Improved sheet for the production of improved vent assemblies

Abstract

There is provided a sheet comprising a membrane, and a first layer of a cured material, the first layer being provided on a first side of the membrane, the first layer comprises a first array of apertures, wherein the membrane comprises a retracted material and the sheet is configured such that a portion of the sheet comprising one aperture cut from the sheet corresponds to a vent assembly.

Description

[0001] Improved Sheet for the Production of Improved Vent Assemblies

[0002] Field

[0003] The present disclosure provides improved sheets of vent assemblies and improved vent assemblies formed from the same.

[0004] Background

[0005] Acoustic vent assemblies are used to protect acoustic transducers such as microphones and speakers in electronic devices from particulates and liquids. Typical acoustic vent assemblies comprise a membrane that protects the acoustic transducer whilst minimising the impact of acoustic vent assemblies on the acoustic performance of the acoustic transducers.

[0006] The manufacture of acoustic vent assemblies often involves multiple acoustic vent assemblies being formed in a sheet of material to then be cut from the sheet to form the individual acoustic vent assemblies.

[0007] The conversion process of cutting individual vents from a sheet of material can yield vents with variability of performance from one vent to the next. Therefore, a common practice is to subject the resulting array of converted vents to thermal treatment where the vents are subjected to elevated temperatures. The structure of the membrane may change when subjected to such elevated temperatures and can result in shrinkage of the membrane, for example, with the aim to obtain more consistent performance across vents.

[0008] However, uncontrolled or lack of shrinkage within the membrane can be uneven and can still result in the acoustic performance of individual acoustic vent assemblies that vary significantly and can lead to a lower yield of acoustic vent assemblies from a given sheet of material as acoustic vent assemblies. The lower yield results because more variable acoustic performance of vents makes it difficult to apply global adjustments to a microphone response on a particular device.

[0009] Accordingly, there remains a need for more consistent performance of acoustic vent assemblies that are produced from a sheet of material such that the yield of acoustic vent assemblies from a given sheet of material is higher than previously provided.

[0010] Therefore, the present disclosure is aimed at providing improved sheets of vent assemblies. Summary

[0011] According to a first aspect there is provided a sheet comprising a membrane, and a first layer of a cured material, the first layer being provided on a first side of the membrane, the first layer comprises a first array of apertures, wherein the membrane comprises a retracted material and the sheet is configured such that a portion of the sheet comprising one aperture cut from the sheet corresponds to a vent assembly.

[0012] As used herein, the term “retracted material” refers to material that has been exposed to elevated temperatures that induce a structural change in the material. The retracted material may be a fibrillated material that has had the tension of the fibrillated material increased or the fibrils of the fibrillated material may be retracted into non-linear configurations. For example, the non-linear configurations maybe include serpentine configurations where at least a majority of fibrils of the structure of the retracted material change sense or wind back on themselves. The thickness of the membrane comprising a retracted material may be more consistent from the edge of the vent to the middle of the vent compared to the thickness of a membrane of a vent that does not comprise a retracted material. Therefore, the difference in thickness of the membrane comprising a retracted material between the edge of the vent and the middle of the vent may be smaller than for a membrane that does not comprise a retracted material. The thickness of the membrane comprising a retracted material may be thinner at the edge of the vent compared to the thickness of a membrane that does not comprise a retracted material. The thickness of the membrane comprising a retracted material may be thinner at the middle of the membrane compared to the thickness of the membrane that does not comprise a retracted material.

[0013] The membrane may have a thickness difference from the edge of the membrane to the center of the membrane of less than 2 pm. The membrane may have a thickness difference from the edge to the center of the membrane of less than 1 pm. The thickness difference (Ax) may be defined as the thickness of the center (Xc) minus the thickness of the edge (xe) of the membrane:

[0014] Ax = xc_xe (1)

[0015] The thickness difference of the membrane comprising a retracted membrane may be at least 0.5 pm less than the thickness difference for a corresponding membrane that does not comprise a retracted membrane. The thickness difference of the membrane comprising a retracted membrane may be at least 1 pm less than the thickness difference for a corresponding membrane that does not comprise a retracted membrane. The thickness difference of the membrane comprising a retracted membrane may be at least 0.5 pm less than the thickness difference for a corresponding membrane that comprises a retractable membrane. The thickness difference of the membrane comprising a retracted membrane may be at least 1 pm less than the thickness difference for a corresponding membrane that comprises a retractable membrane.

[0016] The percentage change between the thickness difference of a membrane comprising a retracted membrane and the thickness difference of a membrane comprising a membrane that does not comprise a retracted material or that comprises a retractable material may be at least 10%. The percentage change may be at least 20%. The percentage change may be at least 30%. The percentage change may be a percentage decrease.

[0017] The percentage change between the thickness difference of a membrane comprising a retracted membrane and the thickness difference of a membrane comprising a membrane that does not comprise a retracted material or that comprises a retractable material normalised to the thickness of a membrane that does not comprise a retracted material (normalised percentage change) may be at least 40%. The normalised percentage change may be at least 50%. The normalised percentage change may be defined as:

[0018] % diffn = ((Axi - AX2) / AXI) x 100 (2)

[0019] Typically, a membrane is planar and extends in two dimensions in a plane. The provision of a first layer of a cured material on a first side of a membrane comprising a retracted material may act to restrain the retracted material of the membrane to thereby result in a restrained retracted material. Accordingly, without being bound by theory, the first layer of cured material may restrain or pin the material at the periphery of each aperture of the first array of apertures such that when the material within each aperture of the first array of apertures is exposed to elevated heat or pressure. This may result in the material being less free to macroscopically retract in the in-plane direction, and will instead retract predominately in the direction perpendicular to the plane of the membrane, resolve slack in the material, or reorient its internal structure. As a result, structural inconsistencies within an array of venting material, such as slack, thickness variations, and differences in tension, are significantly lessened in the restrained retracted material. Therefore, the provision of a first layer of cured material on a membrane comprising a retracted material ensures greater consistency between each vent assembly that is formed from the sheet. For example, the structure of each vent assembly may be more consistent when formed from the sheet of the present aspect. The acoustic performance of each vent assembly formed from the sheet of the present aspect may be more consistent.

[0020] The sheet may comprise a second layer of a cured material and the second layer may be provided on a second side of the membrane. The second layer may comprise a second array of apertures and each aperture of the first array of apertures may form an aperture pair with an aperture of the second array of apertures and the aperture pair may occlude each other such that the membrane is exposed on both the first side and the second side.

[0021] Each aperture pair may correspond to a vent assembly when cut from the sheet. The first layer of a cured material may be provided on the periphery of the vent assembly on a first side of the membrane. The second layer of a cured material may be provided on the periphery of the vent assembly on a second side of the membrane. The aperture pair may expose the membrane such that the membrane is free to vibrate and / or equalize pressure across the vent assembly.

[0022] The sheet may comprise at least one support portion. The at least one support portion may be a continuous portion of the sheet that does not comprise an aperture. The at least one support portion may extend along at least one side of the sheet. The at least one support portion may be provided between sub groups of the first array of apertures and, where present, the second array of apertures. The provision of at least one support portion may provide structural stability to the sheet. The at least one support portion may regulate the thermal physical properties of the sheet. The at least one support portion may regulate the thermal physical properties of the membrane.

[0023] The at least one support portion may extend over at least 20% of the surface area of the sheet. The at least one support portion may extend over at least 30% of the surface area of the sheet. The at least one support portion may extend over at least 40% of the surface area of the sheet.

[0024] The sheet may comprise at least two support portions. The at least two support portions may be provided along sides of the sheet. A first support portion of the at least two support portions may be provided along a first side or edge of the sheet. A second support portion of the at least two support portions may be provided along a second side or edge of the sheet. The second side or edge may be on an opposed side of the sheet to the first side or edge. Accordingly, the first array of apertures and, where present, the second array of apertures may be provided between the first support portion and the second support portion. Therefore, the first support portion and the second support portion may provide structural stability and support to the first array of apertures and, where present the second array of apertures.

[0025] The sheet may comprise at least three support portions. The at least three support portions may be provided along three sides of the sheet. A first support portion and a second support portion of the at least three support portions may be provided along a first side or edge and an opposed second side or edge respectively. A third support portion of the at least three support portions may be provided between the first and second support portions. The first array of apertures, and, where present, the second array of apertures, may be provided in two sub groups, a first sub group between the first and third support portions and a second sub group between the second and third support portions.

[0026] The sheet may comprise at least four support portions. A first support portion may be provided along a first side or edge of the sheet. A second support portion may be provided along a second side or edge of the sheet. A third support portion may be provided along a third side or edge of the sheet. A fourth support portion may be provided along a fourth side or edge of the sheet. The first array of apertures, and where present the second array of apertures, may be provided between the first support portion, the second support portion, the third support portion and the fourth support portion. Accordingly, the first array of apertures, and where present the second array of apertures, may be surrounded by the first support portion, the second support portion, the third support portion and the fourth support portion.

[0027] At least the majority of apertures of the first array of apertures and, where present, of the second array of apertures may be circular. At least a majority of the apertures of the first array of apertures that are circular may have the same or substantially the same diameter.

[0028] At least some of the apertures of the first array of apertures and, where present, of the second array of apertures may have a regular shape. At least some of the apertures of the first array of apertures and, where present, of the second array of apertures may have a polygonal shape. For example, at least some of the apertures may be rectangular, pentagonal, hexagonal, heptagonal, octagonal or a higher order polygon.

[0029] Typically, each aperture of the first array of apertures may have the same shape. Where present each aperture of the second array of apertures may have the same shape. Each aperture of the first array of apertures may have the same or substantially the same maximum dimension. Where present each aperture of the second array of apertures may have the same or substantially the same maximum dimension. For example, where the apertures are circular, each aperture of the first array of apertures and / or second array of apertures, where present, may have the same or substantially same diameter. At least one aperture of the first array of apertures may have a different size or shape to the remaining apertures. At least one aperture of the second array of apertures may have a different size or shape to the remaining apertures, where present.

[0030] Each aperture within an aperture pair may have the same shape and may completely or substantially completely occlude one another. Accordingly, an aperture pair may define a venting conduit that is spanned by the membrane.

[0031] The first array of apertures may be a regular array. Each aperture within the first array of apertures may be spaced equally or substantially equally from one another. Each aperture within the first array of apertures may be arranged in a regular pattern. The first array of apertures may be arranged in a rectangle. The first array of apertures may be arranged in a square or oblong. The first array of apertures may be arranged in a circle. The first array of apertures may be arranged in another regular shape, such as a pentagon, or an ellipse, for example.

[0032] The first array of apertures may be provided on a regular grid. The first array of apertures may be provided on a square grid. The first array of apertures may comprise at least four apertures in a two by two grid. The first array of apertures may comprise at least nine apertures in a three by three grid. The first array of apertures may comprise at least sixteen apertures in a four by four grid. The first array of apertures may comprise at least twenty five apertures in a five by five grid. The first array of apertures may be provided in an oblong grid or other regular grid. For the avoidance of doubt, the above arrays are examples only and it will be appreciated that the first array of apertures may be provided in any suitable pattern that can be readily determined by the person skilled in the art.

[0033] In embodiments comprising a second array of apertures, the second array of apertures may be a regular array. Each aperture within the second array of apertures may be spaced equally or substantially equally from one another. Each aperture within the second array of apertures may be arranged in a regular pattern. The second array of apertures may be arranged in a rectangle. The second array of apertures may be arranged in a square or oblong. The second array of apertures may be arranged in a circle. The second array of apertures may be arranged in another regular shape, such as a pentagon, or an ellipse, for example. The second array of apertures may be provided on a regular grid. The second array of apertures may be provided on a square grid. The second array of apertures may comprise at least four apertures in a two by two grid. The second array of apertures may comprise at least nine apertures in a three by three grid. The second array of apertures may comprise at least sixteen apertures in a four by four grid. The second array of apertures may comprise at least twenty five apertures in a five by five grid. The second array of apertures may be provided in an oblong grid or other regular grid. For the avoidance of doubt, the above arrays are examples only and it will be appreciated that the first array of apertures may be provided in any suitable pattern that can be readily determined by the person skilled in the art.

[0034] The sheet may comprise one or more locating features. In embodiments comprising a second array of apertures, the one or more locating features may allow the sheet to be assembled such that the first array of apertures and the second array of apertures may be readily and repeatably aligned. The one or more locating features may comprise a plurality of apertures. The plurality of apertures of the one or more locating features may extend through the membrane.

[0035] The membrane may comprise a fluoropolymer. The membrane may comprise polytetrafluoroethylene (PTFE), polyvinlidene fluoride (PVDF), or fluorinated ethylene propylene (FEP) or combinations thereof. The membrane may comprise a non-fluoropolymer. The membrane may comprise polyparaxylylene (PPX), polyethylene (PE), poly(tetramethyl-p- silphenylenesiloxane) (PTMPS), polyamide 6, polyurethane, thermoplastic polyurethane, polypropylene, polyimide, or polyacrylonitrile (PAN) or combinations thereof.

[0036] Accordingly, the membrane may comprise PTFE, PPX, PE, PTMPS, polyamide 6, polyurethane, thermoplastic polyurethane, PVDF, FEP, polypropylene, polyimide, or PAN or combinations thereof. The membrane may comprise PTFE, PE, polyamide 6, or polyurethane or combinations thereof.

[0037] It will be understood that a cured material is a material that has been through a curing process in which the material has been stiffened or solidified. Typically, the curing process forms new bonds within the material. For example, a cured polymeric material may be a polymeric material that has been cured to induce the formation of cross-links between polymer chains within the polymeric material. The elastic modulus of the cured material may be greater than the elastic modulus of the curable material before the curing process. The cured material may comprise a thermoset adhesive. Accordingly, the cured material may have been exposed to elevated temperatures to thereby cure the material in place on the membrane. The thermoset adhesive may comprise a nitrile phenolic adhesive, epoxy adhesive, polymeric adhesive, acrylic adhesive, silicone adhesive, or polyurethane adhesive, or combinations thereof. For example, the thermoset adhesive may comprise a polyester resin or epoxy resin, a phenol formaldehyde resin or phenolic resin, a polyimide adhesive, nitrile rubber and composites or mixtures of the same.

[0038] The cured material may comprise a catalyst that induces curing when exposed to a specific wavelength of light. For example, the catalyst may induce curing when exposed to ultraviolet (UV) light. The UV curing adhesive may comprise an acrylic adhesive, epoxy adhesive, polyester adhesive, silicone adhesive, vinyl adhesive or composites or mixtures of the same.

[0039] The cured material may comprise an additive or hardener that induces curing of the material. The additive or hardener may induce cross-linking in a curable material to form the cured material. For example, the additive may be a polyamine or sulfur.

[0040] Typically, the cured material is sufficiently stiff to restrain the retracted material during the process of retraction to form the sheet. The cured material may be sufficiently stiff to restrain the retracted material during the process of retraction where the sheet is exposed to elevated temperatures. The cured material may have a melting temperature significantly above the processing temperatures used to produce the sheet and / or to induce retraction of the membrane material to form the retracted material.

[0041] The cured material may have an elastic modulus of at least 5 x 107Pa at a shear deformation frequency of 1 Hz. The cured material may have an elastic modulus of at least 1 x 108Pa at a shear deformation frequency of 1 Hz. The cured material may have an elastic modulus of at least 5 x 108Pa at a shear deformation frequency of 1 Hz. The cured material may have an elastic modulus of at least 1 x 109Pa at a shear deformation frequency of 1 Hz.

[0042] Typically, the membrane comprises a material that retracts at or above a retraction temperature. Exposure to temperatures at or above the retraction temperature may increase the tension of the membrane, and therefore the membrane comprising a retracted material may have a higher tension than a membrane that does not comprise a retracted material.

[0043] The cured material may have an elastic modulus of at least 5 x 107Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of at least 1 x 108Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of at least 5 x 108Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of at least 1 x 109Pa at a shear deformation frequency of 1 Hz at the retraction temperature.

[0044] The cured material may have an elastic modulus of about 5 x 107Pa to about 1 x 1011Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of about 1 x 108Pa to about 1 x 1011Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of about 5 x 108Pa to about 1 x 1011Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of about 1 x 109Pa to about 1 x 1011Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of about 5 x 107Pa to about 5 x 101° Pa at a shear deformation frequency of 1 Hz at the retraction temperature. The cured material may have an elastic modulus of about 5 x 107Pa to about 1 x 101° Pa at a shear deformation frequency of 1 Hz at the retraction temperature.

[0045] The sheet may comprise a first protective layer provided on the first layer of a cured material. The first protective layer may protect the first layer of a cured material during formation of the sheet. The first protective layer may act as a processing aid to prevent bonding between the first layer of a cured material and processing equipment during formation of the sheet. For example, if pressure is applied to the sheet during processing to produce the sheet, pressure may be applied to the first protective layer to thereby indirectly apply pressure to the first layer of cured material or curable material. Pressure may be applied indirectly to the first layer of cured material prior to curing of the curable material to form the cured material. The first array of apertures may be formed through the first protective layer such that the membrane is exposed through the first array of apertures.

[0046] In embodiments comprising a second layer of cured material, the sheet may comprise a second protective layer provided on the second layer of a cured material. The second protective layer may protect the second layer of a cured material during formation of the sheet. The second protective layer may act as a processing aid to prevent bonding between the first layer of a cured material and processing equipment during formation of the sheet. For example, if pressure is applied to the sheet during processing to produce the sheet, pressure may be applied to the second protective layer to thereby indirectly apply pressure to the second layer of cured material or curable material. Pressure may be applied indirectly to the second layer of cured material prior to curing of the curable material to form the cured material. The second array of apertures may be formed through the second protective layer such that the membrane is exposed through the second array of apertures.

[0047] The first protective layer may comprise a substantially inert material. The substantially inert material may not react or significantly deform when subjected to pressure or elevated temperatures. The substantially inert material may not react or significantly deform when subjected to the pressure or elevated temperature used to form the sheet. The substantially inert material may not react or significantly deform when subjected to pressures of at least 50 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of at least 100 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of at least 250 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of at least 500 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of at least 1000 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of from 10 kPa to 5000 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of from 50 kPa to 5000 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of from 100 kPa to 5000 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of from 250 kPa to 5000 kPa. The substantially inert material may not react or significantly deform when subjected to pressures of from 500 kPa to 5000 kPa. The substantially inert material may not react or significantly deform when subjected to temperatures of at least 100°C. The substantially inert material may not react when subjected to temperatures of at least 150°C. The substantially inert material may not react or significantly deform when subjected to temperatures of at least 200°C. The substantially inert material may not react or significantly deform when subjected to temperatures of at least 250°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 50°C to 500°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 50°C to 400°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 50°C to 300°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 100°C to 500°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 150°C to 500°C. The substantially inert material may not react or significantly deform when subjected to temperatures of from 200°C to 500°C. The first protective layer and / or the second protective layer, where present, may comprise polyimide, polyetheretherketone (PEEK), polyamideimide (PAI), poly aryl ether ketone (PAEK), polyethersulfone (PES), polyether ketone ketone (PEKK), or polyphenylene sulfide (PPS / PPSU) or combinations thereof.

[0048] The vent assemblies cut from the sheet may have a mean insertion loss of less than 7 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a mean insertion loss of less than 5 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a mean insertion loss of less than 2 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a mean insertion loss of less than 1.0 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a mean insertion loss of less than 0.8 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a mean insertion loss of less than 0.6 dB as measured at 1 kHz.

[0049] In embodiments where a vent assembly cut from the sheet is used with low noise microphones it is especially important that the vent assembly has low insertion loss. Accordingly, for these embodiments the vent assemblies cut from the sheet may have a mean insertion loss of less than 1 dB as measured at 1 kHz.

[0050] The vent assemblies cut from the sheet may have a standard deviation from the mean insertion loss of less than 0.4 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a standard deviation from the mean insertion loss of less than 0.3 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a standard deviation from the mean insertion loss of less than 0.2 dB as measured at 1 kHz. The vent assemblies cut from the sheet may have a standard deviation from the mean insertion loss of less than 0.1 dB as measured at 1 kHz.

[0051] The vent assemblies cut from the sheet of the present aspect may have a smaller variation in acoustic performance between vent assemblies compared to the variation in acoustic performance between vent assemblies cut from a sheet of the art. The vent assemblies cut from the sheet may have a smaller standard deviation in mean insertion loss as measured at 1 kHz compared to vent assemblies cut from a sheet of the art. The vent assemblies cut from the sheet may have a mean insertion loss that is substantially the same or less than the mean insertion loss of vent assemblies cut from a sheet of the art.

[0052] The vent assemblies cut from the sheet of the present aspect may have a smaller variation in acoustic performance between vent assemblies compared to the variation in acoustic performance between vent assemblies cut from a sheet in the art without substantially impacting the insertion loss.

[0053] The sheet may be a compressed sheet that has been subjected to a compression force. The compressed sheet may have been subjected to pressures of at least 50 kPa. The compressed sheet may have been subjected to pressures of at least 100 kPa. The compressed sheet may have been subjected to pressures of at least 250 kPa. The compressed sheet may have been subjected to pressures of at least 500 kPa. The compressed sheet may have been subjected to pressures of at least 1000 kPa. The compressed sheet may have been subjected to pressures of from 10 kPa to 5000 kPa. The compressed sheet may have been subjected to pressures of from 50 kPa to 5000 kPa. The compressed sheet may have been subjected to pressures of from 100 kPa to 5000 kPa. The compressed sheet may have been subjected to pressures of from 250 kPa to 5000 kPa. The compressed sheet may have been subjected to pressures of from 500 kPa to 5000 kPa.

[0054] In a second aspect there is provided a method of preparing a sheet of the first aspect, the method comprising the steps: providing a first layer of a curable material and a membrane; forming a first array of apertures in the first layer; laminating the first layer onto a first side of the membrane to form a laminate; curing the curable material to form a cured material; and applying an elevated temperature to the laminate to retract the material of the membrane to form the sheet of the first aspect.

[0055] The step of curing the curable material may include one or more of applying pressure and / or heat to the laminate, adding a curing agent or to exposing the curable material to a curing fluid or a curing wavelength of light. It will be appreciated that the specific curing conditions will depend on the specific curable material.

[0056] In embodiments where the curing step comprises applying heat to the laminate, the application of heat may cure the curable material and retract the material of the membrane. The step of curing the curable material to form a cured material may also include the step of applying an elevated temperature to the laminate to retract the material of the membrane to form the sheet of the first aspect. Therefore, a separate step of applying an elevated temperature to the laminate to retract the material of the membrane may not be required. In some embodiments the step of curing the curable material includes applying pressure to the laminate and heating the laminate at the same time.

[0057] The membrane of the present aspect may be a membrane as defined in the first aspect.

[0058] The method may further comprise the steps: providing a second layer of curable material; forming a second array of apertures in the second layer; and laminating the second layer onto a second side of the membrane, wherein each aperture of the first array of apertures forms an aperture pair with an aperture of the second array of apertures and the apertures of each aperture pair substantially overlap.

[0059] The step of curing the curable material may cure the curable material of the first layer of curable material and the second layer of curable material. The curable material of the first layer may be the same material as the curable material of the second layer. The curable material of the first layer may be a different material to the curable material of the second layer. The method may comprise a step of curing the first layer of curable material and an additional second step of curing the second layer of curable material.

[0060] The method may comprise the step of applying a pressure of about 65 kPa (~10 psi) to about 1400 kPa to the laminate to form the sheet. The method may comprise the step of applying a pressure of about 65 kPa to about 1000 kPa to the laminate to form the sheet. The method may comprise the step of applying a pressure of about 65 kPa to about 750 kPa to the laminate to form the sheet. The method may comprise the step of applying a pressure of about 65 kPa to about 500 kPa to the laminate to form the sheet.

[0061] The step of applying an elevated temperature to the laminate may comprise applying a temperature to the laminate that is above the glass transition temperature of the material of the membrane.

[0062] In some embodiments, the step of applying an elevated temperature to the laminate may comprise applying a temperature of at least 50°C. A temperature of at least 100°C to the laminate. A temperature of at least 150°C may be applied to the laminate. A temperature of at least 200°C may be applied to the laminate. A temperature of at least 250°C may be applied to the laminate. A temperature of from 50°C to 300°C may be applied to the laminate. A temperature of from 100°C to 300°C may be applied to the laminate. A temperature of from 150°C to 300°C may be applied to the laminate. A temperature of from 200°C to 300°C may be applied to the laminate. A temperature of from 250°C to 300°C may be applied to the laminate.

[0063] A first protective layer may be placed on the first layer of curable material prior to the step of curing the curable material to form the first layer of cured material. The first protective layer may be placed on the first layer of curable material before the step of forming the first array of apertures such that the first array of apertures is formed through the first layer of cured material and through the first protective layer.

[0064] In embodiments comprising the step of providing a second layer of curable material, a second protective layer may be placed on the second layer of curable material prior to the step of curing the curable material to form the second layer of cured material. The second protective layer may be placed on the second layer of curable material before the step of forming the second array of apertures such that the second array of apertures is formed through the second layer of cured material and through the second protective layer.

[0065] The first protective layer may protect the first layer of curable material during the step of curing the curable material. The first protective layer may protect the first layer of curable material during the step of applying an elevated temperature to the laminate to retract the material of the membrane.

[0066] In embodiments comprising a second protective layer, the second protective layer may protect the second layer of curable material during the step of curing the curable material. The second protective layer may protect the second layer of curable material during the step of applying an elevated temperature to the laminate to retract the material of the membrane.

[0067] According to a third aspect there is provided a method of forming a vent assembly, the method comprising the steps: providing the sheet prepared by the method of the second aspect; and cutting around a pair of apertures to release a vent assembly.

[0068] The step of cutting may be carried out using die cutting. Each vent assembly may be cut from the sheet individually. Each vent assembly may be cut from the sheet simultaneously using an array die cutter, for example. A sub-set of vent assemblies may be cut from the sheet simultaneously. Furthermore, each vent assembly may be cut from the sheet using a continuous process such as a rotary die cut process or a typical step and repeat process which uses pins to both index or advance the material and accurately position it for subsequent die cut processes. The sheet may be in the form of a roll. A sub-set of vent assemblies may be cut and produced on a continuous basis from the roll.

[0069] Each vent assembly may comprise a membrane exposed through a single aperture surrounded by a perimeter of cured material on at least one side of the membrane. In embodiments where the sheet comprises at least a first protective layer, the single aperture may be surrounded by a perimeter of cured material covered by a protective layer on at least a first side of the membrane.

[0070] In a fourth aspect there is provided a vent assembly formed by the method of the third aspect comprising a membrane and a solid or stiff boundary on at least one side of the membrane, wherein the solid or stiff boundary comprises cured material, wherein the membrane comprises a retracted material.

[0071] The vent assembly of the present aspect may have a decreased standard deviation in acoustic insertion loss compared to vent assembly not formed from the sheet of the present disclosure.

[0072] For the avoidance of doubt, features of the sheet of the first aspect are features of the sheet referred to in the subsequent aspects. Features of the vent assemblies described in the first aspect are features of the vent assemblies of the subsequent aspects.

[0073] Brief Description of the Figures

[0074] Embodiments of the present invention will now be described, by way of non-limiting example, with reference to the accompanying drawings.

[0075] Figure 1 : A top-down view of a sheet according to an embodiment;

[0076] Figure 2: A cross-section side view of the sheet of Figure 1 along the line A-A;

[0077] Figure 3: A side view of a vent assembly of an embodiment;

[0078] Figure 4: A top-down view of a sheet according to an embodiment;

[0079] Figure 5: A cross-section side view of the sheet of Figure 4 along the line B-B;

[0080] Figure 6: A top-down view of a sheet according to an embodiment;

[0081] Figure 7: A cross-section side view of the sheet of Figure 6 along the line C-C;

[0082] Figure 8: A top-down view of a sheet according to an embodiment;

[0083] Figure 9: A cross-section side view of the sheet of Figure 8 along the line D-D; Figure 10: A side view of a portion of a sheet according to an embodiment indicating individual vent assemblies that can be cut from the sheet;

[0084] Figure 11 : A side view of a portion of a sheet according to an embodiment indicating individual vent assemblies that can be cut from the sheet;

[0085] Figure 12: Plots of mean insertion loss in dB as a function of frequency for heat treated vent assemblies and non-heat treated vent assemblies comprising a pressure sensitive adhesive (BRN) and a heat cured adhesive or heat activated film (HAF);

[0086] Figure 13: Plots of mean insertion loss in dB as a function of frequency for heat treated vent assemblies and non-heat treated vent assemblies comprising a pressure sensitive adhesive (BRN) and a heat cured adhesive or heat activated film (HAF) before and after compression (ON vs 10N);

[0087] Figure 14: Plots of mean insertion loss in dB as a function of frequency for heat treated vent assemblies comprising a heat cured adhesive or heat activated film (HAF) when heated at 140°C, 180°C, and 220°C for 10 seconds and 60 seconds;

[0088] Figure 15: Plots of storage modulus and tan (delta) as a function of angular frequency (rad / s) for an vent assembly having a curable material (HAF58471) before and after curing;

[0089] Figure 16: Plots of mean insertion loss in dB as a function of frequency for a vent assembly with heat treatment or no heat treatment;

[0090] Figure 17: Plots of mean insertion loss in dB as a function of frequency with cured material on one side only before and after heat treatment;

[0091] Figure 18: Plots of mean insertion loss in dB as a function of frequency with cured material and an espun PTFE membrane with and without heat treatment;

[0092] Figure 19: Plots of mean insertion loss in dB as a function of frequency with and without heat treatment for comparative vent assemblies comprising dense polyethylene and layers of cured material;

[0093] Figure 20: Plots of mean insertion loss in dB as a function of frequency with and without heat treatment for comparative vent assemblies comprising polyethylene and layers of cured material;

[0094] Figure 21 : A scanned electron microscope (SEM) image cross section of a vent that does not comprise a membrane comprising a retracted material, vent heat treated after die cut;

[0095] Figure 22: A scanned electron microscope (SEM) image cross section of a vent that comprises a membrane comprising a retracted material, vent heat treated before die cut;

[0096] Figure 23: Plots of thickness of membranes for a variety of membrane materials at the center and edge of the vent / membrane for heat treated (Y) and not heat treated (N) examples; and Figure 24: Plots of thickness of membranes for a variety of membrane materials at the center and edge of the vent / membrane for heat treated (Y) and not heat treated (N) examples. Detailed Description

[0097] While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

[0098] To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

[0099] General Examples

[0100] With reference to Figure 1 to Figure 3, a vent sheet 1 (acting as a sheet) comprises a membrane 2 comprising expanded polytetrafluoroethylene (ePTFE), a first layer 4 of a phenolic resin / nitrile rubber composition (acting as a first layer of a cured material) and a second layer 6 of a phenolic resin / nitrile rubber composition (acting as a second layer of a cured material). The first layer 4 is provided on a first side of the membrane 2 and the second layer 6 is provided on a second side of the membrane 2. A first protective layer 8 comprising polyimide is provided on the first layer 4 and a second protective layer 10 comprising polyimide is provided on the second layer 6.

[0101] A first array of vent apertures 12 (acting as a first array of apertures) is formed in the vent sheet 1 through the first layer 4 and the first protective layer 8 such that the membrane 2 is exposed through the first array of vent apertures 12. A second array of vent apertures 14 (acting as a second array of apertures) is formed in the vent sheet 1 through the second layer 6 and the second protective layer 10 such that the membrane 2 is exposed through the second array of vent apertures 14.

[0102] Additional apertures 16 are formed through the vent sheet 1 and aid with the alignment of the apertures of the first layer 4 and first protective layer 8 with the apertures of the second layer 6 and second protective layer 10. The location of the additional apertures 16 formed through the vent sheet 1 can be determined based on the type of manufacturing process selected such as rotary or step and repeat processing. Other considerations might include part design throughput advantages. A first border portion 18 (acting as a first support portion) runs along a first side 20 of the vent sheet 1 . The first border portion 18 does not comprise any apertures. A second border portion 22 (acting as a second support portion) runs along a second side 24 of the vent sheet 1 . The second border portion 22 does not comprise any apertures. The first border portion 18 and the second border portion 22 provide structural stability to the vent sheet 1 .

[0103] Vent assemblies 26 are cut from the vent sheet 1 and fixed to a release liner 28 with adhesive 30 (see Figure 3).

[0104] With reference to Figure 4 and Figure 5 a vent sheet 50 (acting as a sheet) comprises a membrane 52 comprising expanded polytetrafluoroethylene (ePTFE), a first layer 54 of a phenolic resin / nitrile rubber composition (acting as a first layer of a cured material). The first layer 54 is provided on a first side of the membrane 52.

[0105] A first array of vent apertures 56 (acting as a first array of apertures) is formed in the vent sheet 50 through the first layer 54 such that the membrane 52 is exposed through the first array of vent apertures 56.

[0106] A first border portion 58 (acting as a first support portion) runs along a first side 60 of the vent sheet 50. The first border portion 58 does not comprise any apertures. A second border portion 62 (acting as a second support portion) runs along a second side 64 of the vent sheet 50. The second border portion 62 does not comprise any apertures. A third border portion 66 (acting as a third support portion) runs along a third side 68 of the vent sheet 50. The third border portion 66 does not comprise any apertures. A fourth border portion 70 (acting as a fourth support portion) runs along a fourth side 72 of the vent sheet 50. The fourth border portion 70 does not comprise any apertures.

[0107] The border portions 58, 62, 66, 70 form a continuous border around the first array of apertures 56 and provide structural stability to the vent sheet 50.

[0108] Vent assemblies are cut from the vent sheet 50 as described for the previous example.

[0109] With reference to Figure 6 and Figure 7 a sheet 100 comprises a membrane 102 comprising expanded polyethylene (ePE), a first layer 104 of a phenolic resin / nitrile rubber composition (acting as a first layer of a cured material). The first layer 104 is provided on a first side of the membrane 102. A first protective layer 106 comprising polyetheretherketone (PEEK) is provided on the first layer 104.

[0110] A first array of vent apertures 108 (acting as a first array of apertures) is formed in the sheet 100 through the first layer 104 and the first protective layer 106 such that the membrane 102 is exposed through the first array of vent apertures 108. The first array of vent apertures 108 is divided into a first group of apertures 110 (acting as a first sub group of apertures) and a second group of apertures 112 (acting as a second sub group of apertures).

[0111] A first support portion 114 runs along a first side 116 of the sheet 100. The first support portion 114 does not comprise any apertures. A second support portion 118 runs along a second side 120 of the sheet 100. The second support portion 118 does not comprise any apertures. A third support portion 122 runs between the first group of apertures 110 and the second group of apertures 112 to thereby divide the first group of apertures 110 from the second group of apertures 112. The third support portion 118 does not comprise any apertures. The border portions 114, 118 and 122 provide structural stability to the sheet 100.

[0112] Vent assemblies are cut from the sheet 100 as described for the first example above.

[0113] With reference to Figure 8 and Figure 9 a vent sheet 150 (acting as a sheet) comprises a membrane 152 comprising expanded polytetrafluoroethylene (ePTFE), a first layer 154 of a phenolic resin / nitrile rubber composition (acting as a first layer of a cured material) and a second layer 156 of a phenolic resin / nitrile rubber composition (acting as a second layer of a cured material). The first layer 154 is provided on a first side of the membrane 152 and the second layer 156 is provided on a second side of the membrane 152. A first protective layer 158 comprising polyimide is provided on the first layer 154 and a second protective layer 160 comprising polyimide is provided on the second layer 156.

[0114] A first array of vent apertures 162 (acting as a first array of apertures) is formed in the vent sheet 150 through the first layer 154 and the first protective layer 158 such that the membrane 152 is exposed through the first array of vent apertures 162. A second array of vent apertures 164 (acting as a second array of apertures) is formed in the vent sheet 150 through the second layer 156 and the second protective layer 160 such that the membrane 152 is exposed through the second array of vent apertures 164.

[0115] Vent assemblies are cut from the vent sheets 1 , 50, 150 and sheet 100 using die cutting. Figure 10 shows how a vent sheet 172 is cut to form vent assemblies 170, where the dashed- line boxes 174 delimit individual vent assemblies 170 in the vent sheet 172 for an example where the vent sheet 172 comprises a membrane 176, a first layer of a heat activated cured material 178, a first protective layer 180 provided on the first layer 178, a second layer of a heat activated cured material 182 and a second protective layer 184 provided on the second layer 182. Figure 11 shows how an alternative sheet 190 is cut to form vent assemblies 192, where the dashed line boxes 194 again delimit individual vent assemblies 192 in the sheet 190. The sheet 190 comprises a membrane 196 and one layer of a heat activated cured material 198.

[0116] Test Methods

[0117] Insertion Loss Measurement

[0118] A vent assembly is constructed on a steel plate having a 1.5 mm diameter circular aperture, such that the aperture in the steel plate is aligned with a substantially impermeable acoustic membrane; permeable layers having apertures larger than the apertures in the steel plate are adhered with adhesive to the face of the acoustic membrane and the steel plate, aligning the aperture of the steel plate and permeable layers. Inside a B&K type 4232 anechoic test box, a face plate is located at a distance of about 6.5 cm from an internal driver or speaker, said face plate having a central aperture in communication with a Knowles SPU0410LR5H MEMS measurement microphone. The speaker is excited to produce an external stimulus at 1 Pa of sound pressure (94 dB SPL) over the frequency range from 100 Hz to 10 kHz. The acoustic response is measured under the following conditions: (a) with the aperture uncovered (b) with the aperture covered with the venting assembly sample, the transducer of the measurement microphone in fluid communication with the circular aperture of one of the steel plates of the venting assembly, and the attachment between that steel plate and the face plate in solid contact. The difference in the response between (a) and (b) is reported in dB (at a specified frequency) as the acoustic loss due to the venting assembly. In other tests, the permeable layers in the above construction may be replaced with impermeable layers, and / or the PET ring may be omitted, and / or openings in the PET ring may be introduced, and / or a permeable acoustic membrane may be substituted for the substantially impermeable acoustic membrane, and / or a support plate may be included, such conditions being tested with the purpose of comparison.

[0119] Thickness Measurement

[0120] The thickness of the membrane portion of the venting assembly sample was imaged using confocal laser scanning microscopy, without physical cross-sectioning by the following method. First, the sample is placed onto an imaging petri dish with the non-pressure sensitive adhesive side facing down (for imaging by an inverted microscope). The sample was initially wet with IPA solution by pipetting pure isopropyl alcohol (IPA) solution onto the sample. The I PA solution was sufficient to wet and clarify the signal for samples constructed with Membranes A, B and C as defined below. For the Membrane D samples, the wetting fluid is then exchanged for at least 24 hours with an excess of mineral oil with 0.03 mg per mL Nile red florescence dye to sufficiently stain the membrane samples. For the samples, a cover slip was then placed on top of the sample and transferred to an inverted microscope (Nikon A1 laser scanning confocal microscope) for imaging.

[0121] A Z-stack of the sample is captured by recording the reflection channel (443 nm laser) and, in the case of Membrane D only, fluorescence (532 nm) channel. Reflection channel records the refractive indices differences and the fluorescence channel records the fluorescently dyed oil trapped inside the Membrane D film. A 3D reconstruction of the Z-stack is then employed to extract the optical slices (optical sectioning) of the sample virtual cross section. Sample thickness was then measured and labeled by the imaging software (NIS-elements AR). PNG files of the cross section with measurement labels are exported from the original nd2 imaging file (Nikon).

[0122] Elastic Modulus Measurement

[0123] The viscoelastic properties of the adhesives were measured using small amplitude oscillatory shear (SAGS) on a rotational rheometer. Rectangular-shaped heat treated and non-heat treated specimens of the adhesives were prepared by layering multiple layers of HAF 58471 together such that the total thickness of the specimens were 0.5 mm and adhered using a hand roller. Bubbles between layers were removed using a vacuum bag. The heat treated condition was then generated by applying 140°C heat and 20 psi (138 kPa) of pressure for 60 seconds using a heat press. The resulting test specimens were then tested using a Torsion Fixture on a rotational rheometer.

[0124] The small amplitude oscillatory shear (SAGS) procedure used was an Oscillation Frequency Sweep. It consists of applying a small amplitude sinusoidal strain and measuring the corresponding sinusoidal stress. The phase shift (phase angle delta) between the input (in this case strain) and output (in this case stress) signals is also measured. The frequency range applied was between 102to 10'2rad / s at 5 points per decade.

[0125] Figure 15 shows results from the oscillation frequency sweep experiments. Plotted on the left vertical axis is the Elastic Modulus (triangle marker), a measure of elasticity or resistance to deformation of the material. The measured phase angle, represented in terms of its tangent, is plotted on the left vertical axis (square marker). The tangent of delta represents the ratio of the viscous (represented by the viscous shear modulus G”) to elastic (represented by the elastic shear modulus G”) responses of a viscoelastic material as shown below. tan (delta) = G” / G’ (3)

[0126] The cured material shows higher modulus over the set frequency range. This indicates a partial cure of the adhesive based on its process conditions. The tan(delta, 6) peak also supports this conclusion as its peak shifts to lower frequencies, this is due to the increase of relaxation time (a time scale that is inverse of the frequency applied). Increase of the cure state shifts the glass transition to lower frequencies (or higher time scales).

[0127] Table 1 below summarizes the measured elastic modulus of the heat treated and non-heat treated adhesive specimens. Table 1 illustrates the difference in elastic modulus for the thermoset with enough curing to obtain reduced acoustic variability of the resulting vents as demonstrated by Figure 14, while the no heat treat condition or insufficiently cured condition (140°C, 10secs) represents the uncured thermoset adhesive

[0128] Heat Treatment

[0129] Elastic modulus

[0130] Pa

[0131] 140°C, 60secs 5.31 x 107

[0132] No heat treatment 2.14 x 107

[0133] Table 1 - Effect of heat treatment of sheet on measured elastic modulus of the adhesive layer at an angular frequency of 6.31 rad / s.

[0134] Examples

[0135] Sheets of vent assemblies, corresponding to sheets of the present disclosure, were prepared as follows:

[0136] Examples were prepared including the following phenolic resin nitrile rubber thermoset adhesives: Examples 1 and 2, HAF58469 (obtained from Tesa), Examples 3 to 8, HAF58471 (obtained from Tesa), and Examples 9-12, LTR 8711 (obtained from Tesa), and were assembled through the following procedure. First, the thermoset adhesive was laminated onto 1 mil (25.4 pm) thickness polyimide (Kapton, Dupont) sheet with a heated roll laminator at 4 bar nip pressure, feed rate 175 cm / min, at a temperature of 100°C for HAF58471 and HAF58469 and 80°C for LTR 8711. Then, the hole pattern shown in Figure 1 was cut into the laminated sheet using a flex die process with holes labelled 16 consisting of a 5 x 5 circular grid pattern with circles of 1.6 mm diameter. The laminated sheet with holes was then laminated to a membrane by a heated roll laminator at 4 bar of nip pressure for HAF58471 and HAF58469 and at 2 bar of nip pressure for LTR 8711 , feed rate 50 cm / min, and a temperature of 140°C for HAF58471 and HAF58469 and 80°C for LTR 8711. The membranes used for example generation include Membrane A (Part number: GAW337, expanded PTFE, W. L. Gore & Associates, Inc., Examples 1-6, and 13-14), Membrane B (described in following paragraph, expanded PE, W. L. Gore & Associates, Inc., Examples 11 and 12), Membrane C (Part number: Filtriq 232756, electrospun PTFE, Zeus Industrial Products, Inc., Examples 7 and 8), and Membrane D (Part number: DT007, electrospun polyurethane, Lime Co., Ltd, Seoul, Republic of Korea, Examples 9 and 10).

[0137] Membrane B is a porous, ultrahigh molecular weight (UMW) expanded polyethyene produced using the gel expansion process. UMW polyethylene refers to polyethylene having an average molecular weight of about 1 ,000,000 g / mol to about 10,000,000 g / mol. A starting resin was used to make the membrane that had a molecular weight of 4,300,000 g / mol, according to the supplier. In this process, the resin is mixed with a hydrocarbon liquid and other additives. This mixture is heated over the polymer melt and extruded into a sheet. This sheet can then be orientated biaxially before and / or after the hydrocarbon liquid is extracted, producing a microporous membrane. Various process details are known, such as those disclosed in US 5,248,461 ; US 4,873,034; US 5,051 ,183; and US 6,566,012; each of which are hereby incorporated-by-reference in their entirety.

[0138] The resulting membrane had a mass per area of 2.44 g / m2, an airflow of 4.9 L / hr at 12 mbar and 2.99 cm2, a thickness of 9.6 pm, an ultimate tensile strength in a first direction of 76 MPa, an ultimate tensile strength in a second direction (orthogonal to the first direction) of 65 MPa was used as in the membrane component of Examples 11 and 12.

[0139] The protective layer-thermoset adhesive-membrane assembly was then attached to another protective layer-thermoset adhesive assembly with the adhesive facing the membrane to form a protective layer-thermoset adhesive-membrane-thermoset adhesive-protective layer assembly with an additional heat lamination process step using a heat press at a temperature of 165°C for HAF58471 and HAF58469 and 82°C for LTR 8711 , 90 psi (621 kPa), and 15 seconds. This step was not performed for Examples 5 and 6 to create an assembled structure where the adhesive-protective layer was only present on one side of the membrane. Examples in Table 2 that are retracted (Examples 1 , 3, 5, 7, 9, 11 , and 13) were given an additional heat and curing step at this stage. Examples utilizing HAF58471 or HAF58469 and polytetrafluoroethylene (Membrane A or C) were treated with a heat press at 220°C and 20 psi (138 kPa) for one minute. Examples utilizing LTR 8711 and Membrane B were treated with a heat press at 130°C and 58 psi (400 kPa) for 5 minutes. Examples utilizing LTR 8711 and Membrane D were treated with a heat press at 100°C and 58 psi (400 kPa) for 5 minutes.

[0140] Examples that contained 5606BRN bonded to the membrane (i.e., Example 13 and 14) were assembled by directly bonding layer of pressure sensitive adhesive to 1 mil (25.4 pm) sheet polyimide (Kapton) using a hand roller. Then the assembly was flex die cut to have the pattern shown in Figure 1. The resulting assembly was bonded to the membrane using a hand roller, utilizing holes 16 to ensure alignment between the top and bottom adhesive layers.

[0141] After the optional heat treatment, examples were then adhered to a layer of pressure sensitive adhesive 5605 BRN (Nitto) with holes cut into the same pattern shown in Figure 1. The circular holes 12 had a diameter of 2 mm. The alignment holes 16 were utilized such that the holes of the assembled layers matched the holes of the pressure sensitive adhesive by assembling the sheets together on a fixture with alignment pins and hand rolling the layers together. Finally, the assembled sheet was die cut according to Figure 10 such that the outer diameter of the resulting vents was 3.3 mm. The individual vents were used for the following acoustic tests.

[0142] A retracted membrane for the purposes of the Examples of Tables 3 and 4 below was determined to be a membrane that has been heat treated. The retraction of the membrane was confirmed by measurement of the difference in thickness of the membrane at the edge and in the center or middle of the membrane compared to the membrane before the heat treatment as measured using the method described above and is shown in Table 2 and Figures 23 and 24. Membranes comprising a retracted material are seen to have a more consistent thickness from the edge to the center of the membrane, I.e. the difference in thickness between the edge and center of the membrane is smaller than for the membrane that has not been heat treated to retract the material.

[0143] Table 2 - Change in thickness (Ax) between center and edge both before and after heat treatment, difference between the change in thickness between membranes without heat treatment and with heat treatment, and percentage difference

[0144] Figure 22 shows a side cross-sectional view of a vent that was made using the methods described above. Heat treatment of the sheet before die cutting the individual vents from the sheet ensures that the edges of the vent are uniform and that the membrane does not extend beyond the first and (in this example) the second layer of cured material. This is in stark contrast to a vent that is heat treated after the vent is die cut from the sheet (see Figure 21, for example).

[0145]

[0146] Table 3- Example sheets that comprising a membrane with one or two adhesive layers where the membrane is retracted (heat treated, Examples 2, 4, 6, 8, 10, and 12) or not retracted (Examples 1, 3, 5, 7, 9, and 11).

[0147] The insertion loss (“IL”) in dB across the vent assemblies formed from an example sheet comprising two layers of a pressure sensitive adhesive (BRN) and an example sheet comprising two layers of a heat cured adhesive (“heat activated film”, HAF) was measured as a function of frequency before and after heat treatment of the sheet using the method described above.

[0148] Figure 16 shows how insertion loss (IL) and standard deviation from the mean of insertion loss varies as a function of frequency for Example 1 (dotted line) and Example 2 (solid line). Figure 17 shows how insertion loss (IL) and standard deviation from the mean of insertion loss varies as a function of frequency for Example 5 (dotted line) and Example 6 (solid line). Figure 18 shows how insertion loss (IL) and standard deviation from the mean of insertion loss varies as a function of frequency for Example 11 (dotted line) and Example 12 (solid line). Figure 19 shows how insertion loss (IL) and standard deviation from the mean of insertion loss varies as a function of frequency for Example 7 (dotted line) and Example 8 (solid line). Figure 20 shows how insertion loss (IL) and standard deviation from the mean of insertion loss varies as a function of frequency for Example 9 (dotted line) and Example 10 (solid line).

[0149] As shown in Figure 12, a significant reduction in both the mean insertion loss and noise in the insertion loss, as measured by the standard deviation of the measured insertion loss, was observed for the sheet comprising HAF (Examples 4 and 3 as defined in Table 3 above). However, a significant increase in the mean insertion loss and standard deviation of the measured insertion loss was observed for the sheet comprising BRN (examples 14 and 13 as defined in Table 3 above).

[0150] With reference to Figure 14, the insertion loss in dB as a function of frequency was measured for a sheet comprising a heat cured adhesive layer when the sheet was heated to 140°C, 180°C and 220°C. It was found that heating the sheet to 140°C for at least 60 seconds provided a consistently low mean insertion loss and greatly reduced standard deviation in that insertion loss compared to the samples with less heat treatment duration.

[0151] With reference to Figure 13 and Table 4 below, a similar experiment was carried out where in addition to heat treatment, the insertion loss in dB was measured with and without an application of a compression force of 10 N. Both the mean insertion loss (“mean IL”) and standard deviation of the measured insertion loss (“St Dev IL”) were not significantly changed for the retracted membrane example comprising cured HAF (Example 3) after the application of a compressive force. In addition, the St Dev IL was low compared to the comparative examples. In contrast, examples that did not comprise a retracted membrane or utilized a non-curable pressure sensitive adhesive (Examples 4, 13, and 14) either showed significant increase in mean insertion loss after the application of a compressive force or had high St Dev IL compared to Example 3.

[0152] Table 4 - Example sheets that comprising an ePTFE membrane (Membrane A) with first and second adhesive layers where the membrane is retracted (heat treated, Examples 3 and 13) or not retracted (Examples 4 and 14) before and after compression. “Ret ?” refers to whether or not a membrane is retracted.

[0153] Accordingly, it has been shown that the provision of sheets comprising membranes having a retracted material and at least a first layer of cured material show a reduced mean insertion loss and standard deviation in that mean insertion loss when compared to vent assemblies produced from sheets where the membrane does not have a retracted material and or does not comprise at least a first layer of cured material.

[0154] While there has been hereinbefore described approved embodiments of the present invention, it will be readily apparent that many and various changes and modifications in form, design, structure and arrangement of parts may be made for other embodiments without departing from the invention and it will be understood that all such changes and modifications are contemplated as embodiments as a part of the present invention as defined in the appended claims.

Claims

Claims1. A sheet comprising a membrane, and a first layer of a cured material, the first layer being provided on a first side of the membrane, the first layer comprises a first array of apertures, wherein the membrane comprises a retracted material and the sheet is configured such that a portion of the sheet comprising one aperture cut from the sheet corresponds to a vent assembly.

2. The sheet of claim 1 , wherein the sheet comprises a second layer of a cured material and the second layer is provided on a second side of the membrane, the second layer comprises a second array of apertures, each aperture of the first array of apertures forms an aperture pair with an aperture of the second array of apertures and the aperture pair occlude each other such that the membrane is exposed on both the first side and the second side.

3. The sheet of claim 1 or claim 2, wherein the sheet comprises at least one support portion, the at least one support portion being a continuous material and does not comprise an aperture.

4. The sheet of any one preceding claim, wherein the sheet comprises a first protective layer provided on the first layer of a cured material.

5. The sheet of any one of claim 2 to claim 4, wherein the sheet comprises a second protective layer provided on the second layer of a cured material.

6. The sheet of claim 4 or claim 5, wherein the first protective layer and / or the second protective layer comprises polyimide, polyetheretherketone (PEEK), polyamideimide (PAI), poly aryl ether ketone (PAEK), polyethersulfone (PES), polyether ketone ketone (PEKK), or polyphenylene sulfide (PPS / PPSU).

7. The sheet of any one preceding claim, wherein the membrane comprises polytetrafluoroethylene (PTFE), polyparaxylylene (PPX), polyethylene (PE), poly(tetramethyl-p-silphenylenesiloxane) (PTMPS), polyamide 6, polyurethane, thermoplastic polyurethane, polyvinlidene fluoride (PVDF), polypropylene, polyimide, or polyacrylonitrile (PAN) or combinations thereof.

8. The sheet of any one preceding claim, wherein the cured material has an elastic modulus of at least 5.3 x 107Pa at a shear deformation frequency of 1Hz.

9. The sheet of any one preceding claim, wherein the cured material comprises a thermoset adhesive.

10. The sheet of claim 9, wherein the thermoset adhesive comprises a nitrile phenolic adhesive, epoxy adhesive, polymeric adhesive, acrylic adhesive, silicone adhesive, or polyurethane adhesive, or combinations thereof.

11. A method of preparing a sheet of any preceding claim, the method comprising the steps: providing a first layer of a curable adhesive cured material and a membrane; forming a first array of apertures in the first layer; laminating the first layer onto a first side of the membrane to form a laminate; curing the curable adhesive to form a cured material; and applying heat to the laminate to thereby retract the material of the membrane to form the sheet of any preceding claim.

12. The method of claim 11 , wherein the method further comprises the steps: providing a second layer of cured material; forming a second array of apertures in the second layer; and laminating the second layer onto a second side of the membrane, wherein each aperture of the first array of apertures forms a pair of apertures with an aperture of the second array of apertures and the apertures of each pair apertures substantially overlap.

13. The method of claim 11 or claim 12, wherein a pressure of at least 68.9 kPa (10 psi) to at least 689 kPa (100 psi) is applied to the laminate to form the sheet.

14. The method of any one of claim 11 to claim 13, wherein a pressure of from 68.9 kPa to 1379 kPa is applied to the laminate to form the sheet.

15. The method of any one of claim 11 to claim 14, wherein a temperature of at least 100°C is applied to the laminate to form the sheet.

16. The method of any one of claim 11 to claim 15, wherein a temperature of from 100°C to 300°C is applied to the laminate to form the sheet.

17. The method of any one of claim 11 to claim 16, wherein a protective layer is placed on the first layer of curable material and, where present on the second layer of curable material, prior to the step of curing the curable material.

18. A method of forming a vent assembly, the method comprising the steps: providing the sheet prepared by the method of any one of claim 11 to claim 17; and cutting around a pair of apertures to release a vent assembly.

19. A vent assembly formed by the method of claim 18 comprising a membrane having a solid boundary on at least one side of the membrane, wherein the solid boundary comprises the cured material, wherein the membrane is retracted.

20. The vent assembly of claim 19, wherein the vent assembly has a decreased acoustic insertion loss standard deviation when installed in a device compared to vent assemblies not formed by the method of claim 18 installed in the device.21 . The vent assembly of claim 20, wherein the vent assembly has a decreased acoustic insertion loss standard deviation, whilst at least maintaining the insertion loss of the vent assembly compared to vent assemblies not formed by the method of claim 18.

22. The vent assembly of claim 19 to claim 21 , wherein the vent assembly has a decreased insertion loss compared to vent assemblies not formed by the method of claim 18.

23. The vent assemblies of claim 21 , wherein the vent assembly has a standard deviation from the mean insertion loss of less than 5 dB as measured at 1 kHz.

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