EP3384486B1 - Reflective acoustic metamaterial - Google Patents

Description

The invention relates to the field of acoustic insulation. In particular, the invention relates to an elementary cell of an acoustic metamaterial, and an acoustic screen comprising such a cell.

Noise pollution in everyday life, for example from the external environment, such as the proximity of a road or air axis, or interior such as the noise of household appliances, are stress factors which deteriorate the comfort of life.

Noise pollution also exists in the building sector and in various industrial fields.

In order to regain comfort, it is often necessary to acoustically isolate the noise source. To do this, solutions for attenuating the propagation of sound waves exist. However, the acoustic insulators known from the state of the art are based on the use of intrinsic characteristics of materials in terms of absorption or reflection of sound waves. The materials conventionally used for this purpose are typically porous materials, such as metallic foams or polymer materials, rock wool, glass wool, cotton wool, cork or agglomerated wood fibers.

A problem with the use of such materials resides in the fact that the choice of the material to be used is dictated by the intrinsic characteristics of the material, which limits the possibility of choosing the material with respect to a given application. In addition, relying on the intrinsic properties of the material also limits the frequency range of material response, as well as manufacturing techniques.

In addition, acoustic panels made from such materials are heavy and bulky, especially those used for low frequencies.

The document of the prior art XP012202298 discloses an acoustic metamaterial consisting of elementary cells each comprising a channel which passes through a solid body, forming a Fabry-Perot type cavity, and a lateral Helmholtz type resonant cavity in communication with said cavity of Fabry-Pérot type.

The objective of the present invention is to solve the problems of sound insulators known from the state of the art. In particular, the object of the invention is to provide an acoustic insulation solution that is efficient and allows flexibility in the choice of material and the range of frequencies.

The invention also aims to reduce the size and weight of acoustic panels.

• un corps en matériau solide,
• au moins une cavité, de type Fabry-Pérot dans ledit corps, et
• au moins une cavité résonante latérale communicant avec ladite cavité de type Fabry-Pérot.

To this end, the invention relates to an elementary cell of acoustic metamaterial comprising:

• a body made of solid material,
• at least one cavity, of the Fabry-Pérot type in said body, and
• at least one lateral resonant cavity communicating with said Fabry-Pérot type cavity.

By Fabry-Pérot type cavity is meant a cavity having two open ends. Such a cavity allows high confinement of the acoustic energy. This cavity being coupled to one or more lateral resonant cavity (s), a series of antiresonances / resonances replace the conventional resonances of single Fabry-Perot type cavities, inducing an increased reflection effect of sound waves. . The Fabry-Pérot type cavity is a duct passing through said body and the lateral resonant cavity is a Helmholtz type acoustic resonator which is a cylindrical cavity concentric with the through duct and communicating with said duct via a neck. Such an effect is obtained independently of the nature of the solid material, which therefore makes it possible to to free oneself from the nature of the material.

In other words, even using a solid material whose intrinsic reflective properties do not are not important, the fact of structuring it so as to have a metamaterial comprising a Fabry-Pérot type cavity coupled to one or more lateral resonant cavity (s) considerably improves the wave reflection sound by this material.

In order to have a significant reflection, it is preferable that the dimensions of the opening of the Fabry-Pérot type cavity, for example the diameter of a cylindrical cavity, are less than the wavelength or the range of lengths d waves that we want to reflect.

Avec Z_sff une impédance effective dépendant de la forme du taux de remplissage de la cavité, et « l_eff » la longueur de la cavité (qu'elle soit repliée ou non).
La résonance s'opère à la fréquence de $f_{\mathit{FP}}=\frac{c}{2l_{\mathit{eff}}}.$

A cette résonance, il est possible de coupler un autre résonateur latérale (de type Helmholtz), dont la fréquence de résonance proche de ƒ_FP permet de générer une forme asymétrique caractérisée par une antirésonance à une fréquence égale ou inférieur à ƒ_FP. La forme du profil obtenue est alors modulée par l'expression suivante : $${\sigma }_F=\frac{A{\left(\frac{w^2-{w_R}^2}{{W_R+w_R)}^2-{w_R}^2}+q\right)}^2}{\left({\left(\frac{w^2-{w_R}^2}{{W_R+w_R)}^2-{w_R}^2}\right)}^2+1\right)}$$

Avec w_R la fréquence de résonance du résonateur latéral, W_R la largeur de bande de la résonance, A un paramètre d'amplitude et q un paramètre d'asymétrie. Ces paramètres dépendent de la forme et du positionnement du résonateur latéral.
Ce qui donne : $$T_F=T_{\mathit{FP}}{\sigma }_F$$

The lateral resonant cavity can have any shape, the only condition for having a reflection of sound waves and that the lateral resonant cavity communicates with the Fabry-Perot type cavity.
The Fabry-Pérot type cavity makes it possible to generate a transmission whose profile can be approximated by: $$T_{\mathit{FP}}=\frac{1}{1+0.25{\left(\frac{Z_{\mathit{eff}}}{Z_0}\right)}^2{\mathit{sin}}^2\left(k_{\mathit{eff}}{\mathrm{the}}_{\mathit{eff}}\right)}$$

With Z _sff an effective impedance depending on the shape of the filling rate of the cavity, and “ l _eff ” the length of the cavity (whether it is folded or not).
Resonance takes place at the frequency of $f_{\mathit{FP}}=\frac{\mathrm{vs}}{2{\mathrm{the}}_{\mathit{eff}}}.$

To this resonance, it is possible to couple another lateral resonator (of the Helmholtz type), whose resonance frequency close to ƒ _FP makes it possible to generate an asymmetric shape characterized by an antiresonance at a frequency equal to or less than ƒ _FP . The shape of the profile obtained is then modulated by the following expression: $${\sigma }_F=\frac{\mathrm{TO}{\left(\frac{w^2-{w_R}^2}{{W_R+w_R)}^2-{{\mathrm{<figure-callout\; id="2"\; label="w\; R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">w</figure-callout>}}_R}^2}+q\right)}^2}{\left({\left(\frac{w^2-{w_R}^2}{{W_R+w_R)}^2-{{\mathrm{<figure-callout\; id="2"\; label="w\; R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">w</figure-callout>}}_R}^2}\right)}^2+1\right)}$$

With w _R the resonant frequency of the lateral resonator, W _R the bandwidth of the resonance, A an amplitude parameter and q an asymmetry parameter. These parameters depend on the shape and the positioning of the lateral resonator.
Which give : $$T_F=T_{\mathit{FP}}{\sigma }_F$$

The Fabry-Pérot type cavity is a duct passing through said body of solid material, which is preferably cylindrical. The advantage of having a through duct lies in the fact that it allows air to circulate and promotes heat exchange between the two media that the elementary cell separates.

Preferably the cylindrical duct has a circular section with a diameter smaller than the wavelengths to be reflected. This makes it possible to have a high energy confinement.

Advantageously, the cell body is parallelepipedal or cylindrical. Such a form is advantageous, because it makes it possible to easily include the cell in a matrix, for example in a matrix of a absorbent panel or else a panel comprising other acoustic elements.

Advantageously, the through duct is coaxial with the cylindrical cell.

Advantageously, the Helmholtz resonator is a cylindrical cavity concentric with the duct passing through and communicating with said duct passing through by a neck. This configuration promotes the interaction between the Fabry-Pérot type cavity and the lateral cavity to perform the function of reflector. Reflection is particularly favored by the presence of the neck connecting the two cavities.

Advantageously, the body of solid material comprises two or more Helmholtz resonators, having different dimensions. Having several Helmholtz resonators with different dimensions makes it possible to increase the frequency band in which the cell is reflective, through couplings occurring at different frequencies. Indeed, as the resonators have different dimensions, each one has a different resonant frequency. As a result, the coupling of such Helmholtz resonators makes it possible to widen the frequency band in which the cell is reflective.

According to one embodiment of the invention, the Fabry-Pérot cavity and / or the lateral resonant cavity is (are) folded up so as to present at least one inlet, at least one outlet and at least two folds.

These folds are made by a technique called, space folding technique, which reduces the thickness of the cell. Thus, thanks to this folding it is possible to manufacture acoustic cells according to the invention, of low thickness.

The invention also relates to an acoustic screen in the form of a panel, comprising at least one elementary cell according to the invention. Such a screen can only comprise elementary reflecting cells according to the invention, but it can also include other acoustic elements, for example absorbing acoustic cells.

Advantageously, said acoustic screen comprises a multitude of elementary cells, said cells being arranged in a plane of the panel so that each cell is able to act on another neighboring cell to widen the reflection frequency band.

In this configuration, the fact of increasing the number of cells per unit area makes it possible to increase the reflection of sound waves.

By plane of the panel is meant in the sense of the present application the surface of the panel which may be flat or curved.

Advantageously, said elementary cells are arranged in said plane of the panel periodically, for example according to a particular network of triangular, square, hexagonal, or honeycomb-shaped type.

• Les figures la à 1c représentent un premier exemple de réalisation d'une cellule élémentaire selon l'invention, comprenant une cavité de type Fabry-Pérot et une cavité latérale ;
• Les figures 2a et 2b représentent des vues de coupes de deux autre exemples dans lesquels la cellule comprend une seule cavité latérale ;
• La figure 3 représente un exemple de réalisation dans lequel la cellule élémentaire comprend en plus de la cavité de type Fabry-Pérot, trois cavités latérales ;
• Les figures 4a à 4c représentent un premier exemple de réalisation dans lequel la cavité latérale est repliée ;
• Les figures 5a à 5d représente un deuxième exemple de réalisation dans lequel la cavité de type Fabry-Pérot est repliée ;
• La figure 6 montre la réponse en transmission d'une cellule élémentaire conforme à l'invention.

The invention will be better understood on reading the following description of preferred embodiments given by way of illustrative, non-limiting examples, with reference to the drawings, among which:

• Figures la to 1c show a first embodiment of an elementary cell according to the invention, comprising a Fabry-Perot type cavity and a lateral cavity;
• The figures 2a and 2b represent sectional views of two other examples in which the cell comprises a single lateral cavity;
• The figure 3 represents an exemplary embodiment in which the elementary cell comprises, in addition to the Fabry-Perot type cavity, three lateral cavities;
• The figures 4a to 4c show a first embodiment in which the lateral cavity is folded back;
• The figures 5a to 5d shows a second embodiment in which the Fabry-Pérot type cavity is folded;
• The figure 6 shows the transmission response of an elementary cell in accordance with the invention.

FIG. 1a represents an isometric view of an elementary cell 1 of acoustic metamaterial in accordance with the invention. The figures 1b and 1c represent respectively, a top view and a view of a section along the axis AA of the elementary cell 1.

The cell 1 comprises a solid cylindrical body 2 comprising a coaxial and traversing cylindrical duct 3, forming a Fabry-Pérot type cavity. The figure 1c shows that the body 2 also comprises in its interior a lateral cavity 4 which is cylindrical and coaxial with the duct 3 to which it is connected through a neck 5, so as to form a Helmoltz type cavity. As can be seen on the figure 1c , the neck 5 is located halfway up the cylindrical cavity 4.

By mid-height is meant a distance located in the middle between the two ends of the through duct forming the Fabry-Pérot cavity.

Such a structuring of the metamaterial cell 1, in particular thanks to the presence of the neck 5 which connects the two cavities, promotes the interaction between the Fabry-Perot type cavity and the lateral cavity to perform a reflector function.

The neck 5 can however be replaced by an opening of another type between the through cavity 3 and the lateral cavity 4.

The figures 2a and 2b represent sections similar to the section shown in figure 1c , two other exemplary embodiments of elementary cells 1 'and 1 "which differ from the exemplary embodiment shown in Figures 1a to 1c by the shape of the lateral cavity.

In the example of figure 2a , the lateral cavity 4 'is substantially similar to the cavity 4 of the figure 1c , nevertheless the neck 5 'is not arranged in the body of solid material 2' at mid-height of the duct 3 'as in cell 1 of the figure 1c , but rather offset towards one end of the conduit 3 '. This offset has the effect of controlling the coupling to optimize the reflection.

In the exemplary embodiment illustrated on figure 2b , the side cavity 4 "has a width (that is to say the distance between the inner wall I and the outer wall E of the body 2" made of solid material) is much greater than the width of the cavity 4 of the figure 1c . The neck 5 ", connecting the lateral cavity 4" to the through duct 3 ", is arranged at mid-height of the duct 3".

By modifying the dimensions of the lateral cavity, its resonant frequency and its factor of quality. In this way, the frequency of reflection can be adjusted.

The embodiment of the figure 3 differs from the examples of figures 2a and 2b in that the cell body 20 comprises three lateral cavities 41, 42, 43 concentric with the through duct 30 to which they are connected. The cavities 41 and 42 are cylindrical and each is connected to the through duct 30 by a neck 51, 52.

As the three cavities 41, 42, 43 have different shapes and / or dimensions, their resonant frequencies are also different. Therefore, the frequency band of reflection of the elementary cell 10 is greater than that of the elementary cell 1, which comprises a single lateral cavity 4 as shown in the figure. figure 1c .

The figures 4a, 4b and 4c represent respectively an isometric view, a top view and a view of a section along the axis BB, of an elementary cell 100 according to another embodiment.

The figure 4c shows that the body 200 of the elementary cell 100 comprises a lateral cavity 400 folded over so as to have several folds, connected to a cylindrical duct 300 passing through the neck 500.

The folding of the lateral cavity 400 makes it possible to considerably reduce the thickness of the cell 100, while keeping the reflection efficiency.

The figure 5a shows an isometric view of an elementary cell 100 ′ parallelepiped, comprising a solid cell body 200 ′ parallelepiped and a duct 300 ′ forming a Fabry-Perot cavity. The figures 5b to 5d respectively represent a view of above, a view of a section along the axis CC and a view of a section along the axis DD, of cell 100 '.

The figure 5c shows that the duct 300 'is folded back and communicates via a neck 500' with a cylindrical cavity 400 'coaxial with a segment of said duct 300' as shown in figure 5d .

The parallelepipedal shape of the elementary cell 100 'has the advantage of allowing better filling of the surface of an acoustic panel.

According to one variant, the Fabry-Pérot type cavity and the lateral cavity are folded over.

The invention also relates to an acoustic screen comprising one or more elementary cells in accordance with the invention and as described above.

The figure 6 illustrates the transmission response of an elementary cell in accordance with the exemplary embodiment presented in FIGS. 1a to 1c. Said cell is cylindrical with a radius of 14.5 mm and comprises a through-cavity of the Fabry-Pérot type with a length of 60 mm and a radius of 2 mm. A lateral neck of 4 mm in width and concentric with the Fabry-Pérot type cavity is positioned halfway between the two ends of the Fabry-Pérot type cavity. To this neck is connected a lateral cavity 36 mm long and 4 mm wide, also concentric with the Fabry-Pérot type cavity.

• Densité (g/cm) : 1.02 (liquide, à 80°)
• Module d'Young : 1463 MPa
• Contrainte-flexion : 49 MPa

Said cell was manufactured by the Project SD3500 3D printer, whose characteristics of the Visijet Crystal resin used are presented below:

• Density (g / cm): 1.02 (liquid, at 80 °)
• Young's modulus: 1463 MPa
• Bending stress: 49 MPa

The characterization presented and which makes it possible to study the acoustic properties of said cell for the audible frequencies, is obtained by means of a standing wave tube provided with 4 microphones. We used a 4206-T type transmission tube kit from Brüel & Kjær.

The diameter of the transmission tube used is 29 mm, which allows measurements to be made for the frequency interval of [500: 6400] -Hz.

A loudspeaker, placed at one end of the tube, generates white noise on the frequency band that interests us.

Pressure measurements are performed using two terminations with different impedance conditions.

As shown by the transmission spectrum presented in figure 6 , we obtain an attenuation centered around 3kHz reaching 40dB (sometimes 50dB), over a relative bandwidth of 133%.

This result represents an improvement of 15 to 20 dB compared to the reflection of a cell simply provided with a Fabry-Pérot type cavity, and this for a frequency ranging from 1 to 5 kHz.

In the description below, we denote the body of solid material indifferently by body of solid material, solid body, cell body or simply body.

Claims (8)

1. Elementary cell (1; 1'; 1"; 10; 100; 100') made of acoustic metamaterial, comprising:

   - a body made of solid material (2; 2'; 2"; 20; 200; 200'),

   - at least one Fabry-Perot cavity (3; 3'; 3"; 30; 300; 300') in said body (2; 2'; 2"; 20; 200; 200'), and

   - at least one lateral resonant cavity (4; 4'; 4"; 41, 42, 43; 400; 400'), which is a Helmholtz acoustic resonator, and communicates with said Fabry-Perot cavity (3; 3'; 3"; 30; 300; 300'),

   characterized in that the Fabry-Perot cavity (3; 3'; 3"; 30; 300; 300') is a channel which passes through said body (2; 2'; 2"; 20; 200; 200'),
   and in that the Helmholtz resonator is a cylindrical cavity (4; 4'; 4"; 41, 42, 43; 400; 400') which is concentric with the through-channel (3; 3'; 3"; 30; 300; 300') and communicates with said channel (3; 3'; 3"; 30; 300; 300') via a neck (5; 5'; 5"; 50; 500; 500').
2. Cell (1; 1'; 1"; 10; 100; 100') according to claim 1, wherein the through-channel (3; 3'; 3"; 30; 300; 300') is cylindrical.
3. Cell (1; 1'; 1"; 10; 100; 100') according to either claim 1 or claim 2, wherein the cell body (2; 2'; 2"; 20; 200; 200') is parallelepiped or cylindrical.
4. Cell (10) according to any of claims 1 to 3, wherein the body made of solid material (20) comprises two or more Helmholtz resonators (41, 42, 43) having different dimensions.
5. Cell (100; 100') according to any of claims 1 to 4, wherein the Fabry-Perot cavity (300') and/or the lateral resonant cavity (400) is/are folded so as to have at least one inlet, at least one outlet and at least two folds.
6. Acoustic screen in the form of a panel comprising at least one elementary cell (1; 1'; 1"; 10; 100; 100') according to any of claims 1 to 5.
7. Acoustic screen according to claim 6, comprising a plurality of elementary cells (1; 1'; 1"; 10; 100; 100'), said cells being arranged in a plane of the panel such that each elementary cell (1; 1'; 1"; 10; 100; 100') is capable of acting on another adjacent elementary cell (1; 1'; 1"; 10; 100; 100') in order to widen the reflection frequency band.
8. Acoustic screen according to claim 7, wherein the elementary cells (1; 1'; 1"; 10; 100; 100') are arranged in said plane of the panel at regular intervals.

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