Sound-permeable cover or protective cover arrangement for acoustic equipment

Abstract

Sound-permeable cover (120) for an acoustic device (102), wherein the cover comprises: a shrunk membrane material having an acoustic insertion loss of less than 6 dB at 1 kHz and a water inlet pressure (WEP) of at least 9.8 kPa, wherein the shrunk membrane material has a microstructure comprising serpentine fibrils and wherein the serpentine fibrils have a width of about 1.0 micron or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONSThis application claims priority to U.S. Provisional Application No. 62 / 411,470, filed Oct. 21, 2016.TECHNICAL FIELDThe present disclosure relates generally to acoustic membranes. More particularly, but not by way of limitation, this disclosure relates to a protective cover assembly that includes a shrunk membrane material.BACKGROUNDAcoustic cover technology is used in many applications and environments to protect sensitive components of acoustic devices from environmental conditions. Various components of an acoustic device function best when they do not contact foreign objects, water, or other contaminants from the outside environment. In particular, acoustic transducers (e.g. microphones, loudspeakers) can react sensitively to soiling. For these reasons, it is often necessary to enclose working parts of an acoustic device with an acoustic cover.Known acoustic protective covers include nonporous films and porous membranes such as expanded polytetrafluoroethylene (ePTFE). Acoustic protective covers are also described in U.S. Pat. No. 6,512,834 B1 or DE 600 21 079 T2 and U.S. Pat. No. 5,828,012 A. A protective cover can transmit sound in two ways: one, by transmitting sound waves, known herein as a resistive protective cover, and the other, by vibrating to generate sound waves, referred to herein as a vibroacoustic or reactive protective cover.Japanese Patent Publication No. 2015-142282 A discloses a waterproof component provided with a waterproof sound transmitting sheet. On the surface of at least one side of the waterproof sound-transmitting sheet, a support layer is adhered. The backing layer is a foam of a polyolefin resin system having a loss modulus of less than 1.0×10 7 Pa.U.S. Pat. No. 6,188,773 B1 discloses a waterproof type microphone including a microphone housing provided with a unit accommodation chamber having a sound receiving opening portion, a microphone unit accommodated in the unit accommodation chamber, and a waterproof membrane air-tightly mounted on the sound receiving opening portion.The U.S. Publication No. 2014 / 0270273 A1 discloses a system and a method for controlling and adjusting a low-frequency response of a MEMS microphone. The MEMS microphone includes a membrane and a plurality of vent openings. The membrane is configured such that sound pressures acting on the membrane cause movement of the membrane.The U.S. Publication No. 2015 / 0163572 A1 discloses a loudspeaker or a microphone module which contains an acoustic membrane and at least one pressure vent.US 2014 / 0138181 A1 discloses a further sound-permeable membrane which is also porous.US 2014 / 0048351 A1 discloses a PTFE membrane which is in turn porous in order to be as transparent as possible to sound waves.WO 2013 / 10937 A1 relates to an expanded polyetherfluoroethylene membrane with serpentine fibrils. The serpentine fibrils impart high extensibility to the membrane while maintaining the strength of the membrane. US 2013 / 0183515 A1 discloses a further fluoropolymer membrane comprising serpentine fibrils.A continuing problem is that many acoustic cover membranes prove unsuitable for some environments. For example, increasing the resistance of a membrane in an acoustic protection assembly to water ingress may reduce the ability of the assembly to properly transmit sound.BRIEF SUMMARY OF SOME EXEMPLARY EMBODIMENTSEMBODIMENTSAccording to one embodiment of the present invention, a protective cover assembly for an acoustic device is disclosed. The protective cover assembly includes a housing having an opening for passing sound waves between an exterior of the housing and an acoustic cavity therein; and a sound transmissive cover connected to the housing and separating the acoustic cavity and the exterior of the housing.According to a further embodiment, a sound-permeable cover for an acoustic device comprises a shrunk membrane. The shrunk membrane has an acoustic insertion loss of less than 6 dB at 1 kHz and resists water ingress, e.g., with a water ingress pressure (WEP) of at least 9.8 kPa. The shrunk membrane material has a microstructure comprising serpentine fibrils. The serpentine fibrils have a width of about 1.0 micron or less.In certain embodiments, the sound transmissive membrane may have an ATEQ airflow rate of at least 10 L / hr per 2.0 cm 2 working surface area of the membrane.In various embodiments, the sound transmissive membrane of the protective cover assembly includes an expanded fluoropolymer membrane. In some specific embodiments, the sound transmissive membrane is an ePTFE membrane or an ePTFE composite. In cases where the sound-transmissive membrane is formed by shrinkage (i.e., thermal shrinkage), the sound-transmissive membrane of a shrunk membrane material may have an area ratio of less than 75% of an associated precursor membrane.In at least one embodiment, a protective, sound transmissive cover for an acoustic device is disclosed. The sound transmissive cover may have a microstructure including serpentine fibrils. In some cases, the sound-transmissive membrane may have an inner diameter of 1.6 mm or more. In some embodiments, the sound-transmissive membrane may be less than 1.6 mm, e.g. less than 1.5 mm, e.g. less than 1.25 mm, e.g. less than 1 mm, or even less. In some embodiments, the sound transmissive membrane may have an insertion acoustic attenuation of, for example, less than 6 dB at 1 kHz, less than 5 dB at 1 kHz, less than 4 dB at 1 kHz, less than 3 dB at 1 kHz, or less than 1 dB at 1 kHz. In various embodiments, the sound transmissive membrane may have an ATEQ airflow rate of at least 10 L / hr per 2.0 cm 2. In some embodiments, the sound transmissive cover is an expanded fluoropolymer membrane. The expanded fluoropolymer membrane may have an insertion acoustic attenuation of less than 4 dB at 1 kHz, less than 3 dB at 1 kHz, or less than 1 dB at 1 kHz.In another embodiment, a sound transmissive cover includes an expanded fluoropolymer membrane. The expanded fluoropolymer membrane has an acoustic insertion loss of less than 6 dB at 1 kHz and resists water ingress, e.g., with a water ingress pressure (WEP) of at least 9.8 kPa. Further, the expanded fluoropolymer membrane has a microstructure comprising serpentine fibrils. The serpentine fibrils have a width of about 1.0 micron or less. The membrane may have an area density greater than or equal to 6 g / m 2 and a thickness less than 15 μm, in accordance with some embodiments. The sound-transmissive cover may have an area density greater than or equal to 7 g / m 2 and a thickness less than 15 μm. The expanded fluoropolymer membrane may have an ATEQ air flow rate of at least 15 L / hr per 2.0 cm 2. In certain embodiments, the membrane may be water impermeable to at least a pressure of 9.8 kPa.In at least one embodiment, a sound transmissive membrane for an acoustic device is disclosed that comprises a shrunk membrane material. The shrunk membrane material may have an acoustic insertion loss of less than 5 dB at 1 kHz and a WEP of at least 9.8 kPa. In some embodiments, the shrunk membrane material may have an ATEQ air flow rate of at least 10 L / hr per 2.0 cm 2. In certain embodiments, the shrunk membrane material may have an area ratio of less than 0.75:1 (i.e., less than 75%) relative to an original area of a precursor membrane. In some embodiments, the shrunk membrane material may be thermally shrunk. In some other embodiments, the shrunk membrane material may be solvent shrunk.In some embodiments, the shrunk membrane material has a mass per area of at least 6.9 g / m 2 and a thickness of less than 16 μm. The shrunk membrane material may be air permeable and water impermeable. For example, the shrunk membrane material may be water impermeable at least up to a pressure of 9.8 kPa.These and other embodiments, as well as many of their advantages and features, will be described in more detail in connection with the following description and the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will be better understood with reference to the accompanying, non-limiting figures. FIG. 1 shows a cross-sectional view of an acoustic device with a protective acoustic cover assembly including a shrunk membrane material, according to embodiments; FIG. 2 shows a series of scanning electron microscope (SEM) images showing a shrunk membrane material precursor, the shrunk membrane material precursor in a shrunk state, and the shrunk membrane material in a partially relaxed state, in accordance with some embodiments; FIG. 3 shows an SEM image of a first comparative example of a membrane; FIG. 4 shows an SEM image of a second comparative example of a membrane; FIG. 5 shows an SEM image of a first example of a shrunk membrane material, in accordance with some embodiments; FIG. 6 shows an SEM image of a second example of a shrunk membrane material, in accordance with some embodiments; FIG. 7 shows an SEM image of a third example of a shrunk membrane material according to embodiments; FIG. 8 is a side sectional view illustrating a test arrangement for testing acoustic diaphragm performance characteristics; and FIG. 9 is a graph graphically illustrating insertion loss (i.e., the difference in sound pressure level versus a non-populated microphone) for various embodiments of an acoustic protective cover.DETAILED DESCRIPTIONVarious embodiments described herein relate to an acoustic protective cover assembly for an acoustic device. More particularly, some embodiments herein relate to a shrunk acoustic membrane including a membrane material having serpentine fibrils.The acoustic protective cover assembly includes a shrunk acoustic membrane that protects against moisture and water ingress. In one embodiment, an acoustic cover comprises a shrunk membrane material for high depth of dip applications. Advantageously, the membrane provides moisture resistance and protects the acoustic device from possible damage from the external environment. The sound transmissive membrane assemblies as described herein have improved sound transmission performance with comparable or improved protection properties compared to conventional assemblies that do not use shrunk membrane materials.Protective Cover AssemblyFIG. 1 shows a cross-sectional view of a protective cover assembly 100 for an acoustic device 102, according to embodiments. The acoustic device 102 may be an electronic device for generating and / or receiving acoustic waves, e.g., at a transducer 104. An acoustic cavity 114 adjacent transducer 104 directs acoustic waves toward or away from the transducer. In some embodiments, transducer 104 may be a microphone or other acoustic sensor, a speaker, a pressure sensor, or other comparable type of sensor. In some embodiments, transducer 104 may be a micro-electric mechanical (MEMs) device, such as a microphone, acoustic sensor, or acoustic speaker. The acoustic device 102 may be an electronic circuit board, such as a flexible circuit, on which the transducer 104 is provided. In some embodiments, the acoustic device 102 may be a sensor module or control circuit for a portable electronic device, such as a mobile phone, smart phone, tablet, portable microphone, handheld computing device, or other comparable device.The acoustic device 102 is at least partially enclosed by a housing 106 protecting the acoustic device 102 from an external environment 108, and may be at least partially sealed and / or watertight. The housing 106 may be a plastic or metal housing. The housing 106 includes an interior environment 110 that at least partially surrounds the acoustic device 102.A sound path 112 is partially defined by an opening 116 in the housing 106. Although a single aperture is shown in FIG. 1, in other embodiments there may be a plurality of apertures in the housing that collectively define a sound path or paths. The opening 116 in the housing 106 serves to pass sound waves between the exterior environment 108 of the housing 106 and an acoustic cavity 114 therein. In an embodiment, the sound path 112 is configured such that pressure waves, i.e., acoustic waves, may propagate from the exterior of the housing 106 to the transducer 104 of the acoustic device 102 when sound is detected. Similarly, in other embodiments, the sound path 112 is configured to allow pressure waves generated by the acoustic device 102 to propagate to the exterior of the housing 106. The acoustic path 112 is traversed by a shrunk membrane material 120 that further defines an acoustic cavity 114. Because the shrunk membrane material 120 crosses the sound path 112, the shrunk membrane 120 may also be referred to herein as a sound transmissive membrane or acoustic membrane. Acoustic cavity 114 is disposed between shrunk membrane material 120 and a portion of acoustic device 102 that includes transducer 104. To ensure sufficient acoustic coverage, the minimum diameter of the shrunk membrane material 120 is at least as large as or greater than the maximum diameter of the aperture 116. The maximum diameter of the opening 116 may vary depending on the application and construction of the housing 106. The acoustic protection arrangement is suitable for any opening size and is not particularly limited. In an exemplary embodiment, the diameter of the aperture 116 ranges from 0.1 mm to 500 mm, e.g., 0.3 mm to 25 mm, e.g., 0.5 mm to 10 mm. Based on these exemplary diameters of the opening, the minimum diameter of the shrunk membrane material is at least 0.1 mm, e.g. at least 0.3 mm, e.g. at least 0.5 mm, e.g. at least 1.0 mm, e.g. at least 1.5 mm or e.g. at least 1.6 mm. Such a size relationship allows the shrunk membrane material 120 to fully traverse the acoustic path 112 and prevent the ingress of liquid or moisture into the acoustic cavity 114. The interior environment 110 of the housing 106 is also at least partially sealed from ingress of liquid or moisture from the exterior environment by the shrunk membrane material 120.The shrunk membrane material 120 may be secured between the acoustic device 102 and the housing 106, for example, by a first layer 118 (or layers) and a second layer 122 (or second layers). The first and second layers 118, 122 may include any suitable means for attaching a membrane, such as adhesive layers (i.e., double-sided adhesive layers), breathable layers for promoting air exchange between the acoustic cavity 114 and the interior environment 110 of the housing 106 (e.g., the second layer or layers 122 may include a breathable layer). In some embodiments, the shrunk membrane material 120 may be held between the housing 106 and the acoustic device 102 by mechanical clamping force with or without the aid of adhesives. In certain embodiments, the first layer 118 may be an extension of the housing 106.In some embodiments, a total thickness of the shrunk membrane material 120 and the first and second layers 118, 122 may be on the order of about 25 μm to about 2500 μm. In an embodiment, a total thickness of the stack may be on the order of about 100 μm to less than 1000 μm. Without limitation, in some example applications, an acoustic device may be used in combination with a comparatively small thickness MEMs transducer, e.g., on the order of 100 μm to 1000 μm. Thus, an acoustic device including the shrunk membrane material 120 may be very thin, on the order of 0.2 to 1.2 mm, which is suitable for use in many small form factor applications, such as portable electronic devices.In some embodiments, the shrunk membrane material may be a layer of polymeric composite material, which may alternatively be nonporous, microporous, or porous. Various nonporous materials that can be used in a shrunk membrane material can include polymeric films (e.g., TPU, PET, silicone, polystyrene block copolymer, FEP, and the like) or polymeric composites. Porous materials, which may include expanded polytetrafluoroethylene (ePTFE) materials and ePTFE composite materials, provide a balanced ratio of acoustics and water protection. Various porous and nonporous materials have excellent acoustic transmission and provide excellent water protection as well as very low thickness and light weight. In some cases, a membrane material may be treated. For example, a membrane material may include an oil repellent coating, e.g., by an oil repellent polymer, before or after the membrane material is shrunk.In various embodiments, the shrunk membrane material has a microstructure including serpentine fibrils. As used herein, the term "serpentine fibrils" means multiple fibrils that first flex or rotate in one direction and then into another. The serpentine fibrils may generally have a width of about 1.0 micron or less. The serpentine fibrils may be connected by nodes. Serpentine fibrils can be formed, for example, by controlled shrinkage, as described in the publication US 2013 / 018351 A1. Controlled shrinkage can be achieved by causing articles to shorten in length in at least one direction by the action of heat, wetting with a solvent, or by other suitable means or combinations thereof to prevent the buckling, folding or wrinkling of the resulting article from being detectable by the naked eye. A shrunk membrane may be formed by shrinking a precursor membrane to convert a substantial portion of fibrils contained therein to serpentine fibrils. In some cases, articles shrunk in accordance with the teachings of the present disclosure may require pulling apart in the shrink direction to make up the serpentine fibrils.In one embodiment, the shrunk membrane may have a thickness of no more than 100 μm, no more than 50 μm, or no more than 20 μm. In some embodiments, the shrunk membrane may have a thickness of no more than 16 μm, no more than 14 μm, no more than 12 μm, or no more than 10 μm. The shrunk membrane is sufficiently thick to resist bursting under pressures caused by fluctuating external pressure and / or temperature within the acoustic cavity, while at the same time being sufficiently thin to minimally impede acoustic energy passing through the shrunk membrane. A shrunk membrane is sufficiently thick to resist excessive deformation of the membrane that would impair acoustic performance. The areal density (i.e. mass per unit area) of a shrunk membrane may vary depending on the embodiment and may be, for example, less than 15 g / m 2, less than or equal to 12 g / m 2 or less than or equal to 10 g / m 2.Adhesive layers such as the first and second layers 118, 122 may be formed from any suitable layer having an adhesive surface on each side for joining two parts. Thus, an adhesive layer can be, for example, a polymer layer which has been impregnated with adhesive within the scope of a surface treatment, similar to a two-sided plastic tape. Adhesive layers may include a double-sided self-adhesive tape comprising a PET backing and an acrylic tacky adhesive (e.g., TESA® 4972). Adhesive layers may have different thicknesses depending on the desired thickness of a pressure compensation arrangement. Exemplary adhesive layers may take any thickness on the order of 5 to 1000 μm. In certain embodiments, multiple adhesive layers may be overlaid to provide additional height, e.g., to increase the volume of the acoustic cavity 114, to increase offset between the housing 106 and the shrunk membrane material 120, or both. Specific examples of adhesive layers are about 30 μm thick or 48 μm thick. Generally, an adhesive layer is waterproof and nonporous. In one embodiment, an adhesive layer adjacent to the external environment is waterproof, while other adhesive layers may also be waterproof.The present invention will be better understood from the following non-limiting examples and test results.Example MembranesFIG. 2 shows a series 200 of SEM images showing a shrunk membrane material precursor 202, the shrunk membrane material precursor in a shrunk state ( 204), and the shrunk membrane material in a partially relaxed state ( 206), in accordance with some embodiments of the present disclosure. Precursor 202 is a microporous expanded functional tetrafluoroethylene polymer (TFE) copolymer or an ePTFE copolymer having a microstructure characterized by nodes interconnected by fibrils similar to the functional TFE copolymers described, for example, in U.S. Patent 3,953,566.It will be understood that comparable fibrillated polymeric materials may also be suitable precursors for a shrunk membrane material, including membrane materials containing a greater or lesser ratio of nodes to fibrils, or having widely differing microstructures, as described, for example, in U.S. Pat. Nos. 5,814,405 and 4,902,423. The specific precursor membrane 202 is explained in more detail below with reference to Comparative Example 2, which is also shown in FIG. 4. This membrane is available from W. L. Gore and Associates under part number GAW 333.Precursor 202 may be shrunk using an elevated temperature while controlling the degree of shrinkage, thereby forming a thermally shrunk membrane. For example, the shrunk state 204 may be achieved by shrinking the precursor 202 to an area ratio of about 0.58:1 relative to the original area of the precursor 202 while heating at 300° C. Shrinkage deforms fibrils to assume a serpentine structure and appearance.A precursor 202 may be shrunk by methods other than thermal shrinkage to form a membrane with serpentine fibrils. For example, in some embodiments, precursors may be shrunk by applying a solvent (e.g., isopropyl alcohol), which is then allowed to dry out of the membrane while not constrained, forming a solvent shrunk membrane.The fully shrunk precursor 204 may also be partially relaxed or restretched by applied tension. The partially relaxed, shrunk membrane material 206 shown in FIG. 2 was partially shrunk to an area ratio of about 0.575:1 relative to the original precursor 202.Further examples of precursor membranes and shrunk membranes are explained below with reference to Figures 3-7 and Table 1.Comparative Example 11,005,116,410,126,1Example 10,746,912,916,83,4Example 20,73714,217,33,9Example 30,3813,31011,93,8Example 40,569,111,4202,4Example 50,4511,31415,23,6Example 60,618,41416,54,1Comparative Example 21,001,2Example 70,586,91218,50,7The end face / original area ratio is a measure of the shrinkage of a membrane compared to its respective precursor membrane. The ATEQ air flow rate is a measure of the air permeability of a membrane, as measured by the A TEQ Air Flow Test described below, in liters of air flow through a particular membrane per unit time. In each case, the specimens were diaphragms formed into circular acoustic covers with inner diameters of 1.6 mm. Insertion loss is a measure of the degree by which a particular membrane obstructs the passage of sound through the membrane as measured in an insertion loss detection test as described below. The insertion loss is measured as the difference in sound pressure detected by a test assembly due to the presence of blockage by the membrane. Comparative Examples 1 and 2 are non-shrunk membrane materials also used herein as precursors for the various example membranes. The example membranes of Examples 1 to 17 are shrunk membrane materials prepared by shrinking a precursor membrane.Formation of the serpentine fibrils can be accomplished by, but is not limited to, thermally induced controlled shrinkage of expanded polytetrafluoroethylene (ePTFE), wetting the article with a solvent such as isopropyl alcohol or Fluorinert® (a perfluorinated solvent commercially available from 3M, Inc., St. Paul, MN), or a combination of these two techniques.Comparative Example 1:FIG. 3 shows an SEM image of the first comparative example 300 of a membrane. The first comparative example membrane 300 was prepared according to U.S. Patent No. 7,306,729 B2 with a mass per unit area of 5.2 g / m 2, a bubble point of 924 kPa, an ATEQ air flow rate of 10.1 L / hr through 2.0 cm 2 at a differential pressure of 12 mbar, a thickness of 16.4 microns, and longitudinal and cross matrix tensile strengths of 314 MPa and 419 MPa, respectively.Comparative Example 2:FIG. 4 shows an SEM image of a second comparative example 400 of a membrane. The second membrane serving as a comparative example was produced according to the document U.S. Pat. No. 3,953,566 A, as explained above with reference to the precursor membrane 202 which was described with reference to FIG. 2.Example 1:FIG. 5 shows an SEM image of a first example 500 of a shrunk membrane material, in accordance with some embodiments. The first shrunk membrane material example 500 was prepared using the first comparative example 300 as a precursor. The precursor was biaxially held in a frame and placed in an oven at 320°C for 30 seconds, shrunk transversely to an area ratio of about 0.74:1 with respect to its original area, and then cooled to room temperature. The resulting shrunk membrane material 400 had a mass per unit area of 6.9 g / m 2, an ATEQ air flow rate of 16.8 L / hour, and a thickness of 12.9 microns.In particular, the first shrunk membrane material example 500 has improved ATEQ air flow rates compared to its precursor, the first comparative example 300. At the same time, the first example 500 showed a significantly reduced insertion loss of 3.4 dB compared to an insertion loss of 6.1 dB of the precursor. These results indicate that improvements were achieved in both sound transmission (inverse of insertion attenuation) and air flow (ATEQ airflow) simultaneously. This result is particularly surprising because it would normally be expected that an increased density of the membrane would be associated with a loss of airflow performance and / or a loss of sound transmission.Example 2:FIG. 6 shows a SEM image of a second example 600 of a shrunk membrane material, in accordance with some embodiments. The second shrunk membrane material example 600 was prepared using the first example membrane 500 as a precursor. The precursor was shrunk to an area ratio of about 0.73:1 relative to its original area. The resulting second example membrane 600 showed a mass per unit area of 7.0 g / m 2, an ATEQ air flow rate of 17.3 L / hour, and a thickness of 14.2 microns.The second example membrane 600 achieved similar performance to the first example membrane 500. For example, the second example membrane 600 achieved an even higher ATEQ air flow rate than the first example membrane 500 (17.3 L / hr vs 16.8 L / hr) with only slightly increased insertion loss (3.9 dB vs 3.4 dB). Both performance characteristics are significantly improved over the original precursor, the first membrane 300 serving as a comparative example.Example 3: Example 3:A third example of shrunk membrane material was derived from a precursor based on the first comparative example 300. The third example membrane was shrunk by an area ratio of 0.38:1 in the transverse direction at a temperature of 360° C. This gave a membrane having a mass per unit area of 13.3 g / m 2, an ATEQ air flow rate of 11.9 L / hour, and a thickness of 10 microns. The third example of shrunk membrane material achieved an insertion loss of 3.8 dB and an ATEQ air flow rate of 11.9 L / hr.Example 4: Example 4:A fourth example of shrunk membrane material was derived from a precursor based on the first comparative example 300. The fourth example membrane was shrunk by an area ratio of 0.56:1 in the transverse direction at a temperature of 360° C. This gave a membrane having a mass per unit area of 9.1 g / m 2, an ATEQ of 20 L / hour and a thickness of 11.4 microns. The fourth example of shrunk membrane material achieved an insertion loss of only 2.4 dB and an ATEQ air flow rate of 20.0 L / hr.Example 5: Example 5:A fifth example of shrunk membrane material was derived from a precursor based on the first comparative example 300. The fifth example membrane was shrunk at a temperature of 320° C. by an area ratio of 0.45:1 in the transverse direction. This gave a membrane having a mass per unit area of 11.3 g / m 2, an ATEQ of 15.2 L / hour and a thickness of 14 microns. The fifth example of shrunk membrane material achieved an insertion loss of 3.6 dB and an ATEQ air flow rate of 15.2 L / hr.Example 6:A sixth example of shrunk membrane material was derived from a precursor based on the first comparative example 300. The sixth example membrane was shrunk at a temperature of 320° C. by an area ratio of 0.61:1 in the transverse direction. This gave a membrane having a mass per unit area of 8.4 g / m 2, an ATEQ air flow rate of 16.5 L / hour, and a thickness of 14 microns. The fifth example of shrunk membrane material achieved an insertion loss of 4.1 dB and an ATEQ air flow rate of 16.5 L / hr.Example 7:FIG. 7 shows an SEM image of a seventh example 700 of a shrunk membrane material according to embodiments. The seventh example membrane 700 is different from Examples 1-6 in that it is derived from a precursor based on the second comparative example membrane 400 as shown in FIG. 4. The seventh example membrane 700 is also shown with its precursor in FIG. 2 (e.g., precursor 202, shrunk precursor 204, and shrunk membrane material 206). As described above with reference to FIG. 2, the seventh example membrane 700 is shrunk while heating to 320° C. to an area ratio of 0.57:1 with respect to its precursor membrane. The seventh example membrane 700 was shrunk in the transverse direction to give a membrane material having a mass per unit area of 6.9 g / m 2, an ATEQ air flow rate of 18.5 L / hr, and a thickness of 12 microns. As with previous examples of shrunk membrane materials, the seventh example membrane 700 achieved improvements in insertion loss over its precursor membrane (0.7 dB vs. 1.2 dB) at 18.5 L / hr airflow performance in the ATEQ airflow test.Test MethodsTest for Detecting Insertion LossFIG. 8 illustrates an exemplary test arrangement 800 for detecting insertion loss. Insertion loss can be detected by connecting each example membrane (comparative example membranes and shrunk membrane materials) to an opening 848 of a specimen holder plate 842, fully enclosing the assembly, and then measuring sound generated by a speaker after passing through the opening and the assembly.Each test specimen 802 was formed into a circular acoustic cover having an inner diameter of 1.6 mm, for example, via an adhesive ring 804, and applied over a circular opening 848 having a diameter of 1 mm to a test specimen holder plate 842 via an adhesive layer 844. The test specimen was placed at a distance of 6.5 cm from an internal driver or loudspeaker in a reflection-free test box B&K type 4232. A MEMS sensing microphone 846 type Knowles® SPA2410LR5H was attached to the probe holder plate 842, e.g., by soldering.The loudspeaker was excited to generate an external stimulus 806 at 1 Pa sound pressure (94 dB SPL) over the frequency range of 100 Hz to 11.8 kHz. The sensing microphone 846 measured the acoustic response as a sound pressure level in dB over the frequency range. The readings were obtained with the aperture 848 uncovered, i.e., without the test specimens 802 present, to calibrate the loss detection test. The measurement values were obtained by incorporating each respective example as test specimen 802.FIG. 9 is a graph 900 graphically illustrating the insertion losses for Comparative Examples 1-2 and Examples 1-7 described above and tabulated in Table 1. The graph 900 shows the insertion loss (i.e., the difference in sound pressure level versus a non-populated microphone) at a frequency of 1 kHz for the various embodiments of a pressure compensation arrangement described above with reference to FIGS. 2-7 and in conjunction with a test arrangement similar to the test arrangement 800 shown in FIG. 8. Diagram 900 shows error bars reflecting a standard deviation in the measurement of insertion loss.ATEQ Airflow TestATEQ airflow is a test method for measuring laminar airflows through membrane specimens. For each example, a test piece in membrane form having an area of 2.0 cm 2 was prepared. The specimens were clamped between two plates such that the specimens were sealed across a flow path. A Premier D Compact Flow Tester from ATEC Corp. was used to measure the air flow rate (L / hr) through each acoustic membrane test specimen by inducing it to pass through the flow path via a differential pressure of 1.2 kPa (12 mbar).Testing of Water Inlet Pressure (WEP)The WEP was determined in a similar manner to the ATEQ Airflow Test by exposing the specimens in a test set to water at differential pressures of 34.5 and 500 kPa across a flowpath.Bubble Point TestBubble point was determined with a capillary flow porometer, CFP-1500, from Porous Materials Inc.Thickness: ThicknessSpecimens were measured with a digital micrometer Keyence LS-7010M.The invention has now been described in detail for the sake of clarity and understanding. However, it will be understood by those skilled in the art that certain changes and modifications may be made within the scope of the appended claims.In the foregoing description, for purposes of explanation, numerous details have been set forth in order to provide an understanding of the various embodiments of the present invention. However, it will be apparent to those skilled in the art that certain embodiments may be practiced without some of these details or with additional details.Having disclosed several embodiments, it will be apparent to those skilled in the art that various modifications, alternative constructions and equivalent solutions may be used without departing from the spirit of the embodiments. Moreover, a number of well-known approaches and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the foregoing description should not be taken as limiting the scope of the present invention or the claims.When a range of values is given, it will be understood that any intermediate value up to the smallest fraction of the unit of the lower limit, unless the context dictates otherwise, between the upper and lower limits of this range is also expressly disclosed. Any narrower range between any stated values or nonstated intermediate values in a particular range and any other stated or intermediate value in that particular range is included. The upper and lower limits of these smaller ranges may be independently included in or excluded from the range, and any range in which either one, none, or both limits are included in the smaller ranges is also encompassed by the present invention subject to a specifically excluded limit in the specified range. When the specified range includes one or both of the limits, ranges except one or both of these included limits are also included.As used herein and in the appended claims, the singular forms "a", "an" and "the / s" also include plural referents unless the context dictates otherwise. Also, the words "comprise," "comprising," "includes," "including," "comprising," and "comprises," when used in this specification and in the following claims, are intended to specify the presence of stated features, integers, components, or steps, but do not preclude the presence or addition of one or more other features, integers, components, steps, operations, or groups.Further examples are described below in order to facilitate understanding of the disclosure:E1. A sound transmissive cover for an acoustic device, the cover comprising: an expanded fluoropolymer membrane having a microstructure comprising serpentine fibrils, the expanded fluoropolymer membrane having an insertion acoustic attenuation of less than 6 dB at 1 kHz and a water entry pressure (WEP) of at least 9.8 kPa.E2. The covering of Example 1, wherein the expanded fluoropolymer membrane has an ATEQ air flow rate of at least 10 L / hr per 2.0 cm 2.E3. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an insertion acoustic attenuation of less than 5 dB at 1 kHz.E4. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an insertion acoustic attenuation of less than 4 dB at 1 kHz.E5. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an insertion acoustic attenuation of less than 3 dB at 1 kHz.E6. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an insertion acoustic attenuation of less than 1 dB at 1 kHz.E7. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an areal density greater than or equal to 6 g / m 2 and a thickness less than 15 μm.E8. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an areal density greater than or equal to 7 g / m 2 and a thickness less than 15 μm.E9. The covering of any preceding example, wherein the expanded fluoropolymer membrane has an ATEQ air flow rate of at least 15 L / hr per 2.0 cm 2.E10. A waterproof, sound-transmissive cover for an acoustic device, the cover comprising: a shrunk membrane material having an acoustic insertion loss of less than 6 dB at 1 kHz and a WEP of at least 9.8 kPa.E11. The cover of Example 10, wherein the shrunk membrane material has an ATEQ air flow rate of at least 10 L / hr per 2.0 cm 2.E12. The covering of any preceding example, wherein the shrunk membrane material has an area ratio of less than 0.75:1 relative to an original area of a precursor membrane.E13. The cover of any preceding example, wherein the shrunk membrane material has a mass per area of at least 6.9 g / m 2 and a thickness of less than 16 μm.E14. The cover of any preceding example wherein the shrunk membrane material is air permeable.E15. The cover of any preceding example, wherein the shrunk membrane material includes a thermally shrunk membrane.E16. The cover of any of Examples 1-14, wherein the shrunk membrane material includes a solvent shrunk material.E17. The covering of any preceding example, wherein the shrunk membrane material has an area ratio of less than 75% of an associated precursor membrane.E18. The covering of any preceding example, wherein the shrunk membrane material includes an expanded fluoropolymer membrane.E19. The covering of any preceding example, wherein the shrunk membrane material has a microstructure including serpentine fibrils.E20. A protective cover assembly for an acoustic device, the assembly comprising: a housing having an opening for passing acoustic waves between an exterior of the housing and an acoustic cavity therein; and a sound transmissive membrane connected to the housing and separating the acoustic cavity and the exterior of the housing, wherein the shrunk membrane, e.g., sound transmissive membrane, has an acoustic insertion loss of less than 6 dB at 1 kHz and a water entry pressure (WEP) of at least 9.8 kPa.E21. The assembly of Example 20, wherein the sound transmissive membrane has an ATEQ airflow rate of at least 10 L / hr per 2.0 cm 2.E22. The assembly of any preceding example, wherein the sound transmissive membrane has an acoustic insertion loss of less than 5 dB at 1 kHz.E23. The assembly of any preceding example, wherein the sound transmissive membrane includes an expanded fluoropolymer membrane.E24. The assembly of any preceding example, wherein the sound transmissive membrane includes an ePTFE membrane.E25. The assembly of any preceding example, wherein the sound transmissive membrane comprises a shrunk membrane material having an area ratio of less than 75% of an associated precursor membrane.E26. The assembly of any preceding example, wherein the sound transmissive membrane material is thermally shrunk.E27. The assembly of any of Examples 20-25, wherein the sound transmissive membrane material is solvent shrunk.E28. The assembly of any preceding example, wherein the sound transmissive membrane has a microstructure comprising serpentine fibrils.E29. The assembly of any preceding example, further comprising: an acoustic device connected to the acoustic cavity, wherein the acoustic device is capable of generating and / or receiving the acoustic waves.

Claims

A sound transmissive cover (120) for an acoustic device (102), the cover comprising: a shrunk membrane material having an acoustic insertion loss of less than 6 dB at 1 kHz and a water entry pressure (WEP) of at least 9.8 kPa, wherein the shrunk membrane material has a microstructure comprising serpentine fibrils, and wherein the serpentine fibrils have a width of about 1.0 micron or less.The cover (120) of any preceding claim, wherein the shrunk membrane material has an ATEQ airflow rate of at least 10 L / hr per 2.0 cm 2, preferably at least 15 L / hr per 2.0 cm 2.The cover (120) of any preceding claim, wherein the shrunk membrane material has an acoustic insertion loss of less than 5 dB at 1 kHz, preferably less than 4 db at 1 kHz, more preferably less than 3 dB at 1 kHz, and even more preferably less than 1 dB at 1 kHz.The cover (120) according to any one of the preceding claims, wherein the shrunk membrane material has an area density greater than or equal to 6 g / m 2, preferably greater than or equal to 7 g / m 2 and a thickness less than 15 μm.The cover (120) according to any one of the preceding claims, wherein the cover (120) is watertight.The cover (120) of any preceding claim, wherein the shrunk membrane material has an area ratio of less than 0.75:1 relative to an original area of a precursor membrane.The cover (120) of any preceding claim, wherein the shrunk membrane material has a mass per area of at least 6.9 g / m 2 and a thickness of less than 16 μm.The cover (120) of any preceding claim, wherein the shrunk membrane material is air permeable.The cover (120) of any preceding claim, wherein the shrunk membrane material comprises a thermally shrunk membrane.The cover (120) of any preceding claim, wherein the shrunk membrane material comprises a solvent shrunk material.The cover (120) of any preceding claim, wherein the shrunk membrane material comprises an expanded fluoropolymer membrane.A protective cover assembly (100) for an acoustic device (102), the assembly comprising: a housing (106) having an opening (116) for passing acoustic waves between an exterior (108) of the housing (106) and an acoustic cavity (114) therein; and the sound transmissive cover (120) of any preceding claim.The assembly of claim 12, further comprising: an acoustic device (102) coupled to the acoustic cavity (114), wherein the acoustic device (102) is capable of generating and / or receiving the acoustic waves.A sound transmissive cover (120) for an acoustic device (102), the cover comprising: an expanded fluoropolymer membrane having a microstructure including serpentine fibrils, wherein the expanded fluoropolymer membrane has an insertion acoustic attenuation of less than 6 dB at 1 kHz and a water entry pressure (WEP) of at least 9.8 kPa, and wherein the serpentine fibrils have a width of about 1.0 micron or less.

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