EP3167170A2

EP3167170A2 - Sound attenuation device and method

Info

Publication number: EP3167170A2
Application number: EP15736467.0A
Authority: EP
Inventors: Sergio BELLIZZI; Bruno Cochelin; Renaud COTE; Philippe Martin HERZOG; Pierre-Olivier Mattei; Marc Pachebat; Cédric PINHEDE
Original assignee: Aix Marseille Universite; Centre National de la Recherche Scientifique CNRS; Ecole Centrale de Marseille
Current assignee: Aix Marseille Universite; Centre National de la Recherche Scientifique CNRS; Ecole Centrale de Marseille
Priority date: 2014-07-10
Filing date: 2015-07-09
Publication date: 2017-05-17
Also published as: WO2016005489A3; FR3023645B1; WO2016005489A2; US20170249933A1; US10522128B2; FR3023645A1

Abstract

The invention relates, in particular, to an attenuation device intended for attenuating sound waves generated by a source (1) emitting sound waves of which the frequencies are between f1 and f2 and of which the pressure levels are between n1 and n2, the attenuation device comprising at least one sound absorber (11) comprising at least one non-linear membrane; - the attenuation device being configured such that the first face (13) of the absorber (11) is in acoustic communication with the source (1); - the attenuation device comprises at least one coupling element (16) for coupling the second face (14) with the source (1), the coupling element (16) being configured to transmit, to the second face (14), sound waves depending on the sound waves emitted by the source (1), and of which the phase and/or the amplitude results in a pressure differential of the sound waves arriving at the first and second faces respectively at the same time. The invention also comprises a sound attenuation system and method.

Description

"Sound attenuation device and method"

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of attenuation of sounds. It relates in particular to a device for attenuating the sounds generated by a source. It finds for example particularly advantageous application, without being limiting, the field of attenuation of the noise of mouth of a heat engine.

STATE OF THE ART

There are many solutions to attenuate the sounds generated by a source.

Several of these solutions rely on the use of a flexible membrane whose face is set in motion by sound. When the movements of the membrane correspond to an energy pumping effect, the sound is attenuated.

Membrane energy pumping is proven effective when one side of the membrane is coupled with the source emitting the sounds and another side of the membrane is left free. On the other hand, leaving one side of the membrane free generates a secondary sound radiation that propagates outwards and greatly limits the interest of the attenuation device.

For this reason in particular, the attenuation device is in practice housed in part at least in a hood.

Capoter the free face of the membrane then cuts the secondary sound radiation but negatively disturbs the free deformation of the front face of the membrane. Indeed, the hood traps a volume of air that opposes the movement of the membrane.

A hood of very large volume avoids this opposition to free movement of the membrane but poses obvious problems of congestion.

A small hood has two disadvantages: if the static pressure changes, there is a harmful swelling of the membrane; the volume of air trapped opposes movements, especially those at the frequency of the source.

In order to limit the size of the attenuation device and the emission of secondary radiation, a partial solution has been described in the document published under the number EP2172640. This solution is based on a membrane whose rear face is enclosed in a hood to define a dissipation chamber energy. The energy dissipation chamber is equipped with a pipe having a high inertia and damping in order to equalize the average pressures prevailing on both sides of the membrane so as to avoid the appearance of an additional constraint on the membrane, this additional constraint being detrimental to the operation of the system. The effectiveness of this solution is in practice limited.

In order to limit the size of the attenuation device and the emission of secondary radiation, another solution has been described in the Chiu Yu Him thesis, "Noise Attenuation by Vibroacoustic Coupling", The Hong Kong Polytechnic University, 2004. This solution is based on a magnetic system that compensates for the overpressure generated between the cover and the rear face of the membrane during the deformation of the latter.

This solution aims to compensate for the acoustic phenomena generated in the rear chamber. This solution is in practice very complex to implement. There is therefore a need to provide a solution to attenuate the sound of a primary source more efficiently than known solutions and while having a complexity of implementation and a limited footprint.

This is an objective of the present invention.

SUMMARY OF THE INVENTION

To achieve this objective, one aspect of the present invention relates to an attenuation device for attenuating acoustic waves generated by a source emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2,

the attenuation device comprising at least one acoustic absorber comprising at least one membrane, the acoustic absorber having at least a first face and at least a second face distinct from the first face, the acoustic absorber being configured to exhibit a behavior nonlinear deformation when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the range f ₀ 1 - f ₀ 2 covering at least 50% of the range f1 -f2 and whose levels of pressure are between n ₀ 1 and n ₀ 2, the range n ₀ 1 -n ₀ 2 covering at least 50% of the range n1 -n2;

the attenuation device being configured so that the first face of the absorber is in acoustic communication with the source.

Advantageously, the attenuation device comprises at least one coupling element of the second face with the source, the coupling element being configured to transmit to the second face acoustic waves such that IP1 -P2l> k.lP1 1 during at least part of the operating cycle of the source, with P1 and P2 being the instantaneous acoustic pressure of the acoustic waves arriving respectively on the first and second faces at the same time and with k> 0.2. And | P | the absolute value of P

P1 and P2 are here the instantaneous acoustic pressures, measurable in practice. IP1 -P2I represents the absolute value of P1 -P2 and IP1 I represents the absolute value of the instantaneous pressure P1.

The device of the invention thus provides for acoustically coupling each of the two faces of the absorber to the source. It thus makes it possible to exploit the second face of the absorber by coupling it acoustically or electro-acoustically with the source.

The attenuation device is arranged so as to impose a difference in the path between the acoustic waves arriving respectively on the first and second faces of the absorber, thus leading to increase the differential pressure applying to these two faces.

Each of the two faces of the membrane is used and participates in energy pumping.

The invention thus effectively attenuates sounds in a wide range of frequencies and while maintaining a small footprint.

The invention uses the rear face instead of suffering a disadvantage. The invention thus turns away from known solutions based on the formation of a dissipation chamber on the rear face of the membrane.

The invention makes it possible to use a cover, forming a chamber, of limited size without the secondary radiation preventing the deformation of the membrane, and this advantageously whatever the situation.

The solution described in document EP2172640 aims to establish a balancing of the static pressures on either side of the membrane, without transmitting acoustic phenomena on the back of the membrane Acoustic phenomena are removed by the pipe and not felt. Indeed, the pipe has a high inertia and dissipation which prevents it from transmitting acoustic waves (fast vibrations). Thus, the effectiveness of the attenuation is low because the inertia of the high inertia pipeline aims to equalize the static pressures and the dissipation chamber thus equipped prevents optimal deformation of the membrane.

The invention aims, on the contrary, to accentuate and promote the instantaneous acoustic pressure difference between the two faces of the absorber in order to exploit both sides of the membrane. If the membrane moves is that there is presence of a pressure difference and the invention aims to increase this difference. For this, the rear face of the membrane is acoustically coupled to the source and not only balanced in pressure with the front face of the membrane. The acoustic waves can therefore reach the second face of the membrane and promote the dissipation mechanisms.

The present invention aims to accentuate an instantaneous acoustic pressure difference between the two faces of the absorber in order to exploit both sides of the membrane and to optimize the attenuation of sound. In the invention, the coupling element between the source and the rear face has a transmission maximum for a frequency greater than f1.

The invention does not provide for forming a dissipation chamber on the rear face of the membrane as taught by the solutions described in EP2172640 but it On the contrary, it provides a coupling chamber between the rear face of the membrane and the source.

Thus, while the state of the art suggests to the skilled person to remove, reduce or compensate for the acoustic phenomena generated between the rear face and the hood, the invention provides for exploiting these phenomena and adapting them to exploit both sides of the membrane.

The known solutions would have diverted the person from the field of the invention.

Moreover, the device of the invention has a limited complexity.

According to another aspect, the present invention relates to a system comprising a source emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2 and an attenuation device according to the invention. configured to attenuate the acoustic waves of said source.

According to another aspect, the present invention relates to an attenuation method for attenuating acoustic waves generated by a source emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2. ;

selecting at least one acoustic absorber comprising at least one membrane, the acoustic absorber having at least a first face and at least a second face distinct from the first face and preferably opposite to the latter, the acoustic absorber being configured to the membrane exhibits a non-linear deformation behavior when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the range f ₀ 1 -fo 2 covering at least 50% of the range f 1 - f2 and whose pressure levels are between n ₀ 1 and n ₀ 2, the range n ₀ 1 -n ₀ 2 covering at least 50% of the range n1-n2;

arranging the first face of the absorber in acoustic communication with the source;

selecting a coupling element of the second face with the source, the coupling element being selected so as to transmit to the second face a maximum of instantaneous acoustic pressure for acoustic waves whose frequencies are greater than f1 and so as to transmit on the second face acoustic waves of which:

the instantaneous acoustic pressure exerted on the second face is a function of the instantaneous acoustic pressure of the acoustic waves emitted by the source, the properties, of phase and / or amplitude, for example, lead to that IP1 -P2l> k.lP1 1 during at least a part of the operating cycle of the source, with P1 and P2 being the instantaneous acoustic pressures of the waves acoustic signals arriving respectively on the first and second faces at the same time and with k> 0.2.

According to another aspect, the present invention relates to a method of attenuation of noise generated by a source, the method being characterized in that it comprises the following steps:

- determine the acoustic mode (s) of the noises;

selecting at least one acoustic absorber comprising at least one membrane, the acoustic absorber having at least a first face and at least a second face different and often opposite the first face, the selection of the absorber on at least one physical characteristic of the flexible membrane according to the acoustic mode (s) (s), fix the membrane so that the membrane can undergo a large deformation under the action or modes (s) acoustics (s), said physical characteristic the membrane being for example the stiffness, which depends on the thickness, the surface and the Young's modulus of the membrane; arranging the first face of the absorber in acoustic communication with the source;

coupling the second face with the source so as to transmit to the second face a maximum acoustic pressure for acoustic waves whose frequencies are higher than the minimum frequency emitted by the source and so as to transmit acoustic waves to the second face; such that an instantaneous acoustic pressure differential is created on either side of the first and second faces of the absorber.

BRIEF DESCRIPTION OF THE FIGURES

The objects, objects, as well as the features and advantages of the invention will become more apparent from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

Figure 1 is a diagram of an exemplary embodiment of the invention. Fig. 2 is a diagram of another exemplary embodiment of the invention, wherein the absorber is a flared conduit at its ends. Figures 3a and 3b are diagrams of other exemplary embodiments of the invention.

Figure 4a is a diagram of another exemplary embodiment of the invention comprising an acoustic coupling chamber with respect to the second face of the membrane and acoustically coupled to the source.

Figure 4b is a diagram of another exemplary embodiment of the invention in which a plurality of couplers acoustically couple the first face of the membrane to the source.

Figure 4c is a diagram of another exemplary embodiment of the invention in which the coupling element is formed of a plurality of elements including a coupling with the front chamber and a coupling between the front chamber and the source.

Figures 5a and 5b are diagrams of other embodiments of the invention in which the coupling between the second face of the membrane and the source is an electro-acoustic coupling.

Fig. 6 is a diagram of another exemplary embodiment of the invention wherein the acoustic absorber comprises a plurality of membranes.

Figures 7a and 7b illustrate two absorber embodiments that can be used for all embodiments of the invention.

Figures 8a and 8b illustrate an embodiment of the invention, wherein the attenuation device is disposed in the false ceiling of a room.

Figures 9a and 9b are simplified views, respectively of sectional front and side, of the attenuation device used in the embodiment illustrated in Figures 8a and 8b.

The drawings are given by way of examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily at the scale of practical applications. In particular, the relative dimensions of the different elements are not representative of reality. DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail embodiments of the present invention, some reminders and definitions are given below regarding the pressures as well as their levels:

Pressure is an instantaneous physical quantity that can be broken down into two additive components, static pressure and instantaneous sound pressure. The static pressure can be defined as the arithmetic mean of the pressure during the time interval considered, typically of the order of at least 10 periods of the smallest frequency of interest, for example 0.2s for 50Hz. The instantaneous sound pressure represents the pressure component involving acoustic phenomena, typically above 20 Hz.

The instantaneous sound pressure has alternating positive and negative values, and has a zero average.

The pressure level is the mean of the instantaneous acoustic pressure in the least squares sense (rms value). In the general case it is estimated from data collected in the same type of time interval as above. In the present invention, the pressure level is denoted "n".

In the case of harmonic waves, the amplitude of the wave is defined as the maximum value taken by the component of the pressure at the frequency considered. In the general case the amplitude is the maximum value taken by the instantaneous acoustic pressure. There is a proportional relationship between amplitude and level for harmonic waves or of known form. For any given signal there is a quadratic relation between its level, and the amplitudes of its Fourier components.

For example, the following method can be used to determine the instantaneous acoustic pressures P1 and P2 acting on the first and second faces of the absorber: to place microphones adapted to the frequencies and pressure levels targeted, ideally near the center of the membranes at an axial distance from the surface of the membrane. . .

I order d, such that d = and in any case a distance less than π

tenth of the wavelength associated with f2.

For determining the pressure levels n1 and n2 it is possible, for example, to use any device capable of calculating the rms value in a frequency range comprising at least the range from f1 to f2. For example, an analyzer connected to the microphones placed as described above can be used.

The invention provides for forming an acoustic coupling between the rear face of the absorber and the source in order to exploit the instantaneous acoustic pressure on the rear face of the membrane in addition to exploiting the front face of the absorber. The invention thus turns away from known solutions based on the formation of a dissipation chamber on the rear face of the membrane. Indeed, in the context of the development of the present invention, it has been found that in a solution with or without a duct with high pressure equalizing inertia, the volume of air trapped in the rear chamber opposes the movement of the membrane and night attenuation of the sound. More specifically, the attenuation device comprises at least one acoustic absorber configured to exhibit a behavior in nonlinear deformation when it receives acoustic waves whose frequencies are within a wide range of frequencies and pressures emitted by the source. The acoustic absorber allows a transmission of pressure and flow.

The first face of the absorber is in acoustic communication, that is to say it allows a transmission of pressure with the source. In addition, the attenuation device comprises at least one coupling element of the second face with the source. The coupling element is configured to transmit to the second face acoustic waves whose instantaneous pressure is a function of the instantaneous pressure of the acoustic waves emitted by the source. Advantageously, the coupling element is configured to create a difference in the path between the waves arriving at the same time on the first and second faces of the membrane in order to generate and promote an instantaneous pressure difference between these faces of the absorber.

The device of the invention thus provides for acoustically coupling the two faces of the absorber. It thus makes it possible to exploit the second face of the absorber by coupling it acoustically or electro-acoustically with the source. The invention thus effectively attenuates sounds in a wide range of frequencies and while maintaining a small footprint.

Before starting a detailed review of embodiments of the invention, are set forth below optional features that may optionally be used in combination or alternatively:

Preferably, the coupling element is configured to transmit to the second face acoustic waves whose phase and / or amplitude results in IP1-P2l> k.lP1 for at least a part of the operating cycle of the source, P1 and P2 being the instantaneous pressures of the acoustic waves arriving respectively on the first and second faces at the same time and with k> 0.5 and preferably k> 1. IP1 -P2I is the absolute value of P1 -P2 and IP1 I is the absolute value of P1.

Advantageously, k> 1, 5, preferably k> 1, 8, preferably k> 2.

Advantageously, the coupling element is configured to transmit to the second face acoustic waves whose instantaneous pressure exerted on the second face is a function of the instantaneous pressure of the acoustic waves emitted by the source. Advantageously, the coupling element is configured to transmit to the second face a maximum of instantaneous acoustic pressure for acoustic waves whose frequencies are greater than f1.

Advantageously, at least one membrane is configured to exhibit non-linear deformation behavior when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the range f ₀ 1 -fo 2 covering at minus 70% and preferably 100% of the f1-f2 range.

For example, the following method can be used to determine whether a membrane is non-linear within the meaning of the preceding description: The membrane has a non-linear deformation when, subjected to a harmonic excitation at a frequency chosen between f1 and f2 and a level chosen between n1 and n2, it is possible to detect near a response containing terms at frequencies different from that of the excitation.

Advantageously, said second face is opposite to said first face. Advantageously, the at least one coupling element of the second face with the source is configured to transmit acoustic waves to the second face, the phase and / or amplitude of which results in IP1-P2l> k.lP1. during at least X% of the operating cycle of the source, with X = 10. According to one embodiment, X = 30.

According to one embodiment, the acoustic absorber has a linear resonance frequency f _ri , with f _ri <f1. Thus the acoustic absorber, typically a membrane, has a linear resonance frequency strictly less than the minimum frequency to be attenuated.

If the acoustic absorber comprises several membranes, then each membrane has a linear resonance frequency strictly less than the minimum frequency to be attenuated.

The acoustic absorber membrane (s), exhibiting a linear resonance frequency f _ri , with f _ri <f1 and exhibiting non-linear deformation behavior when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the f ₀ 1 -fo 2 range covering at least 50% of the f1-f2 range makes it possible to generate the energy pumping phenomenon, which leads to a particularly effective attenuation of the noise of the source.

According to one embodiment, the at least one coupling element of the second face with the source is configured so that the membrane reaches the acoustic pumping state, preferably at least when the frequency range transmitted by the coupling element is partly greater than the frequency f ₀ 1 and preferably greater than f ₀ 2.

According to one embodiment, the at least one coupling element of the second face with the source is configured so that at a given instant the acoustic system comprising the membrane reaches a state capable of triggering the acoustic pumping, which is attained for example, without limitation, by adding noise at frequencies different from the frequency range of interest for a limited time (see Côte et al., JSV 333 (2014) 5057-5076).

- When the frequency range transmitted by the coupling element is in part greater than the frequency f ₀ 2, the signal transmitted by the coupling element contributes to triggering the desired absorption regime, and the noise generated or transmitted to the High frequencies can easily be attenuated by conventional systems such as porous absorbers. The action of the coupler can be verified by measuring an increase in the sound level when the coupling is broken, for example by the insertion of a rigid wall, or by detecting a non-linear mode change (characterized by its stationarity, its spectrum , and where appropriate the amplitudes and phases of its components). The coupler design can be verified by measuring its transfer function. This must show a maximum of transmission for frequencies higher than f1.

According to one embodiment, the acoustic coupling element, for example an acoustic pipe or pipe, comprises at least one inner wall and at least one acoustically absorbing element disposed on the inner wall.

Advantageously, the acoustically absorbing elements are configured to partially absorb at least the high frequencies produced by the source and / or the attenuation device, for example the 30% or 10% of the highest frequencies produced by the source and or by the attenuation device.

According to one embodiment, the at least one acoustically absorbing element is glass wool.

Advantageously, the coupling element of the second face with the source comprises at least one acoustic duct. According to a non-limiting embodiment, the device comprises a cover forming with said second face a closed volume except for an opening formed by said at least one acoustic duct. According to another embodiment, said second face is housed in said at least one acoustic duct. According to one embodiment, the coupling element of the second face with the source is free of an electro-acoustic coupler. Thus the coupling element of the second face with the source does not include an electro-acoustic coupler.

Advantageously, the coupling element of the second face with the source comprises at least one electro-acoustic coupler. Advantageously, at least one membrane is a loudspeaker membrane, an outer face of the membrane being said first face. Advantageously, the absorber is configured in such a way that the loudspeaker receives an electrical signal that is a function of an acoustic signal from the source.

According to one embodiment, the absorber comprises a microphone arranged to pick up acoustic waves originating from the source and is configured in such a way that the acoustic signal is provided by the microphone. Preferably, the device comprises a cover defining with said second face a closed volume with the exception of an instantaneous pressure balancing capillary prevailing on said first face and said second face.

Alternatively, the absorber is configured so that the acoustic signal is taken on the second face of the membrane. Preferably, the device comprises a cap defining with said second face a closed volume with the exception of an acoustic coupling duct between said second face and the source.

- The hood forms a chamber, also known as a dissipation chamber. Preferably it defines a closed volume with the exception of one or more passages provided for the couplers.

- The attenuation device comprises a plurality of coupling elements of the second face with the source.

Advantageously, the attenuation device is configured to allow bi-directional communication between the source and said first face of the absorber.

According to one embodiment, the attenuation device is configured in such a way that the acoustic communication between the source and said first face of the absorber is a direct acoustic coupling, preferably without an intermediate sound transmission element. That is to say only by the volume of the enclosure in which the source is arranged. According to another embodiment, the attenuation device is configured so that the acoustic communication between the source and said first face of the absorber is an acoustic coupling made in part at least by one or more acoustic ducts. Preferably, the attenuation device is configured so that the acoustic communication between the source and said first face of the absorber is an acoustic coupling made only by one or more acoustic ducts.

Advantageously, the attenuation device comprises an enclosure configured to house the source and the acoustic absorber, the attenuation device being configured so that the first face of the absorber is in acoustic communication with the source by the internal volume of the enclosure.

if n1 and n2 are arbitrary then n01 <n2 <n02, n02 may be possibly infinite (no upper limit to nonlinearity).

Advantageously, the coupling element of the second face with the source has an inlet mouth for the acoustic waves coming from the source, the inlet mouth having a difference of operation with the first face of a value equal to ½ Ac modulo Ac, with Ac = (λ ₂ ^0.5, λ-ι ^0.5 ) and more generally a value between ½ Ac - ¼ Ac and ½ Ac + ¼ Ac, modulo Ac; Ai being the wavelength of the minimum frequency fi to be attenuated and A ₂ being the wavelength of the maximum frequency f ₂ to be attenuated.

Advantageously, the coupling element of the second face with the source has several resonance frequencies.

According to one embodiment, the first and second faces of the absorber extend in parallel planes.

According to one embodiment, the first and second faces of the absorber are mechanically linked.

According to one embodiment, the coupling element of the second face with the source does not comprise a membrane.

The invention will now be described with reference to FIGS. 1 to 6.

The diagram 100 illustrated in the figures comprises a source 1 and a sound attenuation device 10 emitted by the source 1.

The source 1 emits acoustic waves whose frequencies are between the frequencies fi and f ₂ , whose instantaneous acoustic pressures are between pi and p ₂ whose pressure levels are between n-ι and n ₂ . The pressure level is the average of the instantaneous acoustic pressure in the least squares sense (rms value). .

Typically, in the context of the invention, is greater than 20 H _z and is preferably between 20 and 1000 H _z and f ₂ is less than 20000 H _z and is preferably between 40 and 1000 H _z. Advantageously, in the context of the invention n1 is greater than 100 Pa.

Without this being limiting of the invention, the source 1 may for example be the air intake port of the supply circuit of an internal combustion engine. In these engines, the mouth lets out a noise usually called mouth noise. The invention is not limited to the attenuation of mouth noises.

The attenuation device 10 comprises in particular an acoustic absorber 1 1. This absorber 1 1 comprises at least one membrane 12.

In the embodiments which will be described hereinafter with reference to FIGS. 6 and 7 for example, the absorber 11 comprises several membranes 12.

Whatever the number of membranes 12 that the absorber 1 1 has, the latter has a first face 13 and a second face 14. These two faces 13, 14 are distinct and are most often opposed.

The membrane is a flexible membrane. It causes the absorption of said source noise by deforming.

According to the invention, the flexible membrane 12 must be able to deform non-linearly in order to absorb the acoustic energy. It has indeed been shown in the literature (see the article by B. Cochelin, P. Herzog, PO Mattei, "Experimental evidence of energy pumping in acoustics" in Proceedings of the Academy of Sciences, Accounts Rendus Mechanics (CR Mechanical 334, pages 639 to 644, 2006) that a flexible membrane can deform in large amplitude in order to pump the acoustic energy of an acoustic mode irreversibly over a wide frequency band. This acoustic energy pumping phenomenon is due to the non-linear behavior of the membrane (which can undergo large deformations) which can be tuned to the frequency of the acoustic mode to be absorbed, as long as it is sufficiently energetic.

Thus, the acoustic transfer function of the device is non-linear between p1 and p2, in that it is not proportional to the acoustic pressure, between f1 and f2, during at least part of the operating cycle of the device, during at least part of the operating cycle of the source.

The operation of the source and the device are not necessarily periodic. If they are periodic, the operating cycle corresponds to their period.

This type of membrane is often referred to as NES, an acronym for Non-linear Energy Sink meaning in French and then nonlinear energy. For this type of membrane, we can refer to the following publication: AFVakakis, OVGendelman, G.Kerschen, LABergman, MDMcFarland and YS Lee, Nonlinear Targeted Energy Transfer in Mechanical and Structural Systems, vv and v.ll, Springer, 2009.) ch.3 p93.

The membrane 12 has properties that enable it to deform non-linearly in a frequency range covering at least 70% that of the waves emitted by the source 1 and in a range of pressure levels covering at least 70% and preferably 100%. % that of the waves emitted by the source 1.

The following properties, in particular of the membrane 12, are chosen so as to enable it to deform in a non-linear manner: its mass, its elastic modulus, its dissipation and tension properties, its density and its dimensions, in particular its diameter and its thickness.

To choose and arrange the membrane, we can for example refer to the article article by B. Cochelin, P. Herzog, PO Mattei, "Experimental evidence of energy pumping in acoustics" in Proceedings of the Academy of Sciences For example, the non-prestressed flexible membrane 12 is placed in the device at a location allowing it to pump the maximum energy to the acoustic wave, as shown in FIG. 3, pages 639 to 644, 2006). or in other words where it will be the most excited by the acoustic wave. The membrane 12 will therefore advantageously be placed at a pressure belly so that it can vibrate in large amplitudes. However, if it is considered that the acoustic velocity is negligible, the sound pressure can then be considered spatially uniform and in this case the location of the membrane 12 is small. The coupling of the rear face 14 of the membrane 12 with the source 1, as provided by the present invention allows a large deformation of the membrane 12 and good dissipation of the energy pumped by the membrane 12. This dissipation is much more effective than that obtained with the solutions based on a cowling of the rear face 14 with balancing of the pressures between the front 13 and rear 14 faces but without acoustic coupling of the rear face 14.

In order to properly choose the characteristics of the membrane or membranes 12, it is necessary to know the mode (s) acoustic (s) to absorb. When these modes are identified, the characteristics of the membrane (s) 12 can then be determined so as to obtain a behavior in nonlinear deformation and a good energy coupling between the membrane (s) 12 and the acoustic mode (s) (s) ) to absorb. The desired stiffness for the membrane 12, which is a function of the thickness, the surface and the Young's modulus of the membrane 12, is determined primarily. The nonlinear stiffness is dimensioned according to the intensity from the source component to the highest frequency that is to be attenuated, allowing attenuation over a wide frequency band.

To choose the stiffness c¾ of the membrane 12, it is assumed that the transverse displacement w (r, t) of the membrane 12 is approximated by the parabolic form function w (r, t) = q (t) (1 / r). where R is the radius of the membrane 12 (assumed to be circular in shape) and r is the radial coordinate. It is also assumed that the membrane 12 deforms in large amplitudes (which is the goal). This leads to being able to express analytically the stiffness a ₃ of the membrane 12 as a function of its Young's modulus E, its thickness and its radius R.

The stiffness at ₃ can then be expressed using the following equation:

a ₃ = (32/3 (1 - ν ² ) ττ). (E / p _air C). (H ³ / R ⁴ ). (1 / ω). (S _t / S _m )

in which v denotes the Poisson's ratio, E the Young's modulus of the membrane 12, p air density, C the speed of sound, h the thickness of the membrane 12, R the radius of the membrane 12 (which is supposed to be circular), Ω the frequency of the acoustic standing wave system to be absorbed, St the section of the clean air outlet in the air filter and Sm the section of the membrane 12.

The nonlinear energy pumping effect ^is not obtained by pretension of the membrane 12 which may, according to the invention deforming in large amplitudes. This nonlinear energy pumping effect is obtained in particular by the configuration of the coupling of the two faces of the membrane 12 and by the capacity of the membrane to deform non-linearly.

The membrane 12 of the absorber can be passive. It can also be motorized or active. This is typically the case when it equips a speaker.

The first face 13 of the absorber is acoustically coupled to the source 1, preferably in a purely acoustic manner. This acoustic coupling makes it possible to transmit the sound to attenuate between the source 1 and the first face 13.

The acoustic coupling between the source 1 and the first face 13 of the absorber 1 1 can be achieved by at least one acoustic connection 15 '.

Acoustic connection is a connection that allows a correlation in the two opposite directions of propagation of the acoustic wave.

Coupling means acoustic power transmission means between several locations, either by an acoustic connection, or via any conversion, for example via an electro-acoustic conversion. A coupler is characterized by the existence of a correlation, linear or otherwise, of the pressure at the point identified as being the output of the coupler with respect to the pressure of the identified point. as the coupler input. In the case of an acoustic type connection, a correlation also exists in the other direction.

Thus, by placing a microphone at the input and at the output of an element, it is possible to check whether or not it is a coupler.

Preferably, an acoustic coupler that is purely acoustic or electroacoustic between a source emitting waves of pressure level lying between n1 and n2 in a frequency range f1-f2 and a face of a membrane makes it possible to transmit at least 20 % and preferably at least 70% and preferably as close as possible of 100% of the pressure level or the amplitude of the source to the membrane for a frequency range covering at least 70% of the range f1 - f2.

Preferably, the acoustic or electroacoustic coupler makes it possible to transmit to the membrane 100% of the sound level of the source for a frequency range covering 100% of the f1-f2 range.

An acoustic coupler thus makes it possible to establish an acoustic path between two distinct places, thus allowing the transmission of sound, with a possible transformation.

Preferably, the acoustic or electro-acoustic couplers used in the context of the present invention have a transmission maximum for a frequency higher than the lowest frequency to be attenuated in the primary system. Preferably, each coupler has a transmission maximum for a frequency between the transmission frequencies f1 and f2 of the source.

Preferably, the acoustic couplers of the present invention may be termed broadband.

Broad band coupling enables acoustic transmission over a wide frequency range, in contrast to a Helmholtz resonator which transmits acoustic waves in a narrow range around a single resonant frequency. For example, conical bore tubes (horns) are known to be wideband acoustic transmitters.

The couplers of the present invention may also have a plurality of resonant frequencies within the frequency range of the source 1.

The acoustic coupling between the source 1 and the first face 13 of the absorber 1 1 can be achieved by at least one acoustic coupler 15 made of an acoustic duct 15. In certain embodiments, such as that illustrated in FIG. 4b, this acoustic coupling is made by two couplers 15, 15 each formed of an acoustic duct.

According to another embodiment, the acoustic coupling is carried out by putting the first face 13 opposite the source 1, as illustrated in FIG. 5a, 5b. This is called direct acoustic communication.

Particularly advantageously, the device 10 is configured so that the second face 14 of the absorber 1 1 is also coupled to the source 1. This coupling may be a purely acoustic coupling, as illustrated in the non-limiting examples of FIGS. 1 to 4. It may also be an electro-acoustic coupling as illustrated in the non-limiting examples of FIGS. 5a to 5c.

Typically, the coupling between the source 1 and the second face 14 makes it possible to transmit at least 20% of the instantaneous acoustic pressure during part of the cycle in a frequency range at least equal to 70% and preferably equal to 100% of the range _, from source 1. The definition of acoustic coupling and couplers given above for the first face 13 also applies to the second face 14.

More specifically, the coupling element 16 of the second face 14 to the source 1 is configured to transmit a maximum level of sound for acoustic waves whose frequencies are greater than -

Thus, the two faces 13, 14 of the absorber 1 1 are acoustically coupled to the source 1. Preferably, this pressure difference is such that the instantaneous acoustic pressures P1 and P2 acting at a given instant on the first and second faces 13, 14 respectively and during at least part of the operating cycle of the source 1 satisfy the following condition: | P -P ₂ | > k. Pi, with k> 0.2 and preferably k> 0.5 and preferably k> 1, and preferably k> 1.5, and preferably k> 1.8.

Thus, the first and second faces 13, 14 are operated in a non-linear manner, and participate in the acoustic damping.

The device is configured to impose the waves from the source 1 a difference between those arriving on the first face 13 and those arriving on the second face 14 at the same time.

This difference in path results in a difference in phase and / or amplitude between the acoustic waves arriving on both faces 13, 14 at the same time. For example, the acoustic couplings of the two faces 13, 14 with the source 1 can be chosen so that the acoustic waves of the source 1 reach the second face 14 with an amplitude identical to those arriving at the same time on the first face 13 but with an opposite phase.

More generally, the coupling element of the second face with the source is configured in such a way that the acoustic waves coming from the source and reaching the second face 14 have a difference of direction with respect to those reaching the first face. 13. This difference in operation can be characterized as follows: the coupling element of the second face with the source has an inlet mouth for the acoustic waves coming from the source, the inlet mouth having a difference of operates with the first face of a value equal to ½ Ac modulo Ac, with Ac = (A ₂ ^0.5 λ-ι ^0.5 ) and more generally a value between ½ Ac - ¼ Ac and ½ Ac + ¼ Ac, modulo Ac; Ai being the wavelength of the frequency fi to be attenuated and A ₂ being the wavelength of the maximum frequency f ₂ to be attenuated. The two faces 13, 14 are then used and participate in the attenuation of sounds.

The device is then configured so that this acoustic coupling on the two faces 13, 14 creates a pressure difference during at least part of the operating cycle of the source 1.

The device may comprise a cover 21, facing the second face 14 to prevent the emission of secondary radiation. The cover 21 will not disturb the deformation of the second face 14, the latter being set in motion by the acoustic waves received from the source 1 to which it is coupled.

In order to create a difference in the path between the waves arriving at the same time on the first and second faces 13, 14 of the absorber 11, the device, according to some embodiments, preferably provides, in combination with the coupling elements, a distance "d" between the second face 14 and the input of the coupling element 16 coupling this face 14 to the source 1. "d" is chosen so as to ensure or at least to participate in the creation of the walking difference.

Preferably, d <1/2 A ₂ modulo A ₂ , (ie d <n.1 / 2 λ ₂ , with n any relative integer), A ₂ being the wavelength of the maximum frequency to be attenuated and preferably d <1/4 to ₂ .

Advantageously, the at least one coupling element 16 of the second face 14 with the source 1 is configured to transmit to the second face 14 acoustic waves whose phase and / or amplitude leads to that IP1 -P2l> k .lP1 l during at least X% of the operating cycle of the source 1, with X = 10. According to one embodiment, X = 30.

According to one embodiment, the acoustic absorber 1 1 has a linear resonance frequency f _ri , with f _ri <f1. Thus the acoustic absorber, typically a membrane, has a linear resonance frequency strictly less than the minimum frequency to be attenuated. If the acoustic absorber comprises several membranes, then each membrane has a linear resonance frequency strictly less than the minimum frequency to be attenuated.

The acoustic absorber membrane (s), having a linear resonance frequency f _ri , with f _ri <f1 and presenting a behavior in nonlinear deformation when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the f ₀ 1 -fo 2 range covering at least 50% of the f1-f2 range makes it possible to generate the energy pumping phenomenon, which leads to a particularly effective attenuation of the noise of the source.

Advantageously, the at least one coupling element 16 of the second face

14 with the source 1 is configured so that at a given instant the acoustic system comprising the membrane reaches a state likely to trigger the acoustic pumping, which is attained for example, without limitation, adding a noise to frequencies different from the frequency range of interest for a limited time see Côte et al. JSV 333 2014 5057-5076.

When the frequency range transmitted by the coupling element is partly greater than the frequency f2, the signal transmitted by the coupling element contributes to triggering the desired absorption regime, and the noise generated or transmitted at high frequencies can easily be attenuated by conventional systems such as porous absorbers. The action of the coupler can be verified by measuring an increase in the sound level when coupling is broken, for example by the insertion of a rigid wall, or by detecting a non-linear mode change characterized by its stationarity, its spectrum, and where appropriate the amplitudes and phases of its components. The coupler design can be verified by measuring its transfer function. This must show a maximum of transmission for frequencies higher than f1.

It thus emerges from the foregoing that the invention improves the acoustic comfort and reduces the damage due to vibrations, thanks to the transformation of the sound by means of at least one flexible membrane 12 and exploited on its two faces 13, 14. membrane 12 undergoes large deformations under the action of sound. The non-linearity of the system possibly allows the appearance of non-linear modes of low amplitude for the primary system and the energy pumping effect.

The invention is in part based on a membrane 12 which has properties of mass, elastic modulus, dissipation, and voltage, chosen for the desired operating regime (frequency range and sound intensity). According to one embodiment, the membrane 12 is covered on each of its two faces 13, 14 by a sealed cover 21 forming chambers, at least one of the chambers being connected to the primary system either directly or by an acoustic pipe. The choice of the dimensions of the covers 21 (shape, volume of air contained in the chamber), the parameters of the pipes (shape, length, diameter, positions of the connection to the primary system), make it possible to adapt the system to the intended application . The invention can also exploit the existing volumes in the system of the primary source for the realization of the chambers and pipes.

Compared to EP 2172640 A1 mentioned in the section on the state of the art, the dissipation chamber is replaced by a rear coupling chamber 16 connected to either the primary system or to an acoustic load. The coupling can be acoustic, electro-acoustic or by any other means. The main chamber can be part of the primary system or be connected acoustically.

The specificities of the embodiments illustrated in FIGS. 1 to 7 will now be detailed.

Figure 1 illustrates an embodiment in which the source 1 is coupled to the absorber 1 1 by a unidirectional coupler 15 or bidirectional 15 '. This coupler allows an acoustic coupling between the source 1 and the first face 13 of the absorber 1 1.

A unidirectional coupler is characterized by the fact that it transmits information in a single direction.

A bidirectional coupler is characterized by the fact that it transmits information in two opposite directions of the same path (rectilinear or not), for example in the two opposite directions of the same rectilinear direction.

The coupling element 16 allows acoustic coupling between the source 1 and the second face 14 of the absorber 1 1.

As illustrated, allows an acoustic coupling between the source 1 and the first face 13 of the absorber 1 1 can be a solely acoustic absorber, having as in this example two membranes 12, 12 '.

The face of the membrane 12 which is connected to the source 1 by the coupler 15 serves as the first membrane 14 for the absorber 11. The face of the membrane 12 'which is connected to the source 1 by the coupler 16 acts as second membrane 13 for the absorber 1 1.

As also illustrated in this figure to represent an alternative embodiment, the acoustic absorber may be or include a loudspeaker 19.

FIG. 2 illustrates an embodiment in which the source 1 is housed in an enclosure 30 inside which the absorber 1 1 of the device 10 is placed. The absorber 1 1, in the form of a membrane 12 or a stack of membranes is arranged in a tube. The tube is preferably flared at each of its ends to form a horn.

A first section of the tube disposed between a first end of the tube and a first face 13 of the absorber 1 1 forms an acoustic coupling between this face 13 and the source 1. A second section of the tube disposed between a second end of the tube and a second face 14 of the absorber 1 1 forms an acoustic coupling between this face 14 and the source 1.

In this embodiment as in that of Figures 3 and following, the first 13 and second 14 faces of the membrane 12 extend in parallel planes. In this embodiment as in that of Figures 3 to 6 and 7b and following, the first 13 and second 14 faces of the membrane 12 are mechanically bonded.

The shape and dimensions of the tube, for example its diameter, its flare and its disposition with respect to the source 1 are chosen so as to create the differential pressure on each face of the absorber 1 1, thus allowing the membrane 12 to deform freely and to exploit both sides.

The enclosure 30 makes it possible to confine the device 10 and the source 1 for a better attenuation of the sounds.

FIG. 3a illustrates an embodiment which differs from that of FIG. 2 in that the absorber 11, for example in the form of a membrane 12 or a stack of membranes connected in series or in parallel, is arranged at one end of a tube. The face turned towards the outside of the tube serves as the first face 13 for the absorber 1 1. The coupling between this face and the source 1 is therefore direct and is done through the internal volume of the chamber 30. The one facing the inside of the tube acts as a second face 14 for the absorber 1 1 The coupling of this face 14 and the source 1 is therefore done by the tube 17 forming the coupling element 16. According to one embodiment, the coupling element 16, here the tube 17 comprises at least one acoustically absorbing element preferably disposed on an inner wall of the coupling element 16. Advantageously, the acoustically absorbing elements are configured to absorb in particular at least the high frequencies produced by the source 1 and / or by the attenuation device, for example the 30% or 10% of the highest frequencies produced by the source and / or the attenuation device.

This optional and optional embodiment with absorbent element positioned in the acoustic coupling element is applicable to all other embodiments described in the present invention.

FIG. 3b illustrates an embodiment in which the source 1 is located or opens into the enclosure 30. This enclosure 30 comprises, outside its volume, a pipe inside which the absorber 1 is arranged. 1 typically a membrane 12 or a plurality of membrane connected in series or in parallel. A first section of the pipe 15 forms the coupling a first face 13 of the membrane 12 and the source 1 and a second section of the pipe 17 forms the coupling between a second face 14 of the membrane 12 and the source 1.

This embodiment thus differs from that of FIG. 2 in particular in that the coupling elements 15, 17 of the source 1 to the faces 13 and 14 of the absorber 11 are disposed outside the enclosure 30.

FIGS. 4a, 4b and 4c illustrate embodiments in which the absorber 1 1 comprises a cap 21 arranged facing the first face 13 of the membrane 12.

This cover 21 forms a front coupling chamber 24. It is in fact coupled to the source 1. This coupling is preferably a purely acoustic coupling. It may for example be composed of a plurality of pipes or pipes, for example two as illustrated in FIG. 4b.

The second face 14 of the membrane 12 is also coupled to the source 1 by the coupling element 16. The coupling can be acoustic only (a conduit for example) or be electro-acoustic. The dots used in FIG. 4a illustrate that different types of coupling elements 16 can be used.

Preferably, the device 1 comprises a cover 21 facing the second face 14, defining with the latter a rear coupling chamber 25. The rear chamber 25 is not closed but allows an acoustic connection between the coupling element 16 and the source 1.

Thus, the second face 14 is placed in a coupling chamber 25 and not in a dissipation chamber as provided by the solutions of the state of the art mentioned in the section relating to the prior art.

In the embodiment illustrated in FIG. 4c, the rear coupling chamber is coupled to the source 1 via the front chamber 24. A duct 17 acoustically couples the front 24 and rear 25 chambers, the front chamber 24 being coupled to the source 1 by the coupler 15. This embodiment clearly shows that the coupling element 16 of the second face 14 to the source may comprise several elements, here: a conduit 17, the front chamber 24 and the conduit 15.

In these embodiments, the properties of the membrane or diaphragms 12, covers 21 and couplers 15 and 17 are chosen with respect to the source 1 so as to create the pressure differential across the faces 13 , 14, thus allowing at least one membrane 12 to deform non-linearly and to create energy pumping for each of its faces.

Figures 5a, 5b, 5c illustrate embodiments in which the absorber 1 1 and an electro-acoustic absorber. It comprises a loudspeaker 19. The membrane (s) 12 are then motorized membranes.

In the embodiments of FIGS. 5a and 5b, the primary source is placed in an enclosure 30 and a front face of the loudspeaker 19 is also arranged in or with respect to this enclosure 30. It can, as illustrated, form a portion of the wall of the enclosure 30. Thus, the acoustic coupling between the first face 13 of the absorber 1 1 and the source 1 is a direct coupling.

The second face 14 of the absorber 1 1 or rear face is placed in a hood

21.

The device 10 is equipped with a control module 23 of the loudspeaker 19. This control module controls the membrane 12. This control is performed so as to perform on the membrane, during a part of the operating cycle, a linear function or not the pressure relative to the source 1 including by measuring magnitudes related to the movement of the membrane. Thus, this control notably takes into account information relating to source 1.

In the embodiment of FIG. 5a, this information relating to the source 1 is picked up by a microphone 20. It is preferably placed inside the enclosure 30 and is therefore in direct acoustic coupling with the source 1. Preferably, the cover 21 is then closed with the exception of a static pressure balancing capillary 22. capillary 22 has a high inertia, for example a length-to-diameter ratio, so as to balance the average pressure between the chamber 30 and the chamber 25.

However, this capillary 22 does not allow, by its inertia, to transmit the fast vibrations and thus the acoustic waves of the source 1.

In the embodiment of FIG. 5b, this information relating to the source 1 is picked up on the surface of the second face 14 of the membrane of the loudspeaker 19.

The device 10 taking the information on the surface of the membrane 12 thus measures the dual size of the speaker 19. For example, if the speaker 19 is voltage-controlled, the current is measured, and vice versa. This makes it possible to measure the response of the loudspeaker 19 to all the requests to which it is subjected.

In this embodiment also, the coupling between the second face 14 or the rear of the speaker 19 and the source 1 is indeed an electro-acoustic coupling.

Moreover, in this embodiment, the device 10 has a purely acoustic coupling element 16 between the rear face of the loudspeaker 19 and the source 1. This acoustic coupling comprises, for example, a channel between the enclosure 30 and a cover 21 in which the second face 14 of the absorber 11 is placed. The properties of this channel, in particular its volume, its diameter, its length and its position and its shape allow it to transmit inside the chamber 25 the acoustic waves of the source 1. This coupling element therefore not only allows a pressure equalization unlike the capillary 22 of the previous embodiment.

Thus, in this embodiment, the chamber 25 is an acoustic coupling chamber.

FIG. 5c illustrates an embodiment similar to that of FIG. 5a and which differs in the manner in which the first face 13 of the loudspeaker 19 is coupled to the source 1. This figure illustrates in dotted lines a coupling which may not be a direct acoustic coupling as provided in the embodiment of FIG. 6, but may possibly be an acoustic coupling made by a conduit 15 or by an electro-acoustic coupler.

FIG. 6 illustrates an embodiment in which the absorber 11 comprises a plurality of membranes 12. In this nonlimiting example, it comprises four membranes 12 and a motorized membrane equipping a loudspeaker 19. The two membranes 12 'and 12 "together form an absorber comprising two faces facing each other and two faces 13, 14 facing outwards and acting as first and second faces coupled to the source 1.

The two membranes 12 "'and 12" "each have a face which together define a first face 13 and each have a face which together define a second face 14.

The device comprises as many couplers as necessary in order to couple each of the front and rear faces to the source 1.

Figures 7a and 7b illustrate two embodiments of absorber 11 that can be used for all embodiments of the invention. In each of these two embodiments, the absorber 11 comprises a plurality of membranes 12. Preferably, each of these membranes has properties enabling it to deform non-linearly for at least part of the frequency range. fi-f ₂ and pressure levels nn ₂ of source 1.

In the embodiment illustrated in FIG. 7a, the absorber 11 comprises a plurality of membranes 12 arranged in series. The outwardly facing face of an outer membrane forms the first face 13 coupled to the source 1 and the outwardly facing face of another outer membrane forms the second face 14 coupled to the source 1.

In the embodiment of FIG. 7b, the membranes are associated with each other so that one of their faces together form the first face 13 of the absorber 1 1 coupled to the source 1 and so as to the other of their faces together form the second face 14 of the absorber 1 1 coupled to the source 1. Detailed example of realization:

A non-limiting example will now be detailed with reference to Figures 8a, 8b, 9a and 9b.

This example describes a sound attenuation device 10, also referred to as a choke, intended to be disposed in the intermediate space formed between a false ceiling 60 and the upper wall 51 of a part 50. This arrangement is illustrated in FIG. 8a. In this arrangement, the source 1 emitting acoustic waves is a person or any equipment of the room 50. The entire room thus forms an enclosure 30 within the meaning of the present invention.

The false ceiling 60 is configured to pass the acoustic waves from the source 1 to the attenuation device 10. The attenuation device 10 comprises a box 40 or box enclosing a membrane 12. The box is closed except for two openings from which an acoustic coupler 15, 17 respectively extends. Each of the acoustic couplers 15, 17 forms a flag. In this embodiment, the acoustic couplers 15, 17 are identical.

Each acoustic coupler 15, 17 forms a truncated cone. In this example, the end 152, 172 of smaller diameter has a diameter 4 times smaller than that of the end 151, 171 of larger diameter. Each end 152, 172 of smaller diameter is attached to the box 40 and more precisely to an opening thereof. Each acoustic coupler 15, 17 thus obscures an opening of the box 40.

Acoustic couplers 15, 17 are aligned along the axis of the truncated cone.

Preferably, the attenuation device 10 is oriented along a diagonal of the part 50. Thus the axes of the truncated cones are parallel to the plane containing the false ceiling 60 and are oriented along a diagonal of the latter.

The membrane 12 is carried by a sealed partition 41. Thus, the box 40 combined with the partition 41 defines two chambers each forming a coupling volume.

The membrane 12 used in this example has a diameter of 6 cm. It is consistent with that described in the publication CR Mechanics 334 (2006) already mentioned above: B. Cochelin, P. Herzog, PO Mattei, "Experimental evidence of energy pumping in acoustics" CR Mechanical volume 334, pages 639 to 644 , 2006.

The length of the entire device 10 formed by the caisson 40 and the acoustic couplers 15 is approximately 1.7 m. The device is intended to attenuate sounds whose frequency is around 100 Hz.

The operation of the device 10 is conditioned by the characteristics of the membrane 12 and by the dimensions of the coupling volumes that surround it.

With respect to a solution where only one of the faces of the membrane is exploited and where there would therefore be only one acoustic coupler (a single horn coupled to the membrane), the attenuation device 10 according to the The invention has the following specificities and advantages:

The enclosure of the membrane makes it possible to absorb the high frequency noise while crossing the roof, without this noise being directly radiated in the room 50.

The enclosure of the membrane does not degrade performance because the second pavilion (in addition to the first pavilion) provides acoustic coupling. The invention also allows the simultaneous processing of several resonant modes of the part 50 where the device 10 is implanted. Obviously if their frequency is identical and their difference in operation corresponds to ½ wavelength, difference measured at the level of the membrane. Different modes of possibly immeasurable frequencies can also be attenuated (cf JSV332 (2013) 1639, S. Bellizzi and coauthors, "Responses of a two degree-of-freedom system coupled to a nonlinear damper under multi-forcing frequencies" in Journal of Sound and Vibration, Vol. 332, pp. 1639-1653, 2013).

Some additional details of this exemplary embodiment are given below:

• Room 50: The floor of room 50 forms a square 3.6 meters diagonal. It has a height under the false ceiling 60 is 3m.

• Length of the coupler 15, 17 taken along its main axis: 45 cm;

• Surface S151, S171 of the end of larger diameter: 0.1 1 meters

squares;

• Surface S152, S172 of the end of smaller diameter: 0.0069 meters

squares;

• Box 40 forming a cube of 48 cm side;

• Watertight partition 41 defining with the box 40 two volumes of 0.056

cubic meters each.

From the foregoing description, it is clear that the invention improves the acoustic comfort and reduces the damage due to vibration, through the transformation of sound by means of an absorber such as a flexible membrane. Moreover, the membrane that is operated on both sides has no direct radiation in the room (which provides better comfort), and the implementation of couplers at two distinct points can treat more acoustic modes. The connection of the back of the membrane to the primary acoustic source creates an acoustic charge on the back of the membrane. This acoustic load pumps air from the rear chamber and deforms the membrane in a coordinated manner with the primary source. The non-linearity of the system allows the appearance of non-linear modes of low amplitude for the primary system and the membrane undergoes large deformations under the action of sound. The invention thus avoids the problem of secondary radiation without impairing the attenuation of sound. It also emerges from the foregoing description that the invention provides the following technical advantages due to the coupling of the second face of the membrane with the source and the non-linearity of the operation of the membrane:

- removal of radiation to the outside without canceling the efficiency of sound transformation;

- exploitation of the apparent linear stiffness added to the membrane by the coupling chamber to improve the sound transformation efficiency;

- coordination of effects on both sides of the membrane so that they act in cooperation to improve the sound transformation efficiency;

- Possible coupling of the membrane in several positions of the source to improve the sound transformation efficiency.

- Covering the membrane to deport the invention away from the primary system. This is useful in particular for reasons of space or constraints due to the primary system (eg temperature, flow).

The invention is not limited to the previously described embodiments and extends to all the embodiments covered by the claims.

REFERENCES

1. Source

10. Mitigation device

1 1. Acoustic absorber

12. Membrane

13. First face

14. Second face

15. Acoustic coupler with first face 16. Coupling element

17. Acoustic coupler with the second side

18. Electroacoustic coupler

19. Speaker

20. Microphone

21. hood

22. Static pressure balancing capillary

23. Speaker control

24. Coupling chamber with the first side

25. Coupling chamber with the second side

30. Pregnant

40. Caisson

41. Waterproof partition

50. Room

51. Upper wall

60. False ceiling

70. Intermediate space

151. Acoustic coupler input

152. Output of the acoustic coupler

171. Acoustic coupler input

172. Output of the acoustic coupler

100. System

Claims

1. Attenuation device for attenuating acoustic waves generated by a source (1) emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2:

the attenuation device comprising at least one acoustic absorber comprising at least one membrane, the acoustic absorber having at least one first face and at least one second face; ) distinct from the first face (13), the acoustic absorber (1 1) being configured to exhibit non-linear deformation behavior when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the range f ₀ 1 -fo2 covering at least 50% of the range f1 -f2 and whose pressure levels are between n ₀ 1 and n ₀ 2, the range n ₀ 1 -n ₀ 2 covering at least 50% of the n1 -n2 range;

the attenuation device being configured so that the first face (13) of the absorber (1 1) is in acoustic communication with the source (1);

the attenuation device being characterized in that it comprises at least one coupling element (16) of the second face (14) with the source (1), the coupling element (16) being configured to transmit to the second face (14) a maximum power for acoustic waves whose frequencies are higher than f1 and configured to transmit to the second face (14) acoustic waves of which:

the instantaneous acoustic pressure exerted on the second face (14) is a function of the instantaneous acoustic pressure of the acoustic waves emitted by the source (1),

the phase and / or the amplitude results in IP1 -P2l> k.lP1 1 during at least part of the operating cycle of the source (1), with P1 and P2 being the instantaneous acoustic pressures of the acoustic waves arriving respectively on the first and second faces (14) at the same time and with k> 0.2.

2. Device according to the preceding claim, wherein the coupling element (16) is configured to transmit to the second face (14) acoustic waves whose phase and / or amplitude leads to that IP1-P2l> k .lP1 for at least part of the operating cycle of the primary source (1), with k> 0.5 and preferably k> 1.

3. Device according to the preceding claim, wherein k> 1, 5, preferably k> 1 .8.

4. Device according to any one of the preceding claims, wherein the at least one coupling element (16) of the second face (14) with the source (1) is configured to transmit acoustic waves whose phase and / or amplitude to the second face (14) results in IP1-P2l> k.lP1 being present for at least X% of the operating cycle of the source (1), with X = 10.

5. Device according to the preceding claim, wherein X = 30.

6. Device according to any one of the preceding claims, wherein the acoustic absorber (1 1) has a linear resonance frequency f _ri , with f _ri <f1.

7. Device according to any one of the preceding claims, wherein the at least one coupling element (16) of the second face (14) with the source (1) is configured so that the membrane reaches a state. acoustic pumping, preferably at least when the frequency range transmitted by the coupling element (16) is partly greater than the frequency f ₀ 1 and preferably greater than the frequency f ₀ 2.

8. Device according to any one of the preceding claims, configured to allow two-way communication between the source (1) and said first face (13) of the absorber (1 1).

9. Device according to any one of the preceding claims, wherein at least one membrane (12) is configured to exhibit non-linear deformation behavior when it receives acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the f ₀ 1-fo 2 range covering at least 70% and preferably 100% of the f1-f2 range.

10. Device according to any one of the preceding claims, wherein at least one membrane (12) is configured to exhibit non-linear deformation behavior when it receives acoustic waves whose pressure levels are between n ₀ 1 and n ₀ 2, the range n ₀ 1 -n ₀ 2 covering at least 70% and preferably 100% of the range n1-n2.

1 1. Device according to any one of the preceding claims, wherein the coupling element (16) of the second face (14) with the source (1) comprises at least one acoustic duct (17).

12. Device according to the preceding claim, comprising a cover (21) forming with said second face (14) a closed volume with the exception of an opening formed by said at least one acoustic duct (17).

13. Device according to claim 1 1, wherein said second face (14) is housed in said at least one acoustic duct (17).

14. Device according to any one of the preceding claims, wherein the coupling element (16) comprises at least one inner wall and at least one acoustically absorbing element disposed on the inner wall.

15. Device according to the preceding claim, wherein the acoustically absorbent elements are configured to partially absorb at least the high frequencies produced by the source (1) and / or by the attenuation device.

16. Device according to any one of the two preceding claims, wherein the at least one acoustically absorbing element is glass wool.

17. Device according to any one of the preceding claims, wherein the coupling element (16) of the second face (14) with the source (1) comprises at least one electro-acoustic coupler (18).

18. Device according to the preceding claim, wherein the electroacoustic coupler (18) comprises at least one speaker (19) and wherein at least one membrane (12) is a membrane (12) of the loudspeaker (19).

19. Device according to the preceding claim, wherein an outer face of the membrane (12) of the speaker (19) is said first face (13).

20. Device according to any one of the two preceding claims, wherein the absorber (1 1) is configured in such a way that the loudspeaker (19) receives an electrical signal according to an acoustic signal from the source ( 1).

21. Device according to the preceding claim, wherein the absorber (1 1) comprises a microphone (19) arranged to pick up acoustic waves from the source (1) and is configured so that the acoustic signal is provided by the microphone (19).

22. Device according to the preceding claim, comprising a cover (21) defining with said second face (14) a closed volume with the exception of a capillary

(22) balancing the static pressures prevailing on said first face (13) and said second face (14).

23. Device according to claim 18, wherein the absorber (1 1) is configured to receive an acoustic signal taken from the second face (14) of the membrane (12).

24. Device according to the preceding claim, comprising a cover (21) defining with said second face (14) a closed volume with the exception of a conduit (17) forming acoustic coupling between said second face (14) and the source ( 1).

25. Device according to any one of the preceding claims, wherein the attenuation device comprises a plurality of coupling elements of the second face (14) with the source (1).

26. Apparatus according to any one of the preceding claims, wherein the attenuation device is configured so that the acoustic communication between the source (1) and said first face (13) of the absorber (1 1). either a direct acoustic coupling without intermediate sound transmission element.

27. Device according to the preceding claim, wherein the attenuation device comprises an enclosure (30) configured to accommodate the source (1) and the acoustic absorber (1 1), the attenuation device being configured so that that the first face (13) of the absorber (1 1) is in acoustic communication with the source (1) by the internal volume of the enclosure (30).

28. Device according to any one of claims 1 to 25, wherein the attenuation device is configured so that the acoustic communication between the source (1) and said first face (13) of the absorber (1). 1) is an acoustic coupling made in part at least by one or more acoustic ducts (15).

29. Device according to any one of the preceding claims, wherein the coupling element (16) of the second face (14) with the source (1) has a plurality of resonant frequencies.

30. Device according to any one of the preceding claims, wherein the first (13) and second (14) faces of the absorber (1 1) extend in parallel planes.

31. Device according to any one of the preceding claims, wherein the first (13) and second (14) faces of the absorber (1 1) are mechanically connected.

32. System (100) comprising a source (1) emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2 and an attenuation device according to any one of the claims. previous configured to attenuate the acoustic waves of said source (1).

33. An attenuation method for attenuating acoustic waves generated by a source (1) emitting acoustic waves whose frequencies are between f1 and f2 and whose pressure levels are between n1 and n2, characterized in that the method comprises the following steps:

selecting at least one acoustic absorber (1 1) comprising at least one membrane (12), the acoustic absorber (1 1) having at least one first face (13) and at least one second face (14) distinct from the first face (13), the acoustic absorber (1 1) being configured to exhibit non-linear deformation behavior when receiving acoustic waves whose frequencies are between f ₀ 1 and f ₀ 2, the range f ₀ 1 - fo2 covering at least 50% of the range f1 -f2 and whose pressure levels are between n ₀ 1 and n ₀ 2, the range n ₀ 1 -n ₀ 2 covering at least 50% of the range n1-n2;

arranging the first face (13) of the absorber (1 1) in acoustic communication with the source (1);

selecting a coupling element (16) of the second face (14) with the source (1), the coupling element (16) being selected so as to transmit to the second face (14) a maximum of power for waves acoustic whose frequencies are greater than f1 and configured to transmit acoustic waves to the second face (14) of which:

the instantaneous pressure exerted on the second face (14) is a function of the instantaneous pressure of the acoustic waves emitted by the source

(1).

the phase and / or the amplitude results in IP1 -P2l> k.lP1 1 during at least part of the operating cycle of the source (1), with P1 and P2 being the instantaneous pressures of the acoustic waves arriving respectively on the first and second faces (14) at the same time and with k> 0.2.

34. Method according to the preceding claim, wherein the at least one coupling element (16) of the second face (14) with the source (1) is configured to transmit to the second face (14) acoustic waves whose phase and / or amplitude leads to that IP1 -P2l> k.lP1 1 for at least X% of the operating cycle of the source (1), with X = 10.