ACOUSTIC ABSORBER, ACOUSTIC WALL AND METHOD FOR DESIGN AND PRODUCTION

Abstract

A passive sound absorber includes a cavity opening to the outside via an input direction to form a Helmholtz resonator for a first frequency. The absorber further includes at least one moving element, or wafer, suspended or held by suspensions in a position obstructing the neck in a non-sealed manner. The relative stiffness of the suspensions and the wafer is determined so that the assembly resonates in a vibration in a piston movement at a second frequency different from the first frequency, achieving absorption for this second frequency or frequency range. A hybrid version includes a coil that is controlled to adjust the acoustic impedance of the absorber.

An acoustic wall includes a plurality of such absorbers produced by a repetitive structure opening through perforations, each receiving such a wafer, and a method of designing and manufacturing such an absorber or wall is also provided.

Claims

1. An acoustic absorber device, notably passive absorber, comprising: an enclosure delimiting a cavity opening outwardly into an inlet direction through at least one orifice passing through a front wall of a determined thickness, thereby forming a neck having a determined opening surface and a determined length, the dimensions of said enclosure and said neck being determined to together form a Helmholtz resonator for a first frequency or frequency range, called natural frequency: at least one mobile element, or wafer, is suspended to said enclosure by one or more mechanical connections, or suspensions, in a position partially obstructing said at least one neck, i.e. unsealed on all or part of its stroke; and the stiffness of the suspensions and the stiffness of the wafer are determined in their combination, particularly in their ratio, so that said wafer vibrates in a piston type resonance mode along the direction of the incident wave, at a second frequency or frequency range different from the first frequency, particularly lower, thereby achieving absorption for this second frequency or frequency range.

2. The device according to claim 1, characterised in that the wafer is made of one or more materials chosen from silicon, quartz, alumina, titanium and its alloys, steel, aluminium and its alloys, plastics and notably polymers.

3. The device according to claim 1, characterised in that the suspensions are made of a material and using a geometry providing an elastic behaviour, with stiffness for the movement of the wafer in its periphery of less than 6 N/m, and in particular of less than 2 N/m for a wafer of average diameter between 10 mm and 20 mm.

4. The device according to claim 1, characterised in that the suspensions comprise elongated arms connecting the wafer to the enclosure in a shape extending around said wafer parallel to the edge of the neck and/or the wafer.

5. The device according to claim 4, characterised in that the wafer is made within a plate or a sheet integral with the enclosure, by a part rendered mobile with respect to said enclosure by means of one or more cutouts made in said plate or sheet so as to form suspension arms.

6. The device according to claim 1, characterised in that the wafer is held in the neck by one or more advances protruding from the neck at both ends to extend in front of the periphery of the wafer so as to form a stop preventing said wafer from escaping from the neck.

7. The device according to claim 1, characterised in that the wafer has a periphery which conforms to the inner surface of the neck with a determined deviation over a sufficiently determined length, in combination with said deviation and with the nature of the materials of the neck and the wafer, to allow said wafer to move along the neck without causing its blocking by tilting and arching.

8. The device according to claim 1, characterised in that the wafer is formed by a diaphragm of speaker fixed to an outer frame by a flexible peripheral seal, and in that said seal has one or more cutouts surrounding said wafer over at least 20% of its periphery, and in particular at least 40%.

9. The device according to claim 1, characterised in that the wafer further interacts with the enclosure by an electromagnetic system so as to form the membrane of a loudspeaker, and in that said electromagnetic system is controlled by an electronic circuit: in order to achieve active acoustic absorption, and/or so as to modify the acoustic impedance of said loudspeaker to enhance absorption, shift the absorption frequency, widen the absorption frequency range, or a combination of these effects.

10. A sound absorbing wall comprising a multitude of devices according to claim 1, juxtaposed within a continuous two-dimensional array to provide acoustic absorption in a common direction.

11. The wall according to claim 10, characterised in that it comprises a plate with a honeycomb structure whose housings form a multitude of cavities which are closed on a so-called rear side, and whose cavities are covered on one front side by one or more walls cut to form a multitude of necks each receiving a wafer.

12. A process for the industrialisation of an acoustic absorber according to claim 1, intended to absorb a target frequency, comprising: a step of determining dimensions of a cavity provided with a neck so that said cavity and said neck form a Helmholtz cavity having a first frequency Helmholtz resonance higher than the target frequency; and a step of determining characteristics of a suspended wafer adapted to be arranged in the neck of said cavity so as to produce an absorber tuned to a second frequency corresponding to said target frequency.

13. The method according to claim 12, wherein the suspended wafer is determined so that the suspension of the absorber has its first normal mode of deformation at a frequency lower than the second frequency.

14. The method according to claim 13, characterized in that the wafer of the acoustic absorber is determined so as to have, when it is free, its first normal mode of deformation at a frequency higher than the second frequency.

15. A method of manufacturing an absorber according to claim 1, characterised in that it comprises at least one step of cutting out a sheet or plate so as to form one or more acoustic absorber wafers.

16. The method according to claim 15, characterised in that the plate or sheet is fixed to the surface of a plate having a honeycomb structure, and in that the cutting out step produces a plurality of wafers distributed with respect to the housings of the honeycomb structure so as to form the plurality of wafer of an acoustic wall whose housings form a multitude of cavities which are closed on a so-called rear side, and whose cavities are covered on one front side by one or more walls cut to form a multitude of necks each receiving a wafer.

Description

LIST OF FIGURES

[0097] Other features and advantages of the invention will emerge from the detailed description of an embodiment, which is in no way limiting, as well as the appended drawings where:

[0098] FIG. 1 is a diagram in axial section which illustrates a Helmholtz resonator according to the state of the art;

[0099] FIGS. 2a and b are perspective diagrams that illustrate an acoustic wall according to the state of the art, comprising a multitude of Helmholtz resonators, formed by a honeycomb structure covered with a perforated plate, before and after assembly;

[0100] FIG. 3 is a perspective view of an axial section of an absorber according to the first embodiment of the invention, comprising a cavity of 21 cm.sup.3 with electrodynamic silicon wafer;

[0101] FIG. 4 is a scale perspective view illustrating the cuts forming the suspensions and the absorber wafer of FIG. 3;

[0102] FIG. 5 is a scale perspective view illustrating the wafer of the absorber of FIG. 3, in a version with its electromagnetic coil and stiffeners;

[0103] FIG. 6 is a schematic view in principle, in axial section, of an absorber defined by the invention, in a configuration where the neck is more narrow than the cavity;

[0104] FIG. 7 is a graph illustrating absorption curves experimentally obtained using the absorber of FIG. 3 and for two different cavity volumes, in a configuration with and without a seal around the wafer;

[0105] FIG. 8 is a schematic view in principle, in axial section, of an absorber according to a second example of embodiment of the invention, in a configuration with a neck forming part of the cavity;

[0106] FIG. 9 is a schematic view in axial section which illustrates an acoustic wall according to a third example of embodiment of the invention comprising a multitude of absorbers, formed by a honeycomb structure covered with several perforated plates forming the neck and which enclose a cut plate to form the wafers and their suspensions;

[0107] FIGS. 10a and b are schematic views, in axial section and seen from the left side, which illustrate one of the absorbers within a honeycomb acoustic wall, according to a fourth example of embodiment of the invention, with a free wafer retained by outer layers protruding above the neck;

[0108] FIGS. 11a and 11b are schematic half-views in axial section, which illustrate one of the absorbers of a honeycomb acoustic wall, according to two variations of a fifth example of embodiment of the invention, with a thick unfixed wafer retained by external layers protruding above the neck;

[0109] FIG. 12 is a schematic view in axial section which illustrates a sixth example of the invention embodiment with a conical diaphragm-shaped electrodynamic loudspeaker mounted on perforated peripheral joints; wherein the leakage section is formed by orifices passing through the inside of the membrane, in two half-views presenting different variations;

[0110] FIG. 13 is a schematic view in axial section which illustrates a seventh example of the embodiment with a rigid wafer having a leakage section in its inner part, where the leakage section is formed by orifices passing through the inside of the wafer, presenting two half-views with different variations; [0111] in FIG. 13a, with unsealed suspension, and [0112] in FIG. 13b, with waterproof suspension;

[0113] FIG. 14 is a schematic view in axial section which illustrates an eighth example of a flexible centre wafer embodiment of including a leakage section, wherein the leakage section is formed by orifices passing through the inside of the wafer, in two half-views presenting different variations; [0114] in FIG. 14a, with unsealed suspension, and [0115] in FIG. 14b, with sealed suspension;

[0116] FIG. 15 is a diagram illustrating the difference between a movement: [0117] in FIG. 15a, in piston mode, and [0118] in FIG. 15b, in drum mode;

[0119] FIG. 16 is a schematic axial sectional view which illustrates the neck and the wafer of the absorber of FIG. 3, in a version with its electromagnetic coil and stiffeners as shown in FIG. 5.

DESCRIPTION OF EXAMPLES OF EMBODIMENT

[0120] Single Absorber

[0121] FIG. 1 to FIG. 7 illustrate a first example of embodiment of the invention. For the other examples, only their differences from the first embodiment will be described.

[0122] In this first example of embodiment, the absorber 3 was made and tested in the context of research originally intended to achieve an active reduction system by loudspeaker.

[0123] The absorber 3 has the form of a cylinder delimiting an interior cavity 30. This cavity 30 is surrounded by a cylindrical wall 39, it is entirely closed by a rear flat wall 38 and partially by a front wall 37. The latter is pierced with a central orifice opening in a axial D3 direction towards the cylinder of the cavity 30. This orifice has a cylindrical shape through the thickness of the front wall 37, and thus forms a neck 31 of L31 length and with A31 cross section.

[0124] In the example described here, the used wafer is formed by the silicon membrane of an electrodynamic micro-speaker made using MEMS technology (for Micro Electro Mechanical Systems), as described in Iman Shahosseini's thesis, Towards micro high-performance electroacoustic loudspeakers in silicon technology , PhD thesis, Institute of Basic Electronics, 2012, or in I. Shahosseini et al., Towards high fidelity high efficiency mems microspeakers, IEEE International conference on sensors, pp. 2426-2430, 2010 . . . . These electrodynamic micro-HP silicon has the characteristic feature of being less than one centimetre thick and having a resonance frequency comparable to that of a conventional midrange speaker (500 Hz), which allows good integration into a thin environment, for example into a wall of less than 50 mm.

[0125] As illustrated in FIG. 4, the wafer 32 is formed by an inner part cut out of a rigid plate 320. This cutout is made in a pattern comprising several cutouts 330 which surround the wafer 32 almost entirely. In this example, several essentially linear (i.e. one-dimensional) cutouts 330 are made at angles distributed regularly around the centre C32 of the wafer, here in six identical cuts. Each of these cutouts 330 covers an angular part of the periphery away from the centre 32 by a specific distance, which corresponds to the width of the arms and the distance E31 between the periphery of the mobile wafer 325 and the wall of the neck 31. Each of these cuts extends partially along its neighbours, inward and in one direction (here: in the counter-clockwise direction) and outward in the other direction (here: clockwise). Thus, between each group of two cuts side by side, the remaining material forms a spirally developing arm extending along the periphery of the wafer, over a length L330 that is much larger than the gap E31 between the neck and the wafer. It is thus possible to obtain arms 331 (in grey in the figure), of great length and therefore of low stiffness, despite the rigidity of the material of the initial plate 320.

[0126] In this example, the initial plate 320 is made of silicon with a total thickness of 20 m and outer dimensions of 23 mm28 mm, monocrystalline silicon for example, that can be obtained from an SOI-type substrate. The wafer 32 cut out inside this plate has a diameter of 13 mm, and the cutouts 330 have a width of around 20 m. At both ends, the cutouts 330 widen into a circular shape (in black in FIG. 4 and FIG. 5) to limit the fatigue of the material and to avoid cracks.

[0127] As illustrated in FIG. 5, this wafer also carries stiffeners 34, made using methods known in the field of MEMS, formed by ribs protruding from its surface over a certain height, here 300 m. The total thickness of the wafer, when it comes to its rigidity, is therefore 320 m.

[0128] As part of this experiment, the loudspeaker thus produced further comprises a series of electrical tracks deposited on the periphery of the wafer to form an electromagnetic coil (optional) 324 which are connected to the fixed part by two of the 20 m-thick suspension arms 331 that are also formed by cutting into the initial plate 320.

[0129] As illustrated in FIG. 3, the electromagnetic system of this loudspeaker is completed by a permanent annular magnet 374, fixed inside the neck 31 to interact with the coil 324. This magnet can be composed of two Neodymium Iron Boron ring magnets whose theoretical polarisation value is 1.5T, as described in the Shahosseini thesis.

[0130] FIG. 6 is a schematic diagram illustrating this absorber 3, with a suspension 33 which is not sealed and of a very low stiffness (in dashed rounded lines) which can be considered as negligible compared to the stiffness of the wafer 32 (and therefore favouring the piston mode), despite the fact that the suspension and the wafer are formed by the same initial plate.

[0131] In passive mode, in the tests carried out and illustrated in FIG. 7, the wafer vibrates in piston mode by moving between the extreme positions 32a and 32b (dashed lines in FIG. 6). The amplitude of these movements corresponds to a maximum movement of less than 2 mm from the equilibrium position (solid line), and the suspension allows a movement without breakup to up to approximately 4 mm.

[0132] Initially, the experiments aimed at achieving active reduction by activating the loudspeaker according to an electronic control aimed at attenuating frequencies close to the Helmholtz resonance frequency of cavity 30 on which it was mounted. This work was done in the framework of Alexandre Houdouin's thesis of the IEF in 2014, that has not yet been published. To avoid acoustic short circuits, as it is natural when one seeks to optimise the efficiency of a loudspeaker, it was planned to close the gap E31 by a continuous and sealed peripheral seal. Several types of joints had been considered, for example cast latex or a thermoformed polyethylene film.

[0133] However, various tests were carried out at different completion stages of the envisaged system, including before mounting this seal.

[0134] The following table presents the geometrical values of the cavity 30 and the neck 31, as well as the calculated and measured resonant frequencies, for the two tested cavities and without the wafer.

TABLE-US-00001 Parameters Small cavity Large cavity Units R.sub.neck 0.8 0.8 cm A.sub.neck 2.0 2.0 cm.sup.2 L.sub.neck 1.6 2.0 cm V.sub.cavity 21 169 cm.sup.3 f.sub.Helmholtz 1324 417 mm f.sub.measured 1310 420 mm

[0135] Thus, FIG. 7 shows the absorption results in purely passive mode, in a test carried out within a Kundt tube, with the cavity alone (curves in solid lines) with a speaker with no power and without its seal (curves in dashed lines).

[0136] For a large cavity with a volume of 169 cm.sup.3, the curve R1a shows the absorption coefficient obtained with the cavity alone, with a maximum of approximately 0.42 for the measured frequency of 420 Hz. However, for this same cavity, the curve R3a shows that the absorption coefficient has a greatly increased maximum which rises to 0.86 for a frequency that has shifted downwards to 316 Hz.

[0137] Similarly, for a smaller cavity of 21 cm.sup.3 (with a diameter of 30 mm and a height of 30 mm), the curve R1b shows the absorption coefficient obtained with the cavity alone, with a maximum of approximately 0.58 for the frequency of 1310 Hz. However, for this same cavity, the curve R3b shows that the absorption coefficient has an increased maximum which rises to 0.72, for a frequency that this time greatly shifted down to about 930 Hz.

[0138] Compared to the sealed loudspeaker configuration, the calculation showed that the seal removal reduces the stiffness of the system to a value of 5.8 N/m instead of 819.7 N/m, in addition to implying the presence of acoustic leaks.

[0139] Therefore, in a strictly passive manner, results show that mounting such a wafer on the neck of a Helmholtz cavity, if possible very rigid and mounted in a very flexible and preferably light way, allows to obtain a cavity given an improvement of the absorption as well as a decrease of the absorption frequency.

[0140] In FIG. 8 a diagram of an absorber is presented according to a second example of embodiment of the invention, described only in how it differs from the first one, which has the characteristic of having a neck that is part of the cavity. Such a configuration, combined with the other embodiments presented here, makes it possible to vary the possibilities of configuration and agreement, and to improve the compactness and/or the ease of device manufacture.

[0141] Acoustic Wall

[0142] FIG. 9 illustrates an acoustic wall 5 according to a third example of embodiment of the invention, comprising a multitude of absorbers 3, for example that of FIG. 4. This wall is formed by a plate 500 with a periodic honeycomb structure whose housings are parallel to the inlet direction D3 of its absorbers 3. This plate 500 is sealed on its rear side by a sealing layer 58, e.g. a composite layer, a sheet or a bonded sheet.

[0143] This periodic honeycomb architecture makes it possible, for example, to produce an acoustic wall comprising a very high surface density of absorbers while limiting the thickness of the assembly, even if it means using a honeycomb with large housings transversely to the input direction to obtain a large cavity volume maintaining a small overall thickness, for example less than 100 mm or less than 50 mm.

[0144] On its front side, this honeycomb plate 500 is covered with two layers 511 and 513, which are perforated to form a neck 31 of L31 length and A31 area for each housing 30 of the honeycomb. These two perforated layers 511, 513 enclose between them a plate or sheet 812 which is cut to form the wafers 32 of each absorber 3 and their suspensions 33, for example in patterns 330 as described in FIG. 4 or the like.

[0145] Such an architecture can be achieved for example with a sheet 512 of steel, or aluminium, or titanium alloy, which allows for the much cheaper and faster industrial realisation instead of the MEMS technologies of FIG. 3, which is more suitable for industrial applications of large size and/or large series, for example for jet engines or machine soundproofing.

[0146] FIGS. 10a and b illustrate an absorber 6, according to a fourth example of embodiment of the invention, alternatively within a honeycomb acoustic wall 500 similar to that of FIG. 9, and which will only be described in how it differs from the other embodiment.

[0147] In this example, the neck 61 is formed essentially by the thickness of a perforated layer 612, applied to the front side of the honeycomb. Around the neck and on each side of this thick layer 612, the advances 6140 extend inside the neck 61 and protrude above the wafer 62. These advances are distributed, sufficiently numerous and/or present on sufficiently wide angular sectors, to maintain the wafer 62 inside the neck 61 regardless of the stresses it undergoes and the position in which the absorber is positioned in relation to the force of gravity.

[0148] Inside the neck, the wafer is thus totally free to move in the input direction A3, and can be considered suspended by a zero stiffness connection, which allows to obtain performance that can be interesting in many cases.

[0149] In this example, these holding advances 6140 and 6110 are formed by an outer layer 614 plated on the outer side of the thick layer 613, and by an inner layer 611 plated on its inner side. For example, each of these holding layers 611, 640 is positioned and then cut out to form these advances, or formed by deposit in a pattern respecting the outline of the neck and the advances.

[0150] As illustrated in FIG. 10a, the wafer can be made from a sheet 612 sandwiched between two layers of the front side, and which is cut to form each wafer. This base plate 612 is represented here between the inner holding layer 611 and the thick layer 612, but could also be placed on the outer side or between two thick layers.

[0151] FIGS. 11a and 11b illustrate an absorber 7, of a honeycomb acoustic wall, according to two variations of a fifth example of embodiment of the invention, variation within a honeycomb acoustic wall 500 similar to that of FIG. 10, which will only be described regarding its differences from the other embodiments.

[0152] In this example, the wafer 72, 72 is also free-moving and retained by external layers 711 and 713, which protrude from the thick layer 712 above the neck 71. This wafer presented here is significantly thick in the direction of entry D3 to avoid arching and has a periphery that moulds the walls of the neck 71 so as to allow it to be guided during its movements, while leaving a leakage section to achieve the damper according to the invention.

[0153] In FIG. 11a, the leakage section is defined in the outer periphery of the wafer, as indicated by the arrows f72.

[0154] In FIG. 11b, the wafer 72 is surrounded by a sliding surface 721, forming a linear bearing which guides its movement. For example, this surface is made according to a free or sliding adjustment, i.e. just free enough to allow mobility. Such an adjustment is for instance of the H7g6 to H11d11 type according to the ISO system for metal or plastic parts, or with a clearance of less than 0.5 mm or even less than 0.2 mm or 0.1 mm for less precise manufacturing or composite materials. Such adjusted guidance can be likened to a seal, and can therefore be described as a sliding seal. For instance, this sliding joint is covered using a conventional material such as bronze, or silicone or PTFE; the application is dry or done with a liquid film of lubricant, or a ferrofluid film. In such circumferential sealing conditions, the wafer itself has one or more through holes 731 made in the material of the wafer, which then form a leakage section f72.

[0155] It is thus possible to make a wafer more rigid, and/or with a very small peripheral deviation without risk of jamming, more easily so than with a two-dimensional wafer like the one in FIG. 10 or with different constraints.

[0156] In FIG. 11a, the wafer has a closed volume all around. In FIG. 11b, its two end surfaces are shaped to match the wall of the neck, but are interconnected by a part of the smaller section. Such options allow for a more flexible design thanks to the experimentations with the parameters, for example the friction surface against the neck, the mass of the wafer, and/or its overall rigidity.

[0157] FIG. 13 illustrates a seventh example of embodiment of the invention, which will only be described in how its different from the other embodiments. In this embodiment, the rigid wafer also has one or more through openings 330a in its inner or central part.

[0158] In the half-view on the left (FIG. 13a), these inner openings 330a form a leakage section which is added to the section 330 produced around the arms 31 of the unsealed suspension, which could be similar to that of the FIG. 4.

[0159] In the half-view on the right (FIG. 13b), the suspension is of a sealed type, for example formed by an annular bellows made of a metal sheet or a film of plastic or polymer, for example, a Visaton K16 loudspeaker, the membrane of which forms makes for the wafer, with its seal 33a made out of thermoformed polymer forming the suspension. The inner openings 330a then form the only leakage section.

[0160] FIG. 14 illustrates an eighth example of a embodiment, which will only be described in how its different from the other embodiments.

[0161] In this mode, the wafer 92 also or exclusively includes leakage openings 930a located inside the wafer (i.e. the rigid part).

[0162] In the left half-view, the wafer 9b is formed by a layer 921 of a flexible and elastic material, for example a metal sheet or an elastomer, here of constant thickness. This elastomer can be a PDMS, or polydimethylsiloxane, a polymeric material formed from a cross linking agent and a pre-polymer, particularly with a cross linking ratio: pre-polymer of 1:10, in combination with which it is particularly flexible.

[0163] The wafer is attached to the front wall 37 by a bell-shaped annular part 931a with perforated parts 930a, which provides a non-sealed suspension. Inside the suspension 931a, the wafer 92a has a thickening providing increased rigidity in an annular region 922a surrounding the inner openings 930a. This excess thickness 922a is made of a different and preferably rigid material, for example an over-molding or a polymerised resin. This extra thickness, for example in its material and/or its dimensions, provides localised stiffness and additional mass that play on the characteristics of the moving equipment to obtain a movement in piston mode at the desired absorption frequency.

[0164] In the variation in the half-view on the right, described only in how its different from other embodiments, the wafer 92b is formed by a layer 921b whose thickness is increasing inwards, regarding at least or exclusively the annular extra thickness 922b. In this variation, and interchangeably with the left variation, the sealed suspension 931b is presented.

[0165] In these two variations, the inner part has a certain elasticity but is less stressed by the friction of the air since it carries the openings forming the leakage section.

[0166] The movement in piston mode is obtained by a greater stiffness and/or mass in the area which surrounds the suspension, with respect to the stiffness of the suspension itself, and/or by the fact that the central openings 930a in the central part let the air pass and undergo less effort on the part of the acoustic wave.

[0167] FIG. 15 illustrates the piston mode of operation as intended herein, compared with drum mode operation.

[0168] In FIG. 15a, a membrane or a plate 12 is fixed inside an orifice in a rigid wall 17. This plate 12 vibrates in drum mode when its centre moves along the arrow mT much more than its periphery 123, thus deforming itself by a distance d.sub.t.

[0169] In FIG. 15b, a plate or a wafer 32 is fixed inside an orifice in a rigid wall 37 by a suspension 33. This wafer 32 vibrates in piston mode when its centre moves along the arrow mP almost as much as its periphery 323, for example because the suspension stiffness is very low compared to that of the wafer. For the central area 32, it may be considered that it forms a wafer moving in piston mode when its whole d.sub.p movement is much larger than its deformation d.sub.t, or when: d.sub.p>>d.sub.t.

[0170] In this context, it can be considered that this condition is fulfilled when these two values differ by a factor of at least five, preferably 10, 50 or 100.

[0171] Absorber Variation with Loudspeaker Structure

[0172] FIG. 12 illustrates a sixth example of absorber embodiment.

[0173] This absorber 8 uses a conventional electrodynamic loudspeaker structure, here of a conical diaphragm type 82 and moving coil 824 mounted on a conventional perforated frame 85 carrying a permanent magnet 874. This structure is mounted on a front side 87, and enclosed in a cavity 80 delimited by walls 88 and 89.

[0174] The membrane 82 is connected to the front side 87 by a flexible peripheral seal 83 of a conventional type. However, contrary to what is systematically encountered and naturally expected from a loudspeaker emitting sounds such as words or music, here this seal 83 is completed by openwork cutouts 830 (represented by a dotted rectangle), completed during the manufacture process or afterwards. Similarly and according to the configurations, the seal and/or the membrane 82 and/or spider 84 which connects the top of the cone 82 to the frame 85 may also be perforated by cutouts 840. Alternatively or additionally, (not shown here) the membrane itself comprises perforated parts forming all or part of the leakage sections.

[0175] Such absorber is represented here in a version including the electromagnetic activation system 824, 874. This version can be used passively, by not connecting the coil or disconnecting it from the control unit. It can also be used in a hybrid manner by activating the loudspeaker to achieve active absorption in addition to the modified Helmholtz resonance. It can also be used in multi-function mode, for example to achieve absorption (active or passive) at certain times and use as a classic loudspeaker at other times.

[0176] In its purely passive version, the absorber can also be completed with a speaker structure performed incompletely, i.e. with the same mechanical structure but without the electromagnetic system.

[0177] Such an architecture can be particularly interesting for large rooms, and/or walls of large sizes, in which integration and thickness are less important constraints. It can make it possible to place one or more absorbers in specific locations of the wall or the room, possibly in versions of different sizes and frequencies, and in varying numbers depending on the demand.

[0178] In its complete version with the electrodynamic motor, this absorber can be also used in active mode, acoustic impedance matching and/or active reduction mode.

[0179] FIG. 16 illustrates the MEMs-type loudspeaker shown in FIG. 5, installed with its electrodynamic motor 374, 324 in the neck 31 of the absorber of FIG. 3, e.g. for use in active mode, with adaptation of acoustic impedance and/or active reduction.

[0180] Of course, the invention is not limited to the examples which have just been described and many adjustments can be made to these examples without leaving the scope of the invention.