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) is an amorphous carbon layer 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 an adhesion layer (22a-22b) with a thickness between 0.1 µm and 1 µm and incorporating at least one metallic compound. (See Figure 2 for abbreviations.)

Description

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

[0001] The invention relates to the field of transducers, and more particularly to an acoustic diaphragm for an acoustic transducer. The acoustic transducer may be a loudspeaker or a microphone, for example a ribbon microphone in which the ribbon acts as an acoustic diaphragm. The invention also relates to an acoustic transducer comprising the diaphragm.

[0002] The invention finds multiple applications in fields where it is sought to obtain a transducer with a high bandwidth and / or high dynamics, i.e. a high acceleration and / or high frequency of membrane displacement.

[0003] For example, the invention finds a particularly advantageous application for loudspeakers capable of generating high-frequency sounds, such as tweeters or wideband loudspeakers, for example, drivers for audio headphones. Prior state of the 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 diaphragm, set in motion by a support having a coil displaced under the effect of a magnetic field. The support is fixed to only a part of the diaphragm, typically a central circular portion.

[0006] Since the membrane is displaced by an element fixed on only a part of it, it is important that the membrane be sufficiently rigid so that the displacement force applied to the membrane results in a uniform displacement of the membrane.

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

[0008] To obtain 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 greater the weight of the membrane, the greater the electromagnetic force required to move it, thus increasing energy consumption and leading to problems of membrane inertia.

[0010] A membrane is therefore sought which is rigid yet lightweight in order to limit distortions and inertial phenomena, particularly at high frequencies.

[0011] Many loudspeakers today use diaphragms made of glass, carbon, or aramid fibers, particularly for bass and midrange drivers, with thicknesses on the order of millimeters. However, with the typical diaphragm thicknesses of tweeters and headphone drivers, ranging from 30 to 60 µm, these materials do not allow for high-frequency reproduction while minimizing distortion. To reduce distortion, it would be possible to significantly increase the thickness, but this increase would lead to a substantial 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 of metal or alloy of fairly low density, in particular aluminium or titanium.

[0013] At equivalent thickness, membranes made with these materials are more rigid than 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 with membrane inertia.

[0015] To limit distortions and inertial phenomena, it is possible to use a beryllium membrane, as 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] To attempt to approximate the characteristics of a beryllium membrane, it is known to combine a lightweight but relatively flexible 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-language literature. Indeed, an aluminum substrate alone has a density of 2700 kg / m³ and an average stiffness, characterized by a Young's modulus of approximately 70 GPa. With a typical membrane thickness between 30 and 60 µm, this stiffness characteristic does not allow for high frequencies to be obtained while limiting distortion.

[0018] By forming a multilayer structure integrating a low-rigidity substrate and a layer With stiffening, it is theoretically possible to significantly improve the membrane's stiffness because the stiffening layer can exhibit 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 shearing 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 low-rigidity 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 tack layer between the substrate and the stiffening layer.

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

[0021] These bonding layers allow a stiffening layer of DLC or chromium nitride respectively to be fixed to an aluminum or titanium substrate, typically a DLC layer with a thickness between 1.5 pm and 2.4 pm or a chromium nitride layer with a thickness between 1.4 pm and 5.2 pm.

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

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

[0024] This document describes how the bonding layer allows a DLC stiffening layer with a thickness between 1 pm and 10 pm to be fixed to an aluminum substrate.

[0025] It should be noted that a silicon, germanium, or carbon bonding layer has an atomic coordination number close to that of C in a DLC. Thus, this document seems to suggest that in order to obtain adhesion of a DLC layer to a relatively flexible substrate with a thin tack layer, it is advisable to use a tack layer with an atomic coordination close to that of the stiffening layer.

[0026] However, within the scope of the invention, tests were carried out and these tests show that a silicon, germanium, or carbon adhesion layer does not allow for the bonding of a sufficiently rigid DLC layer, i.e., greater than 300 GPa, with a thickness of 4 µm, to an aluminum substrate. Indeed, it was observed that the stiffening layer undergoes delamination during the membrane manufacturing process.

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

[0028] Generally, the membrane manufacturing methods, which will be detailed later, require a significant energy input to the growing deposit during deposition, resulting 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 membrane temperature, a phenomenon accentuated for thin membranes with low thermal inertia. In addition, the difference in the coefficients of thermal expansion of the various membrane materials generates even greater stress within the membrane.

[0029] These stresses result in the presence of significant elastic potential energy in the DLC layer, proportional to the layer's internal stress 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 cohesive energy is lower than the potential energy relaxation generated by stack rupture at this interface. In order to prevent membrane failure under the effect of the significant load 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 tensile strength in the bonding layer on the other.

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

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

[0032] This solution addresses another technical problem, namely the reduction of the coefficient of friction in a mechanical contact under limiting lubrication conditions. It is intended for application to parts operating primarily in a lubricated environment, subjected to significant forces (approximately 100 daN) and moving at frequencies much lower than those of acoustic membranes, particularly at frequencies below 1 kHz.

[0033] Thus, a person skilled in the field 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 massive steel part several centimeters thick weighing generally 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 proposes 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 on a substrate, thus limiting distortions and inertia phenomena at high frequencies, resisting delamination and the risk of interface shearing, and ensuring the adhesion of the stiffening layer to the substrate thanks to an optimized adhesion layer. Description 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 enhanced by creating a chemical composition gradient within the bonding layer, between the two interfaces, which prevents the formation 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 that it is possible to fix a stiffening layer with a thin tack 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 tack layer.

[0040] Similarly, it helps 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 tack layer with an 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 substrate stiffening layer to improve membrane rigidity.

[0042] The invention is characterized in that said at least one stiffening layer corresponds to an amorphous carbon layer 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 being fixed to said substrate by means of an adhesion layer of thickness between 0.1 pm and 1 pm and incorporating at least one metallic compound.

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

[0044] Thus, this embodiment stems from an observation that a bonding layer incorporating at least one metallic compound, having a stiffness between the stiffness of the stiffening layer and the stiffness of the substrate and a thickness between 0.1 µm and 1 µm, can provide the expected delamination resistance and interface shear stress resistance for a stack comprising a DLC amorphous carbon stiffening layer, 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 Anglo-Saxon literature, is a type of carbon coating which possesses certain properties of diamond, although it does not have the typical crystalline structure of diamond.

[0046] These layers are made of carbon having an amorphous (non-crystalline) structure with mixed atomic sp3 (as in diamond) and sp2 (as in graphite) 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 approximately 3520 kg / m³, while that of graphite is approximately 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 is based on an observation that it is particularly advantageous to have an adhesion layer with a rigidity intermediate between that of the substrate and that of the stiffening layer, in order to improve the adhesion of the stiffening layer to the substrate by limiting delamination and the risk of interface shearing between the layers.

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

[0050] Generally, ta-C type DLC amorphous carbon layers have internal compressive stresses ranging from 2 GPa to over 8 GPa. The DLC amorphous carbon layers of the membrane according to the invention exhibit a minimal 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 substrate rigidity and a limited increase in weight and inertia.

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

[0053] For example, the substrate has a thickness between 5 pm and 70 pm and comprises at least one polymer and / or composite that has a stiffness greater than 2 GPa and a density less than 2800 kg / m³. Preferably, the substrate includes 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 between 20 pm and 60 pm and comprises at least one metal or alloy which has a stiffness greater than 30 GPa and a density less than 2800 kg / m3. For example, the substrate may comprise aluminium 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 amorphous DLC 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 is 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] Besides hydrogenated forms, it is also possible to use ta-C type DLC, 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 a 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 course of the invention, it has become apparent that hydrogenated forms of DLC or ta-C are particularly effective in improving membrane stiffness, even at low thicknesses. According to one embodiment of the invention, a significant improvement in membrane stiffness can be observed with a DLC amorphous carbon layer having a thickness between 3 µm and 6 µm and a stiffness 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 Carbon layer density instead of stiffness. Indeed, it is known that a layer stiffness greater than 300 GPa corresponds to a density greater than 2.6 g / cm³ for a taC:H and a density greater than 2.4 g / cm³ for a ta-C. Thus, by measuring a density greater than 2.4 g / cm³ for the stiffening layer, by weighing it before and after deposition, knowing the surface area of ​​the part and the layer thickness, the stiffness should be greater than 300 GPa.

[0062] For the purposes of this invention, the stiffness of a material or a composite of several materials is defined by its modulus of elasticity, or Young's modulus, which corresponds to a measure of the material's resistance to elastic deformation. It is important to note that the values ​​of the modulus of elasticity can vary slightly depending on the purity of the material and its microstructure, whether dense or columnar, i.e., having highly elongated grains 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 performed using a nanoindentation process, as described in document FR 2 796 150, which consists of applying a measuring load to the surface of the material using a probe to produce a deformation of the material, and then measuring the deformations produced by the probe under different loads to determine the hardness and stiffness of the material. This yields a displacement curve as a function of the load, allowing the hardness and stiffness to be extracted.

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

[0066] According to the invention, the proportion of sp3 bonds is measured by Raman spectroscopy. In particular, Raman spectroscopy makes it possible to obtain the average sp3 hybridization atom content of a layer by measuring spectra at different laser excitation wavelengths (325 nm, 442 nm, 488 nm, and 633 nm). The laser power is limited to avoid modifying the material under irradiation. The intensity spectra as a function of the Raman shift are fitted by two Gaussian curves for the G (stretching mode) and D (vibration mode) peaks of the carbon bonds. Plotting the position of the G peak (in Raman shift) as a function of the excitation wavelength yields a straight line whose slope is the dispersion of the G peak (in cm⁻¹ Vnm). This dispersion is proportional to the sp3 hybridization atom content of the analyzed material.The analysis of the sp3 bond content of a material belonging to the DLC family is described for example 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 a single stiffening layer on one side of the membrane to improve the membrane's rigidity. Preferably, the membrane comprises two stiffening layers arranged on either side of the substrate and attached to the substrate by two adhesion layers. This embodiment makes it possible to improve the membrane's rigidity by using two stiffening layers attached on either side of the membrane.

[0068] The bonding layer(s) may comprise at least one material selected from the group including 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. Preferably, the bonding layer comprises chromium and / or at least one chromium alloy. These materials exhibit particularly effective technical performance for enabling the adhesion of carbon to metal, but also for moderating the shear stress between the DLC layer and the substrate, with very thin 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 tack layer(s) have a thickness between 0.1 µm and 0.3 µm. Such a limited tack layer thickness helps to limit the overall weight of the acoustic membrane. A greater thickness is possible, but increases the weight of the membrane.

[0071] Furthermore, the membrane may include at least one protective layer placed between the substrate and the tack layer of a stiffening layer. Alternatively or in addition, at least one protective layer may be fixed to at least one face of the stiffening layer, opposite the face fixed to the substrate by means of the tack layer.

[0072] According to a second aspect, the invention relates to the acoustic transducer comprising a support, preferably cylindrical in shape, a drive motor configured to generate a magnetic field of displacement of 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 dimensioning 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 above 1 kHz, said transducer having a maximum frequency above 30 kHz, a diameter of less than 6 cm and an acoustic diaphragm exhibiting an equivalent Young's modulus greater than 200 GPa, preferably greater than 250 GPa.

[0075] In another embodiment of the invention, the transducer is a wideband loudspeaker, that is to say, it is configured to emit sounds in a frequency range above 20 Hz, said transducer having a maximum frequency above 30 kHz, a diameter of less than 8 cm and an acoustic diaphragm having an equivalent Young's modulus of more than 200 GPa, preferably more than 250 GPa. Brief description of the figures

[0076] The manner of implementing the invention and the resulting advantages will become clear from the following embodiments, given by way of example but not limitation, with support from the accompanying figures in which:

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

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

[0079] [Fig.3] is a schematic cross-sectional view of a membrane of the acoustic transducer of [Fig.1] according to a second embodiment; and

[0080] [Fig.4] is a schematic cross-sectional view of a transducer membrane acoustics of [Fig.1] according to a third embodiment. Detailed description of the invention

[0081] Figure 1 illustrates an acoustic transducer 11 comprising a chassis 12 intended to be fixed inside a housing to form an enclosure. The chassis 12 supports a drive motor 20 for a diaphragm 14a. To achieve this, the acoustic transducer 11 includes a cylindrical support 13 that is 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 classic positioning of a loudspeaker as illustrated in [Fig. 1], in which the motor 20 is disposed in the lower part while the diaphragm 14a is disposed in the upper part of the acoustic transducer 11. Of course, the acoustic transducer 11 can be reversed without changing the invention.

[0084] The outer annular edge 15 of the membrane is connected to an upper support 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 between 1 and 17 centimeters.

[0087] In one embodiment, the acoustic transducer 11 is a high-frequency loudspeaker, that is, it is configured to emit sounds in a frequency range above 1 kHz. In this embodiment, the acoustic transducer 11 has, for example, a maximum frequency above 30 kHz, a diameter of the upper bearing surface 16 of the chassis 12 of less than 6 cm and a diaphragm 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 wideband loudspeaker, that is to say, it is configured to emit sounds in a frequency range above 20 Hz. In this embodiment, the acoustic transducer 11 has a maximum frequency above 30 kHz, a diameter of the upper bearing surface 16 of the chassis 12 of less than 8 cm and a diaphragm 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 that preferentially allow for a weight of less than 220 mg in order to limit the inertia of the membrane. To achieve 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 to the substrate 21 by an adhesion 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 adhesion 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 stiffness of less than 150 GPa. To achieve this, the substrate 21 can be made of a polymer or composite, having a stiffness greater than 2 GPa, a density less than 2800 kg / m³, and a thickness between 5 µm and 70 µm. In this embodiment, it can be made of a material selected from the group comprising carbon composites, carbon fiber reinforced polymers, graphite and its composites, graphene and its composites, carbon nanotubes, and mixtures thereof.

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

[0093] The acoustic membrane 14a-14c also includes at least one stiffening layer 23a-23b of the substrate 21 to improve the stiffness of the membrane 14a-14c.

[0094] According to the invention, the stiffening layer 23a-23b corresponds to a DLC amorphous carbon layer having a stiffness 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 between 3 µm and 6 µm, a density less than 3200 kg / m³, a stiffness greater than 450 GPa and / or a HIT hardness 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 an adhesion layer 22a-22b incorporating at least one metallic compound and having a stiffness between the stiffness of the stiffening layer 23a-23b and the stiffness of the substrate 21, with a thickness between 0.1 and 1 µm. Preferably, it has a thickness between 0.1 and 0.3 µm and is made from a material selected 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 to 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 spraying. 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 layer deposition can be used to manufacture the acoustic membrane 14a-14c according to the invention.

[0102] For the deposition of the adhesion layers 22a-22b on the substrate 21, it is possible to use an electrolytic deposition, a chemical (autocatalytic) deposition, a physical vapor phase deposition, or a chemical vapor phase 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 adhesion layer to be deposited. By applying an electrical voltage, the ions are reduced and deposited on the surface of the substrate, forming a uniform adhesion 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 Anglo-Saxon literature, consists of vaporizing the constituent material of the adhesion layer under vacuum, and depositing the vaporized particles on the surface of the substrate 21. For example, cathode arc deposition is a type of PVD.

[0106] Chemical vapor deposition, called CVD for "Chemical Vapor Deposition" in the Anglo-Saxon literature, consists of introducing gaseous precursors of the material constituting the adhesion layer 22a-22b which react and decompose, forming the adhesion 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 tack 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 "Catholic Arc deposition" in the Anglo-Saxon literature, can be used to deposit the stiffening layers 23a-23b on the tack layers 22a-22b.

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

[0111] With these deposition techniques, the tack layer 22a-22b can be so thin compared 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 to On either side of the substrate 21, it is possible to use a magnesium substrate 21 with a stiffness of 42 GPa and a thickness of 31 pm associated with two stiffening layers 23a-23b in ta-C with a stiffness of 500 GPa and a thickness of 4 pm and two bonding layers 22a-22b of 0.3 pm thickness.

[0113] With these thicknesses and stiffnesses, the equivalent stiffness 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 factor in predicting the behavior of the acoustic membrane 14a is the speed of sound exiting this acoustic membrane 14a. This speed of sound can be estimated by the square root of the ratio between the stiffness and the density. In the example described above, the speed of sound is estimated at 11683 m / s.

[0120] These values ​​are close to those measured for a beryllium membrane which classically has a stiffness of 287 GPa, a density of 1848 kg / m3 and a speed of sound 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 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 volumetric mass density 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 stiffness 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 thus makes it possible to obtain a rigid and lightweight acoustic membrane 14a-14c by means of attaching a stiffening layer to a substrate 21. This acoustic membrane 14a-14c exhibits limited distortion and inertial phenomena at high frequencies. Furthermore, this acoustic membrane 14a-14c effectively resists delamination and the risk of interface shearing. It follows that the invention makes it possible to provide a particularly effective acoustic membrane 14a-14c for forming a high-resolution acoustic transducer 11.

Claims

Demands

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) for improving the stiffness of the membrane (14a-14c); characterized in that said at least one stiffening layer (23a-23b) corresponds to an amorphous carbon layer 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 to said substrate (21) by means of an adhesion layer (22a-22b) of thickness between 0.1 pm and 1 pm and incorporating at least one metallic compound.

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

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

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

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

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

7. Acoustic membrane for 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 any one of claims 1 to 7, wherein said at least one tack layer (22a-22b) comprises 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.

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

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

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

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

13. Acoustic membrane for 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 any one of claims 1 to 13, wherein the stiffening layer (23a-23b) corresponds to tetrahedral ta-C amorphous carbon.

15. Acoustic membrane for acoustic transducer according to any one of claims 1 to 14, wherein 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 gripping layers (22a-22b).

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

17. Acoustic membrane for acoustic transducer according to any one of claims 1 to 16 wherein 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 to the substrate (21) by means of the tack layer (22a-22b).

18. Acoustic transducer (11) comprising: - a support (13); - a drive motor (20) configured to generate a magnetic field of displacement of the support (13); and - an acoustic membrane (14a-14c), according to any 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 above 1 kHz, said transducer (11) having a maximum frequency above 30 kHz, a diameter of less than 6 cm and an acoustic diaphragm (14a-14c) having an equivalent Young's modulus above 200 GPa, preferably above 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 above 20Hz, said transducer (11) having a maximum frequency above 30 kHz, a diameter of less than 8 cm and an acoustic diaphragm (14a-14c) having an equivalent Young's modulus above 200 GPa, preferably above 250 GPa.

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

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