduit being of the type selected from a tube, a chamber. The entrance opening 112 gives the incident sound wave access to the first layer 101 and the tube-like conduit 111 connects the entrance opening 112 to the chamber-type conduit 116. As shown in the FIG. 1 , at least one of the two tube-type conduits 111 may be straight in the first layer 101. Additionally or alternatively, at least one of the two conduits may be of the tube type 111 comprising one or more bends in the first layer 101. Additionally or alternatively , one of the conduits can simply take the form of a tube 111 of length equal to the thickness of the external wall of the cell 109 thus giving direct access to the conduit which succeeds it, for example to the chamber 116. Additionally or alternatively , at least one of the two conduits can be of the tube 111 type which extends inside a conduit of the tube 111 or chamber 116 type which precedes it or which succeeds.

In the example of the FIG. 1 , the tube 111 extends inside the chamber 116. With the chamber 116, they can represent the first resonator 103 of the first layer 101, in the form of a Helmholtz resonator with a pierced bottom. The central frequency and the bandwidth of such a resonator are linked, among other things, to the ratio length relative to the width of the tube-type conduit 111 and to the volume of the chamber-type conduit 116. Alternatively, the conduit of the first layer 101 can comprise a single conduit of tube type 111. The tube 111 can be dimensioned to constitute only the beginning of a long tube which is given the shape of a folded tube as is illustrated in the example represented by the FIG. 6 . The resonator of the first layer 101 can be dimensioned differently so as to have exactly the same acoustic response with a major difference in the filling ratio of the facade of the cell 109. In fact, the choice of a resonator 103 under the shape of a conduit comprising a small tube type conduit 111 (also called "channel") giving access to a bulky chamber type conduit 116 makes it possible to reduce the ratio between the surface of the inlet opening of the conduit and the surface total facade. The conduit-shaped resonator comprising at least one tube-type conduit folded into layer 101 may have a larger acoustic opening which may give rise to an aesthetic advantage. This last case of sizing the resonator of the first layer has the advantage of being less demanding from the point of view of machining precision. Indeed, the central frequency and the bandwidth can be very sensitive to the ratio of acoustic opening to channel length in the case where the conduit comprises a tube-type conduit 111 giving access to a large chamber 116. In the case of a long folded tube, the central frequency is linked to the effective length of the tube while the bandwidth is linked to the acoustic aperture. These two dimensions can have a high tolerance of error or machining precision.

An elementary cell according to the invention, for example as represented in the example of the FIG.1 , allows for increased performances which go well beyond the simple accumulation of the absorption levels of each of the two layers of acoustic resonators. Indeed, the first layer 101 and the second layer 102 are arranged in such a way that the resonators 104a, 104b, 104c, 104d, 104e of the second layer can receive the incident sound wave after it has passed through the resonator 103 of the first layer 101. Having several resonators 104a, 104b, 104c, 104d, 104e in the second layer constitutes a transverse coupling in the second layer. This second layer, taken alone, has very low absorption as indicated by 1101 in FIG. 11 . The resonator 103 of the first layer, also taken alone, has very low acoustic absorption as indicated by 1102 in FIG. 11 . The first layer 101 and the second layer 102 are arranged in such a way that longitudinal coupling or cascade assembly can be carried out between the resonator 103 of the first layer and the resonators of the second layer 104a, 104b, 104c, 104d, 104e, which produces extensive sound absorption as demonstrated by 1103 in FIG. 11 . The effects of this coupling result in the lengthening of the frequency band where absorption is maintained at a maximum level. To do this, optimal and unique sizing is preferred. A specific dimensioning of the elementary cell 109 can first be carried out to target the central frequency and the desired bandwidth, while respecting certain geometric constraints (for example, the thickness of the layers 101, 102, the characteristic surface of the cell 109, as well as various criteria concerning, in particular, the ratio of acoustic opening to characteristic surface). The resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 can be dimensioned so as to adjust the absorption coefficient over the entire targeted frequency width. The first resonator 103 is an acoustic resonator which may present an impedance with high inertia while one or more resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 may be acoustic resonators with an acoustic impedance which presents an strong compliance.

Thus, by obtaining a longitudinal coupling thanks to the two layers 101, 102, the resonators 103, 104a, 104b, 104c, 104d, 104e of the two layers 101, 102 form an inertial part and a compliant part which neutralize each other to present a close impedance of that of the air (or gas used as a replacement) over the frequency bandwidth targeted by the unit cell 109. Thanks to this design, each of the two layers 101, 102 does not need to be individually sized to maximize the sound absorption coefficient. Consequently, a very significant reduction in the size of the elementary cell 109 is obtained. The first resonator can have a non-zero absorption rate extended over a frequency band Δ f which is centered around a characteristic frequency f ₀ for which the absorption coefficient α ₀ ( f = f ₀ ) is maximum on this band frequency Δ f , the maximum value being, for example, less than 30%. Each of one or more resonators of the second layer 102 can have a non-zero absorption rate extended respectively over a frequency band Δ f _k , narrower than the frequency band Δ f, which is centered respectively around a characteristic frequency f _k for which the absorption coefficient α _k ( f = f _k ) is maximum on this frequency band Δ f _k , the maximum value being, for example, less than or equal to 30%. Each of one or more resonators of the second layer 102 can be dimensioned so as to position its respective characteristic frequency f _k on a frequency interval Δ f ± δ where Δ f is the frequency band of the first resonator 103 and δ represents a difference of minimal frequency. That is to say, δ is a tolerance on the frequency band Δ f , f _k being the respective characteristic frequency on an interval approximate to that of Δ f .

As mentioned previously, these two layers of resonators 101, 102 make it possible to obtain longitudinal coupling to allow optimal impedance matching giving rise to a maximum absorption level α ranging, by example, between 90% and 100%, or between 90% and 99%, for an equivalent attenuation between, for example, -10 dB and -20 dB. One or more resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 can be positioned to include a frequency bandwidth Δ f _k and all of the frequency bandwidths Δ f _k can be selected so that they cover substantially the frequency bandwidth Δ f of the cell with a tolerance of ± δ. Examples of the values of the absorption coefficients α ₁ and α _k are shown in the FIG. 11 And FIG. 16 .

In the FIG. 11 , the curves 1101, 1102, 1103, 1104 correspond to theoretical results of numerical calculation for a single elementary cell for a normal incidence while the curve 1105 corresponds to the measures of reduction of the reverberation time in real situation for a plate of a surface area of 10.8 m ² , (i.e., an assembly comprising at least one panel) made up of 30 panels (see FIG. 9 , which represents a panel 700 of 10 elementary cells 709) of side 600mm × 600mm × 54mm. Curve 1104 is the result of an integration in thirds of octaves of the response 1103 of a single elementary cell. The absorption curves 1101 and 1102 correspond, respectively, to each of the resonators of the second layer and the resonator of the first layer, all measured independently. Curves 1103, 1104 show the result of the longitudinal coupling obtained by the two layers.

In the FIG. 16 , the curves 1601, 1602, 1603, 1604 correspond to theoretical results of numerical calculation for a single elementary cell for a normal incidence while the curve 1605 corresponds to the measures of reduction of the reverberation time in real situation for a plate of a surface area of 10.8 m ² (see Fig. 17 which shows a plate which was used for measurements in a reverberation chamber at the CSTB -Centre Scientifique et Technique du Bâtiment-) made up of 30 panels (see for example, the panel of the FIG. 14 ) of 600mm × 600mm × 111mm side. Curve 1604 is the result of an integration in thirds of octaves of the response 1603 of a single elementary cell. The absorption curves 1601 and 1602 correspond, respectively, to each of the resonators of the second layer and the resonator of the first layer, all measured independently. Curves 1603, 1604 show the result of the longitudinal coupling obtained by the two layers.

In the FIG. 20 , the curves 2001, 2002, 2003, 2004 correspond to theoretical results of numerical calculation for a single elementary cell for a normal incidence. The 2005 curve corresponds to measurements in an impedance tube with an internal cross-section of 7 cm × 7 cm (also called Kundt tube) for a panel consisting of 2 × 3 elementary cells (3 × 23 mm and 2 × 34.5 mm to make a panel with dimensions of 6.9 cm × 6.9 cm). The 2004 curve is the result of an integration in thirds of octaves of the theoretical response 2003 of a single unit cell. The 2005 curve comes from the integration in thirds of octaves of the measurement results of a panel comprising 6 unit cells. The absorption curves 2001 and 2002 correspond, respectively, to each of the resonators of the second layer 102 and to the resonator of the first layer, all calculated independently. The curves 2003, 2004 show the result of the longitudinal coupling obtained by the two layers.

In the FIG. 23 , the curves 2301, 2302, 2303, 2304 correspond to theoretical results of numerical calculation for a single elementary cell for a normal incidence. Curve 2305 corresponds to measurements in an impedance tube (also called Kundt tube) for a panel made up of elementary cells. Curve 2304 is the result of an integration in thirds of octaves of the theoretical response 2303 of a single unit cell. Curve 2305 comes from the integration in thirds of octaves of the measurement results of a panel of 6 unit cells. The absorption curves 2301 and 2302 correspond, respectively, to each of the resonators of the second layer and the resonator of the first layer, all calculated independently. Curves 2303, 2304 show the result of the longitudinal coupling obtained by the two layers.

In FIGS. 11, 16, 20 and 23, we see the curves 1102, 1602, 2002 and 2302 for the theoretical absorption coefficient under normal incidence in the first layer. We also see curves 1101, 1601, 2001 and 2301 for the theoretical absorption coefficient under normal incidence in the second layer. Curves 1103, 1603, 2003 and 2303 show the theoretical performance under normal incidence of the accumulation of the performances of the other curves 1101, 1102, 1601, 1602, 2001, 2002, 2301 and 2302. Each resonator of the two layers does not exceed in These examples have an absorption rate of 30%, which corresponds to an amplitude reflection index of more than -1.5 dB. Still in these examples, the configuration of the elementary cell generates an impedance adaptation giving rise to a maximum absorption level ranging from α≃90% to α≃99% (a variation corresponding to the range of amplitude change of the “ripples” along the passband) for equivalent attenuation between -10 dB and -20 dB, respectively.

The effectiveness of such impedance matching is due to the fact that the first layer can behave like an acoustic resonator with a high acoustic inertance value and a low quality coefficient, while the second layer can react as a resonator. broadband composite with high compliance value. Combined, these two elements can behave as an effective acoustic resonator having a wide resonance range in which the effective impedance is very close to that of air.

The elementary cell thus makes it possible to widen the width of the frequency band where the minimum absorption rate targeted is guaranteed by circumventing the obstacles which are at the origin of the deficiencies of an absorber solution based on transverse coupling. Indeed, with the longitudinal coupling obtained by the first layer and the second layer, and as shown by the FIG. 11 , FIG. 16 , FIG. 20 And FIG. 23 , the level of absorption can be improved considerably. Furthermore, the combination of the two layers makes it possible to link, in an exclusive manner, the bandwidth of the elementary cell to that of the resonator of the first layer.

The two traditionally limiting design points, which are the quality coefficient and the absorption level at resonance, are used to advantage in the present invention. Indeed, considering for example the example of the FIG. 1 , the first layer 101 which directly affects the width of the frequency band can be designed with a first resonator 103 having a very wide acoustic opening 111 without being worried by the low absorption levels which result therefrom and in addition, the deep layer 102 which serves to raise these levels is composed of a series of resonators 104a, 104b, 104c, 104d, 104e with very small acoustic openings which reduces its bulk.

The elementary cell according to the invention makes it possible to overcome the limiting duality which links the bandwidth and the level of absorption of known resonators coupled transversely. Considering, for example, the example of FIG. 1 , the longitudinal coupling obtained according to the invention (that is to say that the resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 are mounted in cascade (one could also say in series) with the resonator 103 of the first layer 101) makes it possible to obtain a coupling in the propagation axis of the incident wave: the resonators 104a, 104b, 104c, 104d, 104e of the second layer 102 are mounted in cascade with the resonator 103 of the first layer 101, and an incident wave first enters the first resonator and then enters the second resonators mounted in cascade. This makes it possible to construct an elementary cell 109 which behaves like a global acoustic resonator, where the effective inertance is mainly imposed by the dimensions of the resonators of the first layer 101 and where the effective compliance is modeled by the resonators of the second deep layer 102 which face the resonator of the first layer 101.

The same acoustic absorption signature such as an elementary cell according to the invention can be obtained with one or more dimensional choices. As shown in the FIG.s 18 And 19A, 19B, 19C and the FIG.s 21 And 22A, 22B and 22C , the elementary cell 1809 has a different construction from that of the elementary cell 2109 since the resonator 1803 of the first layer 1801 in the cell 1809 of the FIG. 18 And 19A, 19B and 19C has the shape of a Fabry-Perot resonator while the resonator 2103 of the first layer 2101 in the cell 2109 of the FIG. 21 and FIGS. 22A, 22B and 22C has the shape of a Helmholtz resonator with a drilled bottom. Despite this difference in dimensions and shapes, the acoustic absorption signature 2003 and 2004 corresponding to the elementary cell 1809 of the FIG. 18 and FIGS. 19A, 19B and 19C is substantially equivalent to the acoustic absorption signature 2303 and 2304 corresponding to the elementary cell 2109 of the FIG. 21 and FIG's. 22A, 22B and 22C.

The α absorption levels obtained can be compared to those of intrinsically absorbent materials such as wool, foam and fibrous materials. Unlike these conventional sound absorption solutions, the unit cell can enable such extended absorption efficiency at low frequencies with sub-wavelength cell thicknesses.

Considering the example of the FIG. 1 , the physical mechanisms inducing acoustic absorption in the elementary cell 109 can be the consequence of the structuring carried out in the mass of the materials constituting the elementary cell 109. That is to say, the acoustic absorption of the elementary cell 109 can be determined only as a function of its geometric configuration rather than the absorbing nature of the materials of which it consists. Any solid material (rigid or soft) can be valid for achieving the solution described in this invention.
The material of the first layer 101 and/or the material of the second layer 102 may be of the type selected from a rigid material, a soft material, a solid material, a structured material, an expanded material, an extruded material, a malleable material , a substantially non-sound or acoustic absorbing material and/or an inert material.

A “ rigid ” material means a material that is not easily compressed, folded and/or manipulated during the operation of unit cell 109. In examples, such material may include, but is not limited to, wood, metals and/or glass. In examples, the entire unit cell 109 may comprise one or more rigid materials. Several rigid materials can be joined without glue (for example, hot, under pressure and/or under stress).

A “ soft ” material means a material that compresses, bends and/or is easily manipulated. A soft material can have properties between those of liquids and solids. It can, for example, have a Young's modulus of elasticity between 0.001 GPa and 0.1 GPa. Examples of such a material may include, without limitation, a polymer. Several soft materials can be joined without glue (for example, hot and/or under pressure).

A “ solid ” material means a material that has no voids or has a low porosity value of less than 20%. In examples, such material may include, without limitation, a metal, an alloy, a ceramic, a terracotta, clay, concrete, a natural or artificial stone slab, plastic, glass and/or a agglomerated textile.

A “ structured ” material designates a material comprising a constituent pattern. In examples, such material may include, but is not limited to, natural wood species and derived woods (for example, MDF (" medium density fireboard ") , and/or chipboard).

An “ expanded ” material designates a material which has undergone a physical and/or thermal and/or chemical operation to increase its volume. In examples, such material may include, but is not limited to, stone, clay, cork, plastic, natural rubber, synthetic rubber and/or resin.

An “ extruded ” material designates a material which has undergone a thermomechanical or mechanical operation forcing it to pass through a die or die. In examples, such material may include, but is not limited to, a metal, an alloy and/or a polymer.

A “ malleable ” material means a material that is easily influenced, or easily bendable. A malleable material capable of being hammered or pressed into shape without breaking or crack. In examples, such material may include, without limitation, metal, plastic, natural wood, natural rubber, synthetic rubber, paper and/or cardboard.

A “ substantially non-sound or acoustic absorbent ” material designates a material in which sound or acoustic dissipation is practically zero or negligible. However, such a material does not prohibit the dissipation of sound or acoustic power at the interfaces between the material and another material (air). The elementary cell is an example of this phenomenon since this cell can be constructed entirely of one or more substantially non-absorbing materials and can nevertheless allow acoustic absorption by thermo-viscous dissipation in the thin layer of air which is in contact with the walls of the construction.

An “ inert ” material means a material that is physically, chemically and/or biologically inactive. In examples, the material of the first layer and/or the material of the second layer may be of the type selected from a material with very slow structural deterioration and very low particle and dust emission.

In examples, the material of the first layer and/or the material of the second layer may be of the type selected from materials which react only very weakly or very slowly to particular conditions of use such as ambient temperature, humidity level, atmospheric pressure and gas mixture in the air.

The elementary cell according to the present invention offers sound absorption better than that of current materials at low frequencies. This, while maintaining the properties of mechanical resistance, physico-chemical integrity and health safety that foams, wools and fibrous materials are unable to guarantee.

THE FIG. 2 to FIG. 6 give examples of the first and second layer of the FIG.1 . The present invention is not limited to these examples.

There FIG. 2 shows a possible generic form among others of the resonators 204 of the second layer 202. The second inlet opening 215 of the tube-type conduit 218 which represents one of these resonators 204 can vary over its entire length. The resonator 204 of the second layer 202 may be of the quarter-wave type (that is to say a Fabry-Perot type resonator with an obstructed bottom). This, starting from the part integrated into the resonator of the first layer (only partially shown in this figure) to the part integrated into the second layer 202. The dimensional variable b _k represents the projection of the acoustic opening on the plane [ xz] of resonator number k among the n resonators of the second layer 202.

There FIG. 3 shows a variation of the shape shown in the FIG. 2 for which the resonator 304 of the second layer 302 comprises a second inlet opening 315 giving access to a tube-type conduit 318 comprising elbows which opens onto the chamber-type conduit 326 of the resonator number k . As mentioned previously, the tube 318 can be folded to increase the length of the conduit in order to to adjust the central frequency and the bandwidth of the resonator 304. With the tube 318, the chamber 326 forms a resonator 304 of the Helmholtz type.

There FIG. 4 shows a possible generic form among others of the resonator 403 of the first layer 401. This first resonator 403 forms a conduit comprising a first inlet opening 412 and an outlet opening 424. This resonator 403 can consist of a conduit tube type 411 giving access to another tube type conduit 416. The two tube type conduits 411, 416 can be folded into the first layer. Together, the two tubes 411, 416 can form a Fabry-Perot type resonator 403. The dimensions of these tubes 411, 416 are variable over all their lengths and can, for example, merge into one another in certain configurations (see FIG. 6 ). The dimensional variables b and B represent, respectively, the projection on the plane [ xz ] of the acoustic opening of the tube 411 and that of the tube 416 which succeeds it. In this case, the resonator 403 of the first layer 401 corresponds to a cascade assembly of two conduits 411, 416, both of the Fabry-Perot type.

There FIG. 5 shows another possible generic form among others of the resonator 503 of the first layer 501. In this case, this resonator 503 can be compared to a Helmholtz type resonator having a pierced bottom (opening towards the resonators of the second layer) which is characterized by a conduit comprising a cascade assembly of a constricted tube type conduit 511 called " neck of the resonator " and a chamber type conduit 516 called " belly of the resonator ". The tube 511 can open (or “extend”) into the chamber 516 of the conduit of the first layer 501. However, the access 524 (also called the exit opening) through the bottom of the belly towards the resonators of the second layer constitutes the point of differentiation with the true Helmholtz resonator which has a background normally totally obstructed. The “ neck ” 511 can be folded with an acoustic opening surface that can be variable over its entire length. The dimensional variables b and B represent, respectively, the projection on the plane [ xz ] of the “ neck ” 511 and the “ belly ” 516.

There FIG. 6 shows another possible generic form among others of the resonator of the first layer 601. In this example, this resonator 603 is of the Fabry-Perot type obtained by assembly and dimensional adjustment of the tube-type conduit 611 and the tube-type conduit 616 , the two conduits forming a folded tube type conduit. The dimensional variables b and B are the projection of the first acoustic input opening 612 on the two axes of the plane [ xz ]. The example illustrated in this figure corresponds to a resonator 603 in the form of a folded acoustic conduit. The tube-type conduit 611 and the tube-type conduit 616 of the first resonator 103 can be combined to form a single conduit whose section can change from one end of the conduit to the other by the variation of its shape, of its dimensions and its tortuosity. In this scenario, the effective length of this conduit can converge towards a quarter of the wavelength value at resonance.

An acoustic panel can consist of at least one elementary cell. The acoustic panel may comprise a single elementary cell. The panel acoustics may include several elementary cells with similar or distinct characteristics. The panel can be flat or curved.
Depending on the manufacturing method, this panel can be monolithic or the result of an assembly. The designation by the terms acoustic metamaterials or absorbent metamaterials of this invention is due to the fact that the latter can be seen as an effective panel which behaves like a much thicker and intrinsically absorbent material. The panel can be sized on the basis of geometric constraints, for example, a characteristic surface, a thickness equal to the sum of the first layer and the second layer as well as various criteria concerning, in particular, the opening ratio on the surface of sign.

There FIG. 10 represents an example of a panel 700 comprising elementary cells 709 with the same shape as the elementary cell 709 shown in section in the FIG. 7 . As shown on the FIG. 7 , the first layer 701 may comprise a Fabry-Perot style resonator 703, formed by the tube-type conduit 711 and the folded tube-type conduit 716. The second layer 702 is made up of a set 704 of several resonators 704a, 704b , 704c, 704d, 704e, which can each and individually take the form of a folded quarter-wave resonator. The material of the first layer and/or the material of the second layer may be of the type selected from a rigid material, a soft material, a solid material, a structured material, an expanded material, an extruded material, a malleable material, a substantially non-absorbent material and/or an inert material. The material used may include, for example, MDF which can be machined with a conventional wood panel furniture manufacturing process (e.g. table saw, corner trimmer, gluing and/or drilling ).

In the FIG. 10 , the segments 741, 742 come from a multiple cut made on two planes perpendicular to the characteristic surface of the acoustic panel. Segment 743 corresponds to a transparent representation of segment 741 with a different perspective which makes it possible to identify and locate the resonators 704a, 704b, 704c, 704d, 704e of the second layer 702 ( FIG. 8 ).

As shown in the FIG.9 , each of the ten elementary cells 709 has a characteristic surface area of 600 mm × 60 mm and a thickness of 54 mm . There FIG. 8 represents a cavalier perspective view of a cell 709 like that of the FIG. 7 . There FIG. 7 is an example of a projection along a plane [xz] of the elementary cell 709, comprising a resonator 703 in the form of a conduit comprising a first inlet opening 712 and an outlet opening 715. The zones of removed material 704a, 704b, 704c, 704d, 704e, 703 of the elementary cell 709 make it possible to identify five resonators 704a, 704b, 704c, 704d, 704e of the second layer 702 connected to the single resonator 703 of the first layer 701 of the elementary cell 709. The cut made 719 provides an overview of the resonators 703, 704a of the first layer 701 and the second layer 702. The FIG. 7 shows a second input opening 715 for one of the resonators 704. The acoustic opening and the length of the resonators 704 of the second layer can be different depending on the position of the cutting mark 719 along the edge 710. This is due to the presence of five different resonators 704a, 704b, 704c, 704d, 704e in the second layer 702. The dimensional variables of the elementary cell 709 of this example are indicated in the FIG. 7 along the x and z axes and in the FIG. 8 along the y axis. A plate (see example FIG. 17 ) can be manufactured by assembling 30 panels 700 totaling a surface area of, for example, 10.8 m ² . The values for the associated dimensions are recorded in the table noted Table 1.

There FIG. 15 shows an example of a panel 1200 using unit cells with the same shape as unit cell 1209 shown in FIG. 12 . There FIG. 12 is an example of a projection along a plane [xz] of the elementary cell 1209, comprising a resonator 1203 in the form of a conduit comprising a first inlet opening 1212 and an outlet opening 1215. FIG. 15 shows the segments 1241, 1242 resulting from a multiple cut carried out on two planes perpendicular to the characteristic surface of the panel 1200. The segment 1243 corresponds to a transparent representation of a segment 1241 with a different perspective which makes it possible to identify and locate the five resonators 1204 of the second layer 1202 and the resonator 1203 of the first layer 1201. There FIG. 12 shows a second input opening 1215 for one of the resonators 1204. FIG. 14 also shows ten elementary cells 1209 assembled in the form of a panel 1200. The acoustic panel is made up of 10 elementary cells, each of the cells having a characteristic surface area of 600 mm × 60 mm and a thickness of 54 mm . There FIG. 13 , gives a cavalier perspective view of an elementary cell 1209. The cut made 1219 provides an overview of the resonators 1203, 1204a of the first layer 1201 and the second layer 1202, respectively. The areas of removed material 1204a, 1204b, 1204c, 1204d, 1204e, 1203 of the elementary cell 1209 make it possible to identify five resonators of the second layer (the deep layer) connected to the single resonator 1203 of the first layer 1201 of the elementary cell 1209 according to this invention. The cut made at mark 1219 provides an overview of the resonators 1203, 1204a of the first layer 1201 and the second layer 1202, respectively. The length and acoustic opening of the resonator 1204a may be different depending on the position of the cutting mark 1219 along the edge 1210. This is due to the presence of five different resonators 1204a, 1204b, 1204c, 1204d, 1204e in the second layer 1202. The dimensional variables of such an example of a panel 1200 are indicated in the FIG. 12 along the x and z axes and FIG. 13 along the y axis. The associated values are recorded in Table 1. A plate can be manufactured by assembling 30 1200 panels totaling a surface area of, for example, 10.8 m ² . The material used may be, for example, MDF which has been machined with a conventional wood panel furniture manufacturing process (e.g. table saw, corner trimmer, gluing and/or drilling ).

There FIG. 19B , gives a cavalier perspective view through transparency of an elementary cell 1809. The zones of removed material 1804a, 1804b, 1804c, 1804d, 1804e, 1803 of the elementary cell 1809 make it possible to identify five resonators 1804a, 1804b, 1804c, 1804d, 1804e of the deep layer 1802 connected to the single resonator 1803 of the first layer 1801 of the elementary cell 1809 according to this invention ( FIG. 18 ). There FIG. 18 is an example of a projection along a plane [xz] of the elementary cell 1809, comprising a resonator 1803 in the form of a conduit comprising a first inlet opening 1812 and an outlet opening 1824. FIG. 18 also shows a second input opening 1815 for one of the resonators 1804. This elementary cell 1809 does not differ from the elementary cells 709 and 1209 represented in the FIG. 7 and the FIG. 12 , respectively, only by the dimensions of the different constituent elements. The dimensional variables of such an example of an elementary cell 1809 are indicated in the FIG. 18 along the x and z axes and FIG. 19A and 19C along the y axis. The associated values are recorded in Table 1. Such an elementary cell 1809 can be manufactured, for example, in PLA with a 3D printer. Such manufacturing can be carried out in several stages where each of the resonators of the first layer and the second layer is produced independently of a single printing phase before a final step of assembling all of the resonators.
There FIG. 22b , gives a cavalier perspective view through transparency of an elementary cell 2109. The zones of removed material 2104a, 2104b, 2104c, 2104d, 2104e, of the elementary cell 2109 make it possible to identify five resonators 2104a, 2104b, 2104c, 2104d, 2104e of the deep layer 2102 connected to the single resonator 2103 of the first layer 2101 of the elementary cell 2109 according to this invention ( FIG. 22A ). The second layer 2102 of this elementary cell 2109 of the FIG. 21 and the FIGS. 22A, 22B and 22C is identical to the second layer 1802 of the elementary cell 1809 of the FIG. 18 and the FIGS. 19A, 19B and 19C . There FIG. 21 is an example of a projection along a plane [xz] of the elementary cell 2109, comprising a resonator 2103 in the form of a conduit comprising a first inlet opening 2112 and an outlet opening 2124. FIG. 21 also shows a second input opening 2115 for one of the resonators 2104. This elementary cell 2109 of the FIG. 21 and the FIGS. 22A, 22B and 22C does not differ from the elementary cell 1809 shown FIG. 18 and the FIGS. 19A, 19B and 19C , only by the first layer 1801 and 2101. In the case of this elementary cell 2109 of the FIG. 21 and the FIGS. 22A, 22B and 22C , the resonator 2103 of the first layer 2101 has the shape of a Helmholtz resonator pierced at its bottom by access openings to the resonators 2104a, 2104b, 2104c, 2104d, 2104e of the second layer 2102. The dimensional variables of such an example of an elementary cell 2109 is indicated FIG. 21 along the x and z axes, and FIGS. 22A and 22C along the y axis. The associated values are recorded in Table 1. Such an elementary cell 2103 can be manufactured, for example, in PLA with a 3D printer. Such manufacturing can be carried out in several stages where each of the resonators of the first layer and the second layer is produced independently of a single printing phase before a final step of assembling all of the resonators.

Another object of the invention is to provide an acoustic plate comprising at least one such acoustic panel. The plate may be able to be fixed to a surface to soundproof, preferably a wall and/or a ceiling. Preferably, the plate may be flat or curved. Two exemplary prototypes of such a plate were made from 2x30 panels, totaling an area of 2x10.8 m ² . In these examples, the two plates consist of panels of the FIG. 9 and some FIG. 14 , respectively. A photograph of one of two plates formed with 30 panels, each panel being a panel of the FIG. 14 , is represented on the FIG. 17 . The manufacturing was carried out using a classic process for manufacturing wooden panel furniture (for example, wood species, OSB, medium, chipboard). The machining process required traditional tools and means for this type of manufacturing (for example, a table saw, angle grinder, gluing, drilling). The required machining precision was only ± 2 mm for elementary cells of 600 mm side. The material used for the manufacture of this full-scale prototype is MDF which was chosen for its significantly lower price than that of other woods. The storage and transport of these prototypes were carried out under the same conditions and with the same means as those of furniture or wooden DIY materials and products. [Table 1] Example #1 ( FIG.9 ) Example #2 ( FIG.14 ) Example #3 Example #4 Unit not 5 5 5 5 - W 600 600 34.5 34.5 mm w 600 600 34.5 34.5 mm D 35.5 78 34 34 mm d 18.5 33 12.5 12.5 mm Z 60 60 23 23 mm z 60 60 23 23 mm L 99 222 100 33 mm L 6 6 1 11 mm _THE 6 6 1 1 mm HAS 570 570 32.5 32.5 mm α 570 570 32.5 ∅ = 7 mm B 13 13 6 21 mm b 13 13 6 ∅ = 7 mm δ 0 0 0 11 mm l ₁ 79 176 126 126 mm a ₁ ( l ) 11 11 6 6 mm _b1 ( l ) 1.5 1.5 1 1 mm l ₂ 89 196 112 112 mm a ₂ ( l ) 111 111 6 6 mm b ₂ (l) 1.5 1.5 1 1 mm l ₃ 100 216 100 100 mm a ₃ (l) 111 111 6 6 mm b ₃ (l) 1.5 1.5 1 1 mm l ₄ 112.5 235 89 89 mm a ₄ ( l ) 111 111 6 6 mm b ₄ (l) 1.5 1.5 1 1 mm l ₅ 126 269 79 79 mm at ₅ (l) 111 111 6 6 mm b ₅ (l) 1.5 1.5 1 1 mm

Claims (14)

Elementary cell (109, 709, 1209, 1809, 2109) of absorbent acoustic metamaterial comprising: - a first layer (101, 401-701, 1201, 1801, 2101) comprising a first resonator (103, 403-703, 1203, 1803, 2103), the first resonator forming a conduit comprising a first inlet opening (112 , 412, 612, 712, 1212, 1812, 2112) and an outlet opening (124a, 124b, 124c, 124d, 124e, 724, 1224, 1824, 2124); - a second layer (102-302, 702, 1202, 1802, 2102) comprising one or more resonators (104a, 104b, 104c, 104d, 104e, 204, 304, 704, 1204, 1804, 2104), each resonator of the second cavity-forming layer (118a, 118b, 118c, 118d, 118e, 218, 318) including a second entrance opening (115a, 115b, 115c, 115d, 115e, 215, 315, 715, 1215, 1815, 2115) connected to the output opening of the conduit of the first resonator so that all or part of an incident sound wave received by the first input opening can penetrate into each resonator of the second layer via this second input opening. Elementary cell according to claim 1, in which: - the first resonator also has a non-zero absorption rate extended over a frequency band Δ f which is centered around a characteristic frequency f ₀ for which the absorption coefficient α ₀ ( f = f ₀ ) is maximum on this frequency band Δ f , the maximum value being, for example, less than 30%; And - each of one or more resonators of the second layer also has a non-zero absorption rate extended respectively over a frequency band Δ f _k , narrower than the frequency band Δ f , which is centered respectively around a characteristic frequency f _k for which the absorption coefficient α _k ( f = f _k ) is maximum on this frequency band Δf _k , the maximum value being, for example, less than or equal to 30%. Elementary cell according to claim 2, in which each of one or more resonators of the second layer is dimensioned so as to position its respective characteristic frequency f _k on a frequency interval Δ f ± δ where Δ f is the frequency band of the first resonator and δ represents a minimal frequency difference. Elementary cell according to any one of claims 1 to 3, in which the conduit of the first layer and/or the cavity of the second layer comprises at least one conduit of the type selected from: - a tube ; - room ; preferably the at least one tube-type conduit comprises one or more elbows. Elementary cell according to any one of claims 1 to 3, in which the conduit of the first layer and/or the cavity of the second layer comprises at least two conduits mounted in cascade, each conduit being of the type selected from: - a tube ; - room ; preferably at least one of the two conduits is of the tube type comprising one or more elbows. Elementary cell according to claim 5, in which at least one of the two conduits is of the tube type extending inside a conduit of the tube or chamber type which precedes it or which succeeds it. Elementary cell according to any one of claims 4 to 6, in which all or part of the conduit of the first layer has substantially the shape of a prism, preferably the prism being a parallelepiped, a cube, a cylinder, a cone or a hexagonal prism. Elementary cell according to any one of claims 4 to 6, in which all or part of the cavity of the second layer has substantially the symmetrical or regular shape of a cuboid, a cylinder, a cone or a hexagonal prism, a parallelepiped , a cube or a cylinder. Elementary cell according to any one of the preceding claims, in which the second entrance opening of at least one of the cavities of the second layer further comprises an extension which extends in the conduit of the first layer, preferably l the extension being straight or folded into the conduit of the first layer. Elementary cell according to any one of the preceding claims, in which the material of the first layer and/or the material of the second layer is of the type selected from: - a rigid material; - a soft material; - a solid material, preferably selected from a metal, an alloy, a ceramic, a terracotta, clay, concrete, a natural or artificial stone slab, plastic, glass and an agglomerated textile; - a structured material preferably selected from natural wood species and derived woods; - an expanded material, preferably selected from a stone, a clay, a cork, a plastic, a natural rubber, a synthetic rubber and a resin; - an extruded material preferably selected from a metal, an alloy and a polymer; - a malleable material, preferably selected from a metal, a plastic, a natural wood, a natural rubber, a synthetic rubber, paper, cardboard; - a substantially non-acoustically absorbing material; and or - an inert material. Elementary cell according to any one of the preceding claims, in which the first layer is in contact with the second layer. Acoustic panel comprising at least one elementary cell according to any one of claims 1 to 11. Acoustic panel according to claim 12, which is flat or curved. Acoustic plate comprising at least one acoustic panel according to claim 13, the plate being able to be fixed on a surface to be soundproofed, preferably a wall and/or a ceiling, preferably the plate is flat or curved.

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