Multilayer Transducers

Abstract

The electrostatic transducer (200) comprises first and second flexible conductive membranes (204, 206) and first and second conductive stators (208, 210). The membranes (204, 206) and stators (208, 210) are assembled in a layered configuration with the membranes (204, 206) between the stators (208, 210) and a mass of trapped air (226) sealed between the first and second membranes (204, 206). The electrostatic transducer (200) is arranged to apply an electrical potential which, in use, creates an electrostatic force between the membranes (204, 206) and the stators (208, 210) causing movement of the membranes (204, 206) relative to the stators (208, 210).

Description

[Technical field]

[0001] The present invention relates to electrostatic transducers, particularly but not exclusively to loudspeakers suitable for reproducing audio signals. [Background technology]

[0002] A conventional electrostatic loudspeaker consists of a conductive membrane placed between two perforated conductive backplates to form a capacitor. A DC bias is applied to the membrane, and an AC signal voltage is applied to the two backplates. This signal exerts an electrostatic force on the charged membrane, which acts to drive air on either side of the membrane.

[0003] Variations of such conventional electrostatic loudspeakers are known. For example, an electrostatic transducer may comprise two membranes. Such a configuration may offer several advantages in terms of the performance of the transducer, such as an increase in sound pressure level (SPL). However, there remains a need to improve the acoustic performance of this type of electrostatic transducer. Summary of the Invention

[0004] Viewed from a first aspect, the present invention provides an electrostatic transducer comprising: first and second flexible conductive films; first and second conductive stators; the membrane and the stators are assembled in a layered configuration with the membrane between the stators; the electrostatic transducer is configured to apply a potential which, in use, generates an electrostatic force between the membrane and the stator causing movement of said membrane relative to said stator; The first and second flexible conductive films have first and second effective compliances, respectively, the first effective compliance being at least 10% greater than the second effective compliance.

[0005] A first aspect of the invention also extends to a method of manufacturing an electrostatic transducer, the method comprising: providing first and second flexible conductive films and first and second conductive stators; assembling first and second flexible conductive membranes and first and second stators in a layered configuration with the membranes between the stators; configuring the electrostatic transducer to apply a potential thereto which, in use, generates an electrostatic force between the membrane and a stator to cause movement of the membrane relative to the stator; Including, Here, the first and second flexible conductive films have first and second effective compliances, respectively, after assembly in a layered configuration, the first effective compliance being at least 10% greater than the second effective compliance.

[0006] As will be appreciated by those skilled in the art, "compliance" is a term of the art that refers to the inverse of the stiffness of a material, i.e., compliance refers to the elongation of the material per unit force applied to the material. When a material (i.e., a membrane in this case) is held under tension, the tension affects the elongation per unit force. As will also be appreciated by those skilled in the art, an effective compliance can be defined that takes into account the tension under which the material is held. Thus, it should be understood that the term "effective compliance" as used herein refers to an effective compliance that takes into account the tension of the membrane when the membrane is attached in a layered configuration. Thus, the effective compliance of each membrane is defined for each membrane (at each tension) when assembled in a layered configuration within the transducer.

[0007] Also, as one skilled in the art will appreciate, the compliance of a material (and thus the effective compliance of a material held under tension) is not necessarily isotropic; for example, the compliance and effective compliance may be anisotropic (e.g., orthogonal). In general, the compliance and effective compliance may each be expressed as a tensor. In the context of the present invention, when it is said that the effective compliance of a first membrane is at least 10% (or any other specified percentage) greater than the effective compliance of a second membrane, this can be understood to mean that, where applicable, each non-zero component in the effective compliance tensor of the first membrane is at least the specified percentage greater than the corresponding component in the effective compliance tensor of the second membrane.

[0008] In general, having two membranes in a layered configuration in a transducer can provide several advantages. For example, by providing a second thin film in an electrostatic transducer, the output of the transducer can be increased, i.e. the SPL (sound pressure level) can be increased for a given input voltage. However, the applicant has noted that this configuration can introduce distortions (especially intermodulation distortions resulting from two membranes resonating together) and that it would be beneficial to improve the frequency response.

[0009] The applicant has realized that by providing membranes with different effective compliances, the intermodulation distortion problem can be reduced or eliminated, and the performance of the transducer can be further improved. A single membrane in a transducer has a unique frequency response that depends on the effective compliance of the membrane. A schematic diagram of a typical frequency response of a transducer with a single membrane is shown in FIG. 1. It can be seen that the amplitude of vibration increases with frequency up to a resonant peak 104, decreases after the peak, and then increases monotonically at higher frequencies. Different effective compliances provide different frequency responses, for example changing the position of the resonant peak.

[0010] Providing a second membrane with a different effective compliance provides an additional contribution to the frequency response, where the additional contribution has a different shape, e.g., a different resonant peak. The frequency response of the entire transducer is therefore a composite frequency response combining the frequency responses of the two membranes. When the difference in compliance is large enough, i.e., at least 10%, the composite frequency response has beneficially improved characteristics. For example, the contribution of two different resonant peaks may provide a flatter frequency response compared to a single membrane or compared to two membranes with the same effective compliance. This may advantageously mitigate the effects of distortion that may result from providing two membranes, while still providing the benefits of increased SPL discussed above.

[0011] It will be understood by those skilled in the art in light of the above disclosure that the difference in effective compliance, i.e., at least 10%, is significantly greater than the difference that occurs due to manufacturing tolerances in a transducer having two membranes. The difference between the nominal effective compliance of the membrane and the actual effective compliance that may occur due to manufacturing tolerances (e.g., membrane dimensions and / or membrane tension) will be 1%-2% or less. Thus, the difference in effective compliance is not inadvertently introduced but is provided intentionally.

[0012] In one set of embodiments, the first effective compliance is at least 15% greater than the second effective compliance, such as at least 20%, at least 25%, or at least 30% greater.

[0013] The difference between the first and second effective compliances can be achieved in any suitable manner, for example, by different membrane materials and / or membrane material properties, by different membrane thicknesses, by different membrane tensions, by other membrane properties or dimensions, or by a combination of two or more of these.

[0014] The first and second membranes may have different thicknesses, such that the first effective compliance can be wholly or partially greater than the second effective compliance as a result of the first and second membranes having different thicknesses.

[0015] The thickness of the second film may be at least 3% greater than the thickness of the first film, such as at least 5%, such as at least 10% greater.

[0016] The thickness of the first membrane may be less than 100 μm, such as less than 50 μm, such as less than 20 μm, such as less than 10 μm. The thickness of the first membrane may be greater than 5 μm, such as greater than 10 μm, such as greater than 20 μm.

[0017] The difference in thickness required to produce a specified difference in effective compliance (e.g., a 10% difference in effective compliance) may depend on the properties of the membrane, e.g., the membrane material, however, in light of the teachings of this disclosure, one of ordinary skill in the art will be able to select an appropriate thickness to achieve the specified difference in compliance.

[0018] The thickness may refer to an average thickness, a minimum thickness, or a maximum thickness.

[0019] Additionally or alternatively, the first and second membranes may be attached to the transducer at different tensile stresses. For example, each membrane may be tensioned differently during manufacture. As a result of the first and second membranes being at different tensile stresses when attached to the transducer, the first effective compliance may be greater in whole or in part than the second effective compliance.

[0020] The membranes may be placed under different tensions during the manufacture of the transducer, for example the first and second membranes may be bonded to the first and second stators, respectively, and / or to the spacer or spacer structure under different tensions, such that the first and second membranes have different tensile stresses.

[0021] The tensile stress of the second membrane may be at least 5% greater than the tensile stress of the first membrane, such as at least 10% greater, such as at least 15% greater, at least 20% greater, such as at least 25% greater.

[0022] The tensile stress of the first membrane may be in the range of 2 MPa to 50 MPa, such as 5 MPa to 30 MPa, such as 10 MPa to 20 MPa. As used herein, "tensile stress" refers to the average tensile stress of the membrane.

[0023] In one non-limiting exemplary embodiment, the transducer has first and second membranes made from 50 μm thick BOPP (biaxially oriented polypropylene), the first membrane having an average tensile stress of 20 MPa and the second membrane having an average tensile stress of 24 MPa.

[0024] Additionally or alternatively, the first and second membranes may be made from different materials, and the first effective compliance may be wholly or partially greater than the second effective compliance as a result of the first and second membranes being made from different materials.

[0025] In one non-limiting exemplary embodiment, the first membrane is made from BOPP and the second membrane is made from BoPET (biaxially oriented polyethylene terephthalate), and the first and second membranes have the same thickness and the same tensile stress, which can provide an effective compliance difference of about 32%, depending on the grade of material.

[0026] In one set of embodiments, each film comprises a laminated structure, for example including a flexible insulating layer and a conductive layer. For example, each film may comprise a flexible insulating layer (e.g., a polymeric material) having a conductive (e.g., metallic) layer on one side thereof, and a further flexible insulating layer (e.g., the same or a different polymeric material) overlaying (e.g., bonded to) the conductive layer. As one particular, non-limiting example, each film may comprise a BOPP layer onto which a gold or aluminum metal coating is vapor-deposited, and a further BOPP layer laminated onto the metal coating and bonded thereto with an adhesive.

[0027] However, this is not required, and in one set of possible embodiments, each membrane comprises or consists of a layer of flexible insulating material (e.g., a polymer) having a conductive layer (e.g., a vapor-deposited metal coating) on ​​one side thereof (e.g., the side of the membrane that faces away from the other membrane when attached to the transducer), without an additional flexible insulating layer laminated onto and bonded to the conductive layer. As one particular, non-limiting example, each membrane may be made from a sheet of BOPP having a conductive layer (e.g., a thin gold or aluminum vapor-deposited coating) on ​​one side.

[0028] Preferably, for each membrane, except for the parameters and / or characteristics specified as different, the membrane properties, materials, and any other parameters related to the attachment are the same within manufacturing tolerances. As mentioned above, more than one parameter or characteristic may be different between the membranes, for example, the membranes may be of different thicknesses and have different tensile stresses, such that a specified difference in effective compliance is achieved. In a preferred embodiment, only one parameter / characteristic selected from thickness, material, and tensile stress differs between the membranes. Thus, in any embodiment, the first and second membranes may have the same thickness. Similarly, the first and second membranes may be made of the same material or the same combination of materials. Similarly, the first and second membranes may be attached to the transducer with the same tensile stress.

[0029] As mentioned above, providing two membranes with a gap between them can potentially improve transducer performance. In one set of embodiments, a first and a second membrane are attached to the transducer with a gap between them.

[0030] However, the applicant has recognized that certain benefits and improvements in the performance of the transducer may be achieved if a trapped air mass is provided between the first and second membranes. For example, such a trapped mass may provide an air cushion that provides acoustic impedance, damping the membrane at high frequencies and providing a greater effective mass at low frequencies. This may enhance low frequencies and flatten high frequencies resulting in an overall flat frequency response. In one set of embodiments, the transducer includes a trapped air mass sealed between the first and second membranes.

[0031] This is novel and inventive in itself and therefore viewed from a second aspect the present invention provides an electrostatic transducer comprising: first and second flexible conductive films; first and second conductive stators; Equipped with the membranes and stators are assembled in a layered configuration with the membrane between the stators and a mass of trapped air sealed between the first membrane and the second membrane; The electrostatic transducer is configured to apply a potential which, in use, creates an electrostatic force between the membrane and the stator causing movement of the membrane relative to the stator.

[0032] A second aspect of the invention extends to a method of manufacturing an electrostatic transducer, the method comprising: providing first and second flexible conductive films and first and second conductive stators; assembling first and second flexible conductive membranes and first and second stators in a layered configuration with the membranes between the stators and a volume of trapped air sealed between the first and second membranes; configuring an electrostatic transducer to apply a potential that creates an electrostatic force between the membrane and the stator, causing movement of the membrane relative to the stator; Includes.

[0033] Where applicable, optional features of the first aspect may also be features of the second aspect, and vice versa.

[0034] It should be understood that when a sealed air mass between a first and a second membrane is said to be "enclosed," this means that the air is surrounded on all sides without gaps. For example, one skilled in the art will understand that a thin membrane may allow the passage of gas therethrough, and therefore, a sealed air mass does not necessarily need to be hermetically sealed between the first and the second membrane. More precisely, "enclosed" means that air cannot move freely in and out of the mass in a manner that transmits acoustic waves. Vents (e.g., provided for static equalization) are not usually necessary, and preferably no vents are provided.

[0035] The first and second membranes may be mounted in the transducer with a spacer or spacer structure located therebetween. The first and second membranes, together with the spacer or spacer structure, may entrap a volume of air. For example, the spacer or spacer structure may be bonded to the first and second membranes, for example to the respective outer edges of each membrane, to seal the entrapped volume of air. Additional spacers or spacer structures may be used to separate the first and second membranes from the first and second stators, respectively. The spacing between the first membrane and the first stator may be any suitable value, for example, 15 μm to 3 mm, for example, 0.1 mm to 1 mm, for example, about 0.5 mm. The spacing between the second membrane and the second stator may be any suitable value, for example, 15 μm to 3 mm, for example, 0.1 mm to 1 mm, for example, about 0.5 mm.

[0036] The transducer may have multiple air pockets sealed between the membranes. In one exemplary embodiment, the spacer comprises a layer of material having multiple large openings separated by walls, with the membranes each bonded to the walls between the openings. Each opening may thus define an air pocket sealed by the membrane. For example, the spacer may have a lattice shape (e.g., a hexagonal or square lattice).

[0037] The spacing between the first and second membranes can have any suitable value. In general, the spacing can be selected based on the application of the transducer, for example, to provide a larger SPL or to enhance lower frequencies. The spacing can be less than 5 mm, for example less than 2 mm. As discussed herein, the spacing between the membranes refers to the spacing when the membranes are in an undistorted position. The term spacing can refer to the vertical distance between the respective center points on the membranes.

[0038] In one set of embodiments, there is no intervening element between the first and second membranes, i.e. there is only an air gap between the membranes. The spacing between the first and second membranes may be at least 5 μm, such as at least 50 μm, such as at least 100 μm. This helps to provide a sufficient air cushion between the membranes to provide the above-mentioned advantages to the transducer performance.

[0039] In general, the first and second membranes may be electrically coupled, but this is not required. In embodiments where there is no intervening element between the first and second membranes, it is preferred that the first and second membranes are electrically coupled.

[0040] In general, the transducer may be electrically biased in any suitable manner to induce movement of the membrane in response to a varying voltage representative of an audio signal. Preferably, the electrostatic transducer comprises a biasing device configured to apply a voltage. As a non-limiting example, in an embodiment without an intervening element between the first membrane and the second membrane, a DC bias V may be applied to the first membrane and the second membrane together with a varying voltage applied to the first stator and the second stator. b (e.g., a voltage V1+V(t) can be applied to the first stator, where V1 is a bias offset and V1 <V b and V(t) is a fluctuating voltage corresponding to the audio signal. A further voltage V2-V(t) can be applied to the second stator, where V2 is a bias offset and V2 <V b (It is.)

[0041] In one set of embodiments, a further electrically conductive stator is provided between the first and second membranes. In such an embodiment, the spacing between the first and second membranes is preferably at least 20 μm, for example at least 50 μm. The further stator preferably comprises perforations that allow air to pass through. Thus, in an embodiment including a volume of enclosed air sealed between the membranes, the volume of air may include a further stator. The further stator may be separated from each of the first and second membranes by respective first and second spacers. The first and second spacers may be joined to the further stator and the first and second membranes (e.g. to the outer edges of the further stator and each membrane), with the joints of the membrane, the first and second spacers and the further stator together enclosing the volume of air.

[0042] The first and second stators preferably comprise perforations to allow air to pass through. The first stator, the second stator and / or the further stators (if provided) may comprise an insulating coating on one or more surfaces facing the membrane.

[0043] In embodiments having a further conductive stator between the first and second membrane, the membranes may be electrically coupled, but preferably the membranes are electrically insulated from each other.

[0044] As mentioned above, the transducer may be electrically biased in any suitable manner. By way of non-limiting example, in an embodiment with a further conductive stator between the first and second membranes, a DC bias V may be applied to the first membrane, the further stator, and the second membrane, respectively, with a varying voltage applied to the first and second stators. a , V b , and V c (For example, V a <V b <V c ) can be applied to the first stator (e.g., a voltage V1+V(t) can be applied to the first stator, where V1 is a bias offset and V1 <V a and V(t) is a fluctuating voltage corresponding to the audio signal. A further voltage V2-V(t) can be applied to the second stator, where V2 is a bias offset and V2 <V c (It is.)

[0045] In some embodiments, the transducer comprises three or more membranes. In addition to the first and second membranes, there may be one or more additional membranes between the first and second stators. For example, the transducer may comprise or consist of a first and second stator with three or more membranes (including the first and second membranes) between the first and second stators. Such a configuration may be electrically biased in any suitable manner. For example, the membranes may all be electrically coupled to each other and a single DC bias may be applied to all of the membranes between the stators with varying voltages applied to the first and second stators, i.e., this is a similar configuration to that described above for the version with two membranes, but with three or more membranes all electrically coupled.

[0046] As mentioned above, in one set of embodiments, the transducer may include a further (i.e., third) stator between the first and second membranes. In a subset of these embodiments, the transducer may include more than three stators and more than three membranes. For example, the transducer may include N stators and N-1 membranes, where N is at least 4. The stators and membranes may be arranged in an alternating layered configuration with the first and second stators being outermost. Such a configuration may be electrically biased in any suitable manner.

[0047] In one non-limiting example of a biasing arrangement, a varying voltage may be applied to each stator and a DC bias may be applied to each membrane. The varying voltage applied to the Nth stator (numbered sequentially through the transducer) may be a voltage V b , which includes a bias offset of N times . This allows a large voltage to be delivered across the transducer (thus improving output power and SPL) while keeping the voltage across any pair of stators well below its breakdown voltage. The magnitude of the voltage amplitude of the fluctuating component is the same for each stator, but the polarity of the fluctuating component for odd numbered stators is opposite to that for even numbered stators. The magnitude of the DC voltage applied to each membrane is the same, but the polarity for odd numbered membranes is opposite to that for even numbered membranes. However, this is only one example and other bias configurations are possible.

[0048] Certain preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0049] [Figure 1] For reference, FIG. 1 shows a schematic diagram of the frequency response of a typical electrostatic transducer having a single membrane. [Figure 2a] FIG. 2a shows a schematic cross-sectional view of a first embodiment of an electrostatic transducer according to the invention. [Figure 2b]FIG. 2b shows a schematic perspective view of the embodiment shown in FIG. 2a. [Figure 2c] FIG. 2c shows a plan view of the third spacer of the first embodiment. [Figure 3a] FIG. 3a shows a schematic cross-sectional view of a second embodiment of an electrostatic transducer according to the invention. [Figure 3b] FIG. 3b shows a schematic perspective view of the embodiment shown in FIG. 3a. [Figure 4a] FIG. 4a shows a schematic diagram of the respective frequency responses of two separate transducers, each fabricated from a single membrane, where the membrane of each transducer has a different effective compliance. [Figure 4b] FIG. 4b shows a schematic diagram of the frequency response of a prior art transducer and a transducer according to the invention, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] For reference, FIG. 1 shows an exemplary schematic diagram of a frequency response 100 of a typical electrostatic transducer with a single membrane. It can be seen that the low frequency portion 102 of the frequency response produces a relatively low SPL (sound pressure level). The SPL has a resonant peak 104 at higher frequencies, followed by a highest frequency portion 106 that then increases with increasing frequency. This frequency response is not ideal, especially due to the poor response at low frequencies and the disproportionately strong response at the resonant peak. To improve the fidelity of the output sound waves based on the input audio signal, a flatter frequency response profile with a stronger low frequency response is desirable.

[0051] Figures 2a and 2b show a first embodiment of an electrostatic transducer 200 according to the invention. The electrostatic transducer 200 comprises a layered arrangement of membranes 204, 206 and stators 208, 210 together with a biasing device 212. Figure 2a shows a schematic cross-sectional view of the layered arrangement. Figure 2b shows a schematic perspective view of the layered arrangement. Figures 2a and 2b are not to scale.

[0052] The layered configuration comprises a first membrane 204 and a second membrane 206 disposed between a first stator 208 and a second stator 210. The first membrane 204 and the second membrane 206 each comprise a flexible conductive layer. The first and second stators 208, 210 each comprise a rigid conductive sheet (an aluminum sheet in this example) having an array of holes 214 therein to allow sound waves generated by the membranes 204, 206 to pass through the stators 208, 210 and into the surrounding environment. In other embodiments, the stators 208, 210 may comprise different materials or combinations of materials.

[0053] The transducer 200 also includes first and second spacers 216, 218. The first spacer 216 is disposed between the first membrane 204 and the first stator 208, thereby holding the first membrane 204 and the first stator 206 in a spaced apart relationship. The first membrane 204 and the first stator 208 are bonded to the first spacer 216 using an adhesive. The second spacer 218 is similarly disposed between and bonded to the second membrane 206 and the second stator 210, thereby holding the second membrane 206 and the second stator 210 in a spaced apart relationship.

[0054] In this example, the spacing between the first membrane 204 and the first stator 208 is 1 mm, and the spacing between the second membrane 206 and the second stator 208 is also 1 mm, although other spacings are possible.

[0055] The transducer 200 further comprises a third spacer 220 disposed between the first membrane 204 and the second membrane 206. FIG. 2c shows a plan view of the third spacer 220. In this example, the third spacer 220 can be seen to have a square shape (although other shapes are possible) with a solid periphery 222 surrounding a central hole 224 on four sides. The position of the third spacer 220 between the first membrane 204 and the second membrane 206 can also be seen in FIG. 2b. The third spacer 220 is bonded to the membranes 204, 206 all around, such that the periphery 222 together with the membranes 204, 206 forms a complete enclosure with no gaps within the enclosure, enclosing a volume of air 226 within the hole 224. The solid periphery 222 may be formed from two or more pieces, such pieces being glued or sealed together without gaps or air holes. As mentioned above and further below, by providing a volume of trapped air between the two membranes, the performance of the transducer, particularly its frequency response, is improved.

[0056] In this example the spacing between the membranes is 0.5 mm, although other spacings are possible.

[0057] Each of the first and second membranes 204 has a respective effective compliance. As mentioned above, the effective compliance may depend on many factors related to the membrane structure, dimensions and / or material, as well as the manner in which it is attached. In the exemplary embodiment of Figs. 2a and 2b, the membranes 204, 206 are identical in structure, material and dimensions. In this example, each membrane is 50 μm thick and comprises a BOPP polymer sheet onto which an aluminum coating has been vapor-deposited, and a further layer of BOPP bonded onto the aluminum layer. In other embodiments, the membranes may comprise different materials or combinations of materials from this particular example and / or from each other, for example, in a variation of Figs. 2a and 2b, the membranes 204, 206 may each comprise a single BOPP sheet with an aluminum coating on one side.

[0058] To provide the difference in effective compliance, the two membranes are attached such that each membrane has a different average tensile stress across its surface: the first membrane 204 has an average tensile stress across its surface of 20 MPa, and the second membrane 206 has an average tensile stress across its surface of 24 MPa.

[0059] In a variation of the embodiment of Figures 2a and 2b, the first and second membranes 204, 206 are made of the same material and are attached in a layered configuration with the same tensile stress, but have different thicknesses. In this variation, the first membrane 204 has a thickness of 50 μm and the second membrane 206 has a thickness of 53 μm. Due to the difference in thickness, the effective compliance of the first membrane 204 is approximately 20% higher than the effective compliance of the second membrane 206.

[0060] In the exemplary embodiment of FIGS. 2a and 2b, the first and second membranes 204, 206 are electrically coupled and a biasing device 212 applies a DC bias V b The bias device 212 also provides a varying voltage to the first and second stators 208, 210. The voltage applied to each stator 208, 210 includes a bias offset (V1 for the first stator 208 and V2 for the second stator 210) and a varying component V(t) corresponding to the audio signal to be reproduced. The varying components have opposite polarities for each stator 208, 210 such that as the voltage V(t) varies, the stators 208, 210 cooperate to push and pull the biased membranes 204, 206 to generate sound waves corresponding to the audio signal.

[0061] Figures 3a and 3b show a second embodiment of an electrostatic transducer 300 according to the invention. The electrostatic transducer 300 comprises a layered arrangement of membranes 304, 306 and stators 308, 309, 310 (with holes 314, 315) together with a bias arrangement 312. Figure 3a shows a schematic cross-sectional view of the layered arrangement. Figure 3b shows a schematic perspective view of the layered arrangement. Figures 3a and 3b are not to scale.

[0062] The first and second membranes 304, 306 and first and second stators 308, 310 of this embodiment have the same structure as the membranes 204, 206 and stators 208, 210 of the first embodiment, including being joined to first and second spacers 316, 318 that hold the first and second membranes 304, 306 in spaced apart relationship relative to the first and second stators 308, 310, respectively. However, in this embodiment, a third stator 309 is provided between the first membrane 304 and the second membrane 306. The third stator 309 has the same structure as the first and second stators 308, 310, i.e., a metal sheet having an array of holes therein.

[0063] Instead of a single third spacer between the first and second membranes 304, 306, there are third and fourth spacers 322, 324. The third and fourth spacers 322, 324 have a shape similar to that shown in FIG. 2c, including a solid periphery and a central hole. The third spacer 322 is disposed between the first membrane 304 and the third stator 309 and is bonded to the first membrane 304 and the third stator 309 around their entire circumference. The fourth spacer 324 is disposed between the second membrane 306 and the third stator 309 and is bonded to the second membrane 306 and the third stator 309 around their entire circumference. The first and second membranes 304, 306 together with the third and fourth spacers 322, 324 and the periphery of the third stator 309 enclose a volume of air 326 between the first and second membranes 304, 306, i.e. the air volume is surrounded on all sides without any gaps. From figure 3a it can be seen that the air volume comprises two regions 328, 330 that are acoustically connected via the hole 315 in the third spacer 309.

[0064] In this example, the spacing between the membranes is 2 mm and the third stator 309 is equidistant from each membrane 304, 306, although other spacings are possible.

[0065] In the exemplary embodiment of FIGS. 3a and 3b, the first and second membranes 304, 306 are electrically isolated from each other, and the biasing device 312 applies a DC bias V to the first membrane 304, the third stator 309, and the second membrane 310, respectively. a , V b , and V c The bias device 312 also supplies a varying voltage to the first and second stators 308, 310. The voltage supplied to each of the first and second stators 308, 310 includes a bias offset (V1 for the first stator 308 and V2 for the second stator 310) and a varying component V(t) corresponding to the audio signal to be reproduced. The varying components have opposite polarities for each stator 308, 310, such that as the voltage V(t) varies, the three stators 308, 309, 310 cooperate to push and pull the biased membranes 304, 306 to generate sound waves corresponding to the audio signal.

[0066] As the membrane vibrates in response to an applied voltage, the trapped air mass 326 serves the benefits discussed above with reference to the first embodiment, namely, enhancing low frequencies and attenuating high frequencies in the transducer response.

[0067] In the example of Figures 3a and 3b, the first and second membranes 304, 306 have the same structure and configuration as the membranes 204, 206 described above with reference to Figures 2a and 2b, i.e., they are formed from the same materials as one another and have the same dimensions, but are attached in a layered configuration with different tensile stresses, such that the first membrane 304 has a higher effective compliance than the second membrane 306.

[0068] As mentioned above, providing two membranes with different effective compliances changes the frequency response of the transducer compared to two membranes with the same effective compliance: since the resonance characteristics depend on the effective compliance of the membranes, providing two membranes with different effective compliances combines the resonance characteristics of both membranes into a single frequency response that is generally flatter than the frequency response of a transducer with a single membrane or with two membranes with the same effective compliance.

[0069] Furthermore, as mentioned above, by having two membranes with a mass of air trapped between them, the acoustic impedance of the membranes is modified so that as a compound vibration element they have a higher effective mass at low frequencies and increased attenuation at higher frequencies, thus flattening the high frequencies in the frequency response of the transducer while enhancing the low frequencies, resulting in an overall flat frequency response.

[0070] 4a and 4b show, by way of example, typical changes observed in the frequency response of a transducer as described with reference to FIGS. 2a, 2b, 3a and 3b above, compared to known configurations.

[0071] Fig. 4a shows an exemplary schematic diagram of the frequency responses 400, 402 of two transducers made with a single membrane, each of which has a different effective compliance. In Fig. 4a, it can be seen that each transducer has a relatively low response at low frequencies, a resonant peak 404, 406 (which depends on the membrane compliance), which gradually increases with frequency after the peak. The frequency response 402 of the membrane with the higher effective compliance has a resonant peak 406 that is shifted downwards in frequency with respect to the resonant peak 404 of the frequency response 400 of the membrane with the lower effective compliance.

[0072] FIG. 4b shows an exemplary schematic diagram of the frequency response of a two-membrane transducer without a seal to enclose a mass of air and with the membranes having the same compliance (dotted line 408), and the frequency response of a two-membrane transducer with a seal and different membrane effective compliances (solid line 410), as described with reference to FIGS. 2a, 2b, 3a and 3b above.

[0073] From Figure 4b it can be seen that when two such different membranes are provided in one transducer, the combination of the two different resonant peaks results in an overall flattening of the frequency response. It can also be seen that by providing two membranes in the same transducer with a volume of air trapped between them, further flattening is provided by enhancing the low frequencies and attenuating the high frequencies. A flatter frequency response is desirable for the transducer to reproduce audio signals with higher fidelity.

[0074] Although only two embodiments have been described, it will be understood that these are merely exemplary and do not limit the scope of the invention as defined by the claims.

Claims

**Claim 1** An electrostatic transducer, comprising first and second flexible conductive films, and first and second conductive stators, wherein the films and the stators are assembled in a layered configuration with the film between the stators and having a mass of enclosed air sealed between the first and second films, the electrostatic transducer being configured to apply a potential that generates an electrostatic force between the film and the stators during use to cause movement of the film relative to the stators. **Claim 2** The electrostatic transducer according to claim 1, wherein the first and second films are mounted within the transducer together with a spacer or spacer structure located therebetween, and the first and second films enclose the mass of air together with the spacer or spacer structure. **Claim 3** The electrostatic transducer according to claim 1, wherein the transducer includes a plurality of masses of air sealed between the films. **Claim 4** The electrostatic transducer according to claim 1, wherein there is no intervening element between the first and second films. **Claim 5** The electrostatic transducer according to claim 4, wherein the distance between the first and second films is at least 5 μm. **Claim 6** The electrostatic transducer according to claim 4, wherein the first and second films are electrically coupled. **Claim 7** The electrostatic transducer according to claim 1, wherein the transducer further comprises one or more additional films between the first and second stators. **Claim 8** The electrostatic transducer according to claim 1, wherein an additional conductive stator is provided between the first and second films. **Claim 9** The electrostatic transducer according to claim 8, wherein the distance between the first and second films is at least 20 μm. **Claim 10** The electrostatic transducer according to claim 8, wherein the additional stator comprises perforations that allow air to pass therethrough. **Claim 11** ​ The additional stators are each separated from each of the first and second membranes by first and second spacers, the first and second spacers are joined to the additional stator and the first and second membranes, and the joined portions of the membranes, the first and second spacers, and the additional stator together enclose the air mass. The electrostatic transducer according to claim 8.

12. The electrostatic transducer according to claim 8, wherein the membranes are electrically insulated from each other.

13. The transducer includes N stators and N - 1 membranes, N is at least 4, and the stators and membranes are arranged in an alternating layered configuration with the first and second stators on the outermost sides. The electrostatic transducer according to claim 8.

14. The electrostatic transducer according to claim 1, wherein the first stator, the second stator, and / or the additional stator includes an insulating coating on one or more surfaces facing the membrane.

15. The first and second flexible conductive membranes each have first and second effective compliances, and the first effective compliance is at least 10% greater than the second effective compliance. The electrostatic transducer according to claim 1.

16. A method of manufacturing an electrostatic transducer, providing first and second flexible conductive membranes and first and second conductive stators; assembling the first and second flexible conductive membranes and the first and second stators in a layered configuration having an enclosed air mass sealed between the first and second membranes with the membranes between the stators; configuring the electrostatic transducer to apply a potential that generates an electrostatic force between the membranes and the stators during use to cause movement of the membranes relative to the stators; A manufacturing method including.

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