ACOUSTIC MEMBRANE FOR ACOUSTIC TRANSDUCER AND ASSOCIATED ACOUSTIC TRANSDUCER

Abstract

The invention relates to an acoustic membrane (14a) for an acoustic transducer, comprising a substrate (21); and at least one stiffening layer (23a-23b) of the substrate (21) for improving the stiffness of the membrane (14a). The stiffening layer (23a-23b) of the substrate (21) corresponds to a layer of amorphous carbon belonging to the DLC family having a stiffness greater than 300 GPa, a density less than 3400 kg / m3 and a thickness between 0.5 µm and 6 µm. The stiffening layer (23a-23b) is fixed to said substrate (21) by means of a bonding layer (22a-22b) with a thickness between 0.1 µm and 1 µm and incorporating at least one metallic compound. Figure for the abstract: Fig 2

Description

Title of the invention: ACOUSTIC MEMBRANE FOR ACOUSTIC TRANSDUCER AND ASSOCIATED ACOUSTIC TRANSDUCER Field of invention

[0001] The invention relates to the field of transducers, and it relates more particularly to an acoustic membrane for an acoustic transducer. The acoustic transducer may correspond to a loudspeaker or a microphone, for example a ribbon microphone whose ribbon acts as an acoustic membrane. The invention also relates to an acoustic transducer comprising the membrane.

[0002] The invention finds multiple applications in the fields for which it is sought to obtain a transducer with a high bandwidth and / or high dynamics, that is to say a high acceleration and / or frequency of displacement of the membrane.

[0003] For example, the invention finds a particularly advantageous application for loudspeakers capable of generating high-frequency sounds, such as tweeters or wide-band loudspeakers, for example drivers for audio headphones. Prior art

[0004] Acoustic transducers include sensors, transforming an acoustic or mechanical quantity into an electrical signal, for example microphones or accelerometers, and sources, transforming an electrical signal into an acoustic or mechanical quantity, for example loudspeakers or headphones.

[0005] A loudspeaker generally comprises a movable membrane, set in motion by a support comprising a coil moved under the effect of a magnetic field. The support is fixed on only a part of the membrane, typically a central circular portion.

[0006] Since the membrane is moved by an element fixed to only part of it, it is important that the membrane is sufficiently rigid so that the displacement force applied to the membrane causes it to move uniformly.

[0007] If the membrane is not rigid enough to withstand its high frequency vibration, distortions of the membrane appear and generate parasitic sounds.

[0008] To achieve high rigidity, it is possible to use naturally rigid materials, or to use materials with medium or low rigidity in a thick layer to form the membrane. However, the use of rigid materials or thick layers of these materials generally results in the formation of membranes with significant weights.

[0009] However, the heavier the membrane, the greater the electromagnetic force required to move it, thus increasing energy consumption and leading to membrane inertia problems.

[0010] A membrane is therefore sought which is rigid while being light to limit distortions and inertia phenomena, particularly at high frequencies.

[0011] A large proportion of loudspeakers today use a membrane made with glass, carbon or aramid fibres, in particular for bass and midrange loudspeakers with thicknesses of the order of a millimetre. However, with the classic thicknesses of tweeter and driver membranes for audio headphones, from 30 to 60 μm, these materials do not allow high frequencies to be obtained while limiting distortions. To limit distortions, it would be possible to significantly increase the thickness, but this increase in thickness would induce a significant increase in weight and, therefore, excessive inertia.

[0012] To obtain the generation of high frequencies while limiting inertia phenomena, it is known to use membranes comprising a substrate made of metal or alloy with a fairly low density, in particular aluminum or titanium.

[0013] At equivalent thickness, the membranes made with these materials are more rigid than the fiberglass membranes but they are not rigid enough to sufficiently limit distortion, particularly for high-fidelity sound reproduction applications.

[0014] When the membranes are made of titanium, they are particularly heavy and cause problems of membrane inertia.

[0015] To limit distortions and inertia phenomena, it is possible to produce a beryllium membrane, because this material combines rigidity (Young's modulus of 287 GPa) and low density (1848 kg / m3). Such a beryllium membrane is described in particular in document FR 2 854 021.

[0016] Alternatives to the use of beryllium are being sought for reasons of cost and availability of this material.

[0017] In an attempt to approximate the characteristics of a beryllium membrane, it is known to combine a light but relatively rigid substrate, for example an aluminum substrate, with a stiffening layer, for example a layer of amorphous carbon from the DLC family, for "Diamond Like Carbon" in the English literature. Indeed, a substrate made of aluminum alone has a density of 2700 kg / m3 and an average rigidity, characterized by a Young's modulus of approximately 70 GPa. With a conventional membrane thickness of between 30 and 60 μm, this rigidity characteristic does not allow high frequencies to be obtained while limiting distortions.

[0018] By forming a multi-layer structure integrating a low-rigidity substrate and a layer stiffening, it is theoretically possible to significantly improve the membrane stiffness because the stiffening layer can have a much higher stiffness than the substrate. However, due to the difference in mechanical properties of the materials, there is a risk of delamination and interface shear between the stiffening layer and the substrate during membrane manufacturing or when the membrane is subjected to high-frequency displacements.

[0019] To maintain a stiffening layer on a less rigid substrate, it is known from the document “Effectiveness of Exotic Vapour-Deposited Coatings on Improving the Performance of Hard Dome Tweeters”, Chapman, Peter John, Audio Engineering Society, to use a bonding layer between the substrate and the stiffening layer.

[0020] More precisely, this document results from experiments according to which, to obtain a stack adhering to the substrate, the bonding layer can be made of chromium nitride with a thickness of between 3.2 μm and 9.4 μm or of chromium with a thickness of between 1.3 μm and 4.7 μm.

[0021] These bonding layers make it possible to fix, on an aluminum or titanium substrate, a stiffening layer respectively made of DLC or chromium nitride, typically a DLC layer with a thickness of between 1.5 μm and 2.4 μm or a chromium nitride layer with a thickness of between 1.4 μm and 5.2 μm.

[0022] With these particularly thick bonding layers, at least 1.3 μm, the membrane resists delamination at high frequencies but the weight of the membrane is also greatly impacted by the weight of the bonding layer, which only contributes very little to the rigidity of the membrane.

[0023] Thus, the presence of the bonding layer increases the weight of the membrane and its inertia, so that the gain obtained by adding the stiffening layer is limited since the characteristics of the membrane are degraded by the increase in inertia. To maintain a stiffening layer with a bonding layer of low thickness, it is known from document DE 10 038 780 to use a bonding layer made of silicon, germanium or carbon to fix a DLC stiffening layer on a less rigid substrate, for example an aluminum substrate. More precisely, this document indicates that the bonding layer made of silicon, germanium or carbon can have a thickness of between 0.001 μm and 10 μm and preferably between 0.002 μm and 4 μm.

[0024] This document describes that the bonding layer makes it possible to fix a DLC stiffening layer with a thickness of between 1 μm and 10 μm on an aluminum substrate.

[0025] It should be noted that a silicon, germanium or carbon bonding layer has an atomic coordination close to C in a DLC. Thus, this document seems to suggest that to obtain a bonding of a DLC layer on a less rigid substrate with a thin bonding layer, it is appropriate to use a bonding layer with an atomic coordination close to that of the stiffening layer.

[0026] However, in the context of the invention, tests have been carried out and these tests show that a bonding layer made of silicon, germanium or carbon does not allow a sufficiently rigid DLC layer, i.e. greater than 300 GPa, having a thickness of 4 μm to be fixed on an aluminum substrate. Indeed, it has been found that the stiffening layer undergoes delamination during the manufacture of the membrane.

[0027] Indeed, a DLC layer has residual stresses, which are internal stresses linked to its manufacture. These stresses are partly linked to the thickness and rigidity of the DLC layer and will stress the bonding layer.

[0028] Generally, the methods of manufacturing membranes, which will be detailed later, require during deposition, a significant energy input to the growing deposit, which results in high internal compressive stresses in the DLC layer. These internal stresses tend to increase with the rigidity of the DLC layer. Furthermore, this energy input generates a significant increase in the temperature of the membrane, a phenomenon accentuated for thin membranes which have a low thermal inertia. In addition, the difference between the coefficients of thermal expansion of the different materials of the membrane generates an even greater stress in the latter.

[0029] These constraints cause the presence of a significant elastic potential energy in the DLC layer, proportional to the internal stress of the layer and its thickness, which can be released by rupture of the weakest zone of the stack. This zone is generally an interface, for which the interface cohesion energy is lower than the potential energy relaxation generated by the rupture of the stack at this interface. In order to prevent the failure of the membrane under the effect of the significant loading due to the internal stresses of a thick and very rigid deposit, it is therefore necessary to optimize the chemical affinity between the materials at the interfaces on the one hand, and to maximize the breaking stress in the bonding layer on the other hand.

[0030] Thus, a stiffening layer combining high stiffness (greater than 300 GPa) and significant thickness (greater than 4 μm), as described in the prior art, will not be able to adhere to a low-rigidity substrate thanks to a silicon, germanium or carbon bonding layer. This is linked to the shear stress which is then greater than the breaking stress of the bonding layer. It follows that membranes which currently use stiffening layers have either adhesion problems with the bonding layer, or a risk of delamination and interface shear, for example with a silicon bonding layer, germanium or carbon, or distortions, typically with a stiffening layer that is too thin, or too much inertia, especially when the bonding layer is too thick.

[0031] In a completely different field, patent FR 3 082 527 describes an industrial mechanical part, such as a piston pin, a cylinder or a pusher, covered by a friction-reducing layer. This friction-reducing layer of non-hydrogenated amorphous carbon, of the ta-C type, is fixed to the industrial mechanical part by a bonding layer based on chromium, carbon and silicon.

[0032] This solution addresses another technical problem, which is that of reducing the coefficient of friction in a mechanical contact in a boundary lubrication regime. It is intended to be applied to parts operating mainly in a lubricated environment, subjected to significant forces (approximately 100 daN) and moving at frequencies much lower than those of acoustic membranes, in particular at frequencies lower than 1 kHz.

[0033] Thus, a person skilled in the art of acoustic membranes seeking to improve the characteristics of an acoustic membrane would not seek a solution used for mechanical friction parts because this solution does not implement the same constraints and does not have the same aim.

[0034] Indeed, it is very different to adhere a friction reduction layer to a solid steel part several centimeters thick generally weighing more than ten grams and a stiffening layer to a membrane with a thickness of less than 70 micrometers weighing a few hundred milligrams, because the thermal properties of the parts and the mechanical characteristics of the substrate are very different.

[0035] The problem that the invention seeks to solve is to overcome the constraints of the state of the art by proposing a rigid and lightweight acoustic transducer membrane by means of fixing a stiffening layer to a substrate, thus limiting distortions and inertia phenomena at high frequencies, resisting delamination and the risk of interface shearing, and ensuring adhesion of the stiffening layer to the substrate thanks to an optimized bonding layer. Statement of the invention

[0036] The invention proposes to address this technical problem by using a thin bonding layer incorporating at least one metallic compound so as to obtain good chemical affinity on the one hand with the substrate, ensured by the presence of at least one metallic compound, and on the other hand with a DLC stiffening layer.

[0037] This chemical affinity is further improved by creating a chemical composition gradient within the bonding layer, between the two interfaces, which makes it possible to avoid the creation of chemical or mechanical discontinuities. Furthermore, to avoid decohesion within the bonding layer, materials with good toughness are required.

[0038] Thus, the combination of these different parameters, namely interface cohesion and toughness, makes it possible to optimize the adhesion of the stiffening layer to the substrate and, consequently, to increase the thickness of the stiffening layer.

[0039] The invention therefore arises from an observation according to which it is possible to fix a stiffening layer with a thin bonding layer incorporating at least one metallic compound, contrary to the technical prejudice of the document “Effectiveness of Exotic Vapour-Deposited Coatings on Improving the Performance of Hard Dome Tweeters”, Chapman, Peter John, Audio Engineering Society, which suggests using a very thick metallic bonding layer.

[0040] Similarly, it makes it possible to limit delamination and the risk of interface shearing of these layers, contrary to the technical prejudice of document DE 10 038 780 which suggests using a bonding layer with atomic coordination close to that of the stiffening layer.

[0041] To this end, according to a first aspect, the invention relates to an acoustic membrane for an acoustic transducer, comprising: - a substrate; and - at least one stiffening layer of the substrate to improve the rigidity of the membrane.

[0042] The invention is characterized in that said at least one stiffening layer corresponds to a layer of amorphous carbon belonging to the DLC family having a stiffness greater than 300 GPa, a density less than 3400 kg / m3 and a thickness of between 0.5 μm and 6 μm; said stiffening layer being fixed to said substrate by means of a bonding layer with a thickness of between 0.1 μm and 1 μm and incorporating at least one metallic compound.

[0043] Advantageously, the substrate has a rigidity of less than 150 GPa and the bonding layer has a rigidity between the rigidity of the stiffening layer and the rigidity of the substrate.

[0044] Thus, this embodiment arises from an observation according to which a bonding layer incorporating at least one metallic compound, having a rigidity comprised between the rigidity of the stiffening layer and the rigidity of the substrate and a thickness comprised between 0.1 μm and 1 μm can provide the expected resistance to delamination and resistance to interface shear stress, for a stack comprising a stiffening layer of amorphous carbon DLC, even though This can have a thickness of up to 6 pm and a high rigidity, greater than 300 GPa.

[0045] A DLC amorphous carbon layer, for "Diamond-Like Carbon" in the English literature, is a type of carbon coating which has certain properties of diamond, although it does not have the typical crystalline structure of diamond.

[0046] These layers consist of carbon having an amorphous (non-crystalline) structure with mixed sp3 (as in diamond) and sp2 (as in graphite) atomic bonds.

[0047] The density of a DLC amorphous carbon layer depends on the proportion of sp3 and sp2 bonds as well as the possible presence of hydrogen, impurities and pores. The density of pure diamond (100% sp3 bonds) is about 3520 kg / m3, while that of graphite is about 2267 kg / m3. If the DLC amorphous carbon layer has a density lower than 3400 kg / m3, this means that it has a specific proportion of sp2 bonds and / or that it contains impurities or pores.

[0048] Thus, this embodiment arises from an observation according to which it is particularly advantageous to have a bonding layer having an intermediate rigidity between that of the substrate and that of the stiffening layer, to improve the adhesion of the stiffening layer to the substrate by limiting delamination and the risk of interface shear between the layers.

[0049] More specifically, the bonding layer is effective for fixing a DLC amorphous carbon layer having a rigidity greater than 300 GPa, a density less than 3400 kg / m3 and a thickness between 0.5 μm and 6 μm.

[0050] Generally, the ta-C type DLC amorphous carbon layers have internal compressive stresses ranging from 2 GPa to more than 8 GPa. The DLC amorphous carbon layers of the membrane according to the invention have a minimum internal stress, close to 2 GPa. Similarly, the order of magnitude of the thermal stresses is also 1 to 2 GPa.

[0051] These specific technical choices make it possible to obtain a membrane with a significant improvement in the rigidity of the substrate and a limited increase in weight and inertia.

[0052] The acoustic membrane according to the invention therefore offers an effective compromise between the needs of rigidity and lightness of the membrane, given that the lightness depends directly on the thickness and the density for a membrane of fixed diameter.

[0053] For example, the substrate has a thickness of between 5 μm and 70 μm and comprises at least one polymer and / or a composite which has a rigidity greater than 2 GPa, a density less than 2800 kg / m3. Preferably, the substrate comprises at least one material selected from the group comprising carbon composites, carbon fiber reinforced polymers, graphite and its composites, graphene and its composites, graphene oxide, carbon nanotubes, and mixtures thereof.

[0054] Alternatively, the substrate has a thickness of between 20 μm and 60 μm and comprises at least one metal or alloy which has a rigidity greater than 30 GPa and a density less than 2800 kg / m3. For example, the substrate may comprise aluminum and / or its alloys and / or magnesium and / or its alloys.

[0055] Preferably, the stiffening layer has a density of less than 3200 kg / m3. A density of less than 3200 kg / m3 means that the DLC amorphous carbon layer has a higher proportion of sp2 bonds, typically greater than 10% for ta-C type DLC, and / or that it contains impurities or pores.

[0056] This definition may include hydrogenated forms of DLC, such as ta-C:H, for “Tetrahedral Amorphous Carbon - Hydrogenated” in the Anglo-Saxon literature, and DLCH, for “Diamond-Like Carbon - Hydrogenated” in the Anglo-Saxon literature.

[0057] Ta-C:H and DLCH coatings are hydrogenated variants of DLC, where a proportion of hydrogen atoms are incorporated into the carbon structure. The addition of hydrogen modifies certain properties of DLC coatings, such as hardness, friction and wear resistance. In general, the incorporation of hydrogen into the carbon structure results in a decrease in density.

[0058] In addition to the hydrogenated forms, it is also possible to use DLC of the ta-C type, for “Tetrahedral Amorphous Carbon” in the Anglo-Saxon literature, because the density of ta-C is classically between 2600 and 3200 kg / m3.

[0059] Ta-C is characterized by an amorphous, i.e. non-crystalline, structure in which a large proportion of the carbon bonds are of the sp3 type (greater than or equal to 50%), similar to those present in diamond.

[0060] This type of bonding which leads to tetrahedral coordination of the first neighbors, hence the "t" in ta-C, gives ta-C certain properties similar to those of diamond.

[0061] In the context of the invention, it has appeared that the hydrogenated forms of DLC or ta-C are particularly effective in improving the rigidity of the membrane, even with small thicknesses. According to one embodiment of the invention, it is possible to note a significant improvement in the rigidity of the membrane with a layer of amorphous carbon DLC having a thickness of between 3 μm and 6 μm, with a rigidity greater than 300 GPa. To detect an acoustic membrane whose stiffening layer is made of ta-C or taC:H, it is possible to search for the density of the carbon layer instead of stiffness. Indeed, it is known that a layer stiffness greater than 300 GPa, corresponds for a taC:H to a density greater than 2.6 g / cm3 and for a ta-C to a density greater than 2.4 g / cm3. Thus, by measuring a density greater than 2.4 g / cm3 for the stiffening layer, by weighing before and after the deposition of the stiffening layer knowing the surface of the part and the thickness of the layer, the stiffness should be greater than 300 GPa.

[0062] For the purposes of the invention, the rigidity of a material or a complex of several materials is defined by its modulus of elasticity or Young's modulus, which corresponds to the measurement of the resistance of the material to elastic deformation. It is important to note that the values ​​of the modulus of elasticity may vary slightly depending on the purity of the material and its microstructure, dense or columnar, that is to say having grains that are very elongated in a crystallographic direction.

[0063] Preferably, the stiffening layer has a stiffness greater than 300 GPa, or even 450 GPa and / or an indentation hardness (HIT) greater than 30 GPa. Preferably, the stiffening layer has a stiffness less than 700 GPa.

[0064] According to the invention, the HIT hardness measurement is carried out according to a nanoindentation method, as described in document FR 2 796 150, consisting of applying a measuring load to the surface of the material using a tip to produce a deformation of the material, then measuring the deformations produced by the tip according to the different loads to determine the hardness and rigidity of the material. A displacement curve as a function of the load is thus obtained, making it possible to extract the hardness and rigidity.

[0065] Preferably, the stiffening layer has a proportion of sp3 bonds greater than or equal to 50%.

[0066] According to the invention, the measurement of the proportion of sp3 bonds is carried out by Raman spectroscopy. In particular, Raman spectroscopy makes it possible to obtain the average content of sp3 hybridization atoms in a layer by means of measurements of spectra carried out at different laser excitation wavelengths (325 nm, 442 nm, 488 nm and 633 nm). The laser power is limited in order to avoid modification of the material under irradiation. The intensity spectra as a function of the Raman shift are adjusted by two Gaussians for the G peaks (elongation mode) and D peaks (vibration mode) of the carbon bonds. The plot of the position of the G peak (in Raman shift) as a function of the excitation wavelength provides a straight line whose slope is the dispersion of the G peak (in cm Vnm). This dispersion is proportional to the content of sp3 hybridization atoms in the material analyzed.The analysis of the sp3 bond content of a material belonging to the DLC family is for example described in the article “Structure of diamondlike carbon films deposited by femtosecond and nanosecond pulsed laser ablation” by Sikora et al., Journal of Applied Physics 108, 113516. (2010).

[0067] It is possible to use only one stiffening layer on one side of the membrane to improve the rigidity of the membrane. Preferably, the membrane comprises two stiffening layers arranged on either side of the substrate and fixed to the substrate by two bonding layers. This embodiment makes it possible to improve the rigidity of the membrane by using two stiffening layers fixed on either side of the membrane.

[0068] The bonding layer(s) may comprise at least one material selected from the group comprising chromium and its alloys, titanium and its alloys, tantalum and its alloys, aluminium and its alloys, tungsten, compounds comprising nitrides, silicides or carbides of these metals, and mixtures thereof. Preferably, the bonding layer comprises chromium and / or at least one chromium alloy. These materials have particularly effective technical performances for enabling the adhesion of carbon to metal, but also for moderating the shear stress between the DLC layer and the substrate, with very low bonding layer thicknesses.

[0069] The rigidity of the bonding layer is advantageously intermediate between the rigidity of the substrate and the rigidity of the stiffening layer.

[0070] Preferably, the bonding layer(s) has a thickness of between 0.1 μm and 0.3 μm. Such a limited thickness of the bonding layer makes it possible to limit the total weight of the acoustic membrane. A greater thickness is possible, but increases the weight of the membrane.

[0071] Furthermore, the membrane may comprise at least one protective layer placed between the substrate and the bonding layer of a stiffening layer. Alternatively or additionally, at least one protective layer may be fixed to at least one face of the stiffening layer, opposite the face fixed with the substrate by means of the bonding layer.

[0072] According to a second aspect, the invention relates to the acoustic transducer comprising a support, preferably of cylindrical shape, a drive motor configured to generate a magnetic field for moving the support, and an acoustic membrane, according to the first aspect of the invention, fixed with said support, said acoustic membrane being configured to generate or capture acoustic waves.

[0073] Preferably, the technical choices for sizing the different layers are made so that the acoustic membrane has a mass of less than 220 mg.

[0074] According to a particular embodiment, the transducer is a high-frequency loudspeaker, that is to say it is configured to emit sounds in a frequency range greater than 1 kHz, said transducer having a maximum frequency greater than 30 kHz, a diameter less than 6 cm and an acoustic membrane having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

[0075] In another variant of the invention, the transducer is a wide-band loudspeaker, that is to say it is configured to emit sounds in a frequency range greater than 20 Hz, said transducer having a maximum frequency greater than 30 kHz, a diameter less than 8 cm and an acoustic membrane having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa. Summary description of the figures

[0076] The manner of carrying out the invention as well as the advantages which result therefrom will emerge clearly from the following embodiments, given for informational but non-limiting purposes, with the support of the appended figures in which:

[0077] [Fig-1] is a schematic sectional view of an acoustic transducer according to a embodiment of the invention;

[0078] [Fig.2] is a schematic sectional view of a membrane of the transducer acoustics of [Fig.l] according to a first embodiment;

[0079] [Fig.3] is a schematic sectional view of a membrane of the acoustic transducer of [Fig.l] according to a second embodiment; and

[0080] [Fig.4] is a schematic sectional view of a membrane of the transducer acoustics of [Fig.l] according to a third embodiment. Detailed description of the invention

[0081] [Fig.l] illustrates an acoustic transducer 11 comprising a chassis 12 intended to be fixed inside a box to form an enclosure. The chassis 12 supports a drive motor 20 of a membrane 14a. To do this, the acoustic transducer 11 comprises a cylindrical support 13 movable in translation by means of the magnetic field generated by the motor 20.

[0082] This cylindrical support 13 is connected to the membrane 14a by an inner annular edge 18 fixed on an upper end of the cylindrical support 13.

[0083] In the description, the relative terms "upper" or "lower" refer to the conventional positioning of a loudspeaker as illustrated in [Fig.l], in which the motor 20 is arranged in the lower part while the membrane 14a is arranged in the upper part of the acoustic transducer 11. Of course, the acoustic transducer 11 can be turned over without changing the invention.

[0084] The annular outer edge 15 of the membrane is connected to an upper bearing surface 16 of the chassis by means of a suspension 17. Thus, the chassis 12 is fixed to the motor 20 at its lower end, while its upper end surrounds the cylindrical support 13 and the membrane 14a.

[0085] In the lower part of the acoustic transducer 11, the cylindrical support 13 preferably has an upper surface 19 provided with a dome, for example an inverted dome.

[0086] In the upper part of the acoustic transducer 11, the upper bearing surface 16 of the chassis 12 has, for example, a diameter of between 1 and 17 centimeters.

[0087] In one embodiment, the acoustic transducer 11 is a high-frequency loudspeaker, that is to say it is configured to emit sounds in a frequency range greater than 1 kHz. In this embodiment, the acoustic transducer 11 has, for example, a maximum frequency greater than 30 kHz, a diameter of the upper support surface 16 of the chassis 12 less than 6 cm and a membrane 14a having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

[0088] In another embodiment, the acoustic transducer 11 is a wide-band loudspeaker, i.e. it is configured to emit sounds in a frequency range greater than 20 Hz. In this embodiment, the acoustic transducer 11 has a maximum frequency greater than 30 kHz, a diameter of the upper bearing surface 16 of the chassis 12 less than 8 cm and a membrane 14a having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

[0089] The membrane 14a is the result of technical choices making it possible, preferably, to obtain a weight of less than 220 mg in order to limit the inertia of the membrane. To do this, according to the invention, the membrane 14a is formed by the association of a substrate 21 and at least one stiffening layer fixed 23a-23b on the substrate 21 by a bonding layer 22a-22b.

[0090] As illustrated in Figures 2, 3 and 4, the membrane 14a-14c preferably has two stiffening layers 23a-23b fixed on either side of the substrate 21 by two bonding layers 22a-22b. Alternatively, the membrane may comprise a single stiffening layer without changing the invention.

[0091] According to the invention, the substrate 21 has a rigidity of less than 150 GPa. To do this, the substrate 21 can be made of polymer or composite, having a rigidity greater than 2 GPa, a density less than 2800 kg / m3 and a thickness of between 5 μm and 70 μm. In this embodiment, it can be made of a material chosen from the group comprising carbon composites, carbon fiber reinforced polymers, graphite and its composites, graphene and its composites, carbon nanotubes and their mixtures.

[0092] According to another embodiment, the substrate 21 may be made of metal or alloy, having a rigidity greater than 30 GPa, a density less than 2800 kg / m3 and a thickness between 20 and 60 μm. For example, the substrate 21 may be made from a material selected from the group comprising aluminum and its alloys or from the group comprising magnesium and its alloys.

[0093] The acoustic membrane 14a-14c also comprises at least one stiffening layer 23a-23b of the substrate 21 making it possible to improve the rigidity of the membrane 14a-14c.

[0094] According to the invention, the stiffening layer 23a-23b corresponds to a layer of amorphous carbon DLC having a rigidity greater than 300 GPa, a density less than 3400 kg / m3 and a thickness between 0.5 pm and 6 pm.

[0095] Preferably, it has a thickness of between 3 μm and 6 μm, a density of less than 3200 kg / m3, a rigidity of greater than 450 GPa and / or a HIT hardness of greater than 30 GPa. For example, the stiffening layer 23a-23b corresponds to tetrahedral amorphous carbon ta-C.

[0096] The stiffening layer 23a-23b is fixed to the substrate 21 by means of a bonding layer 22a-22b incorporating at least one metallic compound and having a stiffness comprised between the stiffness of the stiffening layer 23a-23b and the stiffness of the substrate 21, with a thickness comprised between 0.1 and 1 μm. Preferably, it has a thickness comprised between 0.1 and 0.3 μm and is made from a material chosen from the group comprising chromium and its alloys, titanium and its alloys, tantalum and its alloys, aluminum and its alloys, tungsten, nitride- or carbide-based compounds, and mixtures thereof.

[0097] In the embodiment of [Fig.3], two protective layers 24a-24b are fixed on one face of the two stiffening layers 23a-23b, opposite the face fixed with the substrate 21 by means of the bonding layer 22a-22b.

[0098] In one embodiment, this protective layer is made from a two-component polyurethane varnish and is applied by spray. It has a thickness of between 5 and 20 μm. It is necessary to reduce the thickness of the protective layer to minimize its mass. Different varnishes can be applied depending on the desired aesthetic characteristics.

[0099] [Fig.4] shows an embodiment in which two protective layers 25a-25b are placed between the substrate 21 and the two bonding layers 22a-22b of the stiffening layers 23a-23b.

[0100] In one embodiment, this protective layer is produced by anodizing the surface of the substrate (for magnesium or aluminum alloys) after immersion in an electrolytic bath, to form a protective oxide layer on the surface.

[0101] Different methods of depositing the layers can be used to manufacture the acoustic membrane 14a-14c according to the invention.

[0102] For the deposition of the bonding layers 22a-22b on the substrate 21, it is possible to use electrolytic deposition, chemical deposition (autocatalytic), physical vapor deposition, or chemical vapor deposition as described in patent FR 3 082 526.

[0103] For example, electrolytic deposition, also called electroplating, consists of immersing the substrate in an electrolytic solution containing ions of the bonding layer to be deposited. By applying an electrical voltage, the ions are reduced and deposited on the surface of the substrate, forming a uniform bonding layer.

[0104] In the case of autocatalytic chemical deposition, the substrate is immersed in a solution containing the ions of the metal to be deposited. Unlike electrolytic deposition, the ions are reduced on the surface of the substrate by an autocatalytic chemical reaction through the action of a reducing agent.

[0105] Physical vapor deposition, called PVD for “Physical Vapor Deposition” in the English literature, consists of vaporizing the material constituting the bonding layer under vacuum, and depositing the vaporized particles on the surface of the substrate 21. For example, cathodic arc deposition is a type of PVD.

[0106] Chemical vapor deposition, called CVD for “Chemical Vapor Deposition” in the English literature, consists of introducing gaseous precursors of the material constituting the bonding layer 22a-22b which react and decompose, forming the bonding layer 22a-22b on the substrate 21.

[0107] It is also possible to use plasma-enhanced chemical vapor deposition (PACVD or PECVD), plasma-enhanced physical vapor deposition or reactive PVD, which combines PVD and CVD.

[0108] The deposition of the stiffening layers 23a-23b on the bonding layers 22a-22b can be carried out using the previously described PVD or CVD techniques, or other techniques not mentioned.

[0109] For example, deposition using an electric arc, called CAD for “Cathodic Arc deposition” in the English literature, can be used to deposit the stiffening layers 23a-23b on the bonding layers 22a-22b.

[0110] This technique consists of using an electric arc to evaporate / sublimate a graphite target in an atmosphere of inert gas, generally argon. The carbon ions are accelerated towards the substrate 21 coated with the bonding layer 22a-2 2b, where they form a stiffening layer 23a-23b.

[0111] With these deposition techniques, the bonding layer 22a-22b can be so thin relative to the stiffening layer that it is possible to anticipate the properties of the acoustic membrane 14a-14c by considering only the substrate 21 and the stiffening layer(s) 23a-23b.

[0112] In the example of [Fig.2], with two stiffening layers 23a-23b fixed in on either side of the substrate 21, it is possible to use a magnesium substrate 21 with a rigidity of 42 GPa and a thickness of 31 pm associated with two stiffening layers 23a-23b in ta-C with a rigidity of 500 GPa and a thickness of 4 pm and two bonding layers 22a-22b 0.3 pm thick.

[0113] With these thicknesses and rigidities, the equivalent rigidity of the acoustic membrane 14a can be estimated at 270 GPa according to the following formula:

[0114] [Math.l] 2Eskiti.( hskin'+3.hskin.( hcore+hskin) A +EcoreJicore'l E —-------------:---—j—— -------- (hcore+2.hsk ùt)"

[0115] where E represents the equivalent Young's modulus of the acoustic membrane, Eskin represents the Young's modulus of the stiffening layer, Ecore represents the Young's modulus of the substrate, hskin represents the thickness of a stiffening layer, and hcore represents the thickness of the substrate.

[0116] Furthermore, it is possible to characterize the density of the acoustic membrane 14a at 1980 kg / m3 by considering a density of 2900 kg / m3 for ta-C and a density of 1740 kg / m3 for magnesium, according to the following formula:

[0117] p = Peorehcore+2Pskin.hskin hcvre+2£skin

[0118] where P represents the mass volume of the acoustic membrane, Pskin represents the density of the stiffening layer, Pcore represents the density of the substrate, hskin represents the thickness of the stiffening layer, and hcore represents the thickness of the substrate.

[0119] An important element for predicting the behavior of the acoustic membrane 14a is the speed of sound at the outlet of this acoustic membrane 14a. This speed of sound can be estimated by the square root of the ratio between the rigidity and the density. In the example previously described, the speed of sound is estimated at 11683 m / s.

[0120] These values ​​are close to those measured for a beryllium membrane which typically has a rigidity of 287 GPa, a density of 1848 kg / m3 and a sound speed of 12455 m / s.

[0121] For another example using the structure of [Fig.2], it is possible to use an aluminum substrate 21 with a stiffness of 70 GPa and a thickness of 25 pm associated with two stiffening layers 23a-23b in ta-C with a stiffness of 500 GPa and a thickness of 4 pm. With these thicknesses and these stiffnesses, the equivalent stiffness of the acoustic membrane 14a can be estimated at 313 GPa according to [Math 1]. Furthermore, it is possible to characterize the density of the acoustic membrane 14a at 2750 kg / m3 according to [Math 2] considering a volume mass of 2700 kg / m3 of aluminum. In this example, the speed of sound is estimated at 10670 m / s.

[0122] If the thickness of the ta-C is increased by one micrometer on each face, in the previous example, the equivalent rigidity of the acoustic membrane 14a can be estimated at 342 GPa and the speed of sound at 11147 m / s.

[0123] Thus, it is possible to obtain an acoustic membrane 14a with mechanical properties equivalent to a beryllium membrane.

[0124] The invention therefore makes it possible to obtain a rigid and lightweight acoustic membrane 14a-14c by means of fixing a stiffening layer on a substrate 21. This acoustic membrane 14a-14c has distortions and inertia phenomena limited to high frequencies. In addition, this acoustic membrane 14a-14c effectively resists delamination and the risk of interface shear. It follows that the invention makes it possible to provide an acoustic membrane 14a-14c which is particularly effective for forming a high-resolution acoustic transducer 11.

Claims

Claims

1. Acoustic membrane (14a-14c) for acoustic transducer (11), comprising: - a substrate (21); and - at least one stiffening layer (23a-23b) of the substrate (21) making it possible to improve the stiffness of the membrane (14a-14c); characterized in that said at least one stiffening layer (23a-23b) corresponds to a layer of amorphous carbon belonging to the DLC family having a stiffness greater than 300 GPa, a density less than 3400 kg / m3 and a thickness between 0.5 pm and 6 pm; said stiffening layer (23a-23b) being fixed on said substrate (21) by means of a bonding layer (22a-22b) with a thickness between 0.1 pm and 1 pm and incorporating at least one metallic compound.

2. An acoustic membrane for an acoustic transducer according to claim 1, wherein the substrate (21) has a rigidity of less than 150 GPa and said bonding layer (22a-22b) has a rigidity between the rigidity of the stiffening layer (23a-23b) and the rigidity of the substrate (21).

3. An acoustic membrane for an acoustic transducer according to claim 1 or 2, wherein the substrate (21) has a thickness of between 5 pm and 70 pm and comprises at least one polymer and / or composite which has a rigidity greater than 2 GPa and a density less than 2800 kg / m3.

4. An acoustic membrane for an acoustic transducer according to claim 3, wherein the substrate (21) comprises at least one material selected from the group consisting of carbon composites, carbon fiber reinforced polymers, graphite and its composites, graphene and its composites, graphene oxide, carbon nanotubes, and mixtures thereof.

5. An acoustic membrane for an acoustic transducer according to claim 1 or 2, wherein the substrate (21) has a thickness of between 20 pm and 60 pm and comprises at least one metal or alloy which has a rigidity greater than 30 GPa and a density less than 2800 kg / m3.

6. An acoustic membrane for an acoustic transducer according to claim 5, wherein the substrate (21) comprises at least one material chosen from the group including aluminum and its alloys.

7. An acoustic membrane for an acoustic transducer according to claim 5, wherein the substrate (21) comprises at least one material selected from the group comprising magnesium and its alloys.

8. Acoustic membrane for acoustic transducer according to one of claims 1 to 7, wherein said at least one bonding layer (22a-22b) comprises at least one material chosen from the group comprising chromium and its alloys, titanium and its alloys, tantalum and its alloys, aluminum and its alloys, tungsten, compounds comprising nitrides, silicides or carbides of these metals, and mixtures thereof.

9. Acoustic membrane for acoustic transducer according to one of claims 1 to 8, in which said at least one bonding layer (22a-22b) has a thickness of between 0.1 pm and 0.3 pm.

10. Acoustic membrane for acoustic transducer according to one of claims 1 to 9, in which the stiffening layer (23a-23b) has a density of less than 3200 kg / m3.

11. Acoustic membrane for acoustic transducer according to one of claims 1 to 10, in which the stiffening layer (23a-23b) has a thickness of between 3 pm and 6 pm.

12. Acoustic membrane for acoustic transducer according to one of claims 1 to 11, in which the stiffening layer (23a-23b) has a rigidity greater than 450 GPa.

13. An acoustic membrane for an acoustic transducer according to claims 1 to 12, wherein the stiffening layer (23a-23b) has a proportion of sp3 bonds greater than or equal to 50%.

14. Acoustic membrane for acoustic transducer according to one of claims 1 to 13, in which the stiffening layer (23a-23b) corresponds to tetrahedral amorphous carbon ta-C.

15. Acoustic membrane for acoustic transducer according to one of claims 1 to 14, in which the membrane (14a-14c) comprises two stiffening layers (23a-23b) arranged on either side of the substrate (21) and fixed to the substrate (21) by two bonding layers (22a-22b).

16. Acoustic membrane for acoustic transducer according to one of claims 1 to 15, in which the membrane comprises at least one protective layer (25a-25b) placed between the substrate (21) and the bonding layer (22a-22b) of a stiffening layer (23a-23b).

17. Acoustic membrane for acoustic transducer according to one of claims 1 to 16 in which the membrane comprises at least one protective layer (24a-24b) fixed on at least one face of said stiffening layer (23a-23b), opposite the face fixed with the substrate (21) by means of the bonding layer (22a-22b).

18. Acoustic transducer (11) comprising: - a support (13); - a drive motor (20) configured to generate a magnetic field for moving the support (13); and - an acoustic membrane (14a-14c), according to one of claims 1 to 17, fixed with said support (13), said acoustic membrane (14a-14c) being configured to generate or capture acoustic waves.

19. Acoustic transducer according to claim 18, wherein the transducer is a high-frequency loudspeaker, i.e. it is configured to emit sounds in a frequency range greater than 1 kHz, said transducer (11) having a maximum frequency greater than 30 kHz, a diameter less than 6 cm and an acoustic membrane (14a-14c) having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

20. Acoustic transducer according to claim 18, wherein the transducer is a wideband loudspeaker, i.e. it is configured to emit sounds in a frequency range greater than 20Hz, said transducer (11) having a maximum frequency greater than 30 kHz, a diameter less than 8 cm and an acoustic membrane (14a-14c) having an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

21. Acoustic transducer according to one of claims 18 to 20, in which the acoustic membrane (14a-14c) has a mass of less than 220 mg.

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